15:282 Principles of Genetics - Term Test #3

A man is found to be karyotypically 47, XYY. The presence of an extra Y chromosome most likely results from

Nondisjunction in a paternal meiocyte at meiosis II

If a trait is always transmitted through the phenotypically normal heterozygous mother, often shows skipping generation, and an affected male never transmits the trait to his sons then the trait is

X-linked recessive

It has been found that 220 different alleles exist at a certain locus of the human genome. How many alleles for this gene would be found in a somatic cell of each person

2

How does polyploidy differ from aneuploidy

Polyploidy involves changes in entire chromosome sets while aneuploidy involves less than the entire set of chromosomes

The Philadelphia chromosome is produced by ___ and the resulting Leukemia is caused ___

Translocation, position effect

ABO blood group expression is an example of

Dominance, codominance and multiple alleles

Dr Smith parents have normal hearing. However, Dr Smith has inherited a form of deafness. Deafness is a recessive trait that is associated with the abnormal allele. The allele associated with normal hearing is dominant. Parents could have which of the following genotypes

Dd and Dd

Which offspring will inherit all their mitochondrial DNA from their mother and none from their father

Both sons and daughters

If a rare genetic disease is inherited as a X-linked dominant gene, one would

Affected fathers always have 100% affected daughters

An allotetraploid is crossed to one of the parental species and a sterile progeny is produced. The chromosomal makeup of this progeny will be represented by

2 complete sets of this parental species and a single set of the other species

For loci carried on the differentiated region of the X chromosome, the reciprocal crosses will

Always give different phenotypic ratios among the offspring in the F1 and F2

What are the results of paracentric inversion with single crossing over in the loop

2 normal, one dicentric and one acentric chromosome

A trait that is passed on from affected father to all sons and grandsons, but never to the daughters or granddaughters, must be

Y-linked

Sometimes one gene pair will interact so as to control the expression of a second gene pair in an interaction that is GENERALLY called

Interallelic interaction

A polyploid organism, that receives all of its chromosomal sets from the same species is an

Autopolyploid

What is the mechanism that ensures the law of segregation

Segregation of homologous chromosomes during meiosis I

Genes that are present in hemizygous condition will

Always be passed on to all the males if present on the Y chromosome

An extra finger in humans is rare but is due to a dominant gene. When one parent is normal and the other parent has an extra finger but is heterozygous for the trait, what is the probability that the first child will be normal

50%

Many scientists believe that the evolution of multigene families, such as the genes for hemoglobin, is a result of which type of genetic rearrangement

Duplication

What is the name of the offspring produced when two monosomic gametes (for the same chromosome) fuse during fertilisation

Nullisomic

A phenocopy occurs when

An individual develops a phenotype that is similar to a condition caused by mutant genes but is actually caused by environmental factors and does not have a genetic basis

A Waardenburg syndrome is an autosomal dominant disorder in which patients may exhibit a variety of clinical features, including patches of prematurely grey hair, white eyelashes, a broad nasal root, and moderate to severe hearing impairment. Occasionally, affected individuals display two eyes of different colours and a cleft lip and/or palate. Patients who possess a mutation in the PAX3 gene on chromosome 2 can present with all of these disparate signs and symptoms. Which of the following characteristics of genetic traits is illustrated by this example

Pleiotropy

In an inter chromosomal non reciprocal translocation, a chromosome segment is

Transferred to a region on a different chromosome without a corresponding exchange of genetic material

Males tend to inherit more sex linked conditions because

There is no corresponding allele on their Y chromosomes

Four individuals in a family of five, that has a history of a rare genetic disease, are homozygous recessive for the gene responsible for the disease. Only three of the members had disease symptoms. This indicates that the gene is

Incompletely penetrant

If an individual has 10 gene pairs, how many different gametes can be formed if 5 of the gene pairs are homozygous and the remaining 5 gene pairs are heterozygous

