Week 2 Question 2 - Describe the evidence that the evolution of sex chromosomes is
different from that of typical autosomes and explain the theories that have been used
to account for theses differences?
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Autosomes: long term memory genome (oldest chromosomes)
Allosomes (Sex chromosomes): short term memory genome (younger)
Sex chromosomes are different in size and shape from each other unlike
autosomes (identical).
homogametic XX female / heterogametic XY male
Lahn and Page proposed that sex chromosomes (X & Y) arose from a pair of
identical non-sex determining autosomes, and have diverged gradually from
each other over 300 million years.
Sex determination is due to the presence or absence of Y chromosomes.
Y chromosomes developed from X-like ancestor and then diverged.
First step towards sex determination is when an autosome mutated and
acquired the SRY gene, which is the gene that confirms maleness.
Recombination between X & Y was supressed during evolution i.e. they have
stopped recombining.
Sequential loss of recombination by inversion: four chromosomal inversions
responsible for the evolution of X & Y chromosomes.
As a result, these chromosomes became further apart as they evolved rapidly.
This lead to Y chromosome degeneration or shrinking thus deleterious
mutations accumulate on non-recombine Y chromosome and inability to repair
DNA damage.
X chromosomes however can retain its size and integrity because it can still
recombine.
Sex chromosomes are different in birds as they have evolved independently:
homogametic ZZ male and heterogametic ZW female
Theories that have been used to account for the evolution of sex and the
degeneration of Y chromosomes :
 Advantageous mutation not in LD
 Genetic drift
 Muller’s ratchet explains success of sexual over asexual reproduction.
Accumulation of deleterious mutations in asexual or non-recombining
chromosomes in sexual organism
 Red queen hypothesis- co evolution of host and parasites using snails as
evidence.