Chromosomes X and Y and Sex Determination
- Genetics and Evolution

In a human, the normal chromosomes complement is 46, 44 of which are autosomes while 2 distinct chromosomes are deemed sex chromosomes, which determine the sex of an organism and various sex linked characteristics.

In most animals, those who possess XX chromosomes are female while male animals possess an X and a Y chromosome. However, this is not true of all organisms, as it can be reversed in some species.

Sex Determination

A humans' sex is predetermined in the sperm gamete

The egg gamete mother cell is said to be homogametic, because all its cell possess the XX sex chromosomes. sperm gametes are deemed heterogametic because around half of them contain the X chromosome and others possess the Y chromosome to compliment the first X chromosome.

In light of this, there are two possibilities that can occur during fertilisation between male and female gametes, XX and XY. Since sperm are the variable factor (i.e. which sperm fertilises the egg) they are responsible for determining sex.

Chromosomes X and Y

Chromosomes X and Y do not truly make up a homologous pair. They act similarly in their roles, but they are not homologous (the same). The X chromosome in humans is much longer than the Y chromosome and also contains many more genes.

These genes are said to be sex linked, due to the fact they are present in one of the sex chromosomes. During fertilisation, when the opposing homologous chromosomes come together, the smaller Y chromosome offers no dominance against the 'extra' X chromosomes as indicated below.

The X and Y Chromosomes indicating which genes are sex linked

The arrows indicate sex linked genes in the X chromosome. In this homologous pairing, all those genes are dominant, because there are no opposing genes in the Y chromosome to offer dominance.

So when the organism has an XY chromosome compliment (i.e. a male), these sex linked genes are freely expressed in the organisms phenotype, an example being hairy ears developing in old age.

Sex Linked Characteristics

These sex linked genes on the X chromosome display a number of characteristics. The following are just some examples of phenotypes as a result of these genes in expression;

  • Red-Green colour blindness
  • Haemophilia - A condition which prevents the clotting of the blood
  • Hairy ears in men through advancing age

More information on sex linked characteristics and how they are passed on from generation to generation will be available in new areas of the site soon.

The next page looks at genetic mutations and the consequences as a result of them.

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