Question

You hypothesise that eye lobe discolouration is a sex-linked condition in an insect species with a XY sex determination system, where males are the heterogametic sex. A cross between a normal female and a normal male produce offspring with the following phenotypes:

Normal-eyed females: 254

Normal-eyed males: 126

Discoloured-eyed males: 136

Answer the following multiple-choice questions (Q1-Q2) and then use a chi-square test to provide evidence to support or reject your hypothesis (then answer questions Q3-Q4)

Q1 The condition is?

Q2 What is the expected phenotype ratio of normal females:normal males:discoloured-eyed males from this cross?

Q3 What is your Chi-square value?

Q4 Is the difference between observed and expected phenotype frequencies significant?

Answer 1

1).

The Discoloured-eye is an X-linked recessive trait.

2).

Normal female (XA Xa) x (XA Y) normal male –Parents

	XA	Y
XA	XAXA (normal –eyed female)	XAY (normal –eyed male)
Xa	XAXa (normal –eyed female)	Xa Y (discoloured-eyed male)

Expected phenotypic ratio = 2:1:1

3).

Phenotype	Observed(O)	Expected (E)	O-E	(O-E)2	(O-E)2/E
Normal female	254	258	-4	16.00	0.0620
Normal male	126	129	-3	9.00	0.0698
discoloured male	136	129	7	49.00	0.3798
Total	516	516			0.5116

Chi-square value = 0.5116

4).

Degrees of freedom = number of genotype – 1

Df = 3-1=2

p-value = 5.99

The X^2 value of 0.5116 is less than the critical value of 5.99. So we can accept or support the hypothesis. There is no significant difference between observed and expected phenotype frequencies.