Genetics Notes

DNA and Genetics

• DNA (Deoxyribonucleic Acid):

  • Hereditary material of the cell containing genetic instructions.

  • Essential for the development and functioning of proteins.

• DNA Structure:

  • Double helix structure, resembling a twisted ladder.

  • Composed of nucleotides (sugar + phosphate + base).

  • Backbone: Alternating sugar and phosphate groups.

  • Rungs: Bases that pair A-T (Adenine-Thymine) and C-G (Cytosine-Guanine).

• DNA Replication (Copying):

  • Occurs before cell division and must be accurate.

  • DNA unwinds, and strands separate.

  • New nucleotides pair with exposed bases following the complementary base-pairing rule (A-T, C-G).

  • Enzymes join nucleotides, and DNA recoils.

  • Semi-conservative replication: Half of the chromosome is original DNA, and half is new DNA.

• Chromosome:

  • Organized structure of DNA located in the nucleus of a cell.

• Gene:

  • Segment of DNA on a chromosome that codes for a specific protein, feature, or characteristic.

  • The base sequence determines the sequence of amino acids, which determines the protein made (triplet code).

• Alleles:

  • Alternative forms of a gene, with slight differences in the base sequence.

  • Variations in alleles cause variations in phenotypes, leading to genetic variation between individuals (e.g., red or white flowers).

• Genetic Variation:

  • Differences in phenotypes among individuals due to different genotypes for traits within a population.

• Phenotype:

  • Physical expression of genotype/alleles (e.g., brown or blond hair).

• Genotype:

  • Combination of alleles for a gene; each individual has two alleles, one inherited from each parent.

• Dominant Allele:

  • An allele that masks or hides the recessive allele.

  • Always expressed when an individual has one or two copies of the allele.

  • Represented by a capital letter (e.g., F).

• Recessive Allele:

  • An allele masked by the dominant allele and only expressed if no dominant allele is present.

  • Only expressed if two recessive alleles are present.

  • Represented by a lowercase letter (e.g., f).

• Possible Genotypes:

  • FF: Homozygous dominant

  • Ff: Heterozygous

  • ff: Homozygous recessive

• Phenotype vs. Genotype:

  • FF and Ff genotypes result in the dominant phenotype, while ff results in the recessive phenotype.

• Punnett Square:

  • A tool that gives theoretical probabilities of genetic outcomes.

  • Actual outcomes may not match predicted probabilities, especially in small samples.

  • Random fertilization of eggs by sperm means offspring genotypes may vary unless the sample size is large.

• Phenotype Ratios:

  • Expressed as ratios, fractions, or percentages (e.g., 3 black: 1 white, $3/4$ black & $1/4$ white, or 75% black, 25% white).

• Genotype Ratios:

  • Expressed as ratios, fractions, or percentages (e.g., 1 FF: 2 Ff: 1 ff, or $1/4$ FF & $1/2$ Ff & $1/4$ ff, or 25% FF, 50% Ff, and 25% ff).

• Human Chromosomes:

  • Humans have 46 chromosomes, arranged in 23 pairs.

• Inheritance of Sex:

  • Each fertilization has an equal chance of producing a male or female offspring.

  • Male: XY chromosomes

  • Female: XX chromosomes

  • Half of male gametes carry the X chromosome, and half carry the Y chromosome.

  • Previous offspring do not affect the chance of subsequent offspring being XX or XY.

• Mitosis:

  • Cell division for growth and repair, replacing damaged cells.

  • Produces 2 identical daughter cells.

• Meiosis:

  • Cell division producing gametes/sex cells with half the normal number of chromosomes (haploid) compared to body cells (somatic/diploid).

  • Produces 4 genetically different daughter cells.

  • Shuffles existing alleles through crossing over and independent assortment.

• Crossing Over:

  • Occurs during the first stage of meiosis when homologous pairs of chromosomes line up.

  • Small pieces of DNA are swapped between chromosomes, resulting in a mixing of alleles.

• Gametes:

  • Eggs or sperm have half the normal number of chromosomes to ensure that when a sperm fuses with an egg, the resulting zygote has the correct number of chromosomes.

