BIO1011 Lecture Notes - Lecture 21: Barr Body, Nuclear Membrane, X-Inactivation
4 Jun 2018
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The Chromosomal Basis of Sex
•Humans and other mammals have two types of sex
chromosomes, designated X and Y
•The Y chromosome is much smaller than the X
•A person with two X chromosomes develops anatomy
that we associate with the “female” sex, while a person
with one X and one Y develops “male” properties
•Short segments at either end of the Y chromosome are
the only regions that are homologous with regions on
the X
•The homologous regions allow the X and Y
chromosomes in males to pair and behave like
homologues during meiosis in the testes
•In mammalian testes and ovaries, the two sex
chromosomes segregate during meiosis
•Each egg receives one X chromosome
•In contrast, sperm falls into two categories:
-half the sperm cells that a male produces receive an X chromosome
-the other half receives a Y chromosome
•If a sperm cell bearing an X chromosome fertilises an egg, the zygote is XX and female
•If a sperm cell containing a Y chromosomes fertilises an egg, the zygote is XY and male
•Therefore, in general, sex determination is a fifty-fifty chance
•In humans, the anatomical signs of sex begin to emerge when the embryo is about 2 months old
•Before then, the rudiments of the gonads are generic - they can develop into either testes or
ovaries, depending on whether or not a Y chromosome is present
•A gene on the Y chromosome known as SRY is required for the development of testes
•In the absence of SRY the gonads develop into ovaries, even in an XY embryo
•SRY and all other genes that are located on either sex chromosome is called a sex-linked genes
•The human X chromosome contains approximately 1,100 genes, which are known as X-linked
genes, while genes located on the Y chromosome are known as Y linked genes
•On the human Y chromosome, about 78 genes have been identified, which code for about 25
proteins (some genes are duplicates)
•About half of these are expressed only in the testes, and some are required for normal testicular
functioning and the production of normal sperm
•Because there are so few Y-linked genes, very few disorders are transferred from father to son
on the Y chromosome
Inheritance of X-linked genes
•The fact that males and females inherit a different number of X chromosomes leads to a pattern
of inheritance different from that produced by genes located on autosomes
•Many X-linked genes are not related to sex
•Fathers pass X-linked alleles to their daughters but not to their sons
•In contrast, mothers can pass X-linked alleles to both sons and daughters
•Haemophilia is an X-linked recessive disorder defined by the absence of one or more of the
proteins required for blood agglutination
