3- Genetics (Questions)

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3.1 Define the term gene.

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3.1 Define the term gene.

A gene is a heritable factor that consists of a length of DNA and influences a specific characteristic.

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3.1 Outline the relationship between a gene and a chromosome.

A gene occupies a specific position on a chromosome; this specific position is called locus.

Genes can be linked into groups, and each group = one type of chromosome.

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3.1 Define alleles, and outline how it is formed.

Alleles are the various specific forms of a gene.

New alleles are formed by mutation, and they differ from each other by one or only a few bases.

Most animal have 2 copies of each type of chromosome, and each copy may have same or different alleles; but only one allele can occupy the locus of a gene on a chromosome.

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3.1 Outline the definition of genome.

The genome is the whole of the genetic information of an organism.

The size of a genome is therefore the total amount of DNA in one set of chromosomes in that species. It can be measured in millions of base pairs of DNA.

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3.1 Application: List the number of genes of one plant, one bacterium, one species with more genes and one with fewer genes than a human.

**The number of genes in a species should not be referred to as genome size as this term is used for the total amount of DNA.

Estimated number of protein-coding genes in humans is 21 000.

Escherichia Coli (Bacteria): less genes than humans

~4 200

Oryza Sativa (Rice): plant, more genes than humans

~38 000

Gallus gallus (Chicken): animal, less genes than humans

~1 700

Daphina pulex (water flea): animal, more genes than humans

~31 000

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3.1 Explain the causes of sickle cell anemia.

The cause of sickle cell anaemia is due to the base substitution mutation in the DNA.

-in DNA sense strand gene that codes for hemoglobin protein, GAG is being mutated to GTG (thymine substituted adenine)

-which then codes for valine instead of glutamic acid on the SIXTH amino acid.

-this causes a change to the base sequence of mRNA transcribed from it and a change to the sequence of a polypeptide in hemoglobin.

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3.1 Outline the Human Genome Project and its outcome.

The Human Genome Project began in 1990 with the aim of determining the complete sequence of the human genome and identifying every gene that it contains.

Gene sequencers is a technique used in gene sequencing. The sanger process is used, and fluorescent markers are used to label the DNA fragments in order to find out the order of the DNA sequences.

An optical detector is used to detect the colours of fluorescence along the lane. There is a series of peaks of fluorescence, corresponding to each number of nucleotides, and a computer is used to deduce the base sequences.

Outcomes of the HGP:

knowledge of location of human genes / position of human genes on chromosomes;knowledge of number of genes/interaction of genes / understanding the mechanism of mutations;

evolutionary relationships between humans and other animals;

discovery of proteins / understanding protein function / detection of genetic disease;

leads to the development of medical treatment/enhanced research techniques;

knowledge of the base sequence of genes/study of variation within genome;

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3.2 Distinguish between prokaryotic and eukaryotic chromosomes.

Prokaryotes have one chromosome consisting of a circular DNA molecule, they reproduce asexually through binary fission. Some prokaryotes also have plasmids but eukaryotes do not. Plasmids are used to transfer genetic information from one bacteria to another. They are also used in laboratories to genetically modify a prokaryote.

Eukaryote chromosomes are linear DNA molecules associated with histone proteins.

In a eukaryote species there are different chromosomes that carry different genes, with both coding and non-coding DNA.

Eukaryotes have different types of chromosome with 2 alleles of each type.

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3.2 Describe what homologous pairs are in relationship to diploid and haploid nuclei.

Homologous chromosomes carry the same sequence of genes but not necessarily the same alleles of those genes. A same type of chromosome can be identified by its length and shape (have same length and same position of centromere).

Diploid nuclei have pairs of homologous chromosomes; they have 2 types of chromosomes, meaning they have 2 genes copies (alleles) for each trait. A somatic cells are diploid and divide by mitosis.

Haploid nuclei have one chromosome of each pair, as they only possess a single copy (one allele) for each trait. Sex cells are haploid and they divide by meiosis.

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3.2 State why chromosome number and type is a distinguishing characteristic of a species.

The number of chromosomes is a characteristic feature of members of a species.

In order to reproduce, the species have to have the same number of chromosomes in order to form homologous pairs in zygotes.

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3.2 Describe the process of creating a karyogram, and its uses.

-a karyogram shows the chromosomes of an organism in homologous pairs of decreasing length.

-a cell is "frozen" in metaphase by the application of chemicals that disrupt the mitotic spindle.

-a hypotonic solution is added;

-water enters the cell causing it to swell and burst, separating the chromosomes from each other.

-the chromosomes are stained and viewed with a microscope.