32

If nondisjunction occurs in one of the cells during meiosis II during gametogenesis, what will be the result at the completion of meiosis

¼ of the gametes will be n + 1, ¼ will be n - 1, and ½ will be n

Compared to autopolyploids, allopolyploids are more common in nature because

Chromosomes are more likely to pair as bivalents in allopolyploids

The conversion of a diploid karyotype of 8 acrocentric chromosomes to a diploid karyotype of 4 long metacentric chromosomes would most likely be due to

Robertsonian translocations

A heterozygous chromosomal inversion can be detected

• Through genetic studies indicating a decrease in the frequency of recombination

• By observing a decrease in the number of viable progeny

• By cytogenetic observation of loops in chromosome during prophase I

Genetic Information in Prokaryotes

• Can be transferred vertically and horizontally

Vertical Gene Transfer (VGT)

• Is the transfer of genetic material from parent cell to daughter cell

  • From 1 generation to the next

Horizontal Gene Transfer (HGT)

• Is the transfer of DNA from a donor cell to a recipient cell

  • From same generation

Types of Horizontal Gene Transfer

• Conjugation = Cell-to-cell contact

  • Pilus

• Transduction = By bacteriophages

  • Viruses move DNA from one cell to another

• Transformation = From the environment

  • Plasmids from the environment

Conjugation

• In which genetic information from one bacterium is transferred to another and recombines with the second bacterium’s DNA

Conjugation in E. coli (General)

• Based on the presence of the F plasmid

  • F+ cells contain the plasmid

  • F- cells do not

• F+ cell produce an F pilus that connects it to an F- cell

• Transfer of the F plasmid occurs through the conjugation bridge

  • End result is two F+ cells

Conjugation in E. coli (Steps)

1. F factor directs synthesis of F pilus

2. F+ and F- cells are connected by conjugation tube

   • Bridge is created

3. Single-strand of F- factor DNA is transferred to F- cell by rolling circle replication. Single DNA strand is replicated in F- cell

4. Conjugation tube breaks: F- cell is converted to F+

Conjugation - Integration

• The process by which a mobile genetic element called an integrative and conjugative element is incorporated into the chromosome of a recipient cell

Conjugation - Excision

• Circular form of a conjugation transposon is excised from a bacterial chromosome

Hfr Strain

• Is transferred linearly

• Gene order and distance between genes can be predicted

Interrupted Mating

• Technique used in genetics to map the order genes on a bacterial chromosome

Interrupted Mating (Process)

• Bacterial conjugation is interrupted at specific times during the process of transferring genetic information from a donor cell to a recipient cell

Interrupted Mating Arrow Meaning

• Indicates the order of transfer of Hfr strain

• Flat end - Latest in the transfer process

• Pointed end - Earliest in the transfer process

Types of Transformation in Conjugation

• Natural transformation

• Artificial transformation

Natural Transformation

• Occurs in many bacterial species

  • Including Streptococcus which was studied by Griffith

• DNA that is released from a dead cell is picked up by another live cell

Artificial Transformation

• Accomplished in the lab

• Used to transform E. coli for molecular cloning

• Performed by restriction enzymes

• Bacteria have a system to degrade foreign DNA that does not have benefits or homology

Transformation Steps

1. Donor DNA fragments bind to proteins on cell surface

2. Donor DNA fragments is transported into host cell and becomes single-stranded

3. (Successful) - Single-stranded donor DNA fragment is incorporated into host chromosome

3. (Unsuccessful) - Degradation of donor DNA fragment

Making Transgenics

• Includes Ti-plasmids

  • Microprojectile method/Bombardment

  • Microinjection method

  • Electroporation

Microprojectile Method/Bombardment (Making Transgenics)

• Particles created with DNA and satiate cells

Microinjection Method (Making Transgenics)

• Two arms (move finely)

  • One suction arm

  • One needle arm

Electroporation (Making Transgenics)

• Current runs through tube with DNA and cells present

  • Causes DNA to enter cells

Transduction (Conjugation)