• Fertilization:

  • A random process by which a male gamete fertilizes a female gamete, resulting in new combinations of alleles and a unique zygote.

• Sexual Reproduction:

  • Produces variation between individuals due to random assortment of chromosomes in meiosis and random fertilization.

• Advantage of Variation:

  • Some individuals may survive environmental changes or threatening events (e.g., drought, disease, selection pressures).

  • Ensures that not all individuals will be wiped out by a new disease.

• Survival and Reproduction:

  • Individuals best suited to an environment survive to reproduce and pass on their genes to future generations.

• Selection Pressures:

  • Environmental factors (drought, temperature change, pollution, disease) affect the gene pool of species over time.

• Adaptation:

  • Individual plants/animals do NOT adapt to change; the members of the species most able to withstand the change survive and pass on their beneficial genes.

  • Over time, the species becomes adapted.

• Resistance:

  • Plants don’t become IMMUNE to disease; “Resistant” is a more accurate term.

• Dominant or Recessive Traits:

  • If two normal individuals have affected offspring, the parents must be heterozygous (e.g., Ff).

  • When two f alleles combine, a homozygous recessive ff (affected) offspring forms.

  • If normal were recessive, affected individuals with a dominant allele would not be possible.

Test Cross (Back Cross)

• Definition: Reproduction with a homozygous recessive mate (e.g., ff) to determine the genotype of an individual (FF or Ff, which have the same phenotype).

• Example 1: FF x ff – all offspring are Ff.

• Example 2: Ff x ff – expect approximately a 50/50 mix of phenotypes (half Ff and half ff).

Mutation

• Definition: A change to the base sequence of a gene, resulting in a new protein/characteristic/trait.

• Result: Creation of new alleles within a population.

• Location: A mutation in a gamete can be inherited by offspring, while a mutation in a somatic (body) cell cannot.

Reproduction

• Sexual Reproduction: Uses meiosis, offspring are genetically different, takes longer time, needs 2 parents, but offspring have better disease survivability.

• Asexual Reproduction: Uses mitosis, all offspring are the same, produces identical offspring in large numbers quickly, but offspring have lack of disease survivability.

Mitosis vs. Meiosis

• Mitosis:

  • Growth & repair.

  • Produces 2 genetically identical cells.

  • Cells are diploid (full chromosome set, 2n).

  • Occurs in somatic (body) cells.

• Meiosis:

  • Production of gametes (sperm & egg).

  • Produces 4 genetically different cells.

  • Cells are haploid (half chromosome set, n).

  • Occurs in testes & ovary.

How Meiosis Causes Variation

• Crossing Over: Bits of homologous chromosomes are exchanged during meiosis.

• Independent Assortment: Random distribution of homologous chromosomes into gametes.

DNA Structure & Replication

• Structure:

  • Sugar & phosphate backbone.

  • "Ladder" of bases (A=T and C≡G / complementary base pairing).

  • Nucleotide = sugar & phosphate + base.

• Replication:

  • DNA unwinds and opens up, exposing bases.

  • New nucleotides come in (join A=T and C≡G).

  • Nucleotides are joined by enzymes, and DNA winds up again.

  • Accurate copying ensures daughter cells have the same DNA as the parent cell.

The Big Picture

• Chromosomes are made up of DNA.

• Chromosomes carry genes.

• Genes code for proteins.

• DNA is a triplet code; 3 bases = one amino acid.

• Sequence of bases determines the protein produced.

• Small differences in DNA sequence of a gene = alleles.

Mutation

• Permanent change in DNA sequence or number of chromosomes.

• Difference in base sequence may produce a different amino acid sequence leading to different protein and slightly altered gene.

• Alternative form of same gene is called an allele

Genetics Terms

• Homozygous: 2 alleles are the same (e.g., BB or bb).

• Heterozygous: 2 different alleles (e.g., Bb).

• Pure breeding: Always produces offspring which all resemble the parent / is homozygous (for a trait).

• Dominant: The characteristic that always shows if the dominant allele is present.

• Recessive: The characteristic that only shows if there is no dominant allele present.

• Dominant allele masks the recessive allele

Natural Selection & Disease Resistance

• Genetic diversity is due to meiosis.

