Question

NEED HELP!!!

The traits studied in the experiments involving the crossing of pea plants are seed shape and seed color.

-The shape of the seeds can either be round or wrinkled, and the alleles of these traits are represented by R and r respectively. Allele R is dominant.

-The color of the seeds can be yellow(Y) or green (y). The allele for the yellow seeds is dominant.


A dihybrid cross is preformed between the two pea plants of genotype RrYy. Construct the Punnett square for the corss in the table shown.

QUESTION: Explain how the Punnett square can be used to predict the probability that an offspring from such a cross has a given phenotype (for example yellow, wrinkled seed) Explain your answer with reference to Mendel's Law of independent Assortment.

Answer 1

Hi,

As mentioned in the question, R is for round seeds which is a dominant trait over wrinkled seed r.

Similarly, Yellow color of seeds Y is a dominant trait which is dominant over green color of seeds y.

A dihybrid cross is preformed between the two pea plants of genotype RrYy.

P1:                        RrYy           X           RrYy

Gametes:               RY: Ry: rY: ry

Offspring:               Given in punnet square

So, we have the question: how the Punnett square can be used to predict the probability that an offspring from such a cross has a given phenotype .

The answer lies in the fact that we know which allele is for dominant trait and which allele is for recessive trait. So when we look at any genotype of offspring for example: RRYY, we know it will have round seed and yellow color because it has both alleles of dominant nature. Similarly, if we talk about offspring RrYY we see that it will also have round seed and yellow color because although allele for green color is present but its effect is masked by dominant allele R. In this way we can predict the phenotype of all the offspring.

1  RRYY: Round and green

2  RRYy: Round and green

1  RRyy: Round and green

2: RrYY: Round and yellow

4: RrYy: Round and yellow

2: Rryy: Round and green

1: rrYY: wrinkled and yellow

2: rrYy: wrinkled and yellow

1: rryy: wrinkled and green

So it means that out of 16 offspring:

6  will have Round and green phenotype.

6 will have phenotype round and yellow

3 will have phenotype wrinkled and yellow

1 will have phenotype wrinkled and green

Therefore, we can calculate the probability of offspring by seeing punnet square and if we know the dominant or recessive nature of allele.

Hope it help!