The tabulated the body masses of 17 male White Rock chickens and the means of their male progeny were presented.� Individuals were weighed to nearest gram at 8 weeks of age.� When mature, 17 males were randomly mated and their male progeny weighed when 8 weeks old.�

1.� Estimate the heritability of body mass in the population of chickens from which this sample was obtained.

See attached spreadsheet.�
Heritability = h2 = 0.43, indicating that 43% of the phenotypic variance in the population is heritable. �

2.� What can you say about the environments of fathers and their offspring?

The mean of the offspring is greater than the mean of the fathers.� Assuming that there was no selection, this difference indicates that the progeny�s environment led to greater body mass.

You were provided the number of leaves per plant in 25 F1 and 25 F2 plants from a cross of two cultivated varieties of tobacco, which had mean leaf numbers of 15.0 and 17.9, respectively.

3.� Tabulate and plot the frequency distributions of the F1 and F2 generations.�

See attached spreadsheet.

4.� From each distribution calculate the mean, the variance, and the standard error of the mean.

F1:� mean + S. E. = 15.72 + 0.24, V = 1.46
F2:� mean + S. E. = 15.84 + 0.49, V = 5.97

5.� What are the main differences between the two distributions?

F2 is more variable and has a greater range.

6.� Estimate the degree of genetic determinism in the F2 generation.� What assumptions must you make to do this?

Assume that the varieties crossed were homozygous at all loci.� Tobacco is normally self-pollinating, so this is likely to be true.� If so, the F1 progeny are all genetically identical heterozygoes and their variance is wholly environmental in origin.� The variance of the F2 includes both environmental and genetic components.�

The second assumption is that the environmental variance in the F2 is the same as that in the F1.� With this assumption, the genetic variance is obtained by subtraction.

Population

components

 

variance

 

F2

VG

VE

5.97

F1

 

VE

1.46

, or, 76%.

difference

VG

 

4.51

 

7.� You were presented data from a study of the adult height of people in two West African villages.� Female heights were adjusted to male equivalents so that the means were the same in males and females.� Derive what you regard as the most reliable estimate of the heritability.� (Data from Roberts, D. F. et al. (1978) Ann. Hum. Gen. 42:15-24.)

Because , the heritabilities are obtained by doubling the regression coefficients:

 

Offspring-parent regressions

 

 

Father

Mother

Sons

0.646

0.908

Daughters

0.582

0.840

Mean

0.614

0.874

Standard deviations (inches):� Males 2.5; Females 2.3

 

 

The heritabilities obtained from the regressions on the mother are substantially higher than those from fathers, which can be attributed to a maternal effect.� The regressions on fathers consequently provide the most reliable estimates of heritability.� Because the heights were corrected for the sex differences, there are no reasons to prefer the daughters or the sons, so we take the mean as h2 = 0.614.�

It is interesting to note that the maternal effect can be obtained from the difference between the regressions on mothers and fathers:