• Disease may not affect all individuals because of different combinations of genes.

• Some individuals are more resistant to disease than others.

• These individuals can survive & breed, and resistance may be passed on.

• Survival of the fittest (better suited = survival).

Punnett Squares

• Gametes written at side / top – single letters.

• Genotype – combination of alleles.

• Phenotype – physical appearance/feature.

• Ratio 1 BB : 2 Bb : 1 bb or 3 brown : 1 blue.

• Punnet square predicts “possible” outcomes and probability but NOT exact numbers.

• Fertilisation is random so we don’t EXACTLY get predicted ratios.

• The larger the number of offspring the closer the results will be to the predicted ratios

DNA & Chromosomes

• DNA (deoxyribonucleic acid) acts as a blueprint for how living organisms are built.

• Made up of two long, twisted strands made of nucleotides with complementary base pair sequences (A with T and C with G).

• A gene is a segment of DNA which codes for a protein which determines a trait / characteristic because the base sequence (triplet code) determines amino acid sequence and therefore the protein made which determines the phenotype / characteristic / trait

• A chromosome is a structure which carries many genes and humans have 23 pairs of chromosomes (2n), inheriting one set (n) from the mother, & the other set (n) from the father

Variation

• Variation means differences between individuals and is environmental a n d / o r genetic (inherited)

Environmental Variance

• Plant size may depend on light or water they get or minerals in the soil even if, genetically, they should ALL grow tall.

Genetic Variance

• Genetically unique gametes (sperm/egg & pollen/ova) are formed by meiosis due to crossing over & independent assortment during meiosis

• Only mutations that occurred in a gamete can be inherited

• The random nature of which sperm fertilises which egg (sexual reproduction)

• Mutation also can cause variation

• The (changing) environment determines which variations are more favourable. More favourable characteristics are passed on as those individuals survive to breed!

• The genetic variations that arise in a population happen by chance, but the process of natural selection does not.

• variation IS IMPORTANT in a changing environment

• Individuals in a species show wide range of variation.

• Individuals with characteristics most suited to the environment are more likely to survive and reproduce.

• The genes that allow these individuals to be successful may be passed to their offspring.

• CAUSING “better suited” individuals to survive and pass on their genes to the next generation KNOWN AS survival of fittest / natural selection

Demonstrate understanding of biological ideas relating to genetic variation

• This achievement standard involves demonstrating understanding of biological ideas relating to genetic variation.

  • Achievement = Demonstrate understanding of biological ideas relating to genetic variation.

  • Achievement with merit = Demonstrate in-depth understanding of biological ideas relating to genetic variation.

  • Achievement with excellence = Demonstrate comprehensive understanding of biological ideas relating to genetic variation.

Part 1 – DNA Structure & Cell Division

Variation

• Variation means differences between individuals. Variation arises from inheritance, environmental factors and mutations.

• There are two types of variation:

Continuous

• There is a smooth range of variation between individuals, with all intermediates possible between the extremes. It makes a bell shaped curve when graphed.

  • Examples - skin colour, intelligence, height, and mass. It involves complicated genetics involving many genes and the environmental also has a significant effect. Examples - exposure to sun, nutrition, exercise.

Discontinuous variation

• Variation is of two types (either/or) or more than two types with no intermediates and makes a histogram when graphed. Caused by simple genetics with very little or no environmental effect.

  • Examples - blood group (O, A, B, AB), sex (m/f), the ability to roll tongue* (can/can’t).

    • *Thereis evidence that the abilityto roll a tongue or not may be much more complicated than the simple genetics we see in many text books & examination questions. It is possible to have identical twins where one can roll & the other not; Non-rolling parents can have a child who can tongue roll.

Chromosomes

• Chromosomes are long coiled lengths of DNA (deoxyribonucleic acid)found in the nucleus of a cell. Each chromosome is made up of many genes. Different species of animals and plants contain different numbers of chromosomes. Humans have 46 chromosomes (23 pairs).

  • A pair of chromosomes is called a homologous pair.

Genes

• are sections ofDNA that code for a particular characteristic such as eye colour, hair colour, earshape.

