Question

In daffodil plants, yellow flowers are dominant over white flowers. You discover a yellow plant and want to know whether it is homozygous for the yellow allele or heterozygous. You perform a test cross with a white-flowered plant.
a. Draw two Punnett squares for this test cross, one assuming a homozygous dominant yellow plant and one assuming a heterozygous plant.
b. If the offspring of your test cross all have yellow flowers, what does this tell you about the genotype of the yellow-flowered plant you started with?

Answer 1

Answer:

a: See the attached images for the Punnet squares

b: If the offspring of the test cross are all yellow, it means the genotype of the starting yellow flowered plant is homozygous dominant.

Explanation:

Let the allele for flower color in daffodil be represented by A.

Since A is dominant over a; the two possible genotype for the yellow colour will be:

AA = yellow

Aa = yellow

Test cross involves crossing an organism whose zygosity is unsure with an homozygous recessive version of the trait. Hence, if the yellow plant is AA, the test cross becomes

     AA x aa; all the offspring will be yellow with genotype Aa

If the yellow plant is Aa, the test cross becomes;

      Aa x aa; Aa (yellow), Aa (yellow), aa (white), aa (white) (See the attached images for the Punnet square)

If the offspring of the test cross area all yellow, it means the genotype of the yellow flowered plant I started with is homozygous dominant.