Question

25. A cross is made between two different true-breeding strains of daylilies, both of which have white flowers. All of the F1 generation plants have yellow flowers. When an F, offspring is crossed with either one of the parental strains, half of the offspring have yellow flowers and half of them have white flowers. Propose an explanation for this outcome. In your answer, you should describe the genotypes of the two parental strains and the F1 offspring, and also explain which alleles are loss-of- function and which are needed for yellow-flower color.

Answer 1

In the above question, the type of cross performed is called a test cross.

First some basic definitions,

A monohybrid cross involves mating 2 plants considering only 1 trait at a time (like Tall plants crossed with Dwarf plants).

A dihybrid cross involves mating 2 plants conidering 2 traits at a time (like Tall, Red plants crossed with Dwarf, Green plants)

A cross between 2 parental strains produces offsprings which are called F1 progeny.

Upon selfing F1 progeny, the offsprings obtained are called F2 progeny.

A test cross involves mating the F1 progeny with the recessive parent. This cross is useful to determine whether the F1 offsprings are homozygous or heterozygous.
As Mendel stated, the factors controlling the traits occur in pairs which segregate at the time of gamete formation. (However, we now know that more than 2 alleles for a gene also exist)

Of the 2 alleles in a pair, in a heterozygous condition, 1 dominates while the other gets suppressed.

In the question above, Yellow is the dominant allele while white is the recessive allele.

Lets take Yellow colour allele as Y and white colour allele as y.

Parents were pure breeding.

Since upon crossing F1 progeny with either of the parent we obtained half yellow and half white colour flowers, it means that the offsprings were heterozygous while the parent chosen was recessive. This is a test cross.

P1 x P2: YY (Male) X yy (Female)

F1 progeny: Yy all (All Yellow)

Test Cross -

F1 x P2:    Yy (Male) X yy (Female)

Test cross progeny: Yy Yy yy yy

Yy: yy = Yellow : White = 1: 1