November 1, 2012

II. Mendel's 2 Principles

       A. Principle of Segregation: 2 alleles, for any gene will always separate from one another, when gametes (embryos) are formed.
       B. Principle of Independent Assortment: 2 or more genes will always segregate from one another independently when gametes are formed when they are found on separate chromosome pairs.

A male with the genotype AaBb produces
A_(sperm)
There is a chance that the second gene could be big B or little b. (1/2 chance)

Gene Linkage; when 2, or more, genes are found on the same pair of chromosomes. they won't assort independently. Example: Human Chromosome 17

There are 4 possible sperm cells that can be produced if the two genes are on separte chromosomes.
AB, Ab, aB, ab

Exception: There are 2 possible sperm cells that can be produced if the two genes are on the same chromosomes.
AB, ab. Which is the most common.

Punnet Square
        AB       ab
AB  AABB AaBb

ab   AaBb   aabb

Rule: If you are getting a monohybrid result from a dihybrid cross then the genes are linked.

Exception to the Exception: It is possible for the male to produce an Ab sperm cell, and an aB sperm cell.  But it is very un-common. Known as a recombination cells.

After crossover, if the genes on the chromosome are close together then the sperm cells will most likely be the common sperm cells the , if the two genes are further apart then the male is more likely to produce the un-common sperm cells.