-the images of the chromosomes are then organized in a standard pattern, from longest chromosomes to the smallest;

-with heterosomes at the end

Karyograms can be used to deduce sex and diagnose Down syndrome in humans. The 23rd pair of karyogram reveals the gender.

Down syndrome can be identified as such patients have 3 copies of chromosome 21 (trimosy 21).

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3.2 Distinguish between heterosome and autosomes.

Heterosomes are sex chromosomes, they are the 23rd pair of chromosomes. X is big and long, Y is small and short and contains SRY gene for development of male characteristics.

Heterosomes are homologous in females (XX) but not in males (XY)

Autosomes are chromosomes that do not determine sex (the rest of the somatic cells)

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3.2 Describe Cairns' technique for measuring the length of DNA molecules, his conclusion.

Autoradiography is used through the use of electron microscopes.

1. Allows bacterium to absorb 3H-Thymidine (Tritiated thymidine)

-contains tritium, a radioactive isotope of hydrogen, so radioactively labelled DNA was produced by replication in the E. coli cells.

2. Cells were then placed onto a dialysis membrane and their cell walls were digested using the enzyme lysozyme.

-cells were gently burst to release their DNA onto the surface of the dialysis membrane.

3. A thin film o photographic emulsion was applied to the surface o the membrane

-being left in darkness for weeks

-some o the atoms o tritium in the DNA decayed and emitted high energy electrons, which react with the film.

-each point where a tritium atom decayed there is a dark grain.

The film showed that prokaryotic chromosomes are circular, and the length and width of the chromosomes can be determined.


-chromosome in E. coli is a single circular DNA molecule with a length o 1,100 microm. (the E coli cells is only 2 microm!)

-prokaryotic chromosomes are circular

-measured the lengths of chromosomes.

-he also observed the DNA replication fork.

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3.2 Application: Comparison of genome size in T2 phage, Escherichia coli, Drosophila melanogaster, Homo sapiens and Paris japonica.

*genome size measured in # of base pairs

T2 phage:

170 000 bp

Escherichia coli:

4.6 million bp

Drosophilia melanogaster:

130 million bp

Homo Sapiens:

3.6 billion bp

Paris japonica:

150 billion bp

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3.2 Application: Comparison of diploid chromosome numbers of Homo sapiens, Pan troglodytes, Canis familiaris, Oryza sativa, Parascaris equorum.

Homo Sapiens:


Pan troglodytes:


Canis familiaris:


Oryza sativa:


Parascaris equorum:


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3.2 Skill: Use of databases to identify the locus of a human gene and its polypeptide product.


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3.3 Outline the process of meiosis.

a. meiosis reduces a diploid cell into (four) haploid cell(s);

b. (during prophase I) homologous chromosomes pair up/synapsis;

nuclear membrane degenerates

centrioles move to opposite poles

c. chromatids (break and) recombine / crossing over followed by condensation.

d. (metaphase I) (homologous chromosomes) at the equator of the spindle / middle of cell;

e. (anaphase I) (homologous) chromosomes separate and move to opposite poles;

f. (telophase I) chromosomes reach poles and unwind WTTE;

Separation of pairs of homologous chromosomes in the first division of meiosis halves the chromosome number.

g. (prophase II) chromosomes (condense and) become visible, new spindles form;

h. (metaphase II) chromosomes line up at the centre of the cells/equator;

i. (anaphase II) sister chromatids separate;

j. (telophase II) chromatids reach the poles and unwind;

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3.3 What happens prior to meiosis?

DNA is replicated before meiosis so that all chromosomes consist of two sister chromatids.

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3.3 Explain why meiosis is known as reduction division?

One diploid nucleus divides by meiosis to produce four haploid nuclei. The halving of the chromosome number allows a sexual life cycle with fusion of gametes to form a zygote with 46 chromosomes (not more or less)

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3.3 Explain how sexual reproduction can lead to variation in a species.

allows characteristics from both parents to appear in offspring;

crossing over (during prophase 1) changes chromosome composition;

produces gametes which are all different;

random chance of which sperm fertilizes ovum;

greater variation (resulting from sexual reproduction) favours survival of species through natural selection;

random orientation of homologous pairs during metaphase I.

Accept independent assortment during meiosis from AHL.

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3.3 Application: Explain how non-disjunction can cause Down syndrome and other chromosome abnormalities.

Non-disjunction is when chromosomes fail to separate in in meiosis I / chromatids in meiosis II / anaphase II;

This causes a sex cell to have one less or one more chromosomes, which causes the zygote to have 47 or 45 chromosomes.

Down syndrome can be determined through identifying the trisomy on chromosome 21 on karyogram.

Increased probability with increased age of mother/ages of parents after 35 maternal age

There is a strong correlation between maternal age and occurence of non-disjunction events.

