A histogram is a bar graph that shows the distribution of numeric data (here, the number of dark-skin alleles). To make a histogram of the allele distribution, put skin color (as the number of dark-skin alleles) along the x-axis and predicted number of offspring (out of 64) with each phenotype on the y-axis. There are no gaps in these allele data, so draw the bars next to each other with no space in between.

Human skin color is a polygenic trait which means more than two genes control it. The skin color produced is caused by the additive effect of the genes for the skin color.

Parents heterozygous for the three genes of skin color were considered to study the distribution of dark skin alleles. Each gene has two alleles, one light, and another dark. Thus, six alleles will determine the skin color of the offspring.

The phenotypes of the offspring and the number of dark skin alleles that determine the phenotype were recorded.

A bar graph is made from the data obtained to study the distribution of dark skin alleles. In the graph, the number of dark skin alleles that determine a specific phenotype is an independent variable and is plotted on the x-axis.

The number of offspring with a specific phenotype depends on the number of dark skin alleles. Thus, it is a dependent variable and is plotted on the y-axis of the graph.