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
    Ff
    FFFFF
    FFFFF
  • B
    ff
    FFfFf
    FFfFf
  • C
    Ff
    FFFFf
    FFFFf
  • D
    FF
    FFFFF
    fffff

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 two copies of the allele for cystic fibrosis to be expressed in the phenotype.
  • BCystic fibrosis only affects offspring once every two generations.
  • CA person can carry more than one copy of the allele and not have cystic fibrosis.
  • DA person must have three copies of the allele for cystic fibrosis to be expressed in the phenotype.
  • 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?

Q5:

Duchenne muscular dystrophy is a sex-linked disorder that is characterized by progressive muscle degeneration and weakness in the body. It is caused by recessive alleles carried on the X chromosome.

Which of the following best explains why males are more likely to suffer from this disorder than females?

  • AMales have two Y chromosomes, so a recessive disease will be expressed if there are two copies on each chromosome.
  • BThe X chromosome is smaller than the Y chromosome, so alleles on the X chromosome are more likely to be expressed.
  • CThe chromosomes in males develop slower than those in females, so males are more likely to inherit disorders.
  • DMales only have one X chromosome, so a recessive allele on that chromosome will be expressed.

Assume the allele for Duchenne muscular dystrophy is d. What is the genotype of a female carrier?

  • AXdY
  • BXDY
  • CXDXD
  • DXDXd
  • EXdXd

Assume the allele for Duchenne muscular dystrophy is d. What is the genotype of a female who is homozygous dominant for this allele?

  • AXDXd
  • BXDXD
  • CXdXd
  • DXdY
  • EXDY

Assume the allele for Duchenne muscular dystrophy is d. What is the genotype of a male sufferer?

  • AXDXD
  • BXDXd
  • CXdXd
  • DXdY
  • EXDY

Q6:

Huntington’s disease is caused by a dominant allele (H). A heterozygous male reproduces with a homozygous recessive female.

What is the genotype of the heterozygous male?

  • AHH
  • BHh
  • Chh

What is the probability that a child of this couple will inherit Huntington’s disease? Use a Punnett square.

Which of the following best explains why there are no carriers for diseases caused by dominant alleles, such as Huntington’s?

  • AThe combination of alleles does not allow any offspring to be heterozygous.
  • BIf a person is heterozygous for the allele, they will express it in their phenotype.
  • CIf a person is heterozygous for the allele, they will not express it in their phenotype.
  • DIf a person has the allele, they will not survive to pass it on to their offspring.

Q7:

The pedigree chart provided shows the inheritance of color blindness in a family.

How many members of this family will be color blind?

The couple A and B are having a child. What is the probability that their child will be color blind?

The couple A and B are having a child. What is the probability that their child will be a carrier of the color-blindness allele?

Assume the allele for color blindness is b. What is the genotype of individual C?

  • AXBXB
  • BXBY
  • CXbXb
  • DXBXb

Q8:

Huntington’s diseases is caused by a dominant allele. Which of the following best describes what this means?

  • AA person must have more than one copy of the allele for the disease to be expressed in their phenotype.
  • BA person will express the disease in their phenotype even if only one allele is in their genotype.

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