Levels of Sexual Development. There are 3 levels to sexual development:chromosomal sex: presence or absence of the Y chromosomegonadal sex (primary sex determination): whether the gonads develop as testes or ovaries depends on the presence or absence of the SRY gene, usually found on the Y chromso
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Genetics of Human Sexual Development
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Human karyotype: chromsomes
stained to show bands, from a male
Meiosis, the form of cell division that generates the sperm and eggs, carefully puts exactly 1 copy of each chromosome pair into each cell.
Sometimes meiosis goes wrong and puts 0 or 2 copies of some chromosome into a sperm or egg cell.
the best example of this: Down syndrome, which starts with a sperm or egg with 2 copies of chromosome 21.
Maternal age effect: more frequent in older mothers
The sex chromosomes are quite tolerant of variants.
Most common types involve 45 or 47 chromosomes
There are many other, rarer types, with 48 or even 49 chromosomes, such as 49,XXXXY. Such conditions almost always lead to serious mental deficiencies.
The general rule: if the Y is present, the person is internally and externally male.
46,XX males, with the SRY
gene on the X, have the
1970’s British TV series:
He had an extra Y, which
made him a macho criminal!
Also, a character on the TV show
Nip and Tuck has this condition
CAH is the most frequent cause
of non-standard genitals in
genetically female (XX) children.