Question 1

List three examples of a single gene disorder where there is extremely limited mutational heterogeneity and one example where different mutations in one gene result in a wide range of different diseases.

Accepted Answer

Sickle cell anemia results from a very limited range of mutations that each produce a change at amino acid position 6 in the beta globin chain, converting the normal glutamate to valine. Huntington disease is always caused by a modest expansion in the number of tandem CAG repeats within an array of these repeats located within exon 1. Achondroplasia is caused exclusively by mutation at a single nucleotide, producing a glycine to arginine substitution at amino acid position 380 in the FGFR3 (fibroblast growth factor receptor type 3) protein.
Different mutations in the LMNA (lamin A) gene can result in a wide variety of disorders, including different types of lipodystrophy, muscular dystrophy, cardiomyopathy, and progeria plus a neuropathy, Charcot-Marie-Tooth disease type 2B1.

Question 2

An individual single gene disorder can show different levels of genetic heterogeneity, and different mutations in a single gene can sometimes result in a very wide range of different phenotypes. Which, if any, of the following statements, is false?

Accepted Answer

B,C

Question 3

Which, if any, of the following is incorrect? When used in human genetics, the term heterozygote

Accepted Answer

B,D

Question 4

The human X and Y chromosomes are thought to have evolved from what used to be a pair of autosomes. Unlike homologous pairs of autosomes, the current X and Y chromosomes are very different in many ways, including their DNA composition, DNA sequence classes, and gene content. Comment on the degree of DNA sequence sharing between these two chromosomes, and the consequences of having very different sex chromosomes in males.

Accepted Answer

The answer of The human X and Y chromosomes are...

Question 5

With respect to autosomal dominant inheritance in human genetics, which, if any, of the following statements is incorrect?

Accepted Answer

The answer of With respect to autosomal dominant inheritance in...

Question 6

With respect to X-chromosome inactivation in females, which, if any, of the following statements is incorrect?

Accepted Answer

The process involves epigenetic silencing of each gene on one of the two X chromosomes, either the maternal X chromosome or the paternal X chromosome.

Question 7

With respect to autosomal recessive inheritance, which, if any, of the following statements is incorrect?

Accepted Answer

Heterozygotes can sometimes display symptoms if the disease-causing allele is dominant or if there is a phenomenon known as incomplete penetrance, where not all individuals with the disease-causing allele exhibit symptoms.

Question 8

Which, if any, of the following is incorrect? When used in human genetics, the term hemizygous is a property that

Accepted Answer

Hemizygosity applies to loci on the X chromosome in males because males have only one copy of the X chromosome, and to loci on mitochondrial DNA because they are exclusively maternally inherited.

Question 9

The term phenotype can be applied to a wide range of manifestations. Which of the following properties, if any, do not constitute a phenotypic manifestation?

Accepted Answer

The sequence of a person's beta globin gene is a genetic (genotypic) property, not a phenotypic manifestation. Phenotype refers to the observable physical properties of an organism, which can be influenced by the genotype and the environment, whereas the sequence of a gene is purely genotypic.

Question 10

What is meant by a selfish mutation? Illustrate your answer with reference to achondroplasia.

Accepted Answer

The answer of What is meant by a selfish mutation?...

Question 11

Which, if any, of the following is incorrect? When used in human genetics,

Accepted Answer

The terms dominant and recessive apply to alleles, not phenotypes. Phenotypes are described as dominant or recessive based on the alleles that cause them.

Question 12

Which, if any, of the following is incorrect? When used in human genetics, the term allele

Accepted Answer

The answer of Which, if any, of the following is...

Question 13

Purifying selection removes harmful alleles from the population because a proportion, at least, of people who carry the harmful alleles have reduced biological fitness (manifesting by reduced reproductive success rates). For any inherited disorder, the frequency of mutant alleles in the population is usually stable: mutant alleles that are eliminated from the population (because the people that have them do not reproduce, or reproduce less efficiently), and new mutant alleles are created by de novo mutation. Link individual types of single gene disorder
a) to
e) to one of the values i) to iv) for the percentage of mutant alleles that arise by de novo mutation.

Accepted Answer

The answer of Purifying selection removes harmful alleles from the...

Question 14

With respect to X-linked recessive inheritance, which, if any, of the following statements is false?

Accepted Answer

While women are typically carriers of X-linked recessive disorders, if they inherit two copies of the mutant allele, they can be affected by the disorder. Therefore, statement B is false - women are not always asymptomatic.

Question 15

With respect to mitochondrial inheritance, which, if any, of the following statements is false?
a) Affected individuals can be of either sex.
b) Mitochondrial disorders are transmitted virtually exclusively through the maternal line.
c) Both the sperm and the egg contribute mitochondrial DNA to the zygote.
d) Clinical variability is a common feature of mitochondrial DNA disorders.

Accepted Answer

The answer of With respect to mitochondrial inheritance, which, if...

Question 16

With respect to mosaicism, which, if any, of the following statements, is false?

Accepted Answer

Mosaicism is not solely the result of germline mutations, as somatic mutations can also lead to genetic mosaicism. Therefore, statement D is false. Statements A, B, and C are generally true.

Question 17

Which, if any, of the following is incorrect? When used in genetics, the term character

Accepted Answer

The term character in genetics refers to a phenotype or observable trait that can be inherited or influenced by genes, but it does not apply to intracellular properties like ion concentration or DNA sequences.

Question 18

The Hardy-Weinberg law is an important law of population genetics that relates the frequencies of genotypes to allele frequencies. Summarize in one short sentence what it states about the relationship of genotype frequencies to allele frequencies.

Accepted Answer

The answer of The Hardy-Weinberg law is an important law...

Question 19

With respect to X-linked dominant inheritance, which, if any, of the following statements is false?

Accepted Answer

In X-linked dominant inheritance, each daughter of an affected father will always inherit the affected X chromosome, making the risk of being affected 100%, not 1 in 2.

Question 20

For some single gene disorders, some members of a family who have the same genetic variants at the disease locus as strongly affected family members either show a much milder phenotype or no disease symptoms. List five explanations for why there can be a lack of penetrance or variable expressivity of a single gene disorder.

Accepted Answer

The answer of For some single gene disorders, some members...

Quiz 5: Single-Gene Disorders: Inheritance Patterns, Phenotype Variability, and Allele Frequencies