Homework page 1

1.01 Genotype and phenotype are two terms that are often confused by beginning biology students, but understand them is crucial to interpreting may genetics problems.

(a) Using your own words, explain the difference between genotype and phenotype. You can look up the definitions first in the textbook glossary, but be sure you can state what these terms mean in language that you understand.

(b) A heterozygous tall pea plant is crossed with a dwarf pea plant. What are the phenotypes and genotypes of each parent? Note: To answer this, you will need to look up which of these traits is dominant and which is recessive (if you do not already know).

(c) Considering all seven of the pea traits that Mendel tested, is it possible for a heterozygous plant to have the dominant phenotype? Explain.

(d) Is it possible for a heterozygous plant to have the recessive phenotype? Explain.

1.02 When an individual expresses the dominant phenotype for a trait, two genotypes are possible, homozygous for the dominant allele or heterozygous. In a case where the genotype is not known for sure (because the dominant phenotype is present), it is indicated as A-, meaning that there is at least one dominant allele present, but the second allele may be A or a. Suppose you have a single pea plant with round seeds (R-). What cross would you make to determine if it has genotype RR or Rr? Explain how the results of your cross would enable you to determine the genotype?

1.03 In cucumbers, orange fruit is dominant to cream colored fruit. A true-breeding plant with orange fruit is crossed with a true-breeding plant with cream fruit. The F1 plant are then crossed with each other to produce the F2 generation.

(a) Using allele codes of your choice, show the genotypes and phenotypes of the parents (P generation). You must define what each allele code means.

(b) Which genotypes and phenotypes are possible in the F1 plants?

(c) Using a Punnett square, determine which genotypes and phenotypes are possible in the F2 generation and calculate the expected ratio of each.

(d) What would be the expected genotype and phenotype ratios for a backcross between the F1 and the orange parent?

(e) What would be the expected genotype and phenotype ratios for a backcross between the F1 and the cream parent?

1.04 Maple syrup urine disease is a rare human genetic disease that runs in families. It is so called because a metabolic error causes the urine of affected babies to have an odor like maple syrup. The babies die at an early age if not treated. The parents are almost never affected with these symptoms. If we assume that the disease is caused by a single gene, is it a recessive or a dominant gene? Explain your answer.

1.05 In one of Mendel's experiments, a true-breeding yellow-seeded plant was crossed with a true-breeding green-seeded plant. All F1 plants were yellow-seeded. Mendel then obtained an F2 generation by allowing F1 plants to self-pollinate.

(a) If one of the F1 plants were crossed with a green-seeded plant, what proportion of the progeny would you expect to be yellow?

(b) If a number of the yellow-seeded F2 plants were allowed to self-pollinate, approximately what proportion of these crosseswould you expect to produce only yellow progeny?

1.06 The first child of two normally pigmented parents has albinism, a recessive trait that results from a lack of the pigment melanin.

(a) Given that the normal allele is A and the albino allele is a, show the phenotypes and genotypes of the parents and child.

(b) What is the probability that a second child from these parents will be an albino? (The probability is the fraction of her offspring that you would expect to have PKU. We will define probability and discuss probability calculations in detail later.)

(c) What is the probability that the second child will carry at least one copy of the albino allele?

(d) The couple has a second child who has normal pigmentation. What is the probability that this child is a carrier?

1.07 A cross is made between a homozygous tall pea plant and a homozygous short pea plant.

(a) What types of gametes could each plant produce?

(b) What genotypes and phenotypes would occur in the F1 generation?

(c) An F1 plant is allowed to self-pollinate. Using a Punnett square, calculate the expected ratios of genotypes and phenotypes in the F2 generation.

(d) 756 plants are produced in the F2 generation. Based on your expected ratios, how many of these would you expect to be tall and how many short?

(e) The numbers actually obtained are 571 tall and 185 short. Calculate the actual phenotypic ratio. How well does this agree with your expected ratio?

1.08 A woman with the rare recessive disease phenylketonuria (PKU), who had been treated with a diet having low levels of the amino acid phenylalanine, was told that it was unlikely her children would inherit PKU because her husband did not have it. However, her first child had PKU.

(a) What is the most likely explanation?

(b) Assuming this explanation is true, what would be the probability of her second child having PKU?