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3.3 Application: Description of methods used to obtain cells for karyotype analysis.

Chorionic villus:

-a sampling that enters through the vagina is used to obtain cells from the chorion

-one of the membranes from which the placenta develops.

-the tissue from placenta is collected by entering a tube through the cervix.

-this can be done earlier in the pregnancy than amniocentesis, but whereas the risk of miscarriage with amniocentesis is 1%, with chorionic villus sampling it is 2%.


-involves the removing of amniotic liquid that surrounds the baby through a long needle collected through the mother's abdomen.

-involves passing a needle through the mother's abdomen wall, using ultrasound to guide the needle.

-the needle is used to withdraw a sample of amniotic fluid containing fetal cells from the amniotic sac.

The miscarriage percentage for the two are:

1% amniocentesis and 2% for chorionic villus.

**pre-natal diagnosis by karyotype analysis is usually only carried out in mothers over 35

-until then the risk of miscarriage caused by the procedure is greater than the risk of Down Syndrome.

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3.3 Skill: Drawing diagrams to show the stages of meiosis resulting in the formation of four haploid cells.


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3.4 Outline why Mendel's success is attributed to his use of pea plants.

Mendel discovered the principles of inheritance with experiments in which large numbers of pea plants were crossed.

-his success was due to him obtaining numerical values, rather than just descriptions of outcomes.

-Mendel's use of peas allowed for the observation of easily distinguishable characteristics (i.e. yellow or green pods).

-Also, the peas were able to reproduce quickly allowing for many generations of data to be collected.

-Lastly, the reproduction could be controlled, so Mendel knew exactly which two parent plants were being bred (either cross-bred or self-pollination).

From his experiment he discovered the presence of dominant and recessive alleles through artificial pollination of purebred pea plants.

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3.4 Explain the relationship between meiosis and inheritance.

The two alleles of each gene separate into different haploid daughter nuclei during meiosis.

Gametes are haploid so contain only one allele of each gene.

Fusion of gametes results in diploid zygotes with two alleles of each gene that may be the same allele (homozygous) or different alleles (heterozygous)

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3.4 Explain dominant and recessive allele in inheritance.

Dominant alleles mask the effects of recessive alleles but co-dominant alleles have joint effects; which means (pair of) alleles that both affect the phenotype when present in a heterozygote / both alleles are expressed;

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3.4 List out genetic diseases that are due to autosomal dominant, autosomal recessive, co-dominant, and sex linked.

Autosomal dominant:

Huntington's Disease

Autosomal Recessive:

cystic fibrosis


Sickle cell anemia

ABO Blood groups

Sex linked:

haemophilia (recessive)

red-green color blindness (recessive)

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3.4 Describe patterns that can be seen regarding diseases caused by autosomal dominant, autosomal recessive, and sex linked.

Autosomal dominant:

-every affected individual have at least one affected parent

-present in every generation

-present in both males and females

Autosomal recessive:

-cases where both parent are not affected

-skips generation

-present in both males and females

Sex linked:

-more common in males

-can only inherit from parent of opposite gender

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3.4 Explain the rarity of genetic diseases.

-often times genetic diseases seem to just "appear" in a family without prior history.

-this is usually because the disease is caused by a recessive allele that has been masked by dominant alleles.

-if two carriers, who show no disease symptoms, produce offspring, there is a 1/4 change of the offspring showing the disease characteristics.

Many genetic diseases have been identified in humans but most are very rare.

Most are rare because severe diseases that are caused by homozygous alleles may not survive until reproduction age so they cannot be passed on.

Recessive conditions tend to be more common and dominant conditions.

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3.4 List and explain the factors that increase the mutation rate and can cause genetic diseases and cancer, and apply it to the consequences of nuclear bombing of Hiroshima and accident in Chernobyl.

Radiation and mutagenic chemicals increase the mutation rate and can cause genetic diseases and cancer.


-the high energy wavelengths can have enough energy to cause chemical changes in DNA.

Chemical substances:

-smoke and mustard gas that possesses chemical can change DNA.

-causing thyroid disease after Chernobyl due to release of radioactive iodine.

-250% increase in congenital abnormalities

-Reduced T cell counts and altered immune functions, leading to higher rates of infection

-caused variation in flora and fauna in Chernobyl

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3.5 Explain how gel electrophoresis and polymerase chain reaction are used in DNA profiling.

Gel electrophoresis is used to separate proteins or fragments of DNA according to size, and PCR can be used to amplify small amounts of DNA.

1. DNA (specifically the short tandem repeats) is first cut into smaller, separate fragments by the endonuclease.

2. DNA needs to be copied/amplified for DNS profiling.

3. placed in a block of gel where electric current is applied (different fragments will move different distances because it is negatively charged, and each fragment has different size/weight)

So smaller samples travel faster and further.

