In humans, the dominant allele N causes an abnormal shape of the patella in the knee, whereas n is the recessive normal allele. A separate gene is concerned with finger length, and the dominant allele B causes abnormally short fingers, whereas b is the recessive normal allele. A study focused on people who have both abnormal patellae and short fingers. These individuals were N/n . B/b in genotype, having inherited the N allele from one parent and the B allele from the other parent. These N/n . B/b people mated with normal spouses, producing 40 progeny classified as follows: Normal 3 Abnormal knees and short fingers 2 Abnormal knees only 17 Short fingers only 18 Explain why the four types of children were found in the proportions shown.

Draw the chromosomes of (i) the parents of the N/n . B/b individuals; (ii) the N/n . B/b individuals; and (iii) the four types of children, showing the proper positions of the alleles based on the data above.