9.2.2 Mendelian Inheritance

Question 1

The steps to determining the probabilities of genotypic and phenotypic frequency are

Answer 1

· Identify dominant and recessive alleles
· Determine parental genotypes
· Set up a Punnett square that shows the gametic contributions of the mother and father
· Examine the probable genotypic and phenotypic frequencies in the offspring

Question 2

cystic fibrosis

Answer 2

• Cystic fibrosis is a disease that results from a defective
  plasma membrane protein and causes complications of the lungs, pancreas, and sweat glands.
• A couple has a child with cystic fibrosis. Neither of the
  parents has the disease. What are the odds that their next child will not have cystic fibrosis, but will be a carrier (heterozygous)?
• To solve the problem, follow the steps for determining the probabilities of genotypic and phenotypic frequencies in the offspring.
• Identify dominant and recessive alleles.
• The child has the disease, but the parents don’t; therefore, the cystic-fibrosis trait must be recessive.
  Let N = normal
  n = cystic fibrosis
  nn = cystic fibrosis
• Determine parental genotypes.
  Because both parents are phenotypically normal, each parent
  has at least one dominant allele.
  P = normal X normal
  N_ N_
• Because the child has cystic fibrosis, his genotype must be nn. Therefore the parents each must have one n allele.
• P = normal X normal
  Nn Nn
• Set up a Punnett square that shows the gametic contributions of the mother and father.
• Each side of the Punnett square shows the possible gametes of the mother or father. Each parent can contribute either an N or n allele. The squares inside the Punnett square portray the genotypic frequencies for a single offspring. Each offspring is an independent event with respect to the other offspring.
• At birth the brother is phenotypically normal (does not have cystic fibrosis.) The nn genotype can be eliminated from the Punnett square. The probability that the child is heterozygous is 2/3.

Question 3

What are the chances that the healthy sibling of a child with CF and healthy parents is not a carrier of the CF trait?

Answer 3

• 1/3

Question 4

A plant biologist has recently discovered a population of wild orchids in the Ecuadoran Amazon. The majority of the population has red flowers but a few have pink flowers. To test the inheritance of the trait, she crosses two pink flowers and observes that 1/3 of the offspring are red and 2/3 are pink. What could explain the set of observed frequencies?

Answer 4

• The recessive allele is lethal when homozygous

Question 5

The fictitious variety of hamsters known as “tuxedo” is prized for their black and white coat pattern. You purchase a stud hamster named Henry for $300. All of the progeny sired by Henry in matings with a pure breeding tuxedo female display the black and white phenotype. Matings between certain pairs of offspring, however, produce an undesirable yellow and white coat color 25% of the time. Regarding the situation, which of the following statements is false?

Answer 5

• The pairs of offspring that produce the undesirable coat pattern are both homozygous for a recessive allele

Question 6

Pure-breeding avocado plants with round fruits are crossed with pure-breeding plants with oblong fruits. All of the F1 plants have round fruits. 100 pairs of the F1 plants are interbred and produce 400 F2 offspring. If the F2 plants are test crossed, and each produce two plants, what is the expected phenotypic ratio among the F3 offspring?

Answer 6

• 400 round and 400 oblong.

Question 7

Cystic Fibrosis (CF) is a recessive inherited disease. If two parents without CF have a child with CF, what must be true of the parents?

Answer 7

• They are both heterozygous for the recessive trait.

Question 8

If “N” represents the normal gene and “n” represents the gene for cystic fibrosis, then which of the following represents the genotypes of parents that have a chance of having a child with cystic fibrosis?

Answer 8

• Nn × Nn

Question 9

The Punnett square provides a simple method of calculating the probability of a particular genotypic or phenotypic outcome. In these squares, the dominant allele is often indicated by an uppercase letter, and the recessive by a lowercase letter (in this case the single trait is symbolized by the letters CF (or cf)). What are the genotypes of the offspring represented by cells B and D?

Answer 9

• heterozygous, homozygous recessive