Consider the following dihybrid cross involving two genes that assort independen

Question

Consider the following dihybrid cross involving two genes that assort independently and exhibit simple dominance (s = short-winged; S = long-winged; d = dark-bodied; D = tan-bodied). A short-winged, dark-bodied fly was crossed with a long-winged, tan-bodied fly. All F1 progeny are long-winged and tan-bodied. F1 flies are crossed among themselves to yield:

842 long-winged, tan-bodied flies
271 long-winged, dark-bodied flies
353 short-winged, tan bodied flies
140 short-winged, dark-bodied flies

What is your expected phenotypic ratio for the progeny from this cross?

How many individuals would you expect in each phenotypic class among the progeny?

Calculate the chi-square value rounded to two decimals, and the degrees of freedom.

Do the results observed differ significantly from those expected at the 95% confidence level (p < 0.05)?

Explanation / Answer

The expected phenotypic ratio is 9:3:3:1.

The number of individuals in each phenotypic classes is represented below,

The chi square value is 32.22

Degree of freedom = (number of rows -1)(number of columns -1) = 3

The chi square value is higher than the table value for given degree of freedom and significance level.

Therefore, Null hypothesis is rejected.

So, the observed results differed significantly from the expected results.

Phenotype observed expected O-E (O-E)2 (O-E)2/E Long wing, tan body 842 904 -62 3844 4.25 Long wing, dark body 271 301 -30 900 2.99 Short wings, tan body 353 301 52 2704 8.98 Short wing, dark body 140 100 40 1600 16 1606 1606 32.22

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