Topic 10: Genetics (HL)

Question 1

State the purpose of meiosis

Answer 1

Meiosis is the process by which sex cells (gametes) are made in the reproductive organs. It involves the reduction division of a diploid germline cell into four genetically distinct haploid nuclei.

Question 2

Explain how independent assortment give rise to genetic variety

Answer 2

During metaphase I, the chromosomes line up in the equator randomly meaning that there is an equal chance of gamete containing either maternal/paternal copy of a given chromosome.

Question 3

Explain how crossing over give rise to genetic variety

Answer 3

During prophase I, homologous pairs of chromosomes connect via chiasmata to exchange genetic material. This gene exchange produces new combinations of alleles hence creating genetic variety.

Question 4

Describe the stages of meiosis I

Answer 4

Prophase I: Chromosomes condense, the nuclear membrane dissolves, and homologous chromosomes form bivalents where crossing over occurs.
Metaphase I: Spindle fibres from opposing centromeres connect to bivalents and align them along the middle of the cell.
Anaphase I: Spindle fibres split the bivalents and homologous chromosomes move to opposite sides of the cell.
Telophase I: Chromosomes decondense, the nuclear membrane may reform and cell divides to form two daughter cells.

Question 5

Describe the stages of meiosis II

Answer 5

Prophase II: Chromosomes condense, nuclear membrane dissolves and centromeres move to opposite poles.
Metaphase II: Spindle fibres attach to chromosomes and align them to the middle of the cell.
Anaphase II: Spindle fibres separate sister chromatids and move to opposite poles.
Telophase II: Chromosomes decondense, nuclear membrane reforms, cell divides to form four haploid daughter cells.

Question 6

Define linkage group

Answer 6

A linkage group is a group of genes whose loci are on the same chromosome. Linked genes function as a single heritable unit and do not follow independent assortment.

Question 7

Identify the factor that determines the likelihood of recombination between two linked genes

Answer 7

Distance: the further apart two genes are on a chromosome, the more likely they are to recombine

Question 8

Outline how Morgan discovered non-Mendelian ratios via experimentation with Drosophila

Answer 8

He undertook breeding experiments with Drosophila (fruit flies) and found a clear sex bias in the inheritance of certain traits. Morgan determined that genes located on a shared chromosome would not assort independently and have non-Mendelian ratios.

Question 9

Define polygenic inheritance

Answer 9

Polygenic traits are characteristics that are controlled by more than two gene loci.

Question 10

Distinguish between discrete and continuous variation

Answer 10

Discrete variation is present in monogenic traits. They are only controlled by one gene and cannot be influenced by the environment. Continuous variation is present in polygenic traits. They are controlled by more than one gene and can be influenced by the environment.

Question 11

List examples of polygenic inheritance in humans

Answer 11

Human height, weight, skin colour

Question 12

If x^2 > 0.05, are genes linked or unlinked?

Answer 12

Genes are unlinked, accept null hypothesis

Question 13

If x^2 < 0.05, are genes linked or unlinked

Answer 13

Genes are unlinked, accept alternative hypothesis

Question 14

Write the calculation formula for expected frequencies

Answer 14

Total x ratio

Question 15

Identify the causes of variation

Answer 15

Mutations, genetic recombination, gene flow, genetic drift, natural selection, polygenic traits

Question 16

Define gene pool

Answer 16

Sum total of all genes and their alleles present in an interbreeding population

Question 17

Outline a factor that influences genetic drift and identify two conditions by which this factor may occur

Answer 17

Genetic drift is a change in composition of a gene pool due to random events and is impacted by population size. Size affects stability of allele frequencies (high drift is small populations and small drifts in large populations. Two conditions are population bottlenecks when an event reduces the population size and founder effect when a small group breaks away from a larger population to colonise a new territory.

Question 18

Compare the different types of natural selection

Answer 18

Stabilising selection: the intermediate phenotype is favoured at the expense of both extreme phenotypes such as human birth weights.
Disruptive selection: both of the extreme phenotypes are favoured at the expense of the intermediate phenotype such as the pigmentation of a peppered moth.
Directional selection: only one extreme phenotype is favoured at the expense of the other extreme phenotype and the intermediate phenotype such as antibiotic resistance in bacteria.

Question 19

Discuss the different types of reproductive isolation

Answer 19

Temporal: occurs when two populations differ in their periods of activity or reproductive cycles such as leopard frogs and wood frogs reaching sexual maturity at different times in the spring hence cannot interbreed with each other.
Behavioural: occurs when two populations exhibit different specific courtship patterns such as the same bird species having different mating songs.
Geographic: occurs when two populations occupy different habitats or separate niches within a common region such as lions and tigers that occupy different habitats and do not interbreed (usually) or frogs that live in land and water.

Question 20

Define speciation

Answer 20

Speciation is an evolutionary process that results in the formation of new species from pre-existing species. It occurs when reproductive isolating mechanisms prevent two organisms from producing fertile offspring.

Question 21

Distinguish between allopatric and sympatric speciation

Answer 21

Allopatric speciation occurs when a geographical barrier physically isolates populations and the two populations being to genetically diverge until they can no longer interbreed. Sympatric speciation is a divergence of species within the same geographical location and the reproductive isolation may be a result of genetic abnormalities.

Question 22

Distinguish between phyletic gradualism and punctuated equilibrium

Answer 22

Phyletic gradualism is a speciation that occurs following continuous change at a constant pace over a long period of time. This can be through a gradual accumulation of mutations. Punctuated equilibrium is a speciation that occurs rapidly in bursts with long periods of stability between. In stable periods, characteristics are maintained but this is punctuated by periods of environmental changes

Question 23

Explain how polyploidy can result in a speciation event

Answer 23

Sympatric speciation is most commonly caused as the result of a meiotic failure during gamete formation. If meiotic cells fail to undergo cytokinesis, chromosomal number will double in the gamete (diploid not haploid). This will result in offspring that have additional sets of chromosomes (polyploidy). Speciation will result if the polyploid offspring are fertile but cannot interbreed with the parent population. It is more common in plant species that can self-fertilise and reproduce asexually (vegetative propagation)

Question 24

Compare divergent and convergent evolution

Answer 24

Convergent: Species do not share common ancestry, Adopt similar phenotypic features due to shared selection pressures, Analogous structures
Divergent: Species share a common ancestry, Adopt dissimilar phenotypic features due to different selection pressures, Homologous structures