Question

You cross true-breeding purple and white flowered pea plants. All 202 of the F1 offspring are white. Next you conduct an F1 self cross and yield 312 white and 108 purple F2 offspring.

(a) Based on the data, which trait is dominant?

(b) Diagram the P and F1 crosses and show F2 offspring. Use symbols of your choice to show each genotype and phenotype.

(c) Based on a Chi-Square hypothesis test, do the F2 results fit with what you expect?

(d) How would you determine if a white F2 individual was homozygous dominant or heterozygous?

Answer 1

Answer:

a). Based on the data, white trait is dominant over purple.

b).

Purple (pp) x White (PP) ----Parents (P)

White (Pp)----------------------F1

F1 x F1

Pp x Pp

	P	p
P	PP (white)	Pp (white)
p	Pp (white)	pp (purple)

White : purple = 3:1

Expected white = ¾ * 420 = 315

Expectd purple = ¼ * 420 = 105

Phenotype	Observed(O)	Expected (E)	O-E	(O-E)2	(O-E)2/E
White	312	315	-3	9.00	0.03
Purple	108	105	3	9.00	0.09
	420	420			0.11

Degrees of freed0m = number of phenotypes – 1

Df = 2-1= 1

Critical value at 0.05= 3.84

The chi-square value of 0.11 is less than the p value of 3.84. We accept null hypothesis and the the F2 results fit with what we expect.

d).

By using we can determine the unknow genotype is homozygous or heterozygous

Testcross is a cross between unknown genotype and known recessive genotype.

In testcross, if all the progeny are white, then the genotype of white is homozygous.

PP (white) x (purple) bb ---Testcross.

Pp (all purple)----Progeny

If 50% of progeny are white and 50% are purple, then the genotype is Pp (heterozygous) as below.

Pp x pp ---Testcross

	p
p	pp (purple)-50%
P	Pp (White)-50%