- 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.
- e. 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:
- 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 my 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 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.