Extension of Mendelian Principles and Gene Interactions

Flashcards covering Mendelian extensions, gene interactions, environmental influences on gene expression, and linkage concepts from genetics lecture notes.

Mendelian Principles

The simple pattern of dominant and recessive allele inheritance, which later observations extended.

Incomplete Dominance

A pattern of inheritance where the dominant phenotype is not fully expressed in heterozygous individuals, resulting in an intermediate phenotype.

Overdominance

A condition where heterozygotes exceed the phenotypic expression of both homozygous parents, also known as heterosis or hybrid vigor.

Heterosis (Hybrid Vigor)

Another term for overdominance, where the heterozygote is better than the homozygous parents.

Sickle Cell Anemia

An autosomal recessive disorder where HbAHbS individuals are resistant to malaria and do not suffer from the disease, an example of overdominance.

Co-dominance

Inheritance characterized by the full expression of both alleles in the heterozygote, exhibiting a mixture of phenotypic characters of both homozygotes.

Multiple Alleles

A case in which genes have more than two alternative forms of a gene (allele), such as the ABO blood group system.

ABO Blood Group

An example of multiple allele inheritance, governed by three alleles (IA, IB, i).

Lethal Genes

Genes that can cause death to the individual that carries them, potentially at any stage of development or life.

Dominant Lethal Genes

Lethal genes that are expressed and cause death in either homozygous or heterozygous condition, e.g., Huntington's disease.

Recessive Lethal Genes

Lethal genes that cause death only when present in homozygous recessive condition, e.g., cystic fibrosis, sickle cell anemia, achondroplasia.

Gene Interactions

The phenomenon where two or more different genes influence the outcome of a single trait, leading to modified phenotypic ratios.

Novel Phenotype

A new phenotype that arises from the interaction between dominant and/or homozygous recessive alleles of two different gene pairs.

Recessive Epistasis

A gene interaction where complete dominance exists in both gene pairs, but one gene, when homozygous recessive, hides the effect of the other gene.

Dominant Epistasis Case I

A gene interaction where complete dominance