Genetics Ch. 4 Sex Chromosomes

Chromosome pairs

23 pairs = 46 total chromosomes

Female sex chromosomes

XX

Male sex chromosomes

XY (X is much larger than Y); any person carrying a Y chromosome will be a male

Klinefelter syndrome

rare human males with XXY

Turner syndrome

humans with only one X chromosome (XO) are females

Pseudoautosomal regions (PARS)

the ends of the Y chromosome have nearly identical DNA sequences present at the ends of the X chromosome

X chromosome

most X chromosome genes have nothing to do with sex, they specify proteins needed by both males and females 

Fruit fly chromosomes

number of X chromosomes determine sex, not presence/absence of Y

Moth chromosomes

females = XX; males = XO

Nematode chromosomes

males = XO; but XX = self-fertilizing hermaphrodites, not females

Bird and butterfly chromosomes

female = ZW; male = ZZ

Bee and wasp chromosomes

same kinds of chromosomes but female = diploid; males = haploid

Fish chromosomes

same type and number of chromosomes, but sex is determined by temperature

Germ line

undergo series of mitotic divisions that yield specialized diploid cells that divide via meiosis to produce haploid cells.

Oogenesis 

human egg formation when diploid germ line cells in the ovary multiply via mitosis and produce much primary oocytes that undergo meiosis. 

Primary oocyte

meiosis I division results in 2 asymmetric daughter cells

Secondary oocyte

receives > 95% of cytoplasm while the other small sister cell is called polar body

Meiosis II

secondary oocyte asymmetrical division results in large haploid ovum (functional ovum) and small second polar body

Normal human ovum

22 autosomes and an X chromosome

Human sperm

22 autosomes and either an X or Y chromosome

Spermatogenesis

sperm production where primary spermatocytes undergo symmetrical meiosis I and II resulting in 4 equivalent spermatids

Colorblindness

on the X chromosome and typically affects males more than females; it also typically skips a generation  

Hemophilia

mutations on X chromosome that are rare, recessive trait, resulting in failure to form blood clots. affects males more than females

Hypophosphatemia

X-linked dominant trait resulting in Vitamin D resistant rickets. affects females more than males

Chromosome inactivation

in female cells, XX cells inactivate 1 of their 2 X chromosomes to compensate for having 2 copies of X and male cells having only 1

Barr bodies

each cell chooses one X chromosome at random to inactivate. each embryonic cell decided independently which X chromosome to inactivate (either maternal or paternal) 

Anhidrotic epidermal dysplasia

heterozygous females for X-linked recessive traits have patches of skin that lack sweat glands. patch depends on which X chromosomes is inactivated. 

Sex-limited traits

affect structure or process found in one sex but not the other

Sex-influenced traits

show in both sexes but character of such traits may differ between the 2 sexes because of hormonal differences (i.e. baldness)