Worksheet: Inherited Disorders

In this worksheet, we will practice using genetic diagrams to predict probabilities of offspring inheriting recessive, dominant or sex-linked disorders.

Q1:

A person who is heterozygous for the cystic fibrosis allele (Ff) reproduces with a person who is homozygous dominant (FF). Which of the Punnett squares shows the correct cross?

  • A
    F f
    F FF Ff
    F FF Ff
  • B
    F f
    F FF FF
    F FF FF
  • C
    F F
    F FF FF
    f ff ff
  • D
    f f
    F Ff Ff
    F Ff Ff

Q2:

Cystic fibrosis is an inherited disorder caused by a recessive allele. Which of the following best explains what this means?

  • AA person must have three copies of the allele for cystic fibrosis to be expressed in the phenotype.
  • BA person must have two copies of the allele for cystic fibrosis to be expressed in the phenotype.
  • CCystic fibrosis only affects offspring once every two generations.
  • DA person can carry more than one copy of the allele and not have cystic fibrosis.
  • EA person will express the disease in their phenotype even if only one allele is in their genotype.

Q3:

Polydactyly is caused by a dominant allele.

A female heterozygous for polydactyly reproduces with a heterozygous male. What is the probability that the offspring will have polydactyly?

A male heterozygous for polydactyly reproduces with a female who is homozygous recessive. What is the probability that the offspring will have polydactyly?

Q4:

PKU is an inherited disorder caused by a recessive allele (r). A male who is homozygous for the disorder reproduces with a heterozygous female.

What is the genotype of the male homozygous for PKU?

  • ARr
  • Brr
  • CRR

What is the genotype of the female heterozygous for PKU?

  • ARr
  • Brr
  • CRR

What is the probability that a child of theirs will inherit the disorder PKU?

What is the probability that a child of theirs will be a carrier of the PKU allele?

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