Biology DIHYBRID CROSS

### KEY TOPICS

star Inheritance of two genes
star Law of Independent Assortment

### INHERITANCE OF TWO GENES

● 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".

### LAW OF INDEPENDENT ASSORTMENT

● 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.