Chromosomes and Sex Determination

Drosophila

Attached X chromosomes do not separate in meiosis and can give rise to
flies with extra X chromosomes. Flies that are have 2 sets of autosomes
and 3 X chromosomes (XXX)are sterile females, XXY are fertile females,
XX are fertile females, XY are fertile males. When the number of
autosomes change, intersexes may occur and triploid (XXX/AAA) flies are
female. These observations led Calvin Bridges to develop a model for sex
determination that is dependent on the ratio of the number of X
chromosomes to the number of sets of autosomes. If the ratio is >1, the
fly is a metafemale, if it =1 the fly is a female, if it is 0.5 the fly is a male
etc.
The presence or absence of the Y does not affect sexual development.

At least 3 genes have been found (sxl, traand dsx) that show differential
splicing in male and female development. In this case, the product of the
first affects splicing of the next, etc in a developmental cascade.

Human

Sometimes during meiosis, the paired (MI) or daughter chromosomes (MII)
go to the same pole. This is described as "nondisjunction". If
nondisjunction involves the sex chromosomes, it results in eggs or sperm
with abnormal numbers of X and/or Y chromosomes.

In eggs, a single nondisjunction event leads to XX eggs and O (no X
chromosome) eggs.

Fertilization of an XX egg with an X sperm producesXXX (Triplo X)
females

Fertilization of an XX egg with a Y sperm produces XXY (Klinefelters)
males.

Fertilization of an O egg with an X sperm leads to

XO (Turners)females

Fertilization of an O egg with a Y sperm is lethal