Clinical Genetics Basics

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Questions and Answers

What term is used to describe couples who are related by blood?

Pedigree

Consanguineous (correct)

Siblings

Athletes

Which of the following best describes genetic heterogeneity?

Inheritance patterns are exclusively linear

Phenotypes vary widely without genetic involvement

Same mutations cause different phenotypes

Different mutations cause the same phenotype (correct)

In autosomal dominant inheritance, what is a characteristic feature observed in pedigrees?

The phenotype appears in every generation (correct)

Only males can transmit the phenotype

The phenotype skips generations

Affected individuals have no affected parents

What is the inheritance probability for a child of an affected parent with an autosomal dominant trait?

50% (C) Signup and view all the answers

What does allelic heterogeneity refer to?

Different mutations at the same locus causing a phenotype (D) Signup and view all the answers

Which relationship is NOT considered consanguineous in most societies?

Third cousins (B) Signup and view all the answers

What type of heterogeneity is illustrated by congenital sensorineural deafness?

Nonallelic heterogeneity (B) Signup and view all the answers

Which of the following statements is true regarding males and females in autosomal dominant traits?

There is no difference in inheritance likelihood (B) Signup and view all the answers

What is the likelihood that a daughter of a carrier female will be a carrier?

50% (B) Signup and view all the answers

What is the probability that a son of an affected male will have hemophilia?

0% (D) Signup and view all the answers

According to principles of Lyonization, what is the factor VIII activity level in carrier females compared to non-carrier females?

Approximately 50% (C) Signup and view all the answers

What is true about the daughters of a carrier mother with an affected father?

All will be carriers (D) Signup and view all the answers

What has facilitated carrier detection in females who have not reproduced in recent advancements?

Molecular genetic analysis (A) Signup and view all the answers

What is the main inheritance pattern associated with hemophilia A?

X-linked recessive (C) Signup and view all the answers

If a carrier female has two children, what is the chance that one will be affected by hemophilia?

50% (C) Signup and view all the answers

What unusual genetic occurrence is noted in hemophiliacs regarding their offspring?

They often have children. (A) Signup and view all the answers

What does the term 'alleles' refer to in genetics?

Alternative forms of a gene at the same locus (A) Signup and view all the answers

What is the primary distinction between genotype and phenotype?

Genotype indicates the alleles present, while phenotype indicates observable traits. (C) Signup and view all the answers

In genetics, what defines a homozygote?

An individual with two identical alleles at a locus (C) Signup and view all the answers

What characteristic is true about dominant traits?

They are seen in both homozygotes and heterozygotes. (A) Signup and view all the answers

What is the role of the proband in a pedigree?

The first affected family member seeking medical attention (A) Signup and view all the answers

Which of the following describes hemizygotes?

Individuals with one allele at a locus, often seen in males (A) Signup and view all the answers

How do recessive traits manifest in genetic expression?

Only in homozygotes for autosomal traits and hemizygotes for X-linked traits (B) Signup and view all the answers

What is the function of pedigree symbols in genetics?

To communicate family structure and inheritance patterns (C) Signup and view all the answers

What is the term for the process by which one of the X chromosomes in female somatic cells is inactivated?

Lyonization (A) Signup and view all the answers

In females, how many X chromosomes are typically active in somatic cells?

One (D) Signup and view all the answers

What is represented as the Barr body in interphase cells?

Inactivated X chromosome (C) Signup and view all the answers

How is the X chromosome that remains active determined in female cells?

It is randomly determined. (B) Signup and view all the answers

What does dosage compensation ensure for females with respect to X-linked genes?

They express only one allele despite having two. (B) Signup and view all the answers

What phenomenon explains the variation in expression of X-linked conditions in female heterozygotes?

Lyonization (B) Signup and view all the answers

What year was the Lyon hypothesis proposed by Mary Lyon?

1961 (D) Signup and view all the answers

In X-linked inheritance, what is the significance of males being hemizygous?

They only have one allele for X-linked genes. (B) Signup and view all the answers

What factor contributes to Duchenne Muscular Dystrophy (DMD) being a genetic lethal condition?

