Every individual has two sets of chromosomes-one set from the father and one set from the mother. Our phenotype is the result of the interaction of the alleles on these two sets of chromosomes. The objectives of this exercise are to learn how to use the Punnett square to predict the probability that a child will inherit a certain phenotype and/or genotype from his/her parents and to be introduced to some genetic disorders found in humans.
In the following problems you will practice using the Punnett square to predict the probability of parents with certain genotypes producing children with certain genotypes. Many of the problems will concern genetic diseases many of which are caused by a recessive allele. When a disease or characteristic is caused by a recessive allele, individuals with only a single allele for the disease will usually be normal, but will carry the allele for the trait and therefore possibly pass it on to their children. Individuals with two alleles for the disease or characteristic will actually have the disease or characteristic. Genetic diseases occur because the disease causing allele is unable to produce a specific protein required by the body. In the cases of diseases caused by recessive alleles, an individual who is heterozygous usually appears normal because the one normal allele he receives is able to produce enough of the required protein.
Remember that the Punnett square predicts probabilities-not certainties. Just because statistics predict that the probability of a specific event occurring is 50%, that does not mean that it will always happen 50% of the time. For example, just because statistics predict that ½ of the children a couple has will be boys and ½ will be girls, this does not always happen in real life. As another example, consider flipping a coin. Statistics predict that ½ of the time the coin will land heads up and the other ½ of the time it will land tails up. If you flip the coin enough times this will indeed happen, but if you only flip it a few times you sometimes end up with a predominance of heads or a predominance of tails. This is called luck and it often occurs in genetics as well as gambling. For example, genetics might predict that a couple had 2 out of 4 chances of producing a child with Huntington disease. Suppose this couple then had four children-all of whom were normal. We would say they were lucky-they beat the odds.
Simple Monohybrid Cross-this type of cross involves a single trait controlled by a single pair of alleles. One allele is dominant (designated by a capital letter) and the other is recessive (designated by a small letter).
Phenylketonuria-Individuals with this disorder are unable to produce the enzyme responsible for the conversion of the amino acid phenylalanine to the amino acid tyrosine. Because tyrosine is needed in the production of melanin, PKU sufferers are light-haired and fair -skinned. Phenylalanine is present in protein and in the artificial sweetener aspartamine (such as Equal or Splenda). Phenylalanine builds up in the body and acts as a neurotoxin causing brain damage and mental retardation. Most states require that all newborns be tested for the presence of this disorder. Afflicted individuals are placed on a special diet that is extremely low in phenylalanine. Strict adherence to this diet can prevent the brain damage and mental retardation. As a result of early detection and treatment, adults with this disorder may have a normal phenotype. PKU is caused by a recessive allele (p). Therefore, only individuals homozygous for the allele will develop the disorder.
Assume a man named John is homozygous for the PKU allele and marries a woman named Mary who is heterozygous for PKU and answer the following questions. Using a Punnett square and your formulas for probability will help you solve the problems.
1.
What is John's genotype?
A) pp
B) PP
C) Pp
D) cannot be determined