4. DNA profiling: the banding patterns of a person's DNA can be identified (unique to each individual).

5. Comparing DNA profiles can allow paternity and forensic investigations.

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3.5 Explain how gene modification is carried out.

Genetic modification is carried out by gene transfer between species (the placement of a gene from one species into another and have it expressed).

It is possible because the genetic code is universal.

Gene transfer to bacteria using plasmids makes use of restriction endonucleases and DNA ligase.

1. DNA is isolated from cell via centrifugation and then amplified by PCR.

**Bacterial plasmids are commonly used as vectors (DNA molecule that is used as a vehicle to carry the gene of interest) because they are capable of autonomous self-replication and expression.

2. Restriction endonuclease cleave the sugar-phosphate backbone to generate blunt ends or sticky ends.

3. The gene of interest is inserted into a plasmid vector that has been cut with the same restriction endonucleases

4. DNA ligase joins the vector and gene by fusing their sugar-phosphate backbones together with a covalent phosphodiester bond.

5. The recombinant construct (including the gene of interest) is finally introduced into an appropriate host cell or organism

6. Transgenic cells, once isolated and purified, will hopefully begin expressing the desired trait encoded by the gene of interest.

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3.5 Define what clones are, and the production of cloned embryos.

Clones are groups of genetically identical organisms, derived from a single original parent cell.

Cloned embryos produced by somatic-cell nuclear transfer (nuclear transplantation)- reproductive cloning (not therapeutic cloning with stem cells)

1. Somatic cells removed from adult donor and are cultured.

2. Unfertilised egg is removed from female adult (enucleated - haploid nucleus is removed)

3. Enucleated egg fuses with diploid nucleus from adult donor, forming a diploid egg cell.

4. An electric current is then delivered to stimulate the egg to divide and develop into an embryo

5. The embryo is then implanted into the uterus of a surrogate and will develop into a genetic clone of the adult donor.

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3.5 Describe some natural methods of cloning.

Many plant species and some animal species have natural methods of cloning.

-bacteria reproduce via binary fission - asexual reproduction.

-plants reproduce asexually via:

-stem cutting: a separated portion of plant stem that can regrow into a new independant clone.

-budding: cells split off the parent organism, generating a smaller daughter organism which eventually separates from the parent

e.g. Strawberry plants send out stolons, also known as runners, which are horizontal projections that have new plants on the end that can grow into cloned daughter plants.

-vegetative propagation: small pieces can be induced to grow independently

-identical twins are due to the natural separation of embryo - monozygotic

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3.5 Describe and explain the two methods of cloning in animals.

Animals can be cloned at the embryo stage by breaking up the embryo into more than one group of cells.

-pluripotent cells are separated artificially in the laboratory, each group of cells will form cloned organisms

-separation of embryonic cells can also occur naturally to give rise to identical (monozygotic) twins

-separated groups of cells are then implanted into the uterus of a surrogate to develop into genetically identical clones

-limited by the fact that the embryo used is still formed randomly via sexual reproduction and so the specific genetic features of the resulting clones have yet to be determined

-animals such as hydra create clones through a process of budding.

-a bud develops as an outgrowth due to repeated cell division at one specific site.

-these buds develop into tiny individuals and, when fully mature, detach from the parent body and become new independent individuals.

Methods have been developed for cloning adult animals using differentiated cells.

-involves somatic cell nuclear transfer (SCNT)

-replacing the haploid nucleus of an unfertilised egg with a diploid nucleus from an adult donor

-advantage: it is known what traits the clones will develop (they are genetically identical to the donor)

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3.5 Design of an experiment to assess one factor affecting the rooting of stem-cuttings.

Stem cuttings are typically placed in soil with the lower nodes covered and the upper nodes exposed, where meristematic cells are present to be induced for vegetative propagation.

There are a variety of factors that will influence successful rooting of a stem cutting:

-Cutting position:whether cutting occurs above or below a node, as well as the relative proximity of the cut

Length of cutting (including how many nodes remain on the cutting)

- Growth medium (whether left in soil, water, potting mix, compost or open air)

- The use and concentration of growth hormones

- Temperature conditions (most cuttings grow optimally at temperatures common to spring and summer)

- Availability of water (either in the form of ground water or humidity)

- Other environmental conditions (including pH of the soil and light exposure)

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3.5 Assessment of the potential risks and benefits associated with genetic modification of crops.

Check notebook!

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3.5 Analysis of data on risks to monarch butterflies of Bt crops.

Bt corn is a genetically modified maize that incorporates an insecticide producing gene froma bacterium.

