Question 1

Why are dizygotic twins less likely to show concordance for a monogenic trait than monozygotic twins?

Accepted Answer

Dizygotic twins share only 50% of their genetic material, while monozygotic twins share 100%. Therefore, monozygotic twins are more likely to have the same allele sequences for a monogenic trait.

Question 2

A man whose parents both have brown hair claims that his red beard was inherited from his maternal uncle.Why is this claim incorrect?

Accepted Answer

Choices A, C, and D have no bearing on the correct response. Regardless of whether A, C, or D are true statements or not (hair color actually is an autosomal single-gene trait), a person can only inherit traits from his or her father and mother, not from parents' siblings. Both a person and his uncle may have inherited the same trait from a more remote common vertical ancestor (grandparent, great-grandparent) but not from horizontal ancestors (siblings, aunts, uncles, cousins).

Question 3

Which genetic disorder has both an autosomal dominant and an autosomal recessive form?

Accepted Answer

Retinitis pigmentosa can have both autosomal dominant and autosomal recessive inheritance patterns. Some cases are caused by mutations in a dominant gene, while others are caused by mutations in two recessive genes. This results in different patterns of inheritance within families affected by the disorder.

Question 4

A baby boy is born with six toes (polydactyly)on his right foot,a trait that neither parent has but one that the paternal grandfather has.What is the best explanation for this occurrence?

Accepted Answer

Polydactyly is an autosomal-dominant single-gene trait. Thus, this baby boy did not inherit a recessive allele from each parent to express the trait. It is possible that the baby's grandfather could be his father, but this is not the best explanation. In expressivity, the trait is always expressed, but the degree to which it is expressed can vary. This means that the baby's father might have more than one extra digit, but if expressivity were the actual problem, the father would definitely also have an extra digit. With variation in penetrance, the gene is present but not always expressed. For polydactyly, the penetrance is about 80%. So, the father most likely does have the allele, but it is not expressed in him. However, because the allele is present, he can transmit it (with a probability of 50% for each pregnancy) to his children.

Question 5

In which situation are phenotype and genotype always the same?

Accepted Answer

Autosomal-recessive traits are only expressed in the homozygous state in which the genotype matches the phenotype. Autosomal-dominant traits can be expressed in either the homozygous or the heterozygous state. Thus, with autosomal-dominant expression, phenotype and genotype do not have to match. In X-linked-recessive traits, males express the trait in a dominant manner. In X-linked-dominant traits, females express the trait in either the homozygous or heterozygous state.

Question 6

Which statement regarding inheritance of an autosomal dominant gene allele with known variability in expressivity is true?

Accepted Answer

The degree of expressivity of an autosomal dominant trait with known variability cannot be predicted by analyzing parental expression because expressivity can vary widely even among family members with the same genetic mutation.

Question 7

Which situation most closely represents an example of "regression to the mean?"

Accepted Answer

Regression to the mean refers to the phenomenon where extreme values on a trait tend to be followed by less extreme values in the next generation. In situation C, the mother's IQ is extremely high, and it is unlikely that all three of her children would have the same level of exceptional intelligence. Therefore, the children's IQs regressed to the mean of the population. The other situations do not reflect regression to the mean: A is an example of a recessive trait, B is an example of a genetic predisposition, and D is an example of environmental influences on weight.

Question 8

What is the expected outcome of any pregnancy or child produced by two parents who are each heterozygous for the Huntington disease allele?

Accepted Answer

Huntington disease is an autosomal-dominant disorder with very high penetrance. A heterozygous person always develops the disorder, but the age of onset and the rate of disease progression can vary. This disorder does not show homozygous genotypes because homozygosity appears lethal, with loss at the embryonic or fetal pregnancy stages or within the first 12 months after birth. Thus, even if a child is born who is homozygous for the Huntington disease mutation, he or she will not live long enough to transmit the allele.

Question 9

What can be inferred about a genetic disorder that,when it occurs in monozygotic twins,affects one twin 80% of the time and affects both twins only 20% of the time?

Accepted Answer

The fact that the disorder affects only one twin 80% of the time suggests that nongenetic factors, such as environmental influences, can affect the expression of the disorder even when the twins have identical genetic material.

Question 10

A woman (Adele)has three daughters (Barbara,Brenda,Beverly)with a man (Adam)who is color-blind.Barbara has a daughter (Carol)and a son (Cliff)who have normal color vision.Brenda has no children.Beverly has two sons (Carl,Charlie)who are both color-blind.To which genetic generation do Carl and Charlie belong?

