Lecture 8.docx

3 Pages
Unlock Document

University of Toronto St. George
Ecology & Evolutionary Biology
Stephen Wright

Lecture 8:  Sex chromosomes are not necessary for separate sexes:  Turtles use temperature to determine sex  Coral reef use social environment to determine sex  A single or multiple gene can also be used to determine sex  Evolution of sex chromosomes: 1. Ordinary autosome’s allele evolve that affects gender 2. Reduced recombination between the 2 chromosomes around the locus that determine gender (GDL):  Intra-locus conflict and sexually antagonistic alleles.  Alleles those are good for males and bad for females and vice versa.  Males are Gg and females are gg  If the A and G allele are good for males, then selection will make the A allele on the same loci as G (males) and a allele at the same loci as g (females).  The AG will produce a son with AG without recombination, if there is recombination then the son will also have the a allele  The ag will also produce a daughter and with recombination the A allele might instead be passed on  Non-recombination gametes will produce better offspring than recombination.  D. melanogaster: sexually antagonistic alleles (SAA). Genotypes that were high fitness when expressed in males are often low fitness when expressed in females, and vice versa 3. Suppression of recombination expands as more SAA emerge around GDL 4. Degradation of non-recombining sex chromosomes: x chromosome recombines in females but y chromosome does not, thus the Y-chromosome might degrade 1) Mutation-selection balance (Muller’s Ratchet) :  The population is characterized by some equilibrium distribution (assuming no stochasticity) where mutation increases the amount of deleterious alleles and selection reduces the amount  Muller’s Ratchet: Least Loaded Class is the group of individuals with the fewest numbers of mutations, however they might not reproduce and even if they do chances are that the their offspring have new mutations  LLC might be lost by chance: however they can be easily regenerated by recombination  If there are no recombination then it is hard to recover the LLC or the offspring may even have more mutations, if there are no back mutations or the rate is low  The initial LLC is lost and a new and worse LLC distribution re-establishes itself, and the process repeats and thus a loss of genes  Asexual population: in any of its lines, a load of mutations smaller than that already existing in the present LLC does not exist  The rate of the ratchet depends on: mutation rate, total population size and selection coefficient 2) Beneficial mutations on non-recombining chromosomes  Beneficial mutations might occur at a loci loaded with bad mutations, thus it might get lost if no recombination occurs  Selective sweep (genetic hitchhiking): If It occurs at a loci that is not so bad, then the  allele can increase in frequency, however the baggage AKA  alleles is also taken along with the  allele and thus the  alleles also spread to fixation. Other genes are also degraded  If there is recombination, less variation will be lost and
More Less

Related notes for EHJ352H1

Log In


Don't have an account?

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.