Question

1. In one experiment, Mendel crossed a pea plant that bred true for axial flowers with a pea plant that bred true for terminal flowers. All the F1 plants had axial flowers. Which is the recessive trait? Explain your answer.

2. Green pod color is a dominant allele in pea plants (G). If two plants with green pods are crossed, some of the offspring have yellow pods. What are the possible genotypes of the parents?

3. In garden peas, one pair of alleles controls the height of the plant and a second pair of allele controls flower color. The allele for tall (T) is dominant to the allele for dwarf (t), and the allele for purple (P) is dominant to the allele for white (p).

a. A tall plant with purple flowers crossed with a dwarf plant with white flowers produces 1/2 tall with purple flowers and 1/2 tall with white flowers. What is the genotype of the parents?

b. A tall plant with white flowers crossed with a dwarf plant with purple flowers produces all tall offspring with purple flowers. What is the genotype of the parents?

c. A tall plant with purple flowers crossed with a dwarf plant with white flowers produces 1/4 tall purple, 1/4 tall white, 1/4 dwarf purple and 1/4 dwarf white. What is the genotype of the parents?

Answer 1

1. Terminal flowers is the recessive trait.

Given that the two plants are true breeding plants. This means there are no recessive alleles in parental plants. Let us represent the trait with letter F. According to traditional method let us consider 'F' represents the dominant character and 'f' represents the recessive character.

Consider a cross between the parents.

FF x ff Alleles of Parental Generation

F x f Gametes from parents

Ff F₁ Generation Genotype

So, from this cross it is clear that the produced offspring has one dominant and one recessive allele. Under the dominance of dominant allele the expression of recessive allele suppresses. Hence the trait which is expressed in the F₁ generation is the dominant trait and that which is not expressed is the recessive trait.

Given that F₁ plants had axial flowers and hence it is the dominant trait and terminal flowers is the recessive trait.

2. Gg, Gg

Given that the green colour of the pod is the dominant character (G)

The possible genotypes with dominant recessive alleles are GG, Gg and gg.

Given that two green coloured plants are taken and hence the plants should contain atleast one recessive allele of gene to produce some recessive phenotypes as given in the case.

If one of the parents or both parents contain homozygous dominant alleles. There is no possibility of F₁ recessive individuals i.e., the plants with yellow colour pods. Given that parents have green colour pods, so there is no chance for homozygous recessive condition.

Hence the possible genotypes of parents are Gg and Gg only. With these genotypes 1/4th of the F₁ individuals exhibit recessive phenotype i.e., yellow pods.

3. a TTPp and TTPp

Given that

Tall plant with purple flowers x Dwarf plant with white flowers = 1/2 tall with purple & 1/2 tall with white

As dominant tall character expressed in all F₁ plants, both the parents have homozygous dominant alleles(TT) for height gene.

As half plants have purple and half have white flowers, both the parents are heterozygous(Pp) for the colour gene.

Hence the genotypes of parents are TTPp and TTPp

3. b TTpp and ttPP

In this case one dominant and one recessive character was taken in each parents. When we observe the result all are dominant phenotypes. This is possible only if both individuals are homozygous for their particular phenotypic characters.

Parents

Tall plant with White flowers (TTpp) x Dwarf plant with Purple flowers (ttPP) = Tall plant with Purple flowers (TtPp), Only possible genotype of offspring.

3. c TtPp and TtPp

It was given in the result that all possible combinations of phenotypes are exhibited in F₁ . Because individuals having all possible phenotypes with combination of traits are present each in ratio of 1:4. In order to express all the traits in their F₁, parents must have all different alleles for each trait in their genotype. This is possible only with one genotype i.e., TtPp and TtPp

TtPp x TtPp    Parents

TP Tp tP tp x TP Tp tP tp Gametes

Offsprigs

TTPP - 1/4 Tall with Purple flowers

TTpp - 1/4 Tall with White flowers

ttPP - 1/4 Dwarf with Purple flowers

ttpp - 1/4 Dwarf with White flowers