Accepted Answer

Carl and Charlie belong to the F₂ generation as they are the grandchildren of the original parents, Adele and Adam.

Question 11

What is the most obvious expected finding when analyzing a four-generation pedigree for sex-linked dominant transmission of a trait?

Accepted Answer

In sex-linked dominant transmission, the trait is always transmitted from an affected male to all his daughters, who are obligatory carriers, and then to half of his sons, but never from a father to his son. This is due to the fact that males only inherit one copy of the X chromosome from their mother, while females inherit two. Therefore, if the trait is on the X chromosome, a son can only inherit it from his mother who is a carrier, but not from his father who only passes on his Y chromosome.

Question 12

When calculating the recurrence risk for a complex possibly polygenic disorder with a couple who already have one child with the disorder,which additional factor has the greatest impact on increasing this risk?

Accepted Answer

The fact that the mother has a brother who also has the same disorder indicates that it is more likely that the disorder is caused by a genetic mutation or alteration. This means that the recurrence risk for another child with the disorder is higher compared to other factors listed.

Question 13

Which person is an obligate carrier of an autosomal recessive single gene trait or disorder without expressing the trait or disorder?

Accepted Answer

Attached earlobes (versus free-swinging earlobes) is an autosomal-recessive trait. A person who expresses this trait is homozygous for the allele and transmits the allele to all his or her children regardless of the type of earlobes any of them express. This makes them obligate carriers of the allele for earlobe attachment. The hemophilia gene mutation is on the X chromosome, which a father cannot transmit to his son. Marfan syndrome and widow's peak are autosomal-dominant traits for which there is no carrier status. If the allele is present, it is expressed.

Question 14

Which type of genetic transmission promotes the continued existence of genetic mutations in single genes?

Accepted Answer

Autosomal recessive inheritance requires two copies of the gene mutation to be present for the trait to be expressed. Carriers of the mutation (who have one copy of the mutated gene) do not show symptoms, but can pass on the mutation to their offspring. This allows for the continued existence of the genetic mutation in future generations. Dominant and codominant inheritance patterns require only one copy of the mutation for the trait to be expressed, and sex-linked inheritance involves genes on the X or Y chromosomes, which behave differently than autosomal genes.

Question 15

Which statement or factor is a criterion for autosomal recessive transmission of single gene traits?

Accepted Answer

Because autosomal-recessive traits or disorders are only expressed in the homozygous state, expression of the trait or disorder within a large kindred remains at a relatively constant level of no greater than about 25%. The heterozygous individual is a carrier who has a 50% chance with each pregnancy of transmitting the allele to his or her offspring. There is no evidence to support the presence of the concept of "penetrance" in autosomal-recessive traits and disorders. This issue appears to be associated only with dominant traits and disorders.

Question 16

What is the expected expression of a monogenic trait that has its locus on the Y chromosome?

Accepted Answer

True females have a pair of X chromosomes that determine their genetic sexual identity and do not have a Y chromosome. Therefore, females cannot inherit traits or disorders that are coded only for genes on the Y chromosome.

Question 17

If there are 10 possible alleles for the single gene trait of nose shape,how many alleles can a person with euploid chromosomes inherit from his or her biological parents?

Accepted Answer

A person with euploid chromosomes inherits one allele from each biological parent, so they can inherit a maximum of two alleles for a single gene trait.

Question 18

Which statement or condition best reflects multifactorial inheritance?

Accepted Answer

Multifactorial inheritance involves the interaction of genetic and environmental factors in the development of a trait or disease. Therefore, option B is the best answer as it reflects that susceptibility to a problem is inherited, but environmental conditions play a role in its development. The other options either describe single gene inheritance (option A) or a specific type of late-onset genetic inheritance (option C) or describe a specific example of single gene inheritance (option D) which is not related to multifactorial inheritance.

Question 19

A girl of normal stature is born to two parents with achondroplasia who have very short stature,especially disproportionately short arms and legs.What is the probability (by Punnett Square analysis)that any pregnancy this girl eventually has will result in the birth of an infant with achondroplasia if her partner also has normal stature?

Accepted Answer

The answer of A girl of normal stature is born...

Question 20

Which statement reflects the criterion for autosomal dominant transmission of single gene traits?

Accepted Answer

Autosomal dominant inheritance means that only one copy of the mutant gene is needed for the trait to be expressed. Therefore, if a person is homozygous for the trait, they will always pass it on to their children with each pregnancy.

Quiz 4: Patterns of Inheritance