CM19 - Modes of Inheritance I

Biomedical Sciences I

What is the definition of a species in genetics?

A species is a group of organisms comprised of similar individuals capable of interbreeding

What is a population in genetic terms?

A population consists of individuals of a species occupying a space, interacting, interbreeding, and exchanging genetic material

What is a gene pool?

The gene pool includes all alleles from all individuals within a population

What is allele frequency?

Allele frequency is the proportion of a specific allele among all alleles in a population

What is genotype frequency?

Genotype frequency is the proportion of individuals with a specific genotype in a population

What is the Hardy-Weinberg principle?

It states that allele frequencies remain constant over time in a non-evolving population, maintaining genetic equilibrium

Does Hardy-Weinberg equilibrium apply to X chromosomes in males?

No, it applies to autosomal loci and X chromosomes in females only

What are five factors that disturb Hardy-Weinberg equilibrium?

• Non-random mating

• Small population size

• Mutations

• Natural selection

• Immigration/emigration

What is stratification in genetics?

Subgroups within a population remain genetically separate, causing apparent excess of homozygotes

What is assortative mating?

Assortative mating is mate selection based on a specific trait; it has minor long-term effects on allele frequency

What is consanguinity and its genetic impact?

Mating between relatives, increasing risk of recessive disorders in offspring

What is genetic fitness (f)?

The ability of an organism or a specific genotype to pass its genes to the next generation, measured by its success in surviving and reproducing

What does a fitness value of f = 1 indicate?

The mutant allele is as likely as the wild-type to appear in the next generation

What does a fitness value of f = 0 indicate?

The allele causes death or sterility and is negatively selected against

What determines stable allele frequency in a population?

A balance between removal (selection) and addition (mutation) of mutant alleles

What is gene flow in population genetics?

The slow diffusion of genes across populations, merging gene pools over time

What is genetic drift?

A random change in allele frequency in small populations due to chance

What is the founder effect in genetic drift?

It occurs when a few individuals start a new population, leading to different allele frequencies

What is a bottleneck effect in genetic drift?

It results from a drastic reduction in population size, altering allele and genotype frequencies

What is heterozygote advantage?

It occurs when heterozygotes have increased fitness compared to wild-type homozygotes

Give an example of heterozygote advantage.

Sickle cell anemia provides resistance to malaria

~What are single-gene disorders?

Disorders caused by mutations in a single gene locus, following classical inheritance patterns

~8,000 listed in the OMIM database

What is pleiotropy in genetics?

When a single gene causes multiple phenotypes in different organs or at different times

What is penetrance in genetics?

The probability that a mutant allele will show a phenotype (all-or-none)

What is expressivity in genetics?

The degree of severity of a phenotype among individuals with the same genotype

What is a proband in a pedigree?

The first diagnosed individual in a family, also called the index case

How do you distinguish autosomal vs. sex-linked inheritance in a pedigree?

• Mostly males → likely X-linked

• 50:50 M:F → autosomal

• Male-to-male transmission → autosomal

How do you distinguish dominant vs. recessive inheritance in a pedigree?

One parent affected and offspring is affected→ dominant

Neither parent affected but offspring is affected → recessive

What is autosomal dominant inheritance?

A condition where only one copy of a diseased allele is needed to express the phenotype

What are examples of autosomal dominant disorders?

Huntington’s disease, polycystic kidney disease, familial hypercholesterolemia

What are key characteristics of autosomal dominant inheritance?

• No skipping generations

• 50% recurrence risk for children of affected heterozygotes

• Equal sex distribution

• Often due to structural proteins or transcription factors

What is autosomal recessive inheritance?

A condition where both alleles must be mutated to express the phenotype

What are examples of autosomal recessive disorders?

Albinism, phenylketonuria, sickle cell anemia, cystic fibrosis

What are key characteristics of autosomal recessive inheritance?

• Equal sex distribution

• 25% recurrence risk for offspring of two carriers

• Skips generations

• Parents are usually asymptomatic carriers

• Often involves enzyme deficiencies

What is Y-linked inheritance?

Inheritance of genes on the Y chromosome, affecting only males and passed from father to son

What is X-linked inheritance?

• Males are hemizygotes

• Females are homo or heterozygotes

What is X-linked recessive inheritance?

A condition where males are more frequently affected due to having only one X chromosome

What are examples of X-linked recessive disorders?

Hemophilia A, Duchenne muscular dystrophy, colorblindness

What are key characteristics of X-linked recessive inheritance?

• Higher incidence in males

• Trait skips generations

• No father-to-son transmission

• All daughters of affected fathers are carriers

What is X-linked dominant inheritance?

An x-linked condition expressed in both sexes, even in heterozygotes, with no male-to-male transmission

What are key characteristics of X-linked dominant inheritance?

• Affects both sexes

• No male-to-male transmission

• Affected males → all daughters affected, no sons

• Affected females → 50% sons and daughters affected

• Males often more severely affected (because females have x inactivation)