Define Sex Linkage: Definition, Overview, Examples, Explanation

Sex Linkage definition: Sex linkage refers to the inheritance of genes located on the sex chromosomes (X and Y chromosomes). Traits linked to these chromosomes, like colour blindness and haemophilia in humans, 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, affecting how these traits are expressed. In this article, sex linkage, its historical background, concept of Sex Linkage, and Sex-Linked traits examples are discussed. Sex Linkage is a topic of the chapter Principles of Inheritance and Variation in Biology.

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This Story also Contains

1. What is Sex Linkage?
2. Historical Background
3. Concept of Sex Linkage
4. Sex-Linked Traits Examples

What is Sex Linkage?

Sex linkage refers to the association of particular traits with sex chromosomes whereby the pattern of inheritance becomes different in males and females. The concept is core to genetics in explaining why some genetic conditions are prevalent in one sex over the other. Sex linkage helps unravel the minute details in the genetics of inheritance to 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 bear significantly upon human health. For instance, haemophilia and Duchenne muscular dystrophy are typical X-linked diseases, so males of the population are mostly involved. Thus, the knowledge of sex-linked inheritance allows appropriate genetic counselling, diagnosis, and management to be performed in due time for a given disorder. Besides, it gives insights into an evolutionary process like the development and maintenance of sexual dimorphism.

Historical Background

The historical background of sex linkage lies at the turn of the 20th century, which came from pivotal contributions by scientists like Thomas Hunt Morgan. It developed out of Morgan's work with fruit flies (Drosophila melanogaster), wherein sex-linked traits had their discovery. He observed that some of the traits he was working on, for example, eye colour in fruit flies, did not exhibit the normal Mendelian pattern of inheritance. These traits he then associated with the sex chromosomes, and 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, extending radial roads toward the investigation of genetic disorders, chromosome mapping, and mechanisms of inheritance.

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Concept of Sex Linkage

Sex linkage refers to the genetic concept where certain characteristics related to sex chromosomes result in special patterns of inheritance. These involve essentially, in the case of human beings, mainly X and Y chromosomes. Unlike autosomal genes, sex-linked genes lie on the sex chromosomes themselves. Since males have one X and one Y chromosome (XY), and females have two X chromosomes (XX), there will be differences in how a trait is passed to offspring in males versus females.

There are two basic kinds of sex linkage: X-linked and Y-linked inheritance. X-linked inheritance refers to genes found on the X chromosome. Females have two copies of the chromosome; therefore, they could be carriers of most of the X-linked genes without having the trait. However, males carry a single copy of the X chromosome and hence express the trait 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, whereby the gene is located on the Y chromosome and is passed strictly from father to son. This type of inheritance is much more rare and typically includes traits unique to males.

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 for it to express the trait. Thus, more males are affected with X-linked recessive disorders. On the other hand, under 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.

Y-linked inheritance is rather uncomplicated. Since only males have a Y chromosome, all Y-linked traits show father-to-son transmission. Since relatively few genes are situated on the Y chromosome as compared to the X chromosome, the number of Y-linked traits and disorders is generally rather small.

Sex-Linked Traits Examples

Among the best-documented examples of a group of sex-linked traits are the X-linked traits. There are several sex-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 often affected, as they 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 hemophilia, wherein blood does not coagulate, and continues to bleed from the injured site. It is a historically interesting condition in that it affected many members of European royal families, clearly illustrating this hereditary nature of the traits on the X chromosome.

Duchenne muscular dystrophy is a serious X-linked recessive disorder whereby progressive muscle degenerations lead to weakness. The disorder results from mutations in the DMD gene encoding the protein dystrophin. Therefore, the absence of functional dystrophin will cause progressive muscle damage and loss and eventual loss of muscle function. Treatment has been advanced, but medical features pose a huge challenge to DMD.

Y-linked traits, on the other hand, are rather few but also significant to study. One example is hypertrichosis, an excess growth of body hair, that in some families inherits as a Y-linked trait. Only males inherit the Y chromosome, hence, in general, hypertrichosis and most of the other Y-linked traits are passed directly from father to son.

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