Question

Mary is a typical American girl who drives a jeep and wears jeans with holes in the knees and white t-shirts. She has an image of herself as slightly overweight. She exercises regularly, eats carefully, and tends to drink diet sodas to avoid those extra calories. She meets John, who has a great sense of humor and rarely wears shoes. They soon marry and begin to have children. They appear to be two normal parents, but their first son has some severe developmental delays. Undaunted, they have more children-- two boys and two girls, one of which has the same issue as the first son. Mary thinks that John isn’t telling the truth about his family, who she has never met, and that he has “bad genes”. John thinks it is something Mary does and his ardor for her wanes. So Mary switches to wearing dresses. Since Mary notices that this changes her appearance, she stops dieting, but continues to eat carefully and exercises and gets her whole family to do so. Her image of herself improves, and so does their relationship. They have five more children, none of which are retarded. John feels vindicated and Mary begins to feel guilty. Once again marital troubles arise. Because of their strong commitment to one another, John and Mary decide to visit their minister, who is also a genetic counselor. He has an idea and suggests some tests on the children. The retarded children showed serum phenylalanine concentrations several times the normal. Tests on fragments of the DNA from chromosome 12 indicate that of the first five children, the normal children each have at least one copy of the DNA most people have. (Which allele would these children have?) The developmentally delayed children have two unusual copies. (Which allele would these children have?) In the second set of five children, one of the children has two copies of the normal DNA, two have only one copy, and two have none of the normal sequence of DNA. (What would be the genotypes in each of these cases?) The night the minister is to meet with John and Mary to explain this to them, his car is found on the side of the road with the folder containing this information, two empty cans of diet Dr. Pepper, and a single white rose. The minister was never found. Your team of genetic counselors is called in to explain the cause of the developmental delays to John and Mary and to explain what, if anything, they can do about it. Fill in the information requested below.(5pts) [note: you may need to look outside your text book for help in answering this question. Hint – what warning appears on the label of diet drinks containing aspartame?] You may need to look up some information on aspartame.

1. Based on the information provided in the story, what are John and Mary’s genotypes? John’s __________ Mary’s ____________ How do you know?

2. If John and Mary were to have another child, what is the probably that they would have a child that could possibly have a problem with mental retardation (particularly if they don’t pay attention to diet)? Use the Punnet square below to help with your answer (you must fill it out for full credit!)

3. What can you tell John and Mary about the cause of the developmental delays in their children? Is just having the “right” genotype enough or are there other factors involved?

Answer 1

The parents Mary and John are healthy themselves and do not exhibit any abnormalities. But two of their children do. As mentioned this is associated with phenylalanine levels. Retarded children have higher levels of phenylalanine. Hence they will be homozygous for the recessive allele i.e. they will be genotypically pp. And the normal children have at least one normal allele. Hence they will be Pp or PP.

1. Since John and Mary are both normal and they have children who are homozygous dominant for the locus, both of them have to be heterozygous for the locus. Thus, both Mary and John will be Pp.

2. The probability of their child to have a retarded genotype can be checked by the Punnett square.

	P	p
P	PP	Pp
p	Pp	pp

Only the offspring with pp are at risk for this disease. Hence the probability that their next child would be mentally retarded especially when they don't pay attention to diet will be 1/4 or 25%.

3. The reason for the developmental defects in some of their children is due to the incomplete metabolism of phenylalanine, leading to the accumulation of this amino acid. This further leads to impairment of the developmental pathways thereby causing retardation. This is due to the fact that both the parents are carriers for the disease. However, the disease isn't completely driven by genetics. 25% of their children will be at risk. However, the risk can be eliminated if they ensure that they do not consume diets rich in phenylalanine or aspartame. These diets will induce production of more phenylalanine than these individuals.