GENETICS 5

LINKAGE AND RECOMBINATION

• Morgan carried out several dihybrid crosses in Drosophila melanogaster to study genes that were sex-linked - discovered variation
• Why drosophila (fruitfly)--
• 1. Can be grown in simple medium
• 2. Male female easily distinguishable
• 3. Short life cycle
• 4. Produce large no of progeny
• 5. Various features can be seen with low power microscope.

• Morgan hybridized yellow-bodied, white-eyed females to brown-bodied, red-eyed males and intercrossed those F1

• According to him, two genes did not segregate independent of each other and F2 ratio deviated from 9:3:3:1. 

• This concluded that genes are linked. When they are located on same chromosome. This process is called linkage.
• This forms more parental combinations.

• Some pair are tightly linked - very less recombination
• Some are loosely linked- show slight more recombination

• Recombination is the rearrangement of genetic material. The generation of non-parental gene combination during dihybrid cross is called recombination. 
• When genes are located on same chromosome, they are tightly linked and show less linkage. This is responsible for variation.

Linkage depends on distance between genes on chromosome- helps in genetic mapping

Genetic mapping- knowing the location of various genes in our genome.

Used in HGP human genome project

SEX DETERMINATION

• Different organisms have different types of sex determination.
• Cytological observations in insects -- genetic or chromosomal basis of sex-determination.
• In 1891, Henking traced a specific nuclear structure all through spermatogenesis in few insects.
• He observed specific nuclear structure is located on 50 per cent of sperms only. The discovered X-body but was unable to explain its significance.
• Later- X body - X chromosome

• In insects, 
• XO type of sex determination is present. 
• All the eggs have an additional X-chromosome besides the autosomes. Some sperms bear X-chromosome where as some do not.
• Eggs fertilized by sperm having having X-chromosome become females and those fertilized by sperms that do not have an X-chromosome becomes males.
• For example: grasshopper (males have only one X-chromosome besides autosomes and females have a pair of X-chromosomes)

Found, X chromosome only, helps in sex determination- sex chromosome

Others- autosome

SEX DETERMINATION IN HUMANS

• XY type of sex determination
• Males (XY), Females (XX)
• Drosophila also has XY type of sex determination.

So till here, male forms 2 different gametes, so male- heterogametic

Female- homogametic

SEX DETERMINATION IN BIRDS

• ZW type of sex determination is seen in birds.
• Females have ZW and males have ZZ chromosomes.
• In birds sex is determined by type of ovum.
• In birds, females are heterogametic

SEX DETERMINATION IN HUMAN

SEX DETERMINATION IN BEES