CHAPTER 4 Mendel’s Laws

Peter Maxwell is an aging rock star. Although by day he is a high school music teacher, on the weekends his genetic gifts express themselves on stage. Peter has inherited a lucky combination of traits—hyperextensible fingers (autosomal recessive, genotype hf//hf); a fabulous head of long, red hair just like his paternal grandfather Emmett had; and a high, clear, strong voice that echoes those of the finest rock bands of the 1980s, a trait called rock star voice (RSV), which is autosomal dominant. Over the years Peter has been widely sought after for tribute bands. Each of these traits is inherited on a different autosome.

Alas, Ellie Maxwell, Peter’s wife, is not musical and is, in fact, just the opposite. She has an autosomal dominant condition called congenital amusia or tune (not tone) deafness. Ellie comes from a long line of people who cannot hear the pitch and rhythmic patterns of musical notes as a melody. Instead, they hear a disconnected string of sounds, and this is what they mimic when they attempt to sing. They are always off-key. Her father, an identical twin, was once nicely asked to leave a church choir because he couldn’t correct his off-key notes. Ellie’s paternal grandfather, too, was notorious for his loud, horrible singing. Although she was convinced this lack of talent was inherited, Ellie never sang to her children, for fear that they might imitate her.

Another autosomal dominant trait in the family is distal symphalangism, for which Ellie can blame her tiny toenails and very stiff fingers. Her siblings Jill and Dan luckily escaped both the weird fingers and bad singing voice.

Peter and Ellie have three children (Figure 2). Sean is lead guitarist in a rock band, thanks to his hyperextensible fingers and synesthesia (see Chapter 1). Unfortunately, he has inherited his mother’s tune deafness, which is especially frustrating because he has also inherited rock star voice. He can sing clear, beautiful, soaring notes—but the wrong ones. He has stopped singing since his attempts at harmony made his bandmates ill. Sean hopes he keeps his lustrous hair, which is like his father’s, and doesn’t somehow inherit Uncle Roger’s baldness. How awful it would be to watch those glorious red curls thin and then fall out!

Figure 2 A partial pedigree of the Maxwell family. Different combinations of symbols can be filled in to represent inheritance of specific traits.

Sean’s sister Keri has rock star voice and distal symphalangism. Her stiff fingers don’t bother her, but she always covers her shrunken toenails. Maybe she’ll get fake ones. The only family peculiarity that little sister Anna shares is synesthesia.

Peter’s sister Joan uses her hyperextensible fingers in her career as an eye surgeon. She likes to sing in the operating room, but it is good that her patients are often unconscious because she hasn’t inherited RSV. Peter’s and Joan’s older brother Roger has been unlucky genetically—in addition to the pattern baldness that worries Peter and Sean, Roger has an inherited heart condition called long QT syndrome. The first symptom can be a fatal disturbance of the heart’s rhythm (see extended pedigree, figure i).

MULTIPLE CHOICE

1. In the Maxwell family, the individuals who must be heterozygous (carriers) for hyperextensible fingers are

a. Peter and Sean.

b. Ellie, Anna, and Peter’s parents, Abe and Sarah.

c. Peter’s siblings Joan and Roger and half of their children.

d. all Peter’s grandparents.

e. not clear from the available information.

2. If Keri gets fake toenails, she will be altering

a. her phenotype but not her genotype.

b. her genotype but not her phenotype.

c. her genotype and her phenotype.

d. neither her genotype nor her phenotype.

e. the DNA sequence of the distal symphalangism gene.

3. The probability that Anna inherited normal fingers, assuming that the genes for the two finger traits are on different chromosomes, is___.