Non-Mendelian and Cytoplasmic Inheritance in Genetics

Non-Mendelian Inheritance

Patterns of inheritance that do not follow Mendel's laws. Caused by DNA products not encoded in nuclear chromosomes (cytoplasmic or maternal inheritance) or by transient DNA modifications (epigenetics).

Maternal effect

A Non-Mendelian pattern where the mother's genotype alone determines the offspring's phenotype. Caused by maternal gene products deposited in the egg during oogenesis.

Nurse cells

Diploid cells surrounding the egg that express maternal genes and transfer mRNA or protein products into the egg.

Dextral (right-handed) snails

A dominant trait for right-coiled snail shells; offspring phenotype depends on the mother's genotype, not her phenotype.

Sinistral (left-handed) snails

A recessive trait for left-coiled snail shells; occurs when the mother's genotype is homozygous recessive (dd).

Epigenetic inheritance

Inheritance caused by chemical DNA modifications (like methylation) that alter gene expression without changing the DNA sequence. These modifications can be reversible and passed to offspring.

Dosage compensation

Mechanisms that equalize gene expression between individuals with different numbers of sex chromosomes, such as XX females and XY males.

X-chromosome inactivation

The process in female mammals where one X chromosome is condensed and inactivated to form a Barr body. Causes mosaic expression of X-linked traits.

Barr body

The condensed, inactive X chromosome found in female somatic cells; helps ensure only one active X per cell.

Variegated coat color example

In female mammals, random X-inactivation leads to a mosaic pattern of pigment expression, such as in calico cats or variegated mice.

X-inactivation center (Xic)

The chromosomal region where X-inactivation begins. It contains the Xist gene, which spreads inactivation across the chromosome.

Pseudoautosomal genes

Genes on the X chromosome that escape inactivation; often shared with the Y chromosome and do not require dosage compensation.

Extranuclear (cytoplasmic) inheritance

Inheritance of genes outside the nucleus, found in organelles like mitochondria or chloroplasts. Typically passed maternally since eggs contribute most cytoplasm.

Mitochondrial DNA (mtDNA)

Circular DNA in mitochondria containing genes for rRNA, tRNA, and proteins used in oxidative phosphorylation. In humans, ~17,000 bp long.

Chloroplast DNA (cpDNA)

Circular DNA in chloroplasts involved in photosynthesis; often 10x larger than mitochondrial DNA and contains 110-120 genes.

Maternal inheritance

The transmission of organelle genes (mtDNA or cpDNA) through the cytoplasm of the egg; most offspring inherit these genes only from their mother.

Heteroplasmy

The presence of more than one type of organelle DNA (normal and mutant) in a cell. Important in mitochondrial diseases—symptoms appear when mutant mitochondria exceed a threshold.

Mitochondrial disease

Disorders caused by mutations in mtDNA, often affecting high-energy tissues like muscle or nerve cells. Severity depends on the ratio of normal to mutant mitochondria.

Population genetics

The study of genetic variation within populations and how it changes across generations.

Gene pool

All the alleles of every gene in a population; only individuals that reproduce contribute to the next generation's gene pool.

Population

A group of individuals of the same species that occupy the same area and can interbreed.

Local population

A smaller subgroup within a species that breeds more frequently within itself than with the general population.

Polymorphism

The occurrence of two or more alleles that influence phenotype in a population.

Polymorphic gene

A gene that exists as two or more common alleles in a population.

Monomorphic gene

A gene that exists predominantly as a single allele in a population.

Single-nucleotide polymorphism (SNP)

A variation in a single base pair in the DNA sequence; accounts for most genetic variation among individuals.

Allele frequency

The proportion of a specific allele among all alleles for a given gene in a population.

Genotype frequency

The proportion of individuals with a specific genotype in a population.

Hardy-Weinberg equilibrium (HWE)

A condition where allele and genotype frequencies remain constant over generations in the absence of evolutionary forces.

Hardy-Weinberg equation

p² + 2pq + q² = 1, where p and q represent allele frequencies and genotype frequencies can be calculated from them.

HWE assumptions

No mutations, no genetic drift, no migration, no natural selection, and random mating.

Microevolution

Small-scale genetic changes in a population's gene pool across generations.

Mutation

A random change in DNA sequence that introduces new genetic variation into a population.

Genetic drift

Random changes in allele frequencies due to chance, especially in small populations.

Bottleneck effect

A sharp reduction in population size due to environmental events, leading to reduced genetic variation.

Founder effect

Occurs when a small group breaks off from a larger population to form a new one with limited genetic variation.

Natural selection

The process where individuals with advantageous traits have higher reproductive success, changing allele frequencies over time.

Fitness (Darwinian fitness)

The relative likelihood that a genotype will survive and reproduce, contributing to the next generation's gene pool.

Directional selection

Favors one extreme phenotype that is better adapted to the environment.

Balancing selection

Maintains two or more alleles in a population; includes heterozygote advantage.

Heterozygote advantage

When heterozygous individuals have higher fitness than either homozygote (e.g., sickle cell trait in malaria regions).

