Modes of Inheritance I Flashcards

Flashcards testing terminology, Hardy-Weinberg principle, evolutionary mechanisms, and Mendelian modes of inheritance from lecture notes.

Species

A group of organisms comprised of similar individuals capable of interbreeding.

Population

Individuals of a particular species occupying a definite space, in which individuals interact, interbreed, and exchange genetic material.

Gene pool

All alleles from all individuals within a population.

Allele frequency

The proportion of individuals in a population with a specific allele, which may differ among populations.

Genotype frequency

The proportion of individuals in a population with a specific genotype, which may differ among populations.

Population genetics

The quantitative study of the distribution of allele frequencies in a population and its changes over time and between populations.

Hardy-Weinberg law

A law stating that allele frequencies remain constant over time in a population that does NOT evolve (genetic equilibrium).

Hardy-Weinberg Punnett square

A diagram demonstrating that random mating produces next-generation genotypes in proportions of $p^2(AA)$, $2pq(Aa)$, and $q^2(aa)$.

Hardy-Weinberg allele frequency equation

$p + q = 1$, where $p = f(B)$ is the dominant allele frequency and $q = f(b)$ is the recessive allele frequency at genetic equilibrium.

Hardy-Weinberg genotype frequency equation

$(p + q)^2 = p^2 + 2pq + q^2 = 1$, where $f(BB) = p^2$, $f(Bb) = 2pq$, and $f(bb) = q^2$.

Factors disturbing Hardy-Weinberg equilibrium

Five main conditions: non-random mating, small population size, mutations, natural selection, and immigration/emigration.

Stratification

A non-random mating condition where a population has subgroups that remain genetically separate, leading to an apparent excess of homozygotes in the overall population if a subgroup has a higher allele frequency.

Assortative mating

Choice of a mate based on a particular trait, which has a minor long-term effect on population equilibrium.

Consanguinity

Mating between related parents, which increases the likelihood of rare recessive disorders in offspring.

Tay-Sachs disease prevalence

An autosomal recessive disorder occurring at a rate of $1:3600$ in the Ashkenazi Jew population compared to $1:360000$ in the general population.

Fitness ($f$)

A measure of surviving affected offspring compared to control, determined by the outcome of collaboration between survival and fertility.

Fitness $f = 1$

Indicates that a mutant allele is as likely as the wild-type allele to appear in the next generation.

Fitness $f = 0$

Indicates that a mutant allele causes death or sterility, or is completely negatively selected against.

Stable allele frequency

A balance between the removal (selection) and addition (mutations) of mutant alleles in a gene pool.

Gene flow

The slow diffusion of genes through barriers, involving large populations and slow changes in allele frequency as migrant populations merge into a new gene pool.

CCR5 mutation distribution

A gene variant with the highest frequency in Europe, small frequency in the Middle East and India, and almost absent in Africa.

Genetic drift

Random change in allele frequency in small populations due to chance, where individuals carrying a mutant allele may randomly produce more offspring.

Founder effect and Bottleneck effect

Founder effect occurs when a few isolated individuals start a new population; Bottleneck effect occurs when a critical event drastically reduces population size.

Founder effect

Genetic drift occurring after a few individuals (a fraction of the original gene pool) start a new population in isolation with different allele frequencies.

Bottleneck effect

Genetic drift occurring after an event drastically reduces population size, resulting in altered allele and genotype frequencies in the surviving population.

Heterozygote advantage

A phenomenon where heterozygotes have increased fitness over wild-type homozygotes, such as the sickle-cell allele providing protection against Falciparum malaria.

Single-gene disorders

Mendelian disorders determined primarily by alleles on a single locus that follow classical inheritance patterns.

OMIM

An online database documenting approximately $8000$ single-gene diseases or traits in humans.

Pleiotropy

A phenotypic property where a single abnormal gene produces a variety of phenotypes in different organs, with different signs and symptoms, at different times.

Penetrance

The probability that a mutant allele will have a phenotypic expression, evaluated on an all-or-none basis.

Incomplete penetrance

A scenario occurring when a disease genotype fails to express its expected phenotypic trait (also termed reduced penetrance).

Expressivity

The degree of phenotypic severity among individuals possessing the same disease genotype.

Pedigree

A graphical representation of a family tree using standard symbols to establish the pattern of genetic trait transmission.

Proband

The first individual diagnosed with a disease in a family (also known as propositus/a or index case).

Pedigree symbols key

A standardized chart defining symbols such as squares for males, circles for females, filled shapes for affected individuals, and double horizontal lines for consanguinity.

Pedigree criteria for sex-linked vs. autosomal inheritance

An inheritance pattern affecting mostly males suggests X-linked inheritance, whereas a $50:50$ male-to-female ratio or male-to-male transmission confirms autosomal inheritance.

Pedigree criteria for dominant vs. recessive inheritance

If every affected child has at least one affected parent, the disorder is dominant; if two unaffected parents produce an affected child, it is recessive.

Autosomal dominant inheritance

An inheritance mode requiring only one copy of a mutated allele to exhibit the phenotype, accounting for $>50\%$ of all Mendelian disorders.

Autosomal dominant disease examples

Huntington's disease, polycystic kidney disease, and familial hypercholesterolemia.

Autosomal dominant recurrence risk

The risk for each child of an affected heterozygous parent to inherit the phenotype is $50\%$.

Autosomal dominant gene products

Defective gene products in autosomal dominant traits are usually structural proteins or transcription factors.

Autosomal recessive inheritance

An inheritance mode where the phenotype is exhibited only when both allele copies are mutated.

Autosomal recessive disease examples

Albinism, phenylketonuria, alkaptonuria, sickle cell anemia, and cystic fibrosis.

Autosomal recessive recurrence risk

The risk for offspring of two heterozygous carrier parents is $25\%$, but increases to $50\%$ if a homozygote mates with a heterozygote.

Autosomal recessive gene products

Defective gene products in autosomal recessive disorders are mostly enzymes.

Y-linked inheritance

An inheritance pattern involving genes on the Y chromosome, transmitted strictly from father to son in all generations, affecting only males.

Hemizygote

The genotype status of a male carrying a single X chromosome.

X-linked recessive inheritance

An inheritance pattern characterized by higher incidence in males, unaffected heterozygous females, skipping of generations, and absence of father-to-son transmission.

X-linked recessive disease examples

Hemophilia A, Duchenne muscular dystrophy, and colorblindness.

X-linked dominant inheritance

A rare inheritance mode expressed in heterozygotes, distinguished from autosomal dominant inheritance by a lack of male-to-male transmission and $100\%$ affected daughters from affected fathers.