This insecticide is lethal to certain types of larvae, particularly the European corn borer which would otherwise eat the crop.

Concerns have been raised that the spread of Bt corn may also be impacting the survival rates of monarch butterflies

-wind-borne pollen from Bt corn may dust nearby milkweeds, and monarch butterflies would die eating them.

Caterpillars exposed to Bt pollen were found to have eaten less, grew more slowly and exhibited higher mortality rates

Consider the problem with ecological validity in laboratory experiment:

-there were higher amounts of Bt pollen on the leaves than would be found naturally (e.g. rain would diminish build up)

-Larva were restricted in their diet (in the field, larva could feasibly avoid eating pollen dusted leaves)

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3.4 Explain the causes of cystic fibrosis and Huntington's disease.

Cystic fibrosis is one of the most common genetic diseases. The recessive allele was formed by a mutation in the CFTR gene, which codes for a chloride channel in mucous membranes. The gene has been mapped on chromosome 7 and is involved in the secretion of sweat, mucus and digestive juices.

Huntington's disease is a neurodegenerative disorder that usually starts to affect people between 30 and 50 years of age. It is caused by a dominant allele that has developed through the mutation of the HTT gene found on chromosome 4.

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3.5 Explain how DNA profiling is used in parental and forensic investigation.

from hair/blood/semen/human tissue;DNA amplified / quantities of DNA

increased by PCR/polymerase chain reaction;

satellite DNA/highly repetitive sequences are used/amplified;

DNA cut into fragments;

using restriction enzymes/restriction endonucleases;

gel electrophoresis is used to separate DNA fragments;

using electric field / fragments separated by size;

number of repeats varies between individuals / pattern of bands is unique to the individual/unlikely to be shared;

forensic use / crime scene investigation;

example of forensic use e.g. DNA obtained from the crime scene/victim compared to DNA of suspect / other example of forensic use;

paternity testing use e.g. DNA obtained from parents in paternity cases;

biological father if one half of all bands in the child are found in the father;

genetic screening;

presence of particular bands correlates with probability of certain phenotype / allele;

other example;

brief description of other example;

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3.3 Define meiosis.

Reduction division of diploid nucleus to produce 4 haploid nuclei.

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3.4 State the genotype for all 4 types of blood.

Blood A:

-homozygous: I^A I^A

-heterozygous: I^A i

Blood B:

-homozygous: I^B I^B

-heterozygous: I^B i

Blood AB:

-ONLY heterozygous: I^A I^B

Blood O:

-ONLY homozygous: ii

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3.4 State the gametes for sickle cell anaemia alleles.

dominant allele (no sickle cell gene)


recessive allele (with sickle cell gene)



Hb^A Hb^S

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3.1 Define gene locus.

A gene locus is the location of a gene on a chromosome. Each chromosome carries many genes.

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3.1 Describe an example of a gene with multiple alleles.

Nearly all genes have multiple alleles (multiple versions). For example, in humans the ABO blood type is controlled by a single gene, the isoagglutinogen gene (I for short).

The I gene has three common alleles:

I^A: codes for antigen type A

I^B: codes for antigen type B

i: codes for no antigen

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3.1 State similarities between alleles of the same gene.​

-found at the same locus on homologous chromosomes

-have mostly the same nucleotide sequence and code for the same general type of protein

(for examples the A and B alleles for blood type both code for a membrane embedded protein).

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3.1 State the difference between alleles of the same gene.​

-slightly different from each other in the sequence of nucleotides.

-they can vary by just one base (i.e. A -->T), called a single nucleotide polymorphism (SNP) or by the insertion or deletion of a base.

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3.1 Describe a base substitution mutation.

A gene mutation is a change in the nucleotide sequence of a section of DNA coding for a specific trait.

The new allele that results from the mutation might result in:

Missense - cause one amino acid in the protein coded for by the gene to change

Silent - have no effect on the protein coded for by the gene

Nonsense - code for an incomplete, non-functioning polypeptide for form.

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3.1 State the size in base pairs of the human genome.

The human genome is composed of about 3.2 billion base pairs divided amongst nucleus chromosomes and mitochondrial DNA.

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3.1 Define "sequence" in relation to genes and/or genomes.

Sequence (noun): the order of the nitrogenous bases in a gene or genome.

"The sequence of the gene is ATCCGTA."

Sequence (verb): the process of determining the order of the nitrogenous bases in a gene of genome.

"We are going to sequence the gene to test for a genetic disease."

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3.1 State the aim of the Human Genome Project.

The main aims of the Human Genome Project were to determine the sequence of the ≈ 3.2 billion base pairs and identify the location of the ≈ 20-25 thousand genes in the human genome.

