Define Sex Linkage: Definition, Overview, Examples, Explanation

Sex linkage refers to the inheritance of genes located on the sex chromosomes (X and Y chromosomes). Unlike autosomal genes, sex-linked genes are located on the sex chromosomes and show unique principles of inheritance and variation. Traits linked to these chromosomes, like colour blindness and haemophilia, show different inheritance patterns between males and females. This is because males have one X and one Y chromosome, while females have two X chromosomes.

This Story also Contains

1. What is Sex Linkage?
2. Historical Background
3. Concept of Sex Linkage
4. Sex-Linked Traits Examples
5. Recommended Video on Linkage
6. MCQs on Sex Linkage

Sex linkage helps to understand how genetic variation is influenced by chromosome structure and function. It was first observed by Thomas Hunt Morgan in fruit flies. This concept plays a crucial role in explaining non-Mendelian genetics. Applications of sex-linkage are seen in medicine, animal breeding, and biotechnology.

What is Sex Linkage?

Sex linkage refers to the association of specific traits with sex chromosomes resulting in different patterns of inheritance in males and females. The concept is core to genetics in explaining why some genetic conditions are seen in one sex. Sex linkage helps to understand how genes are passed down. It also shed light on questions relating to genetic disorders and evolutionary biology.

Sex linkage plays a very key role in the transmission of some important traits and disorders that significantly affect human health. For instance, haemophilia and Duchenne muscular dystrophy are typical X-linked disorders that mostly affect males of the population. Pedigree analysis helps to trace inheritance patterns. This allows appropriate genetic counselling, diagnosis, and proper treatment. It also helps scientists learn about evolutionary processes like the development and maintenance of sexual dimorphism (how males and females develop different traits).

Historical Background

The historical background of sex linkage started at the turn of the 20th century. It came from pivotal contributions by scientists like Thomas Hunt Morgan. While studying fruit flies (Drosophila melanogaster), Morgan noticed some traits like eye colour did not exhibit Mendelian genetics. He discovered that these traits were linked to sex chromosomes. Thus, Morgan formulated the theory of sex-linked inheritance. This earned him the Nobel Prize in Physiology or Medicine in 1933 and laid the basis for today's genetic research. Morgan's work established not only the existence of sex linkage but also contributed further to the development of the science of genetics, leading to advances in studying human genetic disorders, chromosome mapping, and mechanisms of inheritance.

Concept of Sex Linkage

Sex linkage refers to the genetic concept where certain characteristics related to sex chromosomes result in special patterns of inheritance. Unlike autosomal genes, sex-linked genes lie on the sex chromosome. Since males have one X and one Y chromosome (XY), and females have two X chromosomes (XX), there are differences in how a trait is passed to males and females.

There are two main types of sex linkage: X-linked and Y-linked inheritance. X-linked inheritance refers to genes found on the X chromosome. Since females have two copies of the chromosome, they are mostly carriers of the X-linked genes without having the trait. However, males carry a single copy of the X chromosome and hence express the trait only if they inherit the X-linked allele. Most of the X-linked disorders like colour blindness, haemophilia, etc are more common in men than women.

The other is Y-linked inheritance, where the gene is located on the Y chromosome and is passed only from father to son. This type of inheritance is very rare and typically includes traits unique to males.Y-linked inheritance is rather simple. Since only males have a Y chromosome, all Y-linked traits show father-to-son transmission. Since a few genes are situated on the Y chromosome as compared to the X chromosome, the number of Y-linked disorders is very rare.

X-linked inheritance can also be divided into two sub-categories: X-linked recessive and X-linked dominant. In X-linked recessive inheritance, males only need a single copy of the recessive allele, while females have to be homozygous to express the trait. Thus, more males are affected with X-linked recessive disorders.

On the other hand, in X-linked dominant inheritance, a single copy of the dominant allele is enough to express the trait in both males and females. This pattern occurs less often than X-linked recessive inheritance, however.

Sex-Linked Traits Examples

One of the best known examples of sex-linked traits are the X-linked traits. There are several X-linked diseases that can be either recessive or dominant in nature. One such X-linked recessive disorder is colour blindness, where a person has difficulty differentiating between two colours, mostly red and green. Since the gene governing colour vision is carried on the X chromosome, males are more likely to be affected. It is because males carry only one X, while females can be carriers but remain asymptomatic for the conditions.

One more example of an X-linked recessive disorder is haemophilia. In this condition blood coagulation does not happen and continues to bleed from the injured site. It is a historically famous condition as it affected many members of European royal families and clearly illustrates how X-linked traits are passed through generations.

Duchenne muscular dystrophy is a serious X-linked recessive disorder. It causes progressive muscle degenerations that lead to weakness. The disorder results from mutations in the DMD gene encoding the protein dystrophin. Therefore, the absence of functional dystrophin causes muscle damage and loss and eventual loss of muscle function. Although treatment has been advanced, managing DMD remains a huge challenge.

Y-linked traits are rather few but also significant to study. One example is hypertrichosis, a condition where there is an excessive growth of body hair. Only males inherit the Y chromosome, hence hypertrichosis and the other Y-linked traits are passed directly from father to son.

Recommended Video on Linkage

MCQs on Sex Linkage

Q1. All genes located on the same chromosome:

1. Form different groups depending on their relative distance.

2. Form one linkage group.

3. Will not be from any linkage groups.

4. Form interactive groups that affect the phenotype.

Correct Answer: 2) Form one linkage group

Explanation:

A linkage group is made up of all the genes that are found on the same chromosome. Due to their proximity, these genes are typically inherited jointly.

Genes that are near each other tend to remain together when the chromosomes are transferred to the progeny. The likelihood of the genes being split during crossing-over decreases with their proximity. When cells divide, this process takes place.

Scientists can better understand how features are passed down from parents to children by using linkage groups. It demonstrates that not every gene is inherited on its own. When researching genetic patterns, this is crucial.

Hence, the correct answer is option 2) Form one linkage group.

Q2. Which of the following will not result in variations among siblings?

1. Independent assortment of genes

2. Crossing over

3. Linkage

4. Mutation

Correct Answer: 3) Linkage

Explanation:

Whenever two genes are nearby and located on the same chromosome, linkage takes place irrespective of the variation. This means the genes tend to be inherited together during meiosis, as they are physically close to each other. The closer the genes are on the chromosome, the less likely they are to be separated by crossing over. However, if the genes are farther apart, recombination events might occur, leading to genetic variation. Linkage violates the independent assortment principle, as the linked genes do not assort independently. The study of linked genes provides insights into gene mapping and inheritance patterns.

Hence, The correct answer is the option (3) Linkage

Also Read:

• MCQs in Sex Linkage

• Linkage and Crossing Over

• Molecular Basis of Mutation