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FIRST CLASS Lecture notes Genetics
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Lecture notes of 8 pages for the course Genetics at UoB (Sex lecture notes)
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SEX Monoecious (=one house): both male & female reproductive structures in
the same organism (also called a hermaphrodite)
Dioecious (=two houses): either male or female reproductive structures
in one organism
HOW IS SEX DETERMINED IN DIOECIOUS
SPECIES?
1.CHROMOSMAL SEX DETERMINATION
2 major different modes dependent on which sex is heterogametic:
Heterogametic sex = the one whose gametes contain different sorts of
sex chromosome
Homogametic sex = has gametes that all contain the same sort of sex
chromosome
E.g. in humans, flies, etc. the male is XY, the female XX = male
heterogamety. In birds & butterflies (and some amphibians, fish, etc.),
the female = heterogametic and male = homogametic. To make the
distinction clear, we refer to the female heterogametic systems as
ZZ/ZW.
Male heterogametic systems come in numerous varieties:
XX-XY system: XX=female, XY=male. Many mammals, flies
XX-XO system: XX=female, XO=male. O implies absence of a
chromosome. E.g. grasshoppers
X1X2-X1X2O system: X1X2 X1X2 =female, X1X2O=male (X1X2 behaves as a
single chromosome, goes into one gamete) E.g. spiders
X1X2X3X4X5 X1X2X3X4X5- X1X2X3X4X5 Y1Y2Y3Y4Y5 system: e.g. duck billed
platypus
Haplo-diploidy:
In all these systems of multiple sex chromosomes there are still plenty of
non-sex chromosomes (autosomes). It is thought that the sex
determination system evolved in ants, wasps & bees by a system with all
autosomes turning to X and no Y.
Unfertilised haploid eggs develop into males (development of eggs without
fertilisation = parthenogenesis)
, Fertilised diploid zygotes develop into females
All full sisters are identical as regards to the alleles inherited from their
father
2.GENIC SEX DETERMINATION
No sex chromosomes, only the sex-determining genes e.g. Yeast gametes
come in 2 types, a & alpha – just allelic differences at one locus define
the 2 types.
Note that chromosomal systems are also controlled by genes: in mammals,
Sry gene on the Y chromosome determines whether we are male or not.
The difference is that the Y, where Sry resides, doesn’t recombine with
the X, while in yeast the chromosome with the a/alpha gene (chr 3) does
recombine.
However, the pseudoautosomal regions on the tips of our Y and X
chromosomes are homologous and do recombine (behave like autosomes as
they don’t determine sex, but on sex chromosomes)
3.ENVIRONMENTAL SEX DETERMINATION
Limpet’s position in the stack:
In Crepidula fornicata, the common slipper limpet, sex is determined by
an environmental factor – the limpet’s position in a stack of limpets.
Temperature in turtles:
Temperature dependent sex determination (TSD) isn’t the same in all
species
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