Question

The "E" ears represent a dihybrid cross in which two unlinked loci govern seed color. (Note that this is a case of 2 loci governing a single phenotypic trait. What's that called?) At locus 1 the allele "D" is dominant and the allele "d" is recessive. At locus 2 the allele "R" is dominant and the allele "r" recessive. A purple seed is produced only when both dominant alleles are present (i.e. at least one of each). Otherwise seeds are yellow. In this particular cross both parents were doubly heterozygous. The observed progeny counts were 340 purple and 266 yellow.

a. Can you determine easily what phenotype ratio this approximates?

b. Do the Punnett square analysis to determine the expected phenotype distribution. Is the observed distribution close to the expected distribution? Do you think it's necessary to do a chi square analysis to determine whether the observed distribution fits the expected one? What if the difference between the two distributions were twice as great? 10 times as great? Do you really know (without doing the chi square test) how large that difference must be before the two distributions are significantly different enough to make you suspect that something other than chance is responsible for the difference?

Answer 1

Answer-

According to the given question-

Here The "E" ears represent a dihybrid cross with two unlinked loci are responsible for seed color.

locus 1 have dominant allele D and recessive allele d while locus 2 have dominant allele R and recessive allele r .

When both dominant alleles are present then there will be purple seed otherwise the deeds are yellow.

So we have both parents were doubly heterozygous i.e. DdRr

when we make a cross between DdRr DdRr then we get

	DR	Dr	dR	dr
DR	DDRR (Purple)	DDRr (Purple)	DdRR (Purple)	DdRr (Purple)
Dr	DDRr(Purple)	DDrr (Yellow)	DdRr (Purple)	Ddrr (Yellow)
dR	DdRR (Purple)	DdRr (Purple)	ddRR (Yellow)	ddRr (Yellow)
dr	DdRr (Purple)	Ddrr (Yellow)	ddRr (Yellow)	ddrr (Yellow)

Phenotypic ratio are 9 : 7 this is a example of Complementary genes ratio where two genes responsible for a single characteristic and both genes have the capability to mask the effect of the other gene.

In complementary genes both dominant allele of both genes are required to represent the dominant characteristic otherwise the recessive characteristic is observed.

Here we have 340 purple and 266 yellow then total number of progeny = 606

If we want to check the probability of expected phenotype then -

For purple = 9 / 16 606 = 0.5625 606 = 340.8 which is approximately same as observed phenotype (340).

For yellow = 7 / 16 606 = 0.4375 606 = 265.12 which is approximately same as observed phenotype (266).

phenotype ratio = 9 : 7

here we do not need do chi square analysis because the observed distribution are similar to expected one.

if the difference between the two distributions were twice or 10 times as great then we need to do the chi square test because without the chi square test we cannot predict the difference between observed and expected phenotype and based on that we can observed the difference and also predict that they are significantly different or not. If they are significantly different this means that there is something which is responsible for this difference