Lecture 7: Sources of Genetic Variation: Sex

Question 1

Sources of Allelic Variation

Answer 1

mutation

Question 2

Sources of Genotypic Variation

Answer 2

Sex (Meiosis)

Question 3

In Sex: No new alleles, only new __

Answer 3

genotypes

Question 4

What process introduces genetic variation during sexual reproduction?

Answer 4

meiosis (crossing over and independent assortment)

Question 5

the exchange of genetic material between homologous chromosomes during meiosis.

Answer 5

crossing over (a form of recombination)

Question 6

changes the composition along chromosome arms, creating new combinations of alleles on chromosomes.

Answer 6

crossing over

Question 7

refers to mating that occurs without regard to the genotypes of individuals.

Answer 7

random mating

Question 8

How does random mating contribute to genetic variation?

Answer 8

increases the likelihood of different genotypes coming together during reproduction, leading to the formation of new combinations of alleles in offspring.

Question 9

produces gametes

Answer 9

meiosis

Question 10

shuffling of gametes

Answer 10

random mating

Question 11

meiosis generate genotypic diversity through two main mechanisms: What is the first mechanism?

Answer 11

The physical exchange of homologous chromosomal regions by homologous recombination.

Question 12

What does homologous recombination result in?

Answer 12

New combinations of DNA within chromosomes.

Question 13

meiosis generate genotypic diversity through two main mechanisms: What is the second mechanism?

Answer 13

The separation of homologous chromosome pairs.

Question 14

What does the separation of homologous chromosome pairs allow?

Answer 14

random and independent shuffling of haploid chromosomes (gametes) during random mating.

Question 15

a process by which a molecule of nucleic acid (usually DNA, but can also be RNA) is broken and then joined to a different one.

Answer 15

Genetic recombination

Question 16

What is the outcome of the crossover process during eukaryotic recombination?

Answer 16

Offspring have different combinations of alleles along the chromosomes compared to their parents.

Question 17

What is the consequence of random mating?

Answer 17

increases genotypic variation

Question 18

Mixing up combinations of alleles at a given locus during reproduction, leading to the formation of new genotypes in the offspring.

This process can also help break up LD on different chromatids by further mixing alleles from different chromosomes.

Answer 18

Random mating

Question 19

What is the consequence of genetic recombination during sexual reproduction?

Answer 19

reduces Linkage Disequilibrium (LD)

Question 20

How does genetic recombination achieve this reduction in LD?

Answer 20

by breaking up associations of alleles along a chromosome.

Question 21

• the non-random association of alleles at two or more loci, not necessarily on the same chromosome.
• It occurs when certain combinations of alleles or genetic markers are more or less frequent than expected based on random formation of haplotypes from allele frequencies.

Answer 21

Linkage Disequilibrium (LD)

Question 22

What can cause Linkage Disequilibrium?

Answer 22

Evolutionary forces such as natural selection and genetic drift

Question 23

__ breaks down LD on the same chromosome by shuffling genetic material between homologous chromosomes, leading to the creation of new combinations of alleles.

Answer 23

Recombination

Question 24

Linkage Disequilibrium (LD)

__ will break down linkage disequilibrium on the same chromosome, and __ __ on different chromosomes

Answer 24

• Recombination
• random mating

Question 25

Can you provide an example of LD caused by natural selection?

Answer 25

In Northern latitudes, lack of sunlight may lead to the evolution of light hair, light skin, and blue eyes, even though these traits are encoded by different genes. These traits become associated with each other due to similar selective pressures.

Question 26

Give an example of LD in many small and isolated populations.

Answer 26

Finnish disease heritage

Question 27

Why do agricultural species often exhibit high levels of LD?

Answer 27

Strong artificial selection by humans

Question 28

Why is LD important to understand?

Answer 28

LD helps identify common allele associations and provides insights into population evolution.

Question 29

How does breaking up LD benefit populations?

Answer 29

Breaking up LD enhances the efficiency of natural selection by increasing genetic variation.

Question 30

How random mating during sex facilitates the combination of favorable mutations from different alleles?

Answer 30

by bringing together haploid chromosomes (sperm and egg)

Question 31

How random mating during sex remove unfavorable mutations more quickly?

Answer 31

bring the deleterious mutations together.
- homozygotes, the deleterious mutations can
now be selected OUT of the population
- no longer masked in the heterozygous

Question 32

How does genetic recombination benefit sex?

Answer 32

It breaks down Linkage Disequilibrium.

Question 33

Random mating during sex benefits (2)

Answer 33

• increase genotypic diversity
• increase the rate of evolution

Question 34

What is the cost of sex in terms of fitness compared to clonal populations?

Answer 34

Sex can result in a loss of fitness relative to clonal populations.

Question 35

How does sex reduce population growth rate?

Answer 35

Sex can reduce population growth rate by half because males, who contribute genetic material but cannot reproduce directly, are present in the population.

Question 36

Why does sex primarily occur in eukaryotes and not in bacteria or archaea?

Answer 36

Bacteria have a high mutation rate and undergo horizontal gene transfer, while viruses can afford mistakes (mutations) since the host produces all offspring.

Question 37

In what organisms is asexuality common?

Answer 37

plants and many invertebrates.

Question 38

Under what conditions do some eukaryotes transition from asexual to sexual reproduction?

Answer 38

Some eukaryotes, such as Daphnia, ciliates, and dinoflagellates, transition from asexual to sexual reproduction in response to a large accumulation of deleterious mutations (mutational meltdown) or stress.

Question 39

Are there eukaryotes with more than two sexes?

Answer 39

Yes, some eukaryotes, such as certain ciliates, have more than two sexes, with some species having as many as 32 sexes.

Question 40

Do sexual species tend to have longer lifespans than asexual species?

Answer 40

Yes

Question 41

Why are asexual species often good early colonizers of novel habitats?

Answer 41

Asexual species often have a rapid growth rate, facilitating quick colonization of new habitats.

Question 42

Asexual species often have a rapid growth rate, facilitating quick colonization of new habitats.

Answer 42

Less than 1%

Question 43

organisms that grow, divide, and can be considered “immortal” as they perpetuate through asexual reproduction.

Answer 43

clonal organisms

Question 44

What distinguishes sexually reproducing organisms from clonal ones?

Answer 44

Sexually reproducing organisms produce unique individuals with novel genetic architectures before eventually dying, in contrast to the immortal nature of clonal organisms.

Question 45

SEX: Benefits (6)

Answer 45

§ Breakdown Linkage Disequilibrium
§ Increase in Genotypic Variation
§ Purge deleterious mutations more easily
§ Bring together favorable mutations
§ Evolution of “individuality”

Question 46

SEX: Costs (4)

Answer 46

§Lower Reproduction Rate (1/2)
§ Have to find mates (not all individuals reproduce)
§ Pass on only ½ of your genome at each reproduction event
§ Death of unique individuals (genome) in the parental generation

Question 47

Relationship between Genetic Variation and Natural Selection

Answer 47

Genetic variation is the raw material upon which natural selection acts.