Question

What genotypes would be expected for the offspring from the following matings? What are the phenotypic ratios for each cross? Explain how you came up with this ratio?

a. AABB x aaBB

b. AaBB x AABb

c. AaBb x aabb

d. AaBb x AaBb

Answer 1

The question above is of a dihybrid cross. We know that in a dihybrid cross two different traits are crossed together.

A is a dominant allele and a is a recessive allele for the same trait .it can be any trait. let say here ,we assume it to be the height of plant. For example, A is for tall ie, dominant allele and a is for small height of plant ie, for recessive allele

Similarly, B is a dominant allele for the another trait let's assume the color of flower, ie yellow and b is recessive allele for color purple.

A.) When the cross between " AABB x aaBB " Is made we get the genotype " AaBB " The cross is shown below :

So in the first cross we get all the genotype as AaBB.

Now, we know that whenever an dominant allele is expressed in homozygous or heterozygous genotype that will result in dominant phenotype. Whereas when allele are expressed in homozygous recessive condition, only then a recessive phenotype is seen.

So here, we have the height of plant expressed in heterozygous condition so dominant trait is expressed ie, tall and color is expressed in homozygous with dominant allele so it will be expressed as all yellow flowers

That means here we have all the plants "tall and yellow" ​​​​​​. We get only one genotype expressing one single phenotype therefore

phenotypic ratio is 1 (or 100%)

B. When the cross between "AaBB × AABb " Is made we get genotype as AABB, AaBB, AABb, AaBb .

These genotype will be expressed as :

AABB (homozygous dominant height, homozygous dominant color) --- tall and yellow

AaBB (heterozygous dominant for height and homozygous dominant for color) -- tall and yellow

AABb (homozygous dominant height, heterozygous dominant color) -- tall and yellow

AaBb (heterozygous dominant height,heterozygous dominant color) --- tall and yellow

We saw that all four genotype will result in one single phenotype.

Therefore the phenotypic ratio is 1 or 100%

C.) When a cross between " AaBb × aabb" Is made we get genotype as AaBb, Aabb, aaBb, and aabb.

now, these genotype will be expressed as flowing phenotype --

AaBb (heterozygous dominant height, heterozygous dominant color) --- Tall and yellow. 4genotype expressing this phenotype

Aabb (heterozygous dominant height, homozygous recessive color) -- Tall and purple. 4 genotype expressing this phenotype

aaBb ( homozygous recessive height, heterozygous dominant color) -- Small and yellow. 4 genotype expressing this phenotype

aabb (homozygous recessive height, homozygous recessive color) -- Small and purple . 4 genotype expressing this phenotype.

so here we can see, we got four different phenotype for four different genotype. Therefore,

phenotypic ratio is 4:4:4:4 or 1:1:1:1 or 25%

D.) When we make a cross between " AaBb × AaBb " We get genotype as AABB, AABb , AaBB, AaBb, AAbb, Aabb, aaBB, aaBb, aabb.

Theese genotype will be expressed as

AABB, AABb, AaBB , AaBb , will expresss phenotype as tall and yellow . And there are total of 9 genotype in chart expressing such phenotype

AAbb and Aabb will express phenotype as tall and purple and there are 3 such genotype expressing this phenotype

aaBB and aaBb will express phenotype as small and yellow. ​​​​​​There are 3 such genotype expressing such phenotype.

aabb will express phenotype as small and purple. there is only 1 genotype expressing this phenotype.

Therefore, phenotypic ratio is 9:3:3:1.