`star` Inheritance of two genes
`star` Law of Independent Assortment


● Mendel also worked with and `color{Violet}"crossed pea plants"` that differed in `color{Violet}"two characters"`, as is seen in the cross between a pea plant that has seeds with `color{Violet}"yellow colour"` and `color{Violet}"round shape"` and one that had seeds of green colour and wrinkled shape.

● Mendel found that the seeds resulting from the `color{Violet}"crossing of the parents"`, had yellow coloured and round shaped seeds.

● Thus, yellow colour was `color{Violet}"dominant"` over green and round shape dominant over wrinkled.

● These results were identical to those that he got when he made `color{Violet}"separate monohybrid crosses"` between yellow and green seeded plants and between round and wrinkled seeded plants.

● Here we use the `color{Violet}"genotypic symbols"` Y for dominant yellow seed colour and y for recessive green seed colour, R for round shaped seeds and r for wrinkled seed shape.

● The genotype of the parents can then be written as `color{Violet}"RRYY and rryy"`.

● The cross between the `color{Violet}"two plants"` can be written down as in Figure showing the `color{Violet}"genotypes"` of the parent plants.

● The gametes RY and ry unite on `color{Violet}"fertilisation"` to produce the `color{Violet}"F1"` hybrid RrYy.

● When Mendel `color{Violet}"self hybridised"` the F1 plants he found that 3/4th of F2 plants had yellow seeds and 1/4th had green.

● The yellow and green colour segregated in a `color{Violet}"3:1 ratio"`.

● Round and wrinkled seed shape also segregated in a `color{Violet}"3:1 ratio"` just like in a `color{Violet}"monohybrid cross"`.


● In the `color{Violet}"dihybrid cross"`, the phenotypes round, yellow; wrinkled, yellow; round, green and wrinkled, green appeared in the `color{Violet}"ratio 9:3:3:1"`.

● Such a ratio was observed for several pairs of characters that Mendel studied.

● The ratio of 9:3:3:1 can be derived as a combination series of 3 yellow: 1 green, with 3 round : 1 wrinkled. This derivation can be written as follows:

`color{Brown}"(3 Round : 1 Wrinkled)"``color{Brown}"(3 Yellow : 1 Green)"` = `color{Brown}"9 Round, Yellow"` : `color{Brown}"3 Wrinkled"`, `color{Brown}"Yellow: 3 Round"`, `color{Brown}"Green : 1 Wrinkled, Green"`.

● Based upon such observations on dihybrid crosses (crosses between plants differing in two traits) Mendel proposed a second set of generalisations that we call `color{Violet}"Mendel’s Law of Independent Assortment"`.

● The law states that `color{Violet}"when two pairs of traits"` are combined in a hybrid, segregation of one pair of characters is `color{Violet}"independent"` of the other pair of characters’.

● The `color{Violet}"Punnett square"` can be effectively used to understand the independent segregation of the two pairs of genes during meiosis and the production of eggs and pollen in the F1 RrYy plant.

● Consider the `color{Violet}"segregation"` of one pair of genes R and r.

● `color{Violet}"Fifty per cent"` of the gamete have the gene R and the other `color{Violet}"50 per cent"` have r.

● Now besides each gamete having either `color{Violet}"R or r"`, it should also have the allele `color{Violet}"Y or y"`.

● The important thing to remember here is that `color{Violet}"segregation of 50 per cent"` R and 50 per cent r is independent from the segregation of 50 per cent Y and 50 per cent y.

● Therefore, 50 per cent of the `color{Violet}"r"` bearing gamete has `color{Violet}"Y"` and the other 50 per cent has y.

● Similarly, 50 per cent of the `color{Violet}"R"` bearing gamete has Y and the other 50 per cent has y.

● Thus there are `color{Violet}"four genotypes"` of gametes (four types of pollen and four types of eggs).

● The `color{Violet}"four types"` are RY, Ry, rY and ry each with a `color{Violet}"frequency"` of 25 per cent or ¼th of the total gametes produced.

● When you write down the `color{Violet}"four types of eggs"` and pollen on the two sides of a `color{Violet}"Punnett square"` it is very easy to derive the composition of the `color{Violet}"zygotes"` that give rise to the F2 plants.