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Summary Sex determination and sex linkage
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BLGY1232 Sex determination and sex linkageSex determination
One way to ensure outcrossing is for a species to have sexually dimorphic individuals
Genotypic sex determination – based on organisms having specific sex chromosomes
that segregate from one another in meiosis Gender is determined by the
genotype of the individual
Mammals, insects and some plants XX (female), XY (male)
Butterflies, birds and amphibians WZ (female), ZZ (male)
Nematodes XX (hermaphrodite), XO (male)
Environmental sex determination – gender is determined by environment conditions
Turtles
Lizards
Shrimps
Some plants
Clown fish and some other species can change sex; the dominant fish in a group
becomes female and the rest remain male
Dominant Y systems;
Presence of a Y chromosome determines maleness Y chromosome gene
called Tdf activates testis development which then produce hormones that
trigger male development
XXY individuals are male (Klinefelter syndrome) the presence of a Y
chromosome suppresses carpel development and promotes stamen
development
Heterogametic sex produces 2 types of gametes, each carrying a different sex
chromosome and Homogametic sex produces 1 type of gamete, all carrying the
same sex chromosome;
Mammals – males (XY) are the heterogametic sex
Drosophila – sperm carry either X or Y chromosome, females (XX) are the
homogametic sex as all eggs have X chromosomes
Birds – males (ZZ) are the homogametic sex
Butterflies – sperm all carry a Z chromosome, females (WZ) are the
heterogametic sex, eggs carry either a W or a Z chromosome
The 1:1 Ratio of males to females in the population is maintained because the sex
chromosomes associate at meiosis and segregate away from each other
Sex-linkage
Sex chromosomes do not recombine over most of their length, genes on the sex
chromosomes ate therefore linked
First observed in Drosophila; Y chromosome carries few genes and in males where
there is a single X chromosome mutations of genes on the X chromosomes can cause
mutant phenotypes that are only visible in males
Male offspring always receive their X-chromosome from their mother
Dosage compensation – because the heterogametic sex only has one copy of one of
the sex chromosomes but the homogametic sex has 2 copies there has to be a
mechanism to prevent overexpression of genes located on the diploid chromosome
due to their extra copy number in mammals its achieved by inactivation of one or
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