Question

Ina certain plant, yellow fruit (Y) is dominant to white fruit (y). A heterozygous plant with yellow fruit crossed with a plant with white fruit. Find the genotype and phenotype ratios.

Answer 1

Genotype ratio: 1:1

Phenotype ratio: 1:1

Explanation:

A heterozygous plant with yellow seed would be having alleles Yy

where Y is dominant yellow colour and y is white fruit.

Yy X yy

   y    y

Y Yy  Yy

y yy   yy

The phenotype ratio from Punnett square is 1:1 as 50 per-cent will be yellow coloured and 50 per-cent will white coloured fruits.

       Genotype ratio will be 1:1

Answer 2

In this case, the genotype and phenotype ratios

• Genotype ratio: 1:1

• Phenotype ratio: 1:1

In the given case, we have three pieces of information to work this out:  

• You are crossing a yellow fruit with a white fruit
• The trait for yellow fruit is dominant over the trait for white fruit
• The yellow fruit used for the cross is heterozygous for the trait

There are three possible genotypes:

1. YY ( purebred Y allele),
2. Yy (heterozygous) and
3. yy (purebred for the y allele).

We know that Y is dominant over y, both YY and Yy genotypes will yield the yellow fruit phenotype. Only the yy genotype will yield white fruit.  

• So you know that the plant with white fruit can only be yy, and you are told in the question that the yellow fruit is Yy.  

Now that you know the genotypes of the parents, you can work out the genotypes of the offspring by drawing a Punnett square like this:  

     Y     y

y   Yy   yy

y    Yy   yy

This tells that, on average, when you produce four offspring you will get two Yy (heterozygous) plants, and two yy (purebred for the y allele) plants.

This simplifies to a 1:1 genotypic ratio and as we know that the Yy genotype gives a yellow phenotype, and a yy genotype gives a white phenotype, which results in a 1:1 phenotypic ratio as well.

Thus,

In this case, the genotype and phenotype ratios

• Genotype ratio: 1:1

• Phenotype ratio: 1:1

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