Sex and Linkage Flashcards
define alleles
A, a, B, b
define genotype
A/a; B/b
define haplotypes
- gametes
- AB, ab
define asexual reproduction
- does not involve fusion of gametes
- offspring inherits all genes from 1 parent
examples of asexual reproduction
- binary fission
- budding
- vegetative
- pathogenesis
asexual reproduction - angiosperms
many can simultaneously reproduce sexually and asexually
asexual reproduction: angiosperms - sexual and asexual examples
- sexual: flowers, fruits, seeds
- asexual: runners, leaflets
asexual reproduction - animals
- some can reproduce by pathogenesis
- where clones develop from unfertilized eggs
define hermaphrodites
- animals that can produce sperm and eggs
- can mate with them-selves (self-fertilization/selfing) or with another individual
pros of asexual reproduction
- 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
explain selfing
they go through meiosis, but not outcrossing (randomly mating with other individuals)
define linkage equilibrium
the genotype of as chromosome at one locus is independent of its genotype at the other locus
define linkage disequilibrium
nonrandom association between a chromosome’s genotype at one locus and its genotype at the other locus
will selection at the A locus interfere with our ability to use HW models to predict evolution at the B locus?
- yes
- only if linked (at equilibrium)
what happens during meiosis
- chromosomes segregate independently of one another
- this means that loci A and B are inherited independently of one another
meiosis - linkage equilibrium
- 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
meiosis - linkage disequilibrium
- after recombination, haplotype frequencies are different than expected
- HW models cannot be used
meiosis - what does it mean when a gene is linked
if two allele physically close, they are less likely to be broken up and tend to be inherited together
what is genetic crossing over (recombination)
- Occurs in prophase of meiosis I (ONLY in sexual reproduction)
- Creates chromosomes with new combination of alleles for all genes one a chromosome
explain the relationship between linkage and distance
- 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)
what are the three mechanisms that can cause linkage disequilibrium
- selection on multilocus genotypes
- migration
- genetic drift/ population admixture
why can the three mechanisms cause linkage disequilibrium
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
selection and linkage disequilibrium example
- 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
selection and linkage disequilibrium example - what are the two allele an example of?
multilocus genotype bc 2 genes both code for the same trait
genetic drift and linkage disequilibrium exmaple
- 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
migration and linkage disequilibrium example
- Population A and B are both LE but in different ways
- When they mix (via migration), it creates LD
what breaks up linkage disequilibrium
- sex
- Sexual reproduction involves meiosis with crossing over and random mating between different individuals
linkage disequilibrium - define random mating
the union of gametes from unrelated parents brings together chromosomes with different haplotypes
linkage disequilibrium - define crossing over
babies from random mating grow up and reproduce with crossing over, which breaks up old combinations of alleles and creates new ones
linkage disequilibrium - define genetic recombination
the creation of new combinations of alleles during sexual reproduction
linkage disequilibrium: genetic recombination - how does genetic recombination breakup disequilibrium
Genetic recombination randomizes genotypes at one locus with respect to genotypes at another → it reduces LD
why do males exist?
- Genetic recombination breaks up linkage disequilibrium
- Genetic recombination pushes populations back towards Hardy-Weinberg Equilibrium
- Muller’s ratchet
- Red Queen
why should asexual reproduction win over sexual reproduction
- 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
asexual always wins, if… (finish sentence)
- a female’s reproductive mode does not affect how many offspring she makes
- a female’s reproductive mode does not affect the probability that her offspring will survive
why do males exist? - explain Muller’s rachet
- 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
muller’s ratchet - define genetic load
the burden imposed by accumulating deleterious mutations
example of Muller’s ratchet in the wild - Timema walking stick
- 6 are asexual and 6 are sexual
- researchers estimated ratio of non-synonymous to synonymous substitutions of important genes
Muller’s ratchet: Timema walking stick example -what is the expectation
- purifying selection eliminates deleterious alleles and leaves a signal
- should make the ratio of nonsynonymous to synonymous mutations low
Muller’s ratchet: Timema walking stick example - what is the result
- 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
what is the Red Queen Hypothesis
individual must continually evolve new adaptations in response to other organisms adaptations to avoid extinction
example for the Red Queen Hypothesis - snail
- snails reproduce asexually or sexually
- a higher proportion of females are asexual in more heavily parasitized population
example of Red Queen Hypothesis - C. elegans
- 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
why is sex adaptive
- 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
when may sex become disadvantageous
- bc asexual wins be numbers
- segregation and recombination break up advantageous genotypes they helped to create
