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LINKAGE

What is linkage? The term linkage describes the situation where genes or certain characters are located on the same chromosome.

Offspring produced by sexual reproduction show only the parental characteristics and only sometimes few new recombinants.

Offspring with combinations of characteristics not found in either of the parents due to crossing over in first prophase of meiosis.

Genes are said to be linked when they are located close together on the same chromosome such that they are always inherited together.

 

Sex linked genes

  • These are genes that are located on the sex chromosomes.
  • Sex-linkage – refers to carrying of the genes on the sex-¬chromosome.
  • Gene for a trait may be present, yet offspring does not show the trait.
  • This happens in human females (XX) where a gene for the trait is recessive.
  • The female acts as a carrier.

 

In human, sex linked characters found on the X chromosome include:

Haemophilia:

What is haemophilia? This is a disease that affects the rate of clotting of blood, leading to excessive bleeding even from a minor cut.

Haemophilia is more common in males than in females.

A female may have the gene for haemophilia and not show the trait because the normal gene is dominant over the gene for haemophilia. Such females are referred to as carriers.

If the carrier female offspring will be carriers while the other half will be normal.

Half the males will be normal and the other heamophilic.

 

Red-green colour-blindness

  • Red-green colour-blindness is caused by a recessive gene found on the X chromosome.
  • It is inherited in the same way as haemophilia.
  • More males 1:10,000, less female 1: 100 million afflicted.
  • It is the inability to distinguish between red and green colours in humans.

 

Genes found on y-chromosome include:

  • Hairy pinna and hairy nose are carried on the Y – chromosome.
  • Premature balding.

 

See also:

COMPLETE AND INCOMPLETE DOMINANCE

STRUCTURE OF DNA

STRUCTURE AND PROPERTIES OF CHROMOSOMES

GENETICS

Growth and Development in Animals

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