• Generalised

  • Occurs via accidents in the lytic cycle

  • Viruses package bacterial DNA and transfer it in a subsequent infection

  • Virtually any gene can be transferred

• Transfer of genetic material through viruses

Bacteriophages

• Are viruses that infect only bacteria

• Exhibit two reproductive cycles

  • Lytic cycle & Lysogenic cycle

Bacteriophage Lytic Cycle

• Adsorption = viral attachment

• Penetration = viral entry

• Synthesis = viral components are made

  • Uses bacteria’s facilities to synthesise its own DNA

• Assembly = components are put together

  • Phage protein and DNA

• Release = viral exit

• APSAR

Bacteriophage Lytic Cycle Steps

1. Phage is adsorbed to bacterial host cell

2. Phage DNA is injected; host DNA is degraded

3. Phage DNA is replicated; phage protein components are synthesised

4. Mature phages are assembled

   • (Alternate 4. for Defective phage) Mature phages are assembled and released

5. Host cell is lysed; phages are released

Lytic Cycle (Bacteriophages)

• Virus kills the host cell

Lysogenic Cycle (Bacteriophages)

• Virus incorporates into the cell’s genome

  • Is why you can be infected for many years without showing symptoms

• Virus integrates into cellular genome as a prophage

• Propagates along with host genome

• Resulting cell is called a lysogen

Lytic Phages

• Are called virulent

Lysogenic Phages

• Are called temperate

Induction

• The switch from the lysogenic cycle to the lytic cycle

Phage Conversion

• Occurs when the prophage alters the bacterial phenotype

  • Vibrio cholerae toxin is viral-encoded

Blending Hypothesis

• Is the idea that genetic material from the two parents blends together

• Blending implies that offspring have a simple mixture of the parent’s characteristics

Pangenesis Theory of Darwin

• Concept of “gemmules” from each organ accumulating in pollen/sperm and egg

Blending Inheritance

• That “hereditary determinants” (genes) were found in gametes

• Physical traits from both parents are blended and the offspring will exhibit an intermediate trait between the two

Particulate Inheritance

• That both hereditary determinants remain in fertilised zygote

• Both parental traits will be exhibited in the offspring

Particulate Hypothesis

• Is the idea that parents pass on discrete heritable units (genes)

  • Mendel documented a particulate mechanism through his experiments with garden peas

Wild Type

• The most common form of any property of an organism, that which is found “in the wild”, or in nature

Mutants

• Heritable variants observed in an organism that differs from the wild type

• Organisms having some abnormal form of a property

Phenotypic Pairs Studied by Mendel

• Round or wrinkled ripe seeds

• Yellow or green seeds

• Purple or white petals

• Inflated or pinched ripe pods

• Green or yellow unripe pods

• Axial or terminal flowers

• Long or short stems

Two Types of Crosses

• Cross-pollination

• Selfing

Cross-Pollination (Mendel)

• Transfer of pollen with brush

• Removal of anthers

Selfing (Mendel)

• Transfer pollen to stigma

Monohybrid Cross

• Involve a single pair of contrasting traits

• Original parents are the P generation, and their offspring are the F1 generation

• Offspring arising from selfing in the F1 generation are the F2 generation

Reciprocal Crosses

• Yield the same outcome

Testcross

• Crossing the progeny back to a known recessive phenotype/genotype

• Is a type of backcross

• Is a way to determine whether an individual displaying the dominant phenotype is homozygous or heterozygous for that trait

Backcross

• Is a genetic breeding technique that involves mating a hybrid organism with one of its parents or a genetically similar organism

Law of Segregation

• Two members of a gene pair segregate from one another into gametes such as that ½ of the gametes carry one member of the pair, while the other ½ of the gametes carry the other member of the pair

Dihybrid Cross

• Involves two pairs of contrasting traits

Law of Independent Assortment

• During gamete formation, segregating pairs of unit factors assort independently of each other

