Question

In pea plants, the allele for purple flower color is dominant to the allele for white flower color. If you performed a test-cross to determine the genotype of a purple-flowered plant, you would expect the percentage of purple-flowered progeny to be _________ if the plant is homozygous and _____________ if the plant is heterozygous.

Answer 1

Answer:

• If the plant is homozygous, the percentage of purple-flowered progeny is expected to be 100%.
• If the plant is heterozygous, the percentage of purple-flowered progeny is expected to be 50%.                

Explanation:

A test-cross is performed between an individual for whom you want to know the genotype (to find out if it is heterozygote or homozygote) and a recessive homozygote individual.

If the first individual is a heterozygote, the progeny phenotypic and genotypic proportions are 50% heterozygote and 50% recessive homozygote. But if the first individual is a homozygote, the whole progeny will be expressing one of the traits.

This is, from the proposed example, we know that:

• The allele for purple flower color is dominant to the allele for white flower color.

• A test-cross was performed to determine the genotype of a purple-flowered plant

We will represent the allele expressing purple color as P, and the allele expressing white color as p.

The purple-flowered plant genotype might be PP or Pp, so the options are:

• Parental) PP    x    pp

     Gametes) P   P     p   p

     Punnet square)   P          P

                           p   Pp      Pp

                           p   Pp      Pp

      F1 Genotype) 100% Purple-flowered plants. The genotype of the tested     individual was homozygote for the purple trait.

• Parental) Pp   x    pp

     Gametes) P   p     p   p

      Punnet square)   P          p

                            p   Pp      pp

                            p   Pp      pp

F1 Genotype) 50% Purple-flowered plants and 50% white-flowered plants. The genotype of the tested individual was heterozygote for the purple trait.

Answer 2

Answer: The percentage of purple-flowered progeny to be 100% if the plant is homozygous and 75% if the plant is heterozygous.

Explanation:

Plants are diploid organisms, that is, they have two copies of each allele. A plant with a purple flower can have a heterozygous (Pp) or homozygous dominant (PP) genotype because the purple allele is dominant. This means that only one copy is needed for it to express itself. If the plant were white, its genotype would be pp, since both alleles are recessive. Gametes are sex cells that fuse during fertilization to form a new organism. These gametes are haploid, meaning that they only have one allele from each gene.

If two plants that are homozygous dominant (PP) are crossed, then their gametes can only carry the P allele. If we cross one gamete with a P allele and another gamete with another P allele, then 100% of the organisms would have a PP genotype and would be purple.

If we cross two plants that are heterozygous (Pp) then their gametes can have either the P or the P allele. Then if we do the punnett square (see attached file), we see that 75% of the plants are purple as there are two Pp genotypes out of a total of four and one PP genotype out of four. And 25% are white since they have a pp genotype.