Alleles

• The name giventooneoftheformsthatagene can have. E.g. the gene for tongue rollinghas2alleles – adominantform (tongue roller) and a recessive form (nontongueroller).

DNA molecule

• DNA (deoxyribonucleicacid)–amoleculethatcontains the instructions to make a new organism. Itisfoundinthenucleusof the cell. It is usually super twisted to form chromosomes.

  • Chromosomes exist as pairs so that individualsinherit two copies of each gene.

Nucelotide

DNA Moleucle Structure

• The backbone of the DNA molecule consists of 2 strands of alternating sugar molecules and phosphate groups, the 2 strands twisted to form a double helix. Each sugar molecule is attached to one of 4 bases called Adenine, Guanine, Cytosine or Thymine (A, G, C & T for short).

• The bases are paired up on opposite strands A always pairs with T and G with C. This is called complementary base pairing – the order of bases in one strand determining the order of bases in the other.

Definition

• A nucleotide is a basic building block for DNA made up of phosphate, sugar and base.

• The 2 strands are held together by weak hydrogen bonds.

• A gene consists of hundreds or thousands of bases. A gene codes for a particular protein by its particular base sequence.

DNA Replication

• DNA must replicate or copy itself before any cell division can take place. The DNA double helix is perfectly suited for replication because each strand can serve as a template (pattern) to produce a strand opposite to itself.

  • First it is “unzipped” – the 2 strands are separated. New nucleotides in the cell line up alongside the unpaired bases, A pairing with a T and G with a C. The new nucleotides are joined together using an enzyme called DNA polymerase.

Semiconservative

• DNA replication is semi conservative i.e. each strand in the original DNA molecule is used as a template to make a new strand of DNA. Each new DNA molecule contains an original strand and a newly made strand.

Cell Division

• Cell division occurs through mitosis & meiosis.

Biological ideas relating to mitosis and meiosis are limited to:

• Purpose where they occur

• Sequence of events (the names of stages are not required)

• Reasonsfor maintenanceor change of chromosome number

• Significance of the number of cells produced

• Cells grow old and die. They need to be constantly replaced. This is done by a type of cell division called mitosis. Mitosis takes place in all cells except the sex organs (testes and ovaries).

Mitosis cell division

• It is used for growth and replacement of cells. If you are going to grow you have to make more cells!

• Daughter cells have the same number of chromosomes as the parent cell.

Meiosis – also called reduction division cell division

• Meiosis – also called reduction division – is the cell division that produces gametes. Meiosis in animals takes place in the testes and ovaries.

• It results in the production of sex cells (sperm and eggs). In plants it results in pollen grains and eggs. Sex cells have half the number of chromosomes as the parent cell. When the sperm and egg cells unite at fertilisation, each contributes 23 chromosomes so the resulting embryo will have the usual 46. Meiosis also allows genetic variation through a process of DNA shuffling while the cells are dividing.

Differecnes between mitosis and meiosis

• To understand how the differences between mitosis and meiosis occur, you must understand the sequence of events that takes place inside the cell for each type of cell division.

  • Remember that the point of mitosis is to produce identical copies of cells for rapid growth and repair.

  • The point of meiosis is to produce sex cells that contain half of the original number of chromosomes.

  • Also, during meiosis genetic information (bits of chromosomes) is swapped between homologous chromosomes when they line up prior to separation.

• Mendel formulated what is now known as Mendel's law of independent assortment. This law states that allele pairs separate independently during the formation of gametes. Therefore, traits (characteristics) are transmitted to offspring independently of one another. It is a random process.

Mitosis

1. Before cell division the chromosomes can’t be seen. As division starts, the chromosomes become fatter and more visible. The plant cell (opposite) and animal cell (below) both only shows 4 chromosomes.

2. The DNA making each chromosome is copied. Each chromosome has been copied. The copies are still attached at the centromere. Each copy is called a chromatid.

3. The chromosomes line up SINGULARLY in the middle of the cell (equator).

4. One chromatid of each chromosome goes to opposite ends of the cell (poles), pulled by the spindle apparatus. The parent cell starts to split into 2.

