You have discovered an interesting new leaf color variant in pea plants. You hav

Question

You have discovered an interesting new leaf color variant in pea plants. You have some wild-type plants with the typical green leaf color and some plants with all yellow leaves. You suspect that a mutation in the chloroplast genome may be responsible for this change in pigment. Describe a crossing experiment that you could conduct to test this hypothesis. What results would you expect to find if your suspicion is correct that the mutation that is responsible for the yellow-leaf phenotype is in the chloroplast genome?

Explanation / Answer

Let the wild type plants with green leaf be GG and plants with yellow leaves be gg

Let us cross wild type with mutant (GG x gg)

All the offspring of F1 generation will have green leaf color

Next we cross male and female from first generation, Gg x Gg

The yellow leaf color trait reappeared in expected 3:1 ratio for recessive Mendelian trait.

To test complementation, let us assume wild type flies have green leaves and color is known to be related to two genes, A and B. Each one of these genes has two alleles, a dominant one that codes for a working protein (A and B respectively) and a recessive one that codes for a malfunctioning protein (a and b respectively which produce yellow leave). Since both proteins are necessary for the synthesis of pigmentation in the green, if a plant is homozygous for either a or b, it will have yellow leaves.

Knowing this, we may perform a complementation test on two separately obtained strains of pure-breeding yellow leaves plants. The test is performed by crossing two plants, one from each strain. If the resulting progeny have green leaves, the two strains are said to complement; if the progeny have yellow leaves, they do not.

If the strains complement, we imagine that one strain must have a genotype aa BB and the other AA bb, which when crossed yield the genotype AaBb. In other words, each strain is homozygous for a different deficiency that produces the same phenotype. If the strains do not complement, they both must have genotypes aa BB, AA bb, or aa bb. In other words, they are both homozygous for the same deficiency, which obviously will produce the same phenotype.

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