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Sexual Differentiation
Sexual differentiation is the process by which an
organism develops into a male or a female. The 23 pairs of
chromosomes in human cells include a pair of sex chromosomes,
so-called because they determine whether a zygote develops into a
male or female. The father’s chromosomes are responsible for sex
determination.
There are two types of sex chromosomes: the X type and the Y type.
A zygote containing a pair of X chromosomes develops into a female,
but a zygote with an X and Y chromosome develops into a male. Thus,
the normal sex chromosome complement for women is XX and for men,
XY. This means that the gametes produced by meiotic cell division
in the female (the ova or oocytes) all contain a single X
chromosome. However, half of the sperms produced by the male will
contain an X chromosome and half will contain a Y chromosome. The
following figure explains how the two types of zygote are
produced.
Y chromosomes are smaller than X chromosomes and
differ in a fundamental way. The genes on the Y chromosome are
concerned solely with sex determination. Their presence ensures a
male, their absence a female. The X chromosome, occurring in both
sexes, contains many genes vital to general development and
functioning. Absence of the X chromosome is incompatible with
life.
Chromosomal Disorders
Disorders can arise if there is an abnormal
number of sex chromosomes. Most people carry two sex chromosomes in
their cell. However, some can carry one more, or one less, X
chromosome. An example of this is Turner’s syndrome, which is
caused by a missing X chromosome and affects females only. Although
full female sexual characteristics never develop, these people are
unmistakably female in appearance and identity. Full female sexual
characteristics develop only in the presence of a second X
chromosome.
A second sex chromosome disorder is Klinefelter’s syndrome, which
only affects males and is caused by one or more extra X
chromosomes. In this case, the condition often passes unnoticed
until puberty, when breast enlargement occurs and the testes remain
small, with the body shape looking female rather than male.
Genes are easily damaged in cell division, but because each cell
contains one chromosome from the mother and one from the father, if
the gene from the mother is damaged or missing, that from the
father can usually be ‘read’ and ensures normal functioning of the
cell. However, because the X chromosome is longer than the Y
chromosome, it contains some genes which are not duplicated in the
Y chromosome. Thus, most other sex-linked traits or disorders are
caused by abnormal genes on the X chromosome.
In females, abnormal or damaged genes on one X chromosome are
usually masked by a normal gene on the other X chromosome; males
have only a single X chromosome, so no such masking takes place. As
a result, X-linked traits or disorders affect males much more
frequently than females. Examples of such conditions include
haemophilia, Duchenne muscular dystrophy and colour vision
deficiency.
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