5. Two daughter cells are made. The number of chromosomes is the same in each daughter cell as the original parent cell.

Meiosis

1. Chromosomes become fatter and more visible. This cell has 4 chromosomes (2 homologous pairs).

2. The DNA making each chromosome is copied. Each chromosome has been copied. The copies are still attached at the centromere. Each copy is called a chromatid. The homologous pairs of chromosomes line up alongside each other in the middle of the cell. Now some crossing over of bits of chromosomes can take place between homologous pairs. This swaps genes from one chromosome to another and leads to variation between offspring.

3. One of each homologous pair goes to opposite ends of the cell. The parent cell starts to split into two, making two daughter cells.

4. The chromosomes in each cell line up in the middle and this time the chromatids separate. We now have 4 daughter cells; each one has half the number of chromosomes as the original parent cell.

   • Also, these chromosomes are not identical to the parent because crossing over took place and independent assortment (also called segregation) occurred (the two chromosomes of each pair (maternal and fraternal) are separated during meiosis and randomly distributed to the daughter cells).

Mutations – when things go wrong with DNA

• It is important that DNA remains unchanged from generation to generation. The complementary base pairing that occurs during DNA replication means this generally occurs. However random, spontaneous mistakes do happen.

  • A mutation is a change in the type or the amount of DNA in a cell.

Type of DNA:

• A mistake made in copying the DNA can produce a slightly different allele of a gene.

• Certain chemicals, ultra-violet light, X-rays, or radiation can increase the chances of changes occurring to DNA even while chromosomes are not being copied.

• Changes to the base sequence are called point or gene mutations and can be

  • (a) insertion (addition of a base)

  • (b) deletion (loss of a base)

  • (c) inversion (2 bases change position)

  • (d) substitution (one base replaces another).

• If the mutation happens in the gametes, then this new allele can be passed on to offspring. Some diseases caused by mutations can be passed on to offspring – these diseases are called inherited diseases eg Cystic fibrosis.

Mutations can be:

• harmful (causing death or disease): To function correctly cells depend on many proteins. Gene mutations may stop one or more proteins from working properly. By changing a gene’s instructions for a protein, the mutation can cause the protein to malfunction or to be missing entirely. If the protein plays a critical role in the body, it can disrupt development or cause a medical condition. Some mutations can cause illnesses such as cancer.

• beneficial (give some advantage or benefit to an organism) e.g. antibiotic resistance in bacteria is beneficial to the bacteria

• silent / neutral – appear to not affect the organism (does not affect phenotype). They may not be evident if the mutation is in a recessive allele. A change to a DNA triplet might not change which amino acid is introduced, or it might change the amino acid to a chemically similar amino acid that works just as well. Silent / neutral mutations do however create the possibility of a future mutation having an effect.

Amount of DNA:

• Sometimes when cells divide, the wrong number of chromosomes ends up in a gamete and it may have a particular chromosome missing, or two copies of a chromosome.

• Down syndrome occurs in about 1 in 900 births. The risk of having a Down syndrome child increases as a woman gets older. Down syndrome is caused each cell in the body having three copies of chromosome 21. Down’s children often have learning difficulties and poor muscle tone, and problems with their sight or hearing, and/or have heart problems.

DNA Structure

• Nucleotide = sugar + phosphate + base

• Double helix, “twisted ladder”, complementary strands held together by hydrogen bonds

How DNA “codes” for proteins

• gene is part of the DNA molecule / chromosome is a sequence of bases / nucleotides

• Sequence of bases must stay the same because they code for an amino acid sequence in a protein.

• Changes in the code are a mutation - cell may no longer be able to carry out its function.

• codes for a specific sequence of amino acids in a protein or codes for a characteristic

• alleles – different forms of a gene - with slight difference in base sequence ⇒ different aa ⇒ different protein ⇒ different properties / characteristic e.g. blue / brown eyes

DNA Replication

• original DNA strands unzips, each side acts as template for new strand

• new nucleotides add on to the original bases (and are joined together by enzymes)

• order of bases in one strand determines the order of bases in the other strand

• bases of old and new strand are complementary to each other, A=T and G≡C ensures new strand is accurate / exact copy of old strand

• Before either type of cell division, the DNA must first be replicated or copied

Cell division

Mitosis

• occurs in somatic cells e.g. toe, liver, skin

• production of new cells for growth & repair

• used in asexual reproduction (mostly plants)

• the two daughter cells genetically identical

• cells are diploid / have full genetic information Cell divides - one copy of each chromosome carrying the same genes, goes into each new body cell. DNA must be accurately copied so daughter cells have same DNA as parent cell.