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3.1 Outline the consequences of the sickle cell mutation on the impacted individual.

-sickle cells are destroyed rapidly in the bodies of people with the disease

-causing anemia, a condition in which there aren't enough healthy red blood cells to carry adequate oxygen to the body's tissues.

-anemia results in fatigue and weakness.

-the sickle cells also block the flow of blood through vessels

-resulting in lung tissue damage that causes acute chest syndrome, pain episodes and stroke.

-it also causes damage to the spleen, kidneys and liver.

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3.1 State the number of genes in the human genome.

There are an estimated 20,000-25,000 genes in the human genome.

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3.1 Describe the relationship between the number of genes in a species and the species complexity in structure, physiology and/or behavior.​

In general, eukaryotes have more genes than prokaryotes. However, within plants and animals there is little correlation between complexity and the number of genes.

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3.1 Explain which gene types are often used to assess the differences in the base sequences of a gene between two species.

-genes that are present in the species being studied must be selected.

-for example, the COX1 gene (which codes for a protein involved in cellular respiration) is present in the majority of eukaryotic species so it is a good choice for comparing sequences between species.

-additionally, the gene has been sequenced for many species and is therefor accessible in genome databases.

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3.1 Outline the use of a computer software tool to create an alignment of the gene sequences between different species.

-a sequence alignment is a way of arranging DNA sequences

-so that similarities and differences between the sequences of different species can be identified.

-computer software programs are able to complete alignments quickly and accurately.

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3.1 Outline the technological improvement that sped the DNA sequencing process.

-the largest advancement in gene sequencing was the automation of the process with computer-assisted technology.

-what used to take humans hours or days can now be done by a computer much more rapidly, more accurately and for less money.

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3.2 Describe the structure and function of nucleoid DNA.​

In prokaryotic cells, the main DNA of the cell is collectively called the nucleoid.

Unlike in eukaryotic cells, the nucleoid DNA is not enclosed in a membrane.

The nucleoid DNA is a double helix that forms a circular loop and is not wrapped around histone proteins (termed "naked.")

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3.2 Compare the genetic material of prokaryotes and eukaryotes. ​

Prokaryotic DNA


-One chromosome


-Plasmids may be present

-No intron sequences

-Found in nucleoid region

-One origin of DNA replication

Eukaryotic DNA


-Multiple chromosomes

-Associated with histones

-No plasmids

-Intron sequences present

-Contained in membrane bound nucleus

-Multiple origins of DNA replication

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3.2 List similarities in the genetic material of prokaryotes and eukaryotes.

In both prokaryotic and eukaryotic cells:

-The DNA is double helix made of two anti

-parallel strands of nucleotides linked by hydrogen bonding between complementary base pairs.

-The replication of DNA is semi-conservative and depends on complementary base pairing.

-DNA is the genetic code for creating proteins through transcription and translation.

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3.2 Describe the structure and function of plasmid DNA.​

-plasmids are extra pieces of DNA found only in prokaryotic cells.

-like nucleoid DNA, plasmid DNA is circular and naked

-however plasmids are much smaller than the main nucleoid DNA

-plasmids replicate independently of the nucleoid DNA.

-plasmids are not found in all prokaryotic cells, can be shared between bacteria and often contain genes for antibiotic resistance.

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3.2 Describe the structure of eukaryotic DNA and associated histone proteins during interphase.

Eukaryotic DNA is linear and associated with histone proteins in a structure called the nucleosome.

During interphase, the DNA is not super-coiled into chromosomes; it is in a loose form called chromatin.

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3.2 Explain why chromatin DNA in interphase is said to look like "beads on a string."

The base unit of chromatin is the nucleosome, a structure composed of DNA wrapped around histone proteins. A chain of nucleosomes gives the appearance of "beads on a string."

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3.2 List three ways in which the types of chromosomes within a single cell are different.

Chromosomes within a cell are different in:

- size (as measured by the # of base pairs)

- the genes they carry

- the sequence of the nitrogenous bases

- the location of the centromere

- the banding pattern when stained

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3.2 State the number of nuclear chromosome types in a human cell.​

There are 24 types of human chromosomes. There are 22 autosomes and 2 types of sex chromosomes.

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3.2 Define homologous chromosome.

Homologous chromosomes a chromosome pair (one from each parent), that carry the same sequence of genes but not necessarily the same alleles of those genes.

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3.2 State a similarity and a difference found between pairs of homologous chromosomes.

Homologous chromosomes have:

-similar length

-the same genes at the same locus

-the majority of the same DNA base sequence

-the same centromere position

-will stain with the same pattern.

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3.2 Define "diploid."

