Gonosomal Inheritance – P.14

Gonosomal inheritance is the inheritance of genes located on sex chromosomes.

It is to be noted that in women there are two copies of an allele located on the X chromosome (one copy on each X chromosome) – the woman is homogametic. In men alleles located on the X chromosome have only one copy – the man is hetrogametic.

*Reminder: An allele occupies the same locus on the homologous chromosomes.

The risk of genetic disease contolled by these genes will be different fot the two genders (Phenotypic expression is dependent on gender).

Holandric inheritance referrs to traits determined by genes present on the Y chromosome only (i.e. are inherited from fathers to sons only).

The X chromosome

  • It does not only contain genes that are important for determining the female gender but also for a number of other traits.
  • Diseases caused and transmitted by the X chromosome are referred to as X-linked diseases.
  • Men are hemizygotes for genes along with the X chromosome.

Alleles can show:

  • complete dominance (dominant)
  • codominance
  • incomplete dominance (recessive)

Where the heterozygote is indistinguishable from one of the homozygotes, the allele involved is said to be dominant to the other, which is said to be recessive to the former.


In men both recessive and dominat alleles inherited with the X chromosome manifest in phenotype.

X-linked recessive hereditary disorders:

Haemophilia A and B – Defective blood clotting
Daltonism – Defective perception of colours
Duchenne muscular dystrophy – Progressive muscular wasting leading up to heart/lung failure
Favism – Haemolytic anaemia induced by consuming fava beans or certain drugs

X-linked dominant hereditary disorders:

Phosphate diabetes – Failure of renal tubular reabsorption of phosphates resulting in phosphate loss and hypophosphataemia (Rickets)
Tooth enamel hypoplasia – Abnormal development of teeth
Incontinentia pigmenti – Developmental disorder associated with changes of skin pigmentation.

Barr Body

Barr bodies are portions of condensed DNA (heterochromatin) belonging to the X chromosome – so it is a deactivated X chromosome.
The process of deactivation is called lyonization and it is a completely random process.
Random inactivation means that one population of cells expresses genes from the maternal X chromosome and the other expresses genes from the paternal X chromosome.

Barr bodies can only be seen in cells belonging to a femeale or a man with Klinefelter’s syndrome.

Haemophilia A and B

Both are medical conditions which result in the unability of the blood to clot because substances known as clotting factors (which combine with platelets) are not present.

Haemophilia A – clotting factor VIII deficiency
Haemophilia B – clotting factor IX deficiency

Read more at: http://www.nhs.uk/Conditions/Haemophilia/Pages/Introduction.aspx

Duchenne Muscular Dystrophy

Is an inherited disorder that involves muscle weakness, which quickly gets worse.

Because of the way it is inherited it usually affects the boys.

Read more at:   https://www.nlm.nih.gov/medlineplus/ency/article/000705.htm



  1. Heterozygote = individual with 2 different alleles of a gene
  2. Homozygote = individual with  2 identical alleles of a gene
  3. Hemizygote = individual with only one allele of a give gene