Question

What do you mean by sex-linked inheritance? Discuss it with reference to colourblindness and haemophilia diseases.

Answer 1

1. Sex-linked Inheritance:

• The human female has XX chromosomes and the human male has XY chromosome along with 44 autosomes in both sexes.
• The genes present on the X chromosome may express in both male and female gender and hence it is called crisscross heredity. While genes present on Y- chromosome found only in the male.
• Genes of approximately 20 characters found on the X chromosome of human are sex-linked characters and a father can not transmit sex-linked traits (of the X chromosome) to his sons, it can be passed on to the daughters only.

Some of the sex-linked hereditary diseases found in human beings are as follows:
Colour blindness:

• It is a hereditary disease. In this disease, the person is unable to differentiate different colours.

The colourblindness is of two types –

1. Red-Green colourblindness
2. Blue colourblindness

• This disease is mainly found in men because males have only one X chromosome. If X – chromosome contains recessive allele of pigment formation gene than no cone formation takes place and man suffers from colour blindness. While two X – chromosomes are found in a woman and hence woman may be a carrier of this disease or may suffer from this disease.

Red-Green colourblindness:

• It is caused due to the absence of red/green cones and it is also called as protanopia/Deuteranopia.
• The person is unable to distinguish red, green, yellow and orange colours. These colours appear to red in the absence of green cones and green in the absence of red cones.

Blue colourblindness:

• It is caused due to the absence of blue cones.
• Such a person is unable to distinguish violet, Indico, Blue and Green colours. They all appear to be green.

2. Inheritance of colourblindness:

• Example 1: Cross between a normal woman and a colourblind man.
• If a colour blind male marries with a normal female the possibilities would be – All children (son and daughters) would be normal, but the daughters would be carriers. Because they are heterozygous, they will not be colourblind.
• The woman acts as a carrier of colourblindness.
• Example 2: Cross between a carrier woman and a colourblind man.
• In this cross, 50% of girls will be colourblind and 50% of girls will be a carrier.
• Similarly, 50% of boys will be normal and 50% of boys will be colourblind.

Conclusion: Following are conclusions regarding heredity of colour blindness:

1. Identification of colours in the vision is a sex-linked character. Its gene is located on the X chromosome and its allele is not found in males.
2. Males are more sufferer from this disease. Because if the normal gene for colour differentiation is absent on X chromosome man than the colour blindness the disease is found.
3. Women have two X-chromosomes and both X chromosomes have the gene for colour identification and expression.
4. While only one chromosome in man has a gene for colour identification and expression. If only one X chromosome carries the recessive gene for colourblindness in woman than the normal woman becomes the carrier of the disease.
5. Gene for the identification of colours is dominant. If both X chromosomes of woman carry this recessive gene then a woman will be colour-blind.
6. Father and son of a colourblind woman are also colour blind. If the husband of a colour blind woman is also colour blind than their daughters will be colour blind. Daughter (normal) of a colour blind father produce 50%sons of normal vision and 50% sons as colourblind.

3. Haemophilia or Bleeder’s disease:

• It is a sex-linked hereditary disease which results in delayed or no blood clotting. It was first discovered by John Otto.
• Haldane discovered haemophilia in the family of Queen Victoria. It is caused due to one recessive gene on the X – chromosome of man.
• It is also called as Royal disease or bleeder’s disease. It is expressed only in men The women act as a carrier of this disease. Recessive genes on both the X – chromosomes cause haemophilia in females but they do not survive.
• Father of haemophilia female is always haemophilic.

Haemophilia is of 2 types –

1. Haemophilia – B or Christmas disease: It is caused due to the absence of factor – IX. It is a weak type of haemophilia.
2. Haemophilia – A: It is caused due to the absence of factor – VIII (AHG).

• This disease is also due to the recessive gene located on the X chromosome. Haemophilic man and normal woman produce 50% carrier daughters and 50% normal sons.
• Carrier woman and normal man produce 25% carrier daughters, 25% haemophilic sons, 25% normal daughters and 25% normal sons.
• This disease is found only in males as males have only one X chromosome. If a woman has both recessive genes on their X chromosomes than such woman cannot survive, because blood flow during menstruation does not stop and hence it is fatal for them.
• When females contain only one recessive gene of haemophilia on the X chromosome and other X chromosomes without such gene, they are a carrier of haemophilia.

