Question

In your work exploring the genetics of the model organism Drosophila, you perform an experiment to test if two recessive mutations, eyeless and vestigial wings, are independently assorting. The allele for eyeless is ey, the allele for vestigial wings is vg. Wildtype alleles for these loci are ey+ and vg+ respectively. You perform a test cross as follows:

Parent 1 - Heterozygous fly with wildtype phenotype, genotype ey+ ey vg+ vg

Parent 2 - eyeless and vestigial wings, genotype ey ey vg vg

The results of this cross give the following numbers of phenotype categories:

Eyes Wing Shape Number

Eyeless Vestigial 25

Eyeless Wild type 18

Wild type Vestigial 14

Wild type Wild type

16

Total 73

Use the method for determining the chi square value to evaluate the hypothesis that these genes are independently assorting. Hint: be sure to calculate expected fraction of offspring and then numbers of each phenotype with this total! Type in the chi square value to the nearest 1 decimal point. There is a buffer around the correct answer so you do not have to worry about rounding differences.

Answer 1

Representation:

Trait 1= eye color

• Wildtype (Eye)= ey+ = E= dominant, genotybe = EE(Homozygous), Ee (Heterozygous)
• Eyeless (mutant) =ey = e= recessive, genotybe = ee

Trait 2= Wing

• Wildtype=vg+ = dominant, genotybe = VV/Vv
• Vestigial (mutant)= recessive, genotybe = vv

Parent 1: Heterozygous

Genotype: EeVv

Parent 2: Recessive

Genotype: eevv

	ev
EV	EeVv
Ev	Eevv
eV	eeVv
ev	eevv

Ratio: 1:1:1:1

Representation:

	O	E	O-E =d	d2	Χ2 (d2/E)
Eyes-vestigial	25	¼ x73=18.25	25-18.25 =6.75	45.5625	45.5625/ 18.25=2.2 (aprox)
Eyeless-vestigial	18	¼ x73=18.25	18-18.25 =-0.25	0.0625	0.0625/ 18.25=0.003 (aprox)
wildtype-vestigial	14	¼ x73=18.25	14-18.25 = -4.25	18.0625	18.0625/ 18.25=0.99 (aprox)
Wildtype-wildtype	16	¼ x73=18.25	16-18.25 =-2.25	5.0625	5.0625/ 18.25=0.3 (aprox)
Total	73	73			3.5 (approx.)

· Critical value for df= 3, p= 7.81

· Since Χ²value in this case 3.5 < 7.81

· Thus, hypothesis is accepted (observed values are almost equal to expected values).