Sex Chromosomes

Sex chromosomes

In women, the 46 chromosomes in the nucleus of each cell are in 23 matched pairs. In men the 46 chromosomes are in only 22 matched pairs, with the 23rd pair consisting of two unmatched chromosomes.

Examination of the 23rd pair of chromosomes in males indicates that one of the pair is similar to the chromosomes of the 23rd pair in females, but the other is much smaller. The large chromosome is known as the X chromosome and the smaller the Y chromosome.

Females have two X chromosomes, and males one X and one Y. These are called sex chromosomes. The 22 pairs of non-sex chromosomes are called autosomes.

Sex determination

The sex of a child is determined by the sex chromosomes inherited from the parents. A female child gets an X chromosome from each parent, whereas a male child gets an X chromosome from the mother and a Y chromosome from the father.

Sex-linked characteristics

When characteristics located on the X chromosome are studied, it is found that the pattern of inheritance is different in the two sexes. Characteristics that show different patterns in the two sexes are called sex-linked characteristics or X-linked characteristics.

When writing the genotypes of sex-linked characteristics, the sex chromosomes are written as capital letters and the allele is written as superscripts for the X chromosome. E.g., an X chromosome with a recessive allele would be represented by ‘X^a’.

Sex-linked characteristics may be dominant or recessive. The dominant form is much rarer. Two common recessive sex-linked traits are red-green colour blindness and haemophilia.

Red-green colour blindness

The ability to discriminate between the colours red and green is controlled by a gene located in the X chromosome. Individuals who are unable to distinguish between the two colours possess the recessive allele of this gene.

A recessive allele is able to be masked by the presence of a dominant allele in females. This is not possible in males, so the frequency of colour blindness is higher in males than in females.

The children of a colour-blind man and a woman homozygous for normal vision would all have normal vision. However, the daughters could produce children who are colour blind, since they are carriers for colour blindness as the carry the recessive allele without showing the phenotype.

Haemophilia

A relatively rare disease in which the blood clots slowly or not at all. The defective allele is recessive to that controlling normal clotting of the blood and is carried on the X chromosome. Males, therefore, can be either normal or haemophiliacs, as they have only one X chromosome. Females can be homozygous normal, haemophiliacs, or heterozygous - carriers of the condition. Females being haemophiliacs is extremely rare.

The pattern of inheritance for haemophilia is similar to that for red-green colour blindness. Haemophiliac fathers pass the recessive gene to their daughters. Carrier mothers may pass a defective gene to their sons, who will be haemophiliacs, or to their daughters, who will then also carry the gene.

Patterns with sex-linked inheritance

Knowledge of sex determination and Mendelian inheritance allows us to understand the patterns that occur in sex-linked inheritance:

• A son’s X chromosome must come from his mother, therefore a father cannot pass a sex-linked trait on to a son. This also means that if a mother has a recessive sex-linked trait, any sons will also have the trait.

• A father can only give his X chromosome to a daughter. Therefore, if a daughter has a recessive sex-linked trait, then her father must also have it as he cannot be a carrier.

• Females may be carriers of a recessive sex-linked trait. If an unaffected mother has an affected child, then she must be a carrier.

• Two people without a recessive sex-linked trait can have a son with the trait, but not a daughter.