Answer 2

HAEMOPHILIA: A SEX-LINKED DISORDER

So far, all the genes we have discussed have had two copies present in all individuals. This is because the individual inherited one from the male parent’s haploid gamete and one from the female parent’s haploid gamete. The two gametes came together during fertilization to produce a diploid individual. There is, however, one exception to this: genes which are present on the sex chromosomes.

In humans, as well as in many other animals and some plants, the sex of the individual is determined by sex chromosomes – one pair of non-homologous chromosomes. Until now, we have only considered inheritance patterns among non-sex chromosomes, or autosomes. In addition to 22 homologous pairs of autosomes, human females have a homologous pair of X chromosomes, whereas human males have an XY chromosome pair. Although the Y chromosome contains a small region of similarity to the X chromosome so that they can pair during meiosis, the Y chromosome is much shorter and contains fewer genes. When a gene being examined is present on the X, but not the Y, chromosome, it is X-linked.

The X chromosome is one of two sex chromosomes. Humans and most mammals have two sex chromosomes, the X and Y. Females have two X chromosomes in their cells, while males have X and Y chromosomes in their cells. Egg cells all contain an X chromosome, while sperm cells contain an X or a Y chromosome. This arrangement means that during fertilization, it is the male that determines the sex of the offspring since the female can only give an X chromosome to the offspring.

Most sex-linked genes are present on the X chromosome simply because it is much larger than the Y chromosome. The X chromosome spans about 155 million DNA base pairs and represents approximately 5 per cent of the total DNA in cells. The X chromosome likely contains 800 to 900 genes. In contrast, the Y chromosome has approximately 59 million base pairs and only 50-60 genes. Sex is determined by the SRY gene, which is located on the Y chromosome and is responsible for the development of a fetus into a male. This means that the presence of a Y chromosome is what causes a fetus to develop as male. Other genes on the Y chromosome are important for male fertility.

Haemophilia is a bleeding disorder that slows the blood clotting process. People with this condition experience prolonged bleeding or oozing following an injury, surgery, or having a tooth pulled. In severe cases of haemophilia, continuous bleeding occurs after minor trauma or even in the absence of injury (spontaneous bleeding). Serious complications can result from bleeding into the joints, muscles, brain, or other internal organs. Milder forms of haemophilia do not necessarily involve spontaneous bleeding, and the condition may not become apparent until abnormal bleeding occurs following surgery or a serious injury.

The major types of this condition are haemophilia A (also known as classic haemophilia or factor VIII deficiency) and haemophilia B (also known as Christmas disease or factor IX deficiency). Although the two types have very similar signs and symptoms, they are caused by mutations in different genes.

Hemophilia A and haemophilia B are inherited in an X-linked recessive pattern. The genes associated with these conditions are located on the X chromosome, which is one of the two sex chromosomes. In males (who have only one X chromosome), one altered copy of the gene in each cell is sufficient to cause the condition. In females (who have two X chromosomes), a mutation would have to occur in both copies of the gene to cause the disorder. Because it is unlikely that females will have two altered copies of this gene, it is very rare for females to have haemophilia. A characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons.

In X-linked recessive inheritance, a female with one altered copy of the gene in each cell is called a carrier. Carrier females have about half the usual amount of coagulation factor VIII or coagulation factor IX, which is generally enough for normal blood clotting. However, about 10 per cent of carrier females have less than half the normal amount of one of these coagulation factors; these individuals are at risk for abnormal bleeding, particularly after an injury, surgery, or tooth extraction.

Colourblindness is another example of a sex-linked trait in humans. The genes that produce the photopigments necessary for colour vision are located on the X chromosome. If one of these genes is not functional because it contains a harmful mutation, the individual will be colourblind. Men are much more likely than women to be colourblind: up to 100 times more men than women have various types of colourblindness.

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