Question

A mutation that causes a yellowing of leaves in snapdragon plants was discovered in the early 1900s. Plants with this condition, termed aurea, have less chlorophyll than green plants.
Crosses between various combinations of green and aurea plants yielded the following results.

green x green = all green
aurea x aurea = 2/3 aurea, 1/3 green
green x aurea = 1/2 green, 1/2 aurea

With respect to snapdragon color, what can be said of the G and GAalleles?

a)The GA is dominant to the G allele

b)The G^A and G alleles exhibit codominance

c)The GA allele is recessive to the G allele

d)The GA and G alleles exhibit incomplete dominance

Answer 1

Looking at the second cross, a 2:1 phenotypic ratio (instead of the typical 3:1) usually indicate a lethal homozygous genotype. Based on the question, it’s sensible to assume that a snapdragon homozygous for the aureal allele, which should have a even lesser chlorophyll count than the heterozygote, can’t even make it to the birth of the plant, thus the GAGA type never existed in reality. Under this assumption, we can deduce that the 2/3 aurea is of genotype GAG and the 1/3 green offspring of genotype GG. If a punnet square was pulled including the lethal genotype, it will be easily seen that the aureal allele is the dominant type. The first cross of genotype GG x GG conform to this conclusion. And the third is therefore a test cross between the homozygous recessive GG and the heterozygous GAG (aurea), with the result abiding our theory. Correct me if I was wrong, the GA allele is termed dominant negative as a single copy of it results in a deficit in chlorophyll amount. But anyway, the explanation above should give you an answer to the relationship between the G and GA allele.