The pattern of sex linked inheritance is very specific. It is in a zigzag manner. The trait passes from maternal grandfather to daughter and from that daughter to her son. It never comes direct from father to son. As a woman is xx, she carries two alleles for any X-linked trait. She can be homozygous or heterozygous for the trait. But a man being XY can carry only one allele of any X-linked trait on his single X-chromosome. His Y-chromosome does not carry its corresponding allele.
He is hemizygous for the trait. As Y-chromosome does not carry any counterpart, even a single recessive allele on his X-chromosome can fully express itself. A sex-linked recessive trait can appear in a woman only when she is homozygous for its alleles. That is why chances for a male to be affected by a sex-linked recessive trait are double that for a female. This is the reason why sex-linked recessive traits are more common in males than females.
X-linked recessive is a trait, which is determined by an X-linked recessive gene. e.g. color-blindness, Hemophilia.
Red-green color-blindness:
Color-blindness is a common hereditary disease. Color-blind person cannot distinguish red from green, because red and green sensitive cone cells are absent in the retina of his eyes. Can you see number 16 in Fig 17.34? If yes, you are normal.
Fig 17.34: A test for red-green -blindness
It is a recessive X-linked trait. Gene for normal trait is ‘C’ and gene for color-blindness is ‘c’ if a color-blind man marries a normal woman (Fig.17.35), he will pass gene for this trait to all his daughters through his X-chromosome. He cannot pass this gene to his sons because sons receive only Y-chromosome from him. His every daughter also receives another X but with the normal dominant allele from her mother. She will be phenotypically normal but heterozygous and a carrier for this recessive gene. When she marries a normal man (Fig. 17.36), she will pass this trait of her father to that half of her sons who inherit grandfather’s X from her. The single recessive allele for color-blindness expresses successfully in a homozygous son because his Y-chromosome does not carry its counter part. Half of her sons will be normal as they inherit grandmother’s X with normal gone. Half of her daughters with both normal X will be normal, but half her daughters will be carries like their mother.
Have you noted in both these crosses, that not even a single female is color-blind?
Have you now understood very well why color-blindness is more common in men than women?
Activity No. 7: Study of inheritance of color-blindness in man
Now look at these crosses more carefully again and search for answers to these two simple question.
Can two normal parents produce a color-blind son?
Can a normal daughter have a color-blind father?
Good, you have found your answers from these crosses. Can you answer two more questions by making their crosses yourself?
- Can two color-blind parents produce a normal son?
- Can a color-blind daughter have a normal father?
Hemophilia
Hemophilia is a rare but more serious hereditary disease. It is an X-linked recessive trait. In this trait the blood fails to clot properly after an injury. It is also called bleeders disease. The affected person may bleed to death ever from minor cuts.
Fig. 17.37: A normal son can have a hemophiliac father
In a normal person, many blood-clotting factors interact in sequence to make blood clot. Hemophiliac’s blood has either a reduction, or malfunction or complete absence of blood clotting factors.
Gene for normal is ‘H’ and gene for hemophilia is ‘h’ being an X-linked recessive, this trait appears more in men than women. A woman has to be homozygous for its recessive allele to show this phenotype, whereas a man with just one recessive allele will display the trait. Like color-blindness it also zigzags from maternal grandfather through a carrier daughter to a grandson. It never passes direct from father to son. So you know why? Can a normal son have a hemophiliac father? Let us get answer by designing a cross (Fig 17.37).
Can a normal son have a hemophiliac mother? No! Why not? A normal son needs an X-chromosome with normal gene; while a hemophiliac mother has hemophilia gene on both of her X-chromosomes.
Activity No. 8: Study of inheritance of hemophilia in man
Now please answer by making crosses your self.
- Can a hemophiliac son have a normal mother?
- Can a hemophiliac son have a normal father?
Sex linkage in man
X and Y chromosomes are different in size. In addition to sex-determining sense they also carry genes for many other traits.
Fig 17.32: the human X chromosome
Many genes are present only on X-chromosome. These have no corresponding alleles on Y-chromosome. These are X-linked genes and their traits are linked traits are also commonly caked sex-linked traits Fig (17.32). A few genes are present only on Y-chromosome, having no counterpart on X-chromosome. These are Y-linked genes and their traits are Y-linked traits. Y-linked traits are found only in males, e.g., maleness. Some genes are present on both X and Y-chromosomes. These are X-and-Y linked genes. As their pattern of inheritance is like autosomal genes, these are also called pseudo autosomal genes.
X being larger has large number of genes, while Y being smaller has small number of genes.
X and Y chromosomes have two distinct regions each due to their linked genes (Fig 17.33).
Fig 17.33: human X and Y chromosomes
- A pairing region with common loci of X-and-Y linked genes due to which they synapse during meiosis.
- A non-pairing of differential region with different gene loci of either X-linked or Y-linked genes.