5.3 Inheritance of Two Genes

Mendel also worked with and crossed pea plants that differed in two characters, as is seen in the cross between a pea plant that has seeds with yellow colour and round shape and one that had seeds of green colour and wrinkled shape (Figure5.7). Mendel found that the seeds resulting from the crossing of the parents, had yellow coloured and round shaped seeds. Here can you tell which of the characters in the pairs yellow/green colour and round/wrinkled shape was dominant?

Thus, yellow colour was dominant over green and round shape dominant over wrinkled. These results were identical to those that he got when he made separate monohybrid crosses between yellow and green seeded plants and between round and wrinkled seeded plants.

Let us use the 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 RRYY and rryy. The cross between the two plants can be written down as in Figure 5.7 showing the genotypes of the parent plants. The gametes RY and ry unite on fertilisation to produce the F1 hybrid RrYy. When Mendel 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 3:1 ratio. Round and wrinkled seed shape also segregated in a 3:1 ratio; just like in a monohybrid cross.