Chromosomal Theory of Inheritance

• States that genes are found at specific locations on chromosomes, and that the behaviour of chromosomes during meiosis can explain Mendel’s laws of inheritance

  • Correlation of Mendel’s postulates with the behaviour of chromosomes formed the foundation of modern transmission genetics

Fork-Line Method

• Visual tool that helps predict the genotypes and phenotypes of offspring from crosses with multiple genes

Chi-Square Analysis

• Is used to test how well the data fit the null hypothesis

• Evaluates the influence of chance on genetic data

  • Chance deviation from an expected outcome is diminished by larger sample sizes

Null Hypothesis

• Assumption that data will fit a given ratio

• Assumes that there is no real difference between the measured values (or ratio) and the predicted values (or ratio)

  • Apparent difference can be attributed purely to chance

Forward Genetics

• A strategy to understanding biological function starting with random single-gene mutants and ending with their DNA sequence and biochemical function

Reverse Genetics

• Starts with genomic analysis at the DNA level to identify a set of genes as candidates for encoding the biological property of interest. Then induces mutants targeted specifically to those genes, and then examines the mutant phenotypes to see if they indeed affect the property under study

Types of Intra-Allelic Interactions

• Incomplete dominance

• Codominance

• Multiple alleles

• Lethal alleles

• Pleiotropy

• Environmental effects

• Penetrance/Expressivity

Intra-Allelic Interactions

• Occurs between alleles of different genes

  • E.g. Coat colour phenotype in mice

Epistasis

• A gene at one locus alters the phenotypic expression of a gene at a second locus

  • E.g. In mice and other mammals, coat colour depends on two genes

  • One gene determines the pigment colour

  • The other determines whether the pigment will be deposited in the hair

Recessive Suppression

• A type of epistasis, where a mutant suppressor allele is recessive to a wild type suppressor allele

Dominant Suppression

• A type of epistasis where a single allele suppresses the expression of another gene

Incomplete Dominance

• Genetic phenomenon where two alleles blend to create a new phenotype, or a mix of two parental traits

• Also known as “partial dominance” or “semi-dominance”

Codominance

• A type of inheritance where two different versions of a gene (alleles) are expressed equally and produce different traits in an organism

  • This means that neither allele is dominant or recessive, and both traits appear/expressed

• E.g. Blood type, Sickle cell, etc

Multiple Alleles

• Term used in genetics to describe a gene that has more than two versions, or alleles

Lethal Alleles

• Are alleles that cause the death of an organism that carries them

• Are usually the result of mutations in genes that are essential for growth or development

• Can be dominant, recessive, conditional, perinatal, or postnatal. Depends on the gene or genes involved

Pleiotropy

• A genetic phenomenon where a single gene influences multiple traits or characteristics

• A single locus affects two or more apparently unrelated phenotypic traits and is often identified as a single mutation that affects two or more wild-type traits

Penetrance

• Refers to the proportion of people with a particular genetic variant who exhibit signs and symptoms of a genetic disorder

  • E.g. Some people with a mutation in the BRCA1 or BRCA2 gene will develop cancer, others will not

Expressivity

• The degree to which a genotype is expressed as a phenotype in an individual

• Qualitative measurement that describes the differences in a phenotype between individuals with the same genotype

  • E.g. Polydactyly - Presence of extra toes in cats. Is dominant but varies widely from cat to cat because the expression of tissues surrounding the developing toe determines the degree of severity

Complete Penetrance

• When a genotype always results in the associated phenotype

  • E.g. Huntington’s disease is 100%, meaning that everyone who has the disease-causing variant will develop the disease

Variable Penetrance

• When some people with a genetic variant do not develop the associated disease, while others do

Variable Expressivity

• Refers to the range of signs and symptoms that can occur in different people with the same genetic condition

• Can cause a trait to range from mild to severe

  • E.g. People with neurofibromatosis type 1 may have mild symptoms like freckling, while others may have more severe symptoms like brain tumours