Affected males do not reproduce (C) Signup and view all the answers

Which of the following statements is true regarding X-linked dominant inheritance?

Heterozygous females have a 50% chance of passing on the trait (A) Signup and view all the answers

What is the expected sex ratio of offspring from heterozygous mothers for conditions lethal in hemizygous males?

2:1 female to male (D) Signup and view all the answers

What is the consequence of the massive size of the dystrophin gene?

It is a prominent target for mutations (A) Signup and view all the answers

In which situation would an affected male not pass the X-linked dominant trait to his offspring?

When he has affected daughters only (D) Signup and view all the answers

Which of the following describes a common characteristic of X-linked dominant conditions?

Affected females show less severe symptoms than males (C) Signup and view all the answers

What percentage of DMD cases can be attributed to new mutations?

One-third (B) Signup and view all the answers

Study Notes

Single-Gene Disorders

Single-gene disorders originate from mutations at a single genetic locus on a chromosome.
A locus refers to the specific position of a gene; alleles are different forms of a gene at the same locus on homologous chromosomes.
Normal alleles are called wild type, while abnormal forms are termed mutant alleles.
Distinction exists between genotype (genetic constitution) and phenotype (observable traits).
Homozygotes have identical alleles, heterozygotes possess different alleles, and hemizygotes have only one allele at a locus (common in X-linked traits in males).
Traits may be dominant (expressed in heterozygotes and homozygotes) or recessive (expressed in homozygotes for autosomes and hemizygotes for X-linked traits).

Pedigree Analysis

Pedigrees illustrate family structures and patterns of single-gene inheritance.
The proband is the first affected family member seeking medical attention.
Siblings are referred to as sibs, and their collective relationship is called sibship.
Relatives are classified based on degree of allele sharing, determined by intervening relatives in the pedigree.
Consanguineous relationships involve couples who are second cousins or closer.

Genetic Heterogeneity

Genetic heterogeneity occurs when the same phenotype results from different genetic abnormalities.
Allelic heterogeneity is when variations at the same locus lead to the same abnormal phenotype (e.g., cystic fibrosis).
Locus or nonallelic heterogeneity describes situations where mutations at different loci can produce the same phenotype (e.g., congenital sensorineural deafness).

Autosomal Dominant Inheritance

Expressed in both heterozygotes and homozygotes; features include:
- Phenotype appears in every generation.
- Each child of an affected parent has a 50% chance of inheriting the trait.
- Phenotypically normal family members do not transmit the trait.
- Both genders transmit the phenotype equally.

X-Linked Inheritance

Female somatic cells have one active X chromosome; the other is condensed as a Barr body.
X-inactivation happens early in embryonic development and can involve either paternal or maternal X.
Lyon hypothesis explains dosage compensation, ensuring females express one allele of X-linked genes.
In X-linked recessive inheritance, a carrier female has a 50% chance of having an affected son and a 50% chance of having a carrier daughter.

Hemophilia A

An X-linked disorder characterized by factor VIII deficiency.
Carrier females exhibit approximately 50% factor VIII activity compared to non-carriers.
Affected males transmit the normal allele to sons and the mutant allele to daughters.

Duchenne Muscular Dystrophy (DMD)

An X-linked recessive disorder caused by mutations in the dystrophin gene, known for being particularly large.
Affected males do not reproduce, resulting in a genetic fitness of 0, leading to new mutations as a source for one-third of DMD cases.

X-Linked Dominant Inheritance

Regularly expressed in heterozygotes, with criteria including:
- Affected males have no affected sons but only affected daughters.
- Offspring of heterozygous females have a 50% chance of being affected.
- Affected females typically exhibit milder symptoms and are more prevalent due to skewed sex ratios in affected offspring.

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Description

Test your understanding of single-gene disorders and the concepts related to genetic loci and alleles. This quiz will cover the definitions and distinctions between wild type and mutant alleles, as well as how these genetic elements are inherited. Dive into the fascinating world of genetics and assess your knowledge!