Diploid mean that the cell contains two complete sets of the chromosomes, one chromosome originating from each parent.

Diploid nuclei have pairs of homologous chromosomes

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3.2 State an advantage of being diploid.​

-being diploid means there are two copies of each chromosome

-therefore two copies of each gene that the chromosome carries.

-so if one of the chromosomes carries a detrimental allele of a gene, there is a second copy of the gene

-whose allele may be able to counter the effects of the mutated version.

-essentially there is a "backup set of genes."

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3.2 Define haploid.

Haploid mean that the cell contains only one set of chromosomes; there are no homologous pairs.

Haploid nuclei have one chromosome of each pair

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3.2 Define "karyogram."

A karyogram is a micro-photograph of all chromosomes of an individual represented in a standard format.

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3.2 Outline the structure and function of the two human sex chromosomes.

The X chromosome is the larger of the two sex chromosomes (a length of about 156 million bp and 1805 genes).

The Y chromosome is much smaller (a length of 57 million bp and about 460 genes)

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3.2 Define "autosome."

An autosome is any chromosome that is not a sex chromosome.

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3.2 Describe the relationship between the genome size of a species and the species complexity in structure, physiology and behavior.​

There is a great variety of genome sizes. In general, eukaryotes have larger genomes than prokaryotes. However, the size of the genome and the number of genes do not appear to correlate to a species "complexity."

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3.2 State the minimum chromosome number in eukaryotes.

The minimum chromosome number in eukaryotes is 2n=2

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3.2 Explain why the typical number of chromosomes in a species is always an even number.

-because of sexual reproduction

-in which each parent gives one set of chromosomes

-resulting in an even number in the offspring.

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3.2 Explain why the chromosome number of a species does not indicate the number of genes in the species.

-its possible to have one large chromosome with many genes

-or many smaller chromosomes with fewer genes.

-likewise, it's possible to have large chromosomes with relatively few genes

-smaller chromosomes that are packed full of genes

-because genes refer to base sequences that would lead to the translation of proteins

-genome contains non-coding sequences that makes a part of the chromosome but not genes

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3.2 Outline the advancement in knowledge gained from the development of autoradiography techniques.​

-autoradiography is used to produce an image of a radioactive substance.

-the technique is used in cellular and molecular biology to visualize structures.

-for example, autoradiography can be used to visualize radioactively stained chromosomes

-bands in DNA electrophoresis gels, tissue samples and single cells.

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3.3 Compare divisions of meiosis I and meiosis II.​

-in meiosis I there is a transition from diploid to haploid

-while in meiosis II the nuclei contain only a haploid number of chromosomes.

-in meiosis I chromosomes remain replicated

-but in meiosis II chromatids of chromosomes are separated.

-meiosis I results in two haploid cells

-while meiosis II results in four haploid cells.

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3.3 List three events that occur in prophase 1 of meiosis.

1. Homologous chromosomes pair up with each other

2. A process called crossing over takes place

3. Chromatids with new combinations of alleles are produced

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3.3 Define bivalent and synapsis.

Bivalent: a pair of homologous chromosomes

Synapsis: the fusion of chromosome pairs in prophase 1 of meiosis

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3.3 Outline the process and result of crossing over.​

-a junction is created where one chromatid in each of the homologous chromosomes breaks

-re-joins with the other chromatid.

-crossing over occurs at random positions anywhere along the chromosomes.

-because a crossover occurs at precisely the same position on the two chromatids involved, there is a mutual exchange of genes between the chromatids.


-chromatids with new combinations of alleles are produced

-(because chromatids are homologous but not identical

-some alleles of the exchanged genes are likely to be different)

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3.3 Describe the attachment of spindle microtubules to chromosomes during meiosis I.

-after the nuclear membrane has been broken down, spindle microtubules attach to the centromeres of the chromosomes.

-aach chromosome is attached to one pole only (not both like in mitosis).

-the two homologous chromosomes in a bivalent are attached to different poles.

-the pole to which each chromosome is attached depends on which way the pair of chromosomes if facing.

-this is called the orientation.

-the orientation of bivalents is random, so each chromosome has an equal chance of attaching to each pole, and eventually being pulled to it.

-the orientation of one bivalent does not affect other bivalents.

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3.3 Describe random orientation of chromosomes during meiosis I.

The orientation of bivalents is random, so each chromosome has an equal chance of attaching to each pole, and eventually being pulled to it.

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3.3 Define non-disjunction

The failure of homologous chromosomes to separate at anaphase.

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3.3 Describe the cause and symptoms of Down syndrome.

Some symptoms include hearing loss, heart and vision disorders as well as mental and growth retardation.

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3.3 Discuss difficulties in microscopic examination of dividing cells.

