How Does Sex Linkage Affect Traits in Males and Females?
Reproduction is essential for the continuity of generation. In different organisms, there are different means of reproduction and just as reproduction, there are different means of sex determination in different animals. At the early stages of life, the process of sex determination holds much importance as it initiates the further process of development of the body. Sex determination is governed by environmental factors such as temperature. In humans and insects, sex determination takes place with the help of genetic factors and sex linkage traits.
Chromosomal Basis of Sex Determination
After understanding the sex linkage meaning, we will now understand sex linkage traits and the chromosomal basis of sex determination. Henking was the one who formulated this theory. He traced the specific nuclear structure in some insects with the help of spermatogenesis. He made an observation that the structures were received by only 50% of the sperms. These structures were then called “X body” by him. After a few years of studies and research, some other scientists concluded that this structure was a chromosome and they named it an X chromosome. The linkage associated with the X chromosome is called X linkage. The further Y chromosome was also discovered and these two together were known as the sex chromosomes. We will learn about different types of sex linkage inheritance below.
Male Heterogamety
In this condition, two different types of gametes are produced by males. So we can say that the sperm helps in determining the sex of the offspring. XO and XY type of methods are there in it.
XO Type: It is also known as the XX-XO type. The insects show this type of sex determination. The males and females have different numbers of chromosomes. The eggs of the female here have an extra X-chromosome besides the number of autosomes. And only 50% of the sperms have the X-chromosomes. The eggs of the grasshopper are fertilized by (A+X) type of sperm and they become females and the males are fertilized by (A+O) type of sperm. So we can conclude that the sperm can helps in determining the sex of the offspring. As the X-chromosome helps in determining the sex, so because of this we have named them as sex chromosomes.
XY Type: This type of sex determination method is present in drosophila and mammals. It is also present in human beings. X and Y are the two types of sex chromosomes that are present in the body. The length of the Y chromosome is shorter than the X chromosome. The number of chromosomes in both males and females is the same.
Sex Determination in Humans
This is also called sex linkage in human. There are 22 pairs of autosomes that are present in human beings. They also have one pair of sex chromosomes. The females are homogametic in nature. This means that all the ova that are formed by the female have similar chromosomes in them that is X chromosomes. The male has an additional Y chromosome and this is present in a 50% ratio and the rest 50% is by the X chromosome. So we can say that males produce two types of chromosomes. And at the time of fertilization, there is an equal probability of either the X or the Y chromosome fusing with the egg that has only the X chromosome. So a girl child is born when both the X chromosomes come together and a boy child is born when the Y chromosome fuses with the X chromosome. So we can say that there is a 50% probability of either a girl child or a boy child. On the Y chromosome, there is a gene named Sry. This is the sex-determining region that helps in coding the testes determining factors. This helps in developing the male characters and it is not present then females would develop. This is how sex linkage in man works.
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Female Heterogamety
In this type, the females produce two different types of gametes. Here the egg helps in determining the sex of the offspring. ZW type and ZO type are the two types of sex-determining mechanisms.
ZW Type: It has two types of chromosomes that are ZZ and ZW type. This type of determination is present in birds. The males have homomorphic sex chromosomes and the females have heteromorphic chromosomes. So, the females are heterogametic in nature.
Sex Determination in Honey Bee
In the sex linkage in honey bees, the sex is determined by the number of sets of chromosomes that an individual receives. When egg and sperms fuse together, they form a female and if the egg is left unfertilized then it develops into a male. So, from this, we can conclude that the males have half the number of chromosomes as that of the females. This type of sex determination is known as haplodiploid sex determination. Now we will study sex linkage in drosophila.
Sex-Linked Inheritance
Here we will study sex linkage in drosophila. In drosophila, sex-linked inheritance was seen. Morgan worked on the inheritance patterns of the eye colour of drosophila. Three types of crosses were made by him.
Cross 1: In the first case he crossed a white-eyed male with red-eyed females. When they reproduced it was found that the first generation has all the flies having red eyes. Then these flies were allowed to self breed and it was seen that both types of traits were seen in the second generation.
Cross 2: Here the white-eyed male was crossed with the red-eyed female. This was almost similar to the test cross. Here he obtained both red and white-eyed females and male and they all were present in equal proportions.
