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MHT-CET : Biology Entrance Exam

MHT - CET : Biology - Sex Determination and Sex Linkage Page 1

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Sex Chromosomes

 

  • The chromosomes that determine the sex of an individual are called sex chromosomes.
  • In man, there are two types of sex chromosomes - X chromosome and Y chromosome. These chromosomes show some structural differences.
  • The X chromosome is longer than the Y chromosome.
  • It is submetacentric and determines the female sex.
  • The Y chromosome is acrocentric and determines the male sex.
  • The portions of X and Y chromosomes that carry similar gene loci are called homologous segments. They can cross over during meiosis.
  • The non-homologous segments are those which do not carry similar gene loci. They do not cross over during meiosis.

Autosomes

 

  • The chromosomes which do not determine the sex of an organism are called autosomes.
  • Autosomes determine the somatic characters of an organism.
  • Autosomes do not carry the genes for sex-linked disorders such as colour blindness, haemophilia, etc.

 

 

Sex Determination in Man

 

  • According to the chromosomal theory of sex determination, the sex of an individual is determined by the sex chromosomes (heterosomes).
  • The mechanism of sex determination in man is XX-XY type.
  • The males are heterogametic and produce two types of sperms, one type containing X chromosome and the other containing Y chromosome.
  • The females are homogametic and produce only one type of eggs, all containing the X chromosome.
  • When a sperm containing X chromosome fertilises an egg, a female child with XX chromosomes is conceived.
  • On the other hand, when a sperm containing Y chromosome fertilises an egg, a male child with XY chromosomes is conceived.
  • Thus, in human beings, the sex of a child is determined by the male due to his heterogametic condition.
  • The total number of chromosomes in human beings are 46 (23 pairs).
  • If 'A' represents the number of autosomes, the male has 44A + XY and the female has 44A + XX chromosomes, respectively.

 

 

Sex Linkage

 

  • The genes that are linked with sex determining genes present on the sex chromosomes are known as sex-linked genes and their linkage is known as sex linkage.

  • There are two main types of sex linkages:
    1. X-linkage
      • X-linked genes are located on the non-homologous part of X chromosome.
      • They do not have a corresponding allele on the Y chromosome. Example: colour-blindness and haemophilia.
      • They are inherited in a criss-cross way.
    2. Y-linkage
      • Y-linked genes are located on the non-homologous part of Y chromosome.
      • They do not have a corresponding allele on the X chromosome.
        Example: hypertrichosis (hairy ear pinna).
      • These characters are found only in males and are also known as holandric.
      • They are inherited in a straight pattern, i.e directly from father to son.

 

 

Sex-linked Inheritance in Man

 

  • Colour-blindness is an example of a disorder produced by X-linked recessive gene.
  • Colour-blindness is a hereditary disease in which the person is unable to distinguish between different colours.
  • This is more common in males because in males only one X chromosome is present, and, therefore, a single recessive gene of the trait is sufficient to cause the disease.
  • On the other hand, since two X chromosomes are present in females, a single recessive gene cannot cause the disease due to the presence of a dominant gene on the other X chromosome.
  • The most common type of colour-blindness is the red-green type, where both the colours appear to be grey to the person affected.
  • Colour-blindness is inherited in a criss-cross pattern, which means its gene is transmitted from father to his daughter and is re-expressed in his grandson.

 

 

 

 

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