Sex-Linked Inheritance: How is the sex of an organism determined? The initial hint of the genetic or chromosomal mechanism of sex determination can be outlined back by some of the experiments carried in insects. In most animals and in some plants, sex is determined by the sex chromosomes/allosomes/heterosomes/idiosomes. In 1891, Henking studied sex determination in insects. He worked on a male bug and noticed that only 50% of the sperm received the nuclear structure and named it as X- body. But, he was not able to explain its significance. Later on, by a further investigation by some scientists, it was found that this X- the body is a chromosome, so it was renamed to X- chromosome. In 1902, Stevens discovered the Y-chromosome. Wilson and Stevens (1905) put forward the chromosomal theory of sex determination with X and Y chromosomes as sex chromosomes.

Sex-linked inheritance is the inheritance of characters (phenotype) that is determined by genes located on one of the sex chromosomes. The genes present on the X-chromosome are known as X-linked genes, while genes that are present on the Y-chromosome are called holandric genes. The inheritance of such X and holandric genes is called sex-linked inheritance.

Characteristics of Sex-Linked Inheritance

It has been witnessed that the X-chromosomes are present in paired form (XX) in human females, and a single X and Y chromosome (XY) is present in human males. The X- chromosome has no counterpart in males, so the genes present, either in dominant or recessive form, are always expressed in males.

Types of Sex-Linked Inheritance

Sex-linked inheritance is of two types can be:

1. X- linked inheritance
2. Y- linked inheritance

1. X-Linked Inheritance

X-linked inheritance refers to a genetic condition showing mutation of genes present on the X- chromosome. It can be either dominant or recessive.

i. X- Linked Recessive Inheritance
It refers to a genetic condition associated with the mutation of genes present on the X chromosome. In males (having one X chromosome), a mutation in the copy of the gene on the single X chromosome causes this condition. Many X-linked genetic disorders in males are pseudodominant. Pseudodominance occurs when only one recessive allele of a gene is inherited, yet it is expressed, e.g., Haemophilia. On the other hand, females must show a mutation on both X chromosomes in order to be affected by the disorder. If only the father or the mother has the mutated X-linked gene, the daughters are usually not affected and are known as carriers because only one of their X chromosomes has the mutation and not both. But on the other hand, sons will get the disorder if they inherit the mutated X-linked gene from their mother. Fathers cannot pass X-linked recessive conditions to their sons. So, in this inheritance, the second generation inherits the trait from the carrier of the first generation. This pattern of inheritance is called a criss-cross pattern of inheritance.

Example:

In fruit fly (Drosophila melanogaster), the gene for white eye colour is X-linked and recessive to another X-linked, dominant gene for red-eye colour.
When a white-eyed male is crossed with a red-eyed female, the F1 flies were all red-eyed. The F2 generation shows a 3: 1 ratio of red and white-eyed flies. But all white-eyed flies of the F2 generation were males only. In F1 Generation, a normal female is crossed with a normal male then, it showed 50% of males as white-eyed and 50% as red-eyed. It shows that the recessive allele is expressed in male-only.

2. Y-linked inheritance

Y-linked inheritance is an inheritance in which the mutated or altered gene is present on the Y-chromosome that means this inheritance occurs only in males. This could be a lethal condition but not always, e.g., Hypertrichosis.

Y linked genes can only be transmitted from male to male, i.e., from father to son. So, this inheritance is also called holandric inheritance (holos- whole; Andros- male).

Conditions:

XY– Normal Male

XY^D– Diseased Male (only one mutated or defected Y- chromosome is enough to cause disease)

Example:

Hypertrichosis (development of hairs on the ear pinna), webbed toes, and porcupine man.

Sex-Linked Disorders in Humans

I. X-Linked Recessive Disorders

a. Colour Blindness

In colour blindness, a person cannot distinguish red and/or green colours. The colourful vision of the eye is due to the cone cells present in the retina of the eye. Cone cells are of three types, red, blue, and green. If these cells show mutation, it can lead to different conditions.

