Lecture 7: Sources of Genetic Variation: Sex Flashcards
Sources of Allelic Variation
mutation
Sources of Genotypic Variation
Sex (Meiosis)
In Sex: No new alleles, only new __
genotypes
What process introduces genetic variation during sexual reproduction?
meiosis (crossing over and independent assortment)
the exchange of genetic material between homologous chromosomes during meiosis.
crossing over (a form of recombination)
changes the composition along chromosome arms, creating new combinations of alleles on chromosomes.
crossing over
refers to mating that occurs without regard to the genotypes of individuals.
random mating
How does random mating contribute to genetic variation?
increases the likelihood of different genotypes coming together during reproduction, leading to the formation of new combinations of alleles in offspring.
produces gametes
meiosis
shuffling of gametes
random mating
meiosis generate genotypic diversity through two main mechanisms: What is the first mechanism?
The physical exchange of homologous chromosomal regions by homologous recombination.
What does homologous recombination result in?
New combinations of DNA within chromosomes.
meiosis generate genotypic diversity through two main mechanisms: What is the second mechanism?
The separation of homologous chromosome pairs.
What does the separation of homologous chromosome pairs allow?
random and independent shuffling of haploid chromosomes (gametes) during random mating.
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.
Genetic recombination
What is the outcome of the crossover process during eukaryotic recombination?
Offspring have different combinations of alleles along the chromosomes compared to their parents.
What is the consequence of random mating?
increases genotypic variation
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.
Random mating
What is the consequence of genetic recombination during sexual reproduction?
reduces Linkage Disequilibrium (LD)
How does genetic recombination achieve this reduction in LD?
by breaking up associations of alleles along a chromosome.
- 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.
Linkage Disequilibrium (LD)
What can cause Linkage Disequilibrium?
Evolutionary forces such as natural selection and genetic drift
__ breaks down LD on the same chromosome by shuffling genetic material between homologous chromosomes, leading to the creation of new combinations of alleles.
Recombination
Linkage Disequilibrium (LD)
__ will break down linkage disequilibrium on the same chromosome, and __ __ on different chromosomes
- Recombination
- random mating
Can you provide an example of LD caused by natural selection?
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.
Give an example of LD in many small and isolated populations.
Finnish disease heritage
Why do agricultural species often exhibit high levels of LD?
Strong artificial selection by humans
Why is LD important to understand?
LD helps identify common allele associations and provides insights into population evolution.
How does breaking up LD benefit populations?
Breaking up LD enhances the efficiency of natural selection by increasing genetic variation.
How random mating during sex facilitates the combination of favorable mutations from different alleles?
by bringing together haploid chromosomes (sperm and egg)
How random mating during sex remove unfavorable mutations more quickly?
bring the deleterious mutations together.
- homozygotes, the deleterious mutations can
now be selected OUT of the population
- no longer masked in the heterozygous
How does genetic recombination benefit sex?
It breaks down Linkage Disequilibrium.
Random mating during sex benefits (2)
- increase genotypic diversity
- increase the rate of evolution
What is the cost of sex in terms of fitness compared to clonal populations?
Sex can result in a loss of fitness relative to clonal populations.
How does sex reduce population growth rate?
Sex can reduce population growth rate by half because males, who contribute genetic material but cannot reproduce directly, are present in the population.
Why does sex primarily occur in eukaryotes and not in bacteria or archaea?
Bacteria have a high mutation rate and undergo horizontal gene transfer, while viruses can afford mistakes (mutations) since the host produces all offspring.
In what organisms is asexuality common?
plants and many invertebrates.
Under what conditions do some eukaryotes transition from asexual to sexual reproduction?
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.
Are there eukaryotes with more than two sexes?
Yes, some eukaryotes, such as certain ciliates, have more than two sexes, with some species having as many as 32 sexes.
Do sexual species tend to have longer lifespans than asexual species?
Yes
Why are asexual species often good early colonizers of novel habitats?
Asexual species often have a rapid growth rate, facilitating quick colonization of new habitats.
Asexual species often have a rapid growth rate, facilitating quick colonization of new habitats.
Less than 1%
organisms that grow, divide, and can be considered “immortal” as they perpetuate through asexual reproduction.
clonal organisms
What distinguishes sexually reproducing organisms from clonal ones?
Sexually reproducing organisms produce unique individuals with novel genetic architectures before eventually dying, in contrast to the immortal nature of clonal organisms.
SEX: Benefits (6)
§ Breakdown Linkage Disequilibrium
§ Increase in Genotypic Variation
§ Purge deleterious mutations more easily
§ Bring together favorable mutations
§ Evolution of “individuality”
SEX: Costs (4)
§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
Relationship between Genetic Variation and Natural Selection
Genetic variation is the raw material upon which natural selection acts.