Cross 3: Red-eyed males were crossed with white-eyed females and a reciprocal cross was taken off the first cross. But here we obtained all red-eyed females and white-eyed males which was surprising as it should have obtained a similar result as that of Mendel.
So after all these experiments, it was concluded by Morgan that the eye colour gene is linked to sex and is present on the X-chromosome. So it was also concluded that these X chromosomes are present in a zig-zag formation and not passed in a direct pattern. This is how sex linkage in drosophila was studied.
FAQs on Sex Linkage in Genetics: Complete Guide for Students
1. What is meant by sex linkage in genetics?
Sex linkage refers to the inheritance of a trait that is determined by a gene located on one of the sex chromosomes (X or Y). Since the gene is physically part of the chromosome that determines sex, its expression and inheritance pattern are linked to the gender of the individual. This is different from traits determined by genes on autosomes (non-sex chromosomes).
2. What are the main types of sex-linked inheritance?
Sex-linked inheritance is primarily categorized based on which sex chromosome carries the gene:
- X-linked inheritance: The gene responsible for the trait is located on the X chromosome. These traits can be recessive (like colour blindness and haemophilia) or dominant.
- Y-linked inheritance (Holandric inheritance): The gene is located on the Y chromosome. Since only males have a Y chromosome, these traits are passed directly from father to son.
3. What are some common examples of X-linked traits in humans?
Some of the most well-known examples of X-linked traits, as per the CBSE syllabus, are genetic disorders caused by recessive alleles on the X chromosome. These include:
- Haemophilia: A disorder that impairs the body's ability to control blood clotting.
- Red-Green Colour Blindness: The inability to distinguish between certain colours, most commonly red and green.
- Duchenne Muscular Dystrophy: A progressive muscle degeneration disorder.
4. Why are males more commonly affected by X-linked recessive disorders than females?
Males are more commonly affected by X-linked recessive disorders because they have only one X chromosome (XY). Therefore, a single recessive allele on their X chromosome is sufficient for the trait to be expressed. Females, on the other hand, have two X chromosomes (XX). For a recessive trait to be expressed in a female, she must inherit the recessive allele on both of her X chromosomes. If she has only one recessive allele, she becomes a carrier but typically does not express the trait.
5. How can you identify a sex-linked trait in a pedigree chart?
You can often identify an X-linked recessive trait in a pedigree by observing specific patterns:
- The trait appears more frequently in males than in females.
- The trait can skip generations, passing from a carrier female to her son.
- An affected father cannot pass the trait to his sons, but he will pass the recessive allele to all his daughters, making them at least carriers.
- An affected female will pass the trait to all of her sons.
6. What is the importance of criss-cross inheritance in understanding sex linkage?
Criss-cross inheritance is a key characteristic of X-linked traits. It describes the pattern where a father passes a sex-linked trait to his daughter, who then passes it to her son (the father's grandson). The trait essentially 'criss-crosses' from the male of one generation to the male of a later generation through a female carrier in the intermediate generation. This pattern is a direct consequence of males passing their X chromosome to daughters and females passing one of their X chromosomes to their sons, making it a crucial concept for tracing X-linked genes.
7. How do sex-influenced and sex-limited traits differ from sex-linked traits?
These terms are often confused but are fundamentally different. The key difference lies in the location of the gene:
- Sex-Linked Traits: The genes are located on the sex chromosomes (X or Y). Their inheritance is directly tied to these chromosomes.
- Sex-Influenced Traits: The genes are on autosomes, but their expression is influenced by the sex hormones of the individual. For example, pattern baldness is an autosomal trait but is dominant in males and recessive in females.
- Sex-Limited Traits: The genes are also on autosomes but are expressed only in one sex. For example, genes for beard growth in males or milk production in females are present in both sexes but are only expressed in one.
8. Is a chromosomal condition like Klinefelter's Syndrome (XXY) considered a sex-linked trait?
No, Klinefelter's Syndrome (XXY) is not a sex-linked trait; it is a chromosomal disorder. A sex-linked trait results from a specific gene on a sex chromosome. In contrast, Klinefelter's Syndrome is an aneuploidy, which is an abnormality in the number of chromosomes. It is caused by an error during meiosis (nondisjunction) that results in a male having an extra X chromosome, leading to an XXY genotype instead of the typical XY.
