Topic Genetic Basis for Variation: Q) Explain what is meant by the Mendel’s Law of Independent Assortment , and how does this give rise to the typical 9 :3 :3 :1 ratio from a cross between two double heterozygotes (i.e. AaBb x AaBb) ? What is the assumption you need to make with regards to the location of the two genes ?

Mendel’s Law of independent assortment states that members of any gene pair segregate from one another independently of the members of the other gene pairs. This independent assortment is related to meiosis since the different pairs of homologous chromosomes can arrange independently of each other on the metaphase plate during metaphase I. As the result, while each gamete will only contain one allele for each gene locus, the alleles of different gene loci are assorted randomly with each other in the gametes.

For two independently assorting pairs of alleles, A/a and B/b, the total no. of different combination is 4. Hence, there are 4 diff possible gametes in equal proportion.

Proportion of gametes of one parent :         1     :      1      :     1     :     1 

When two heterozygotes interbreed, there will be a total of 16 genotypic combinations but only 4 phenotypic classes (i.e. 9A_B_ :3aaB_  :3A_bb  :1aabb).

The assumption – the two genes are found on different chromosomes ! If they are found on the same chromosome (i.e. both genes are linked), then the proportion of the 4 types of gametes will not be equal – more parental gametes and less recombinant gametes (formed by crossing over).

P.S. Unfortunately, I cannot insert the circles representing the gametes I drew using Word Doc :(   but you can refer to N09/P2/4 worked solution