Question

What is the expected phenotypic ratio of a cross between an RrYy and an RRYY individual?

Answer 1

Hint: The cross between two individuals having two pairs of contrasting characters is called a dihybrid cross. The dihybrid cross was first shown by Mendal in a pea plant. With the help of a dihybrid cross, we can understand the inheritance of two contrasting characters at the same time. The trait which is expressed in progeny is called dominant trait while the trait which is not expressed (except when two copies of the trait is present) is called recessive trait.

Complete answer:
The phenotype is the observable characteristic of an individual.
Let us consider RR for round, YY for yellow, rr for wrinkled and yy for green.
According to the question, RrYy is a double heterozygous and RRYY is a homozygous individual and are crossed.
RRYY individual will produce all similar types of gametes i.e. RY while RrYy will produce four different types of gametes i.e. RY, ry, rY and Ry. When a dihybrid cross is done between RRYY and RrYy individuals, all the plants produced will have the same phenotype (observable characters of an individual). All the plants will produce round and yellow seeds. This is because; the yellow colour and round shape are the dominant alleles.
Cross between RRYY and RrYy is shown below:

	RY	Ry	rY	ry
RY	RRYY (Round, yellow)	RRYy (Round, yellow)	RrYY(Round, yellow)	RrYy(Round, yellow)
RY	RRYY(Round, yellow)	RRYy(Round, yellow)	RrYY(Round, yellow)	RrYy(Round, yellow)
RY	RRYY(Round, yellow)	RRYy(Round, yellow)	RrYY(Round, yellow)	RrYy(Round, yellow)
RY	RRYY(Round, yellow)	RRYy(Round, yellow)	RrYY(Round, yellow)	RrYy(Round, yellow)

The phenotypic ratio obtained for round yellow is 1. All the sixteen plants obtained will be round and yellow.

Note:
In a trait, when two similar alleles of the same gene are present, it is called a homozygous condition e.g. AA. In heterozygous conditions, the organisms contain two different alleles of the same gene e.g. Aa. A trait is said to be a dominant trait, if it masks the effect of another while a trait is said to be recessive, which is shown only when two copies of the allele having that trait are present. Recessive trait is not expressed in heterozygous condition.