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Sex Linked Inheritance

by Ramneet Kaur
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Sex-Linked Inheritance:

T H Morgan while working on eye color in Drosophila, found that the gene for eye color did not follow the Mendelian pattern of Inheritance.

  • This was due to the gene being present on the X-chromosome.
  • Genes present on the X-chromosome are known as Sex-linked genes, the characters are sex-linked characters.
  • Sex-linked Inheritance is the study of inheritance of sex-linked characters.
  • Genes present on Y-chromosome are Holandric genes and they are restricted to the male sex only.
  • Females have XX chromosomes while males have XY chromosomes.
  • Males are hemizygous because they have only one allele of X-chromosome. Males will express the character of any gene on the X-chromosome whether it is dominant/recessive.

Examples of Sex-Linked Inheritance:

Haemophilia:

  • Also referred as Bleeder’s disease or the Royal disease.
  • Occurs due to a sex-linked recessive gene.
  • Clotting of blood does not occur even to minor injuries.
  • Haemophilia is of 2 types: Haemophilia A & Haemophilia B/Christmas disease.
  • Haemophilia A occurs due to the absence of Factor VIII, Haemophilia B occurs due to the absence of Factor IX.
  • It expresses in the female when both the X chromosomes have the recessive gene, XhXh, while XXh is a carrier female.
  • In males, it expresses when the single X-chromosome has the recessive gene XhY.

Color Blindness:

  • Color blind persons are unable to distinguish red & green colors.
  • Red color blindness is referred as Protanopia, green color blindness is referred as Deuteranopia.
  • Occurs due to a sex-linked recessive gene.
  • More common in males.

Duchenne’s Muscular Dystrophy:

  • The person is unable to produce Dystrophin in skeletal muscles.
  •  Muscle cells do not release Calcium.
  • Occurs due to a sex-linked recessive gene.

Night Blindness:

  • The person is unable to see well at night and under poor light.
  • Occurs due to a sex-linked recessive gene.
  • Cross I

Normal     Colorblind

XX      x     XcY

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XXc, XXc, XY, XY  → All are normal, but females are carriers.

  • Cross II
  • Carrier        Normal

XXc x     XY

XX, XXc, XY, XcY  → 25% sons are colorblind.

  • Cross III

Colorblind      Normal

XcXc x       XY

XXc, XXc, XcY, XcY  →  All females are carriers & males are colorblind.

  • Cross IV

Colorblind        Colorblind

XcXc x       XcY

XcXc,  XcXc,  XcY,  XcY  → All are colorblind.

Criss Cross Inheritance:

Sex-linked genes show Criss-cross Inheritance, i.e., the gene from the father is passed to the grandson through the sons.

A normal female marries a colorblind male:

Normal     Colorblind

XX      x   XcY

XXc, XXc, XY, XY  → all are normal, but daughters are carriers.

Carrier daughter when marries a normal man

Carrier        Normal

  XXc  x     XY

XX, XXc, XY, XcY  → 50% of grandsons are colorblind.

This shows that the gene from the father has been transferred to the grandson through the daughters.

Such inheritance is known as Criss-cross Inheritance.

ALSO WATCH: Sex-Linked Inheritance

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