Often no cells in meiosis are visible or the images are not clear enough to show details of the process. (Even with prepared slides made by experts it is difficult to understand the images as chromosomes form a variety of bizarre shapes during the stages of meiosis.)

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3.3 Describe the discovery of meiosis.

From the 1880s onwards a group of German biologists carried out careful and detailed observations of dividing nuclei that gradually revealed how mitosis and meiosis occur.

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3.4 Describe conclusions from Mendel's pea plant experiments.

Through selective breeding of pea plants, Mendel discovered:

-that certain traits show up in offspring without blending of the parent's characteristics.

Mendel observed seven traits:

flower color

stem length

seed color

pod color

flower position

seed shape

pod shape.

Mendel concluded:

1. genetic "units" of inheritance are passed from parents to offspring

2. the offspring inherits one "unit" from each parent for each trait.

3. the "unit" may be masked or hidden (i.e. recessive) in an individual but can still be passed on to the next generation.

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3.4 Define gamete, haploid, and zygote.

A gamete is a reproductive cell, egg or sperm. Gametes are haploid; containing a single set of unpaired chromosomes.

Haploid cells contain a single set of unpaired chromosomes and therefore only one allele of each gene.

The zygote is the diploid cell that results from the fusion of two haploid gametes during fertilization.

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3.4 State two similarities and two differences between male and female gametes

Both egg and sperm are haploid (23 chromosomes in humans) cells produced through meiosis.

-the egg and sperm are very different in size and shape. -eggs are large cells; sperm are much smaller.

-sperm have flagella, egg do not.

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3.4 State the maximum number of alleles in a diploid zygote.

Alleles are variations of a single gene. Although there usually are multiple alleles for a gene in the population, any single individual can only have a maximum of two alleles of a gene, one allele on each chromosome of a homologous pair.

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3.4 Define "dominant allele.", "recessive allele", and "co-dominant alleles".

Dominant alleles show their effect even if the individual is heterozygous, they can mask the presence of another allele.

Recessive alleles only show their effect if the individual has two copies (homozygous recessive), otherwise their presence can be masked by a dominant allele.

With codominant alleles, both alleles are expressed equally; there isn't masking of a recessive by a dominant allele.

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3.4 State the usual cause of one allele being dominant over another.

-the cause of allele dominance is complex and can vary between genes.

-in general, the dominant allele codes for a functioning proteins

-whereas the recessive allele codes for a less (or non-) functioning protein.

-sometimes the recessive allele is the "normal" or "healthy" version of the gene.

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3.4 Define "carrier" as related to genetic diseases.

A genetic carrier is an individual that has inherited a recessive allele of a gene but does not display the symptoms of the disease because they also have the dominant (normal functioning) allele. Carriers are heterozygous.

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3.4 Define sex linkage.

Sex linkage refers to genes located on the sex chromosomes, X or Y. The genes expression, inheritance pattern and effect on the phenotype will differ between males and females.

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3.4 Outline Thomas Morgan's elucidation of sex linked genes with Drosophila.

-Thomas Hunt Morgan studied genetics of fruit flies, Drosophila.

-he discovered sex-linked traits; traits that appear to associate differently in males and females.

-flies normally have red eyes, but there was a mutant male with white eyes.

-this white-eyed male was crossed with a red eyed female (P generation).

-all offspring (F1 generation) were red-eyed therefore red is dominant over white.

-then, two of the red-eyed offspring were crossed (F1 X F1).

-in the offspring (F2), only males had white eyes,

-suggesting that the eye-color allele is carried on the X-chromosome.

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3.4 Outline the effects of gene mutations in body cells and gamete cells.

-cell damage and death that result from mutations in somatic cells

-occur only in the organism in which the mutation occurred

-and are therefore termed somatic or non heritable effects.

-cancer is the most notable long-term somatic effect.In contrast, mutations that occur in germ line cells (which become gametes, sperm and egg)

-can be transmitted to future generations and are therefore called genetic or heritable effects.

-genetic effects may not appear until many generations later.

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3.4 Define "mutation" as related to genetic diseases and cancer.

A mutation is the permanent alteration of the nucleotide sequence of the genome of an organism.

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3.4 Define "mutagen", and how it affects cells.

A mutagen is a chemical or physical agent that causes mutations.Mutagens cause mutations in three different ways:

1. Some are mistakenly used as bases when new DNA is synthesized at the replication fork.

2. Some react directly with DNA, causing structural changes that lead to miscopying of the template strand when the DNA is replicated.

3. Some mutagens act indirectly on DNA. They do not themselves affect DNA structure, but instead cause the cell to synthesize chemicals that have a direct mutagenic effect.

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