In most diploid organisms, sex is determined by chromosomes called X and Y.
In the most common form of sex determination, females are XX and males
are XY.
Forms of sex determination:
- Females XX, males XY: This is the most common form in plants
and animals, including humans and other mammals. Y chromosome is
short and serves as a "blank," recessive to all alleles.
Males therefore display the phenotype of the
gene on their single X chromosome without regard to dominance or
recessiveness.
- Females XX, males XO (some insects): X has no partner
- Females WZ, males ZZ (birds, Lepidoptera): reverse of XX-XY
system; males have 2 matching Z chromosomes; females have one Z
chromosome and a blank W chromosome.
- Haplodiploidy in bees and ants: males are haploid for
all chromosomes (not just sex chromosomes); females
are diploid
- Sex determined by single genes in bacteria and many fungi
- Sex determined by environmental temperature or by age in some
fishes and reptiles
Sex-liked traits, carried on the X chromosome in XX/XY species:
Examples: white eyes (w) in fruit flies (recessive to red);
hemophilia and colorblindness (both recessive) in humans
- AA x AY —> females AA,
males AY, both dominant
- AA x aY —> females Aa,
males AY, both dominant
- Aa x AY —> 4 equally frequent genotypes:
AA, Aa, AY, aY
(all females and half of males
with dominant phenotype)
- Aa x aY —> 4 equally frequent genotypes:
Aa, aa, AY, aY
(half of each sex recessive)
- aa x AY —> females Aa (dominant),
males aY (recessive)
- aa x aY —> females aa, males aY (both recessive)
Holandric traits (rare), carried on Y chromosome,
occurring in
males only, passed directly from father to son.
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