Negative frequency-dependent selection

Rare genotypes have higher fitness than common ones, maintaining genetic diversity.

Disruptive (diversifying) selection

Favors multiple distinct phenotypes, often in populations living in diverse environments.

Stabilizing selection

Favors intermediate phenotypes while selecting against extremes, reducing genetic variation.

Migration

The movement of individuals (and their alleles) between populations, altering allele frequencies.

Gene flow

The transfer of alleles from one population to another through migration.

Assortative mating

Nonrandom mating where individuals choose partners based on phenotype.

Positive assortative mating

Individuals mate with others of similar phenotype.

Negative assortative mating

Individuals mate with others of dissimilar phenotype.

Inbreeding

Mating between genetically related individuals; increases homozygosity and can cause inbreeding depression.

Outbreeding

Mating between genetically unrelated individuals; increases heterozygosity.

Inbreeding coefficient (F)

The probability that two alleles in an individual are identical by descent from a common ancestor.

Inbreeding depression

Reduced biological fitness in a population due to increased homozygosity for deleterious alleles.

Mutation rate

The probability that a gene will be altered by a new mutation per generation.

Gene flow (migration)

Movement of alleles between populations that reduces genetic differences and increases diversity.

Population genetics

The study of genetic variation within populations and how it changes across generations.

Gene pool

All the alleles of every gene in a population; only individuals that reproduce contribute to the next generation's gene pool.

Population

A group of individuals of the same species that occupy the same area and can interbreed.

Local population

A smaller subgroup within a species that breeds more frequently within itself than with the general population.

Polymorphism

The occurrence of two or more alleles that influence phenotype in a population.

Polymorphic gene

A gene that exists as two or more common alleles in a population.

Monomorphic gene

A gene that exists predominantly as a single allele in a population.

Single-nucleotide polymorphism (SNP)

A variation in a single base pair in the DNA sequence; accounts for most genetic variation among individuals.

Allele frequency

The proportion of a specific allele among all alleles for a given gene in a population.

Genotype frequency

The proportion of individuals with a specific genotype in a population.

Hardy-Weinberg equilibrium (HWE)

A condition where allele and genotype frequencies remain constant over generations in the absence of evolutionary forces.

Hardy-Weinberg equation

p² + 2pq + q² = 1, where p and q represent allele frequencies and genotype frequencies can be calculated from them.

HWE assumptions

No mutations, no genetic drift, no migration, no natural selection, and random mating.

Microevolution

Small-scale genetic changes in a population's gene pool across generations.

Mutation

A random change in DNA sequence that introduces new genetic variation into a population.

Genetic drift

Random changes in allele frequencies due to chance, especially in small populations.

Bottleneck effect

A sharp reduction in population size due to environmental events, leading to reduced genetic variation.

Founder effect

Occurs when a small group breaks off from a larger population to form a new one with limited genetic variation.

Natural selection

The process where individuals with advantageous traits have higher reproductive success, changing allele frequencies over time.

Fitness (Darwinian fitness)

The relative likelihood that a genotype will survive and reproduce, contributing to the next generation's gene pool.

Directional selection

Favors one extreme phenotype that is better adapted to the environment.

Balancing selection

Maintains two or more alleles in a population; includes heterozygote advantage.

Heterozygote advantage

When heterozygous individuals have higher fitness than either homozygote (e.g., sickle cell trait in malaria regions).

Negative frequency-dependent selection

Rare genotypes have higher fitness than common ones, maintaining genetic diversity.

Disruptive (diversifying) selection

Favors multiple distinct phenotypes, often in populations living in diverse environments.

Stabilizing selection

Favors intermediate phenotypes while selecting against extremes, reducing genetic variation.

Migration

The movement of individuals (and their alleles) between populations, altering allele frequencies.

Gene flow

The transfer of alleles from one population to another through migration.

Assortative mating

Nonrandom mating where individuals choose partners based on phenotype.

Positive assortative mating

Individuals mate with others of similar phenotype.

Negative assortative mating

Individuals mate with others of dissimilar phenotype.

Inbreeding

Mating between genetically related individuals; increases homozygosity and can cause inbreeding depression.

Outbreeding

Mating between genetically unrelated individuals; increases heterozygosity.

Inbreeding coefficient (F)

The probability that two alleles in an individual are identical by descent from a common ancestor.

Inbreeding depression

Reduced biological fitness in a population due to increased homozygosity for deleterious alleles.

Mutation rate

The probability that a gene will be altered by a new mutation per generation.

Gene flow (migration)

Movement of alleles between populations that reduces genetic differences and increases diversity.

Evolutionary genetics

The study of how genetic variation leads to evolutionary change.

Mutation

A change in DNA sequence that creates genetic variation.

Natural selection

The process where advantageous traits become more common in a population.

Genetic drift

Random changes in allele frequencies over time, especially in small populations.

Purifying selection

The removal of deleterious mutations from a population.

Gene duplication

The creation of an extra copy of a gene, providing raw material for evolution.

Homologous genes

Genes derived from a common ancestral gene.

Orthologs

Homologous genes in different species that diverged after speciation.