● 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.
● 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.