Which of the following best explains how the woman inherited Huntingtons disease?

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

Q1:

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?

Q2:

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
• B
• C
• D

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:

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.

Q5:

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.

Q6:

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

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

• AXdY
• BXDXd
• CXDXD
• DXDY
• EXdXd

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

• AXDY
• BXDXd
• CXdXd
• DXdY
• EXDXD

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

• AXdY
• BXDY
• CXdXd
• DXDXD
• EXDXd

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.

Q9:

The genes for yellow-coloured fur in mice and chlorophyll absence in corn are examples of inherited lethal genes. All of the following statements are correct about the two types of lethal genes except .

• Aall yellow mice and all green corn seedlings should be heterozygous to survive
• Brecessive lethal genes can only cause death in the homozygous state
• Cwhen crossing two heterozygotes which carry lethal genes in both corn plants and yellow mice, 100% of white corn seedlings die, whereas only 1/3 of yellow mice in the offspring do not survive
• Dwhen crossing two heterozygotes which carry lethal genes in both corn plants and yellow mice, both types of lethal genes showed lethality (loss) ratio in the offspring of 1 dead : 3 viable

Q10:

A boy inherited one allele of infantile dementia (Tay–Sachs disease) from one of his parents. Which of the following is true regarding the symptoms the boy might develop?

• AThe boy will show symptoms because he is a male.
• BThe boy will show symptoms because this trait is a sex-linked trait.
• CThe boy will show symptoms because his mother is homozygous.
• DThe boy will not show symptoms of the disease.

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