Inheritance of X- Linked Genes

INHERITANCE OF X-LINKED GENES

CHARACTERISTICS OF SEX-LINKED GENES

The X-linked genes exhibit following characteristics patterns of inheritance:

1. The differential region of each chromosome (i.e., Contains genes that have no counterparts on the other kind of sex chromosomes. These genes,whether dominant or recessive show their effects in the male phenotype.Genes in the differential regions are called hemizygous in the males.
2. The X-linked recessive genes show the following two more peculiar features :criss-cross pattern of inheritance that is in criss-cross inheritance,an X-linked recessive gene is transmitted from P₁ male parent to F₂ male progeny through its F₁ heterozygous females which are called carriers and different F₁ and F₂ results in the reciprocal crosses.
3. The X-linked recessive can be detected in human pedigrees through the following clues:
4. The X-linked recessive phenotype is usually found more frequently in the male than in the female.This is because an affected female bear the X-linked recessive allele whereas,an affected male can result when only the mother carries the gene.
5. Usually, none of the offspring of an affected male will be affected ,but all his daughters will carry the gene in masked heterozygous condition so one-half of their sons will be affected.
6. None of the sons of an affected male will inherit the X-linked recessive gene,so not will they be free of the detective phenotype;but they will not pass the gene along to their offspring.
7. Dominant X-linked genes can be detected in human pedigrees through the following clues:
8. It is more frequently found in the female than in the male of the species.
9. The affected males pass the condition on to all of their daughters but to none of their sons.
10. Females usually pass the condition on to one-half of their sons and daughters.
11. An X-linked dominant gene fails to be transmitted to any son from a mother which did not exhibit the trait itself.

In humans, X-linked dominant conditions are relatively rare.

Examples of Inheritance of X-linked Recessive Genes

The crisscross inheritance of recessive X-linked genes can be well understood by following classical examples in Drosophila,man,mouth and chickens etc.:

1. Inheritance of X-linked Gene for Eye Colour in Drosophila

In Drosophila,the gene for white eye colour is X-linked and recessive to another X-linked dominant gene for red eye colour. It is discovered by Morgan in 1910.Following crosses between white eyed and red-eyed Drosophila will make clear the characteristics criss-cross inheritance of gene for white-eyed colour in it:

1. Red-eyed female x White-eyed male.If a wild red eyed female Drosophila is crossed with a mutant white eyed male Drosophila all the F₁ individuals irrespective of their sex have red eyes.When the red eyed male and red eyed female individuals of F₁ are intercrossed,the F₂ progeny is found to include an exclusively red eyed female population and a male population with 50 percent red-eyed individuals and 50 percent white eyed individuals.
2. White-eyed female x Red-eyed male.When a white eyed female Drosophila is crossed with a red eyed male Drosophila,all the female individuals in the F₁ generation are red eyed.When these red eyed female individuals and white eyed male individuals of F₁ are intercrossed the female population of F₂ generation is found to include 50 percent red-eyed and 50 percent white-eyed flies.The F₁ red eyed female with the gene ‘+w’ when crossed with the F₁ red eyed male having the gene ‘+’.The female hybrids produce two types of eggs,50 percent eggs carry the gene ‘+’ and the remaining 50 percent carry the gene ‘w’.The males produce two types of sperms,half carry the ‘+’ and half carry no such gene on Y chromosomes.The union of sperms and ova of F₁ offsprings may produce four possible types of F₂ individuals:

• The eggs with ‘+’ genes if fertilised by sperms with ‘+’genes produce homozygous red eyed females.
• The eggs with ‘+’gene if fertilised by the sperms with ‘Y chromosomes produce the red eyed males.
• The eggs with the gene ‘when fertilised by the sperms having the gene ‘+’produce heterozygous red eyed females.
• The eggs with the gene ‘when fertilised by the sperms having the Y chromosomes white eyed males produced.

The trait of the barred eye in Drosophila is another sex-linked trait which is dominant over the normal eye shape but display inheritance pattern like eye colour.

2.Inheritance of X-Linked Recessive Genes in Humans

In human beings more than 150 confirmed or highly probable X-linked traits are called,most of these are recessive.Certain well-known examples of X-linked recessive genes in humans are those for red-green colour blindness or daltonism,haemophilia and Duchenne’s muscular dystrophy.Some other examples of X-linked recessive traits include 1.deficiency of enzyme glucose -6 phosphate dehydrogenase in erythrocytes causing haemolytic anaemia during allergy reaction of persons for the drugs such as sulfonamides for the broad bean ,called favism;2)two forms of diabetes insipidus;3)one form of anhidrotic ectodermal dysplasia etc……

• Colour blindness.In human beings a dominant X-linked gene is necessary for the formation of the colour sensitive cells,the cones in the retina of the eye.According to trichromatic theory of colour vision,there are three different types of cones,each with its characteristics pigment that react most strongly to red,green and violet light.The frequency of colour blind women is much less than colour blind man.The inheritance of colour blindness can be studied in the following two types of marriages:

1. Marriage between the colour blind man and normal visioned woman.When the colour blind man marries a normal visioned woman,then they will produce normal visioned male and female individuals in F₁.The marriage between an F₁ normal visioned woman and the normal vision male will produce in F₂ two normal-visioned female,one normal visioned male and one colour blind male.
2. Marriage between normal visioned male and colour blind female.If a woman (XX)is colour blind and she happens to marry a normal visioned male (XY),then all F₁ sons will be colour blind and daughters will be normal visioned.Because male receives one X-linked recessive gene for colour blindness from the colour blind mother.

• Haemophilia.Haemophilia is the most serious and notorious disease which is more common in men than women.This is also known as bleeder’s

disease.The person that contains the recessive gene for haemophilia lacks normal clotting substance in blood so minor injuries cause continuous bleeding and ultimate death of the person due to haemorrhages.

3.Inheritance of Z-linked Recessive Genes in Moths

The females are heterogametic and males are homogametic quite unlike that Drosophila and man.Here also sex-linked genes follow the ‘crisscross pattern but from the mother through heterozygous F₁ sons to granddaughters of F₂.

4.Sex linkage in Poultry

In polymath rock chicken the gene for barred feathers is dominant and the gene for black or red unbarred feathers is recessive.Both the genes are Z-linked.A barred male chicken contains two genes for barring because it has two sex chromosomes (ZZ).

1. Inheritance of Y-linked Genes

Genes in the non-homologous region of the Y chromosomes pass directly from male to male.In man,the Y-linked or holandric genes such as ichthyosis hysteria graves hypertrichosis are transmitted directly from father to son.

In the fish Lebistes, the Y chromosome contains a Y-Linked gene maculates that determines a pigment spot at the base of the dorsal fin of male individuals.

Porcupine man.An Englishman by the name of Edward Lambert was born in 1717.His skin was like thick bark which had to be shed periodically.The hairs on his body were quill-like and he subsequently has been referred to as the ‘procupine man’The trait appeared to be transmitted from father to son through generations.None of the daughters ever exhibit the trait.

References

Dhami, P S, and J K Dhami. A textbook of zoology Vol. II and Vol.III. Latest edition. New Delhi: Pradeep publication, n.d.

Kotpal, R L. Modern textbook of Zoology. Meerut, India: Rastogi Publication, n.d.

Rastogi, S C. Cell, and Molecular biology. New Delhi: New Age International (P) Limited, 2001.

Verma, P S, and V K Agrawal. cell biology,Genetics,Molecular Biology,Evolution, and Ecology. New Dehli, India: S. Chand and company Ltd., 2012.