Mendel’s second law is known as the law of independent assortment, which
states that 2 different sets of genes assort independently of each other during
the formation of gametes through meiosis.
The cross conducted taking 2 contrasting pairs of characteristics is known
as the dihybrid cross and it produces 9:3:3:1 in the F2 generation. In Drosophila,
this was stated by Morgan. In this experiment, pairs of contrasting characters,
such as sepia-eye and vestigial-eye mutants were taken to study whether
inheritance patterns follow the Mendelian laws or not. Vestigial wing is characterized
by reduced wings and balancers and the sepia-eye mutant is characterized
by brown eyes.
- Drosophila melanogaster
- Sepia vestigial mutant
- Media bottles
- Anaesthetic ether
Normal D. melanogaster strain and sepia vestigial wing strains were taken in
standard media bottles separately. When the flies were ready to emerge from the
pupa, the original stock were discarded. The newly emerged male and female
virgins were isolated. Flies that were collected were aged for 3–5 days, and were
crossed with each other by conducting a reciprocal cross also. The 2 crosses
also occurred in the following 2 manners.
Normal females × sepia vestigial males.
Normal males × sepia vestigial females.
The progeny produced in F1 were observed for phenotypic expression and
the data were collected and recorded. By taking a few of the F1 flies, inbreeding
was carried out to obtain F2 generation. The phenotype of F2 flies were observed
and the data were recorded.
Parents: sepia vestigial females × normal males
F1: all were normal-eyed and normal-winged
Inbreed: F1 females × F1 males
F2 : normal-winged vestigial-winged normal-winged vestigial-winged
And red-eyed : and red-eyed : and sepia-eyed: and sepia-eyed
Ratio: 177 : 60 : 65 : 20
Phenotype Observed Expected Deviation d2 d2/E
Normal-winged 177 181.12 – 4.12 16.97 0.07
Vestigial-winged 60 60.37 – 0.37 0.137 0.002
Normal-winged 65 60.37 + 4.63 21.43 0.35
Vestigial-winged 20 20.12 – 0.12 0.0144 0.0007
Degree of freedom = 4 – 1 = 3
Sx2 = 0.4427
Parents: normal females × sepia vestigial males
F1: all were normal-eyed (red) and normal-winged.
Inbreed: F1 female × F1 male
F2: red-eyed and normal-winged: red-eyed vestigial-winged: sepia-eyed and
normal-winged: sepia-eyed and vestigial-winged
Phenotype Observed Expected Deviation d2 d2/E = X2
Red-eyed and 212 208.12 3.88 15.05 0.0723 normal-winged
Red-eyed and 70 69.71 0.625 0.390 0.0055
Sepia-eyed and 66 69.375 3.375 11.396 0.164
Sepia-eyed and 22 23.12 1.125 1.265 0.0547
Sx2 = 0.029689
Degree of freedom = 4 – 1 = 3 analysis of result.
At the 5% level of significance, at 3 degrees of freedom, table value = 7.815
and at 2, degree of freedom is 5.991.
At the 4 degree of freedom, it is 9.48.
In direct cross, the X2 value = 0.4427 and is less than deviation. In reciprocal
cross, the X2 value = 0.29689 and is less than table value.
In both cases, the deviation is not significant, so the null hypothesis is