1. If the red cone cells are absent or show mutation, then that condition is called protanopia. It is a common form of colour blindness.
2. If the green cone cells defect or show, the mutation is called deuteranopia.
3. There is one more condition called monochromatic vision, which occurs in the absence of functionality of red, green, and blue cone photoreceptor cells. This condition is a rare one.

Colour blindness, like any other X-linked recessive inheritance, shows criss-cross inheritance, i.e., the male transmits his character to his grandson through his daughter, while the female transmits her character to granddaughter through his son. So, we can say that the transmission of traits takes place between one sex of the offspring of the opposite sex.

Conditions:

XX– Normal female

X^CX– Carrier female (one X chromosome shows mutation or defect which is suppressed by another X chromosome)

X^CX^C– Diseased Female (Both the X- chromosome shows mutation or defect)

XY– Normal Male

X^CY– Diseased Male (only one mutated or defective X- chromosome is enough to cause disease)

Example:

If a colour blind man marries a normal vision female, the daughter will have normal vision but will be a carrier, and all sons will be normal.

b. Haemophilia

It is also known as royal disease as it is present in Queen Victoria’s family. The person suffering from this disease does not have the ability to clot the blood normally, even after a minor injury.

Haemophilia exists in three forms:

1. Hemophilia A: It is caused by a mutation in the Factor VIII genes, so they are able to form AMG (Antihemophilic globulin).
2. Hemophilia B or Christmas disease: This is caused by a mutation in the Factor IX genes, so they are not able to form thromboplastin.
3. Hemophilia C: It is a rare condition caused due to a lack of clotting factor XI.

Conditions:

XX– Normal female

X^hX– Carrier female (one X chromosome shows mutation or defect which is suppressed by another X chromosome)

X^hX^h– Diseased Female (Both the X- chromosome shows mutation or defect)

XY– Normal Male

X^hY– Diseased Male (only one mutated or defective X- chromosome is enough to cause disease)

Example: If a haemophilic man marries a normal female, the daughter will be normal but will be a carrier, and all sons will be normal

ii. X- Linked Dominant Inheritance

A single X- chromosome is enough to cause the mutation in both males and females in X-linked dominant inheritance. The affected male passes its gene to all of their daughters but not to their sons.

Example:

Hypophosphatemia (vitamin D-resistant rickets), Hereditary enamel hypoplasia (hypoplastic amelogenesis imperfecta), in this the tooth enamel is extremely thin and small

Summary

Sex-linked inheritance is the transfer of traits from the sex chromosomes. Female shows XX chromosome while the male has only one X chromosome along with Y chromosome. If the traits are present on the X chromosome, it is called X-linked inheritance, and if it is present on the Y chromosome, it is called Y-linked inheritance. X- linked inheritance can be seen in both males and females. The chances of expression in females are slightly less as to be diseased. Both the X chromosome should defect while Y linked inheritance is expressed only in males as they have Y chromosome and female does not have the Y chromosome.

Frequently Asked Questions (FAQs) on Sex-Linked Inheritance

Q.1. State some examples of sex-linked recessive disorder?
Ans: Colour blindness and haemophilia are examples of sex-linked recessive disorders.

Q.2. What is sex-linked inheritance?
Ans: Sex-linked inheritance is an inheritance in which genes or alleles are inherited through either of the sex-chromosomes, X-chromosome, or Y-chromosome.

Q.3. Which disease is known as a royal disease?
Ans: Haemophilia is known as a royal disease, as shown in Queen Victoria’s family.

Q.4. State a few examples of Y-linked disorder?
Ans: Examples of Y-linked disorder are: Hypertrichosis, webbed toes, and porcupine man.

Q.5. What is criss-cross inheritance?
Ans: In criss-cross inheritance, the second generation inherits the trait from the carrier of the first generation. For example, the traits are transferred from father to daughter, which acts as a carrier and is then transferred to the grandson in which the trait is expressed.

Learn about Mutation in detail here

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