Evolution of Sex Chromosomes
Sex determination has already been covered
o
o Sry gene has already been shown to be important in the development of testis
o β-catenin signalling important in ovary formation
There is a variety of sex determination methods across species
o Bees and wasps can form haplodiploidy offspring with only half as many
chromosomes as they should have
In wasps, egg can be unfertilised which will create a male
If it is fertilised, then barely any sperm is transferred, and it forms
a female
Known as paternal genome elimination
o Reptiles can change their offspring’s sex by changing the temperature of their
nest
o Bluehead wrasse- if the male is removed, the females recognise this as a signal
and change behaviour followed by colour change
Causes a sex change so that there are males which are pregnant
,
There are lots of types of sex determination
o Methods change often, even within families
o These are markers of strong selection
Hermaphrodite ancestors- all individuals are male and female
o Very common in animals
o Doesn’t happen all at once
o Firstly, sex determining gene changes to become more like a sex chromosome
Mutation occurs that is beneficial to one sex but not another
If some hermaphrodites are better at being male than others, it can
spread through the population and create a whole new sex
Mutations can be anything along the lines of making some of the
animals better at carrying offspring or making others better at
fertilisation
o
o The sex determining gene could be a precursor to any of the other genes that
have been determined to influence sex in humans
o For XY to form you need 2 different mutations
One mutation that allows hermaphrodites to be better as female, and
one as male
Intermediate stage (gynodioecy) where there is either a male or female
sex and hermaphrodite
Some species will stay in this stage
Sex determination has already been covered
o
o Sry gene has already been shown to be important in the development of testis
o β-catenin signalling important in ovary formation
There is a variety of sex determination methods across species
o Bees and wasps can form haplodiploidy offspring with only half as many
chromosomes as they should have
In wasps, egg can be unfertilised which will create a male
If it is fertilised, then barely any sperm is transferred, and it forms
a female
Known as paternal genome elimination
o Reptiles can change their offspring’s sex by changing the temperature of their
nest
o Bluehead wrasse- if the male is removed, the females recognise this as a signal
and change behaviour followed by colour change
Causes a sex change so that there are males which are pregnant
,
There are lots of types of sex determination
o Methods change often, even within families
o These are markers of strong selection
Hermaphrodite ancestors- all individuals are male and female
o Very common in animals
o Doesn’t happen all at once
o Firstly, sex determining gene changes to become more like a sex chromosome
Mutation occurs that is beneficial to one sex but not another
If some hermaphrodites are better at being male than others, it can
spread through the population and create a whole new sex
Mutations can be anything along the lines of making some of the
animals better at carrying offspring or making others better at
fertilisation
o
o The sex determining gene could be a precursor to any of the other genes that
have been determined to influence sex in humans
o For XY to form you need 2 different mutations
One mutation that allows hermaphrodites to be better as female, and
one as male
Intermediate stage (gynodioecy) where there is either a male or female
sex and hermaphrodite
Some species will stay in this stage