• chromosomes replicate, becoming visible, nuclear membrane breaks down, line up singularly on equator, chromatids pulled to opposite poles by spindle, cytoplasm divides etc

Meiosis

• occurs in cells in reproductive organs e.g. testis & ovary (animals), ovary & anther (plant)

• produces sex cells / gametes e.g. sperm, egg, pollen, ovule

• cells divide twice

• chromosome number halved (haploid) / half the
  genetic information - need half chromo. no. to enable fertilisation to form zygote & so each new cell has correct no. of chromosomes.

• produces 4 daughter cells, genetically different

• chromosomes replicated - now visible - & line up in homologous pairs on equator crossing over occurs chromosomes pulled to opposite poles. Line up singularly this time, chromatids pulled to opposite poles

How meiosis causes variation

• Homologous pairs line up during meiosis and exchange material during crossing over

• One of each pair of homologous chromosomes goes to a different daughter cell (segregation)

• Meiosis produces gametes. DNA of two gametes is combined during fertilisation - means offspring produced are different from both parents.

Reproduction

Sexual Reproduction

• Sexual (plants & animals) meiosis and variation

  • involves gametes produced by meiosis

  • produces variation

  • advantage of variation - provide offspring with greater chance of survival in successive generations in a changing environment

  • “slow” as offspring have to grow and become sexually mature before breeding can occur

Asexual Reproduction

• Asexual (plants and cloned animals) mitosis and a lack of variation

  • advantage - faster

  • produces genetically identical plants / animals

  • produces / grows plants very quickly / plants
    fruit sooner than from a plant grown from a seed - benefits the grower - always get desired phenotype of plant & produce more plants to make a bigger profit or more plants in a shorter time

• disadvantages – e.g. plants all genetically identical, so susceptible to same diseases. Less variation – may be less suited to future changes in environment.

Asexual reproduction in plants

• vegetative propagation & cloning in plants
  *Asexual reproduction techniques include taking stem cuttings, splitting bulbs or tubers and tissue culture.
  Tissue culture

• E.g. tissue culture: A single plant is divided into small clusters of cells and placed on tissue culture. The single plant can produce many identical plants. Sometimes the original plant has been genetically engineered for a required gene.

Selective breeding – plants / animals

• Breed together individuals that have desirable phenotypes / characteristics e.g. cows with more meat or better milk yields, or a e.g. potato with resistance to disease & potato with a firm texture.

Cloning - Animals

• enucleating - microinjection - surrogate mothers (SCNT)

  • nucleus transplanted into an enucleated cell - electrical current is passed through it so that the cell starts dividing - embryo transplanted into a surrogate mother

  • expensive, low success rate, some problems (eg premature aging), ethics surrounding humans!!

Artificial twinning

• Egg from a desirable cow is fertilised by the sperm from a desirable bull. Fertilised egg is allowed to divide several times. The undifferentiated ball of KEY WORDS - for this part of the standard allele, asexual, characteristic, chromosome, clone, complementary, diploid, fertilisation, gamete, gene, haploid, karyotype, meiosis, mitosis, replication, selective breeding semi-conservative, sexual, trait, variation, zygote Slow process – occurs of numerous generations. At cells is then broken up. Each cell is then grown and each stage individuals are selected with the desired transplanted into surrogate cows. Advantage: best characteristics.
  *Cross pollinate potato plants. Seeds grown / geminated. Potatoes grown from these seeds that show both desired characteristics can then be reproduced asexually to (quickly) produce a crop.

• Both selective breeding & cloning processes (see opposite) reduce genetic diversity of a breed by eliminating certain genes / combinations of genes. characteristics of animals can be selected and new animals produced faster than traditional methods.

• NOTE: Calves are clones of each other but not of the parents.