Sex and Linkage

Question 1

define alleles

Answer 1

A, a, B, b

Question 2

define genotype

Answer 2

A/a; B/b

Question 3

define haplotypes

Answer 3

• gametes
• AB, ab

Question 4

define asexual reproduction

Answer 4

• does not involve fusion of gametes
• offspring inherits all genes from 1 parent

Question 5

examples of asexual reproduction

Answer 5

• binary fission
• budding
• vegetative
• pathogenesis

Question 6

asexual reproduction - angiosperms

Answer 6

many can simultaneously reproduce sexually and asexually

Question 7

asexual reproduction: angiosperms - sexual and asexual examples

Answer 7

• sexual: flowers, fruits, seeds
• asexual: runners, leaflets

Question 8

asexual reproduction - animals

Answer 8

• some can reproduce by pathogenesis
• where clones develop from unfertilized eggs

Question 9

define hermaphrodites

Answer 9

• animals that can produce sperm and eggs
• can mate with them-selves (self-fertilization/selfing) or with another individual

Question 10

pros of asexual reproduction

Answer 10

• don’t have to rely on mate choice
• no competition
• reduced energy need
• Fitness is doubled – all copies of your genes are passed vs sexual reproduction where you get half

Question 11

explain selfing

Answer 11

they go through meiosis, but not outcrossing (randomly mating with other individuals)

Question 12

define linkage equilibrium

Answer 12

the genotype of as chromosome at one locus is independent of its genotype at the other locus

Question 13

define linkage disequilibrium

Answer 13

nonrandom association between a chromosome’s genotype at one locus and its genotype at the other locus

Question 14

will selection at the A locus interfere with our ability to use HW models to predict evolution at the B locus?

Answer 14

• yes
• only if linked (at equilibrium)

Question 15

what happens during meiosis

Answer 15

• chromosomes segregate independently of one another
• this means that loci A and B are inherited independently of one another

Question 16

meiosis - linkage equilibrium

Answer 16

• A and B are physically on the same chromosome, they are inherited independently because of recombination/crossing over during SEXUAL reproduction
• haplotype frequencies are the same as expected
• HW models can be used

Question 17

meiosis - linkage disequilibrium

Answer 17

• after recombination, haplotype frequencies are different than expected
• HW models cannot be used

Question 18

meiosis - what does it mean when a gene is linked

Answer 18

if two allele physically close, they are less likely to be broken up and tend to be inherited together

Question 19

what is genetic crossing over (recombination)

Answer 19

• Occurs in prophase of meiosis I (ONLY in sexual reproduction)
• Creates chromosomes with new combination of alleles for all genes one a chromosome

Question 20

explain the relationship between linkage and distance

Answer 20

• disequilibrium decreases with physical distance between loci
• close = more linked = less likely to be separated (higher disequilibrium, low equilibrium)
• far = less linked = more likely to be separated (lower disequilibrium, higher equilibrium)

Question 21

what are the three mechanisms that can cause linkage disequilibrium

Answer 21

1. selection on multilocus genotypes
2. migration
3. genetic drift/ population admixture

Question 22

why can the three mechanisms cause linkage disequilibrium

Answer 22

These can all produce populations in which some chromosome haplotypes are underrepresented and others are overrepresented compared to what you would expect with linkage equilibrium

Question 23

selection and linkage disequilibrium example

Answer 23

• A and B together code for large body size which confers fitness advantage
• creates linkage disequilibrium because these two alleles are more likely to be inherited together
• they do not move closer to each other – each still has same locus

Question 24

selection and linkage disequilibrium example - what are the two allele an example of?

Answer 24

multilocus genotype bc 2 genes both code for the same trait

Question 25

genetic drift and linkage disequilibrium exmaple

Answer 25

- Small population (drift is most effective) - Only have AB and Ab (no “a” present) - Mutation creates ab - We are missing a possible haplotype: aB - By definition this is LD bc its missing

Question 26

migration and linkage disequilibrium example

Answer 26

- Population A and B are both LE but in different ways - When they mix (via migration), it creates LD

Question 27

what breaks up linkage disequilibrium

Answer 27

- sex - Sexual reproduction involves meiosis with crossing over and random mating between different individuals

Question 28

linkage disequilibrium - define random mating

Answer 28

the union of gametes from unrelated parents brings together chromosomes with different haplotypes

Question 29

linkage disequilibrium - define crossing over

Answer 29

babies from random mating grow up and reproduce with crossing over, which breaks up old combinations of alleles and creates new ones

Question 30

linkage disequilibrium - define genetic recombination

Answer 30

the creation of new combinations of alleles during sexual reproduction

Question 31

linkage disequilibrium: genetic recombination - how does genetic recombination breakup disequilibrium

Answer 31

Genetic recombination randomizes genotypes at one locus with respect to genotypes at another → it reduces LD

Question 32

why do males exist?

Answer 32

1. Genetic recombination breaks up linkage disequilibrium 2. Genetic recombination pushes populations back towards Hardy-Weinberg Equilibrium 3. Muller’s ratchet 4. Red Queen

Question 33

why should asexual reproduction win over sexual reproduction

Answer 33

- within each generation, each sexual female makes 2 daughters and 2 sones, each with 1/2 of her genes - each asexual female makes 4 daughters, each with 100% of her genes - the fraction of individuals in the population that are asexual females increase every generation - and the asexual females have higher fitness

Question 34

asexual always wins, if... (finish sentence)

Answer 34

1. a female's reproductive mode does not affect how many offspring she makes 2. a female's reproductive mode does not affect the probability that her offspring will survive

Question 35

why do males exist? - explain Muller's rachet

Answer 35

- mutation + drift - asexual populations occasionally sustains deleterious mutations - if a zero mutation group is small: chance events may cause its extinction - one mutation group may have highest fitness - but the loss of a group by drift if easier than is replacement by backmutation

Question 36

muller's ratchet - define genetic load

Answer 36

the burden imposed by accumulating deleterious mutations

Question 37

example of Muller's ratchet in the wild - Timema walking stick

Answer 37

- 6 are asexual and 6 are sexual - researchers estimated ratio of non-synonymous to synonymous substitutions of important genes

Question 38

Muller's ratchet: Timema walking stick example -what is the expectation

Answer 38

- purifying selection eliminates deleterious alleles and leaves a signal - should make the ratio of nonsynonymous to synonymous mutations low

Question 39

Muller's ratchet: Timema walking stick example - what is the result

Answer 39

- it is consistent with Muller's ratchet - bc asexual individuals had higher nonsynonymous to synonymous ration than sexual - thus sexual individuals have experienced purifying selection

Question 40

what is the Red Queen Hypothesis

Answer 40

individual must continually evolve new adaptations in response to other organisms adaptations to avoid extinction

Question 41

example for the Red Queen Hypothesis - snail

Answer 41

- snails reproduce asexually or sexually - a higher proportion of females are asexual in more heavily parasitized population

Question 42

example of Red Queen Hypothesis - C. elegans

Answer 42

- reproduce predominately asexually - they first treated all the worms with a chemical mutagen to generate variation - then they reared the populations in an environment w strong selection from pathogens

Question 43

why is sex adaptive

Answer 43

- in a finite population - some will be missing due to sampling error - sex is adaptive bc it recreates these missing genotypes through segregation and recombination

Question 44

when may sex become disadvantageous

Answer 44

1. bc asexual wins be numbers 2. segregation and recombination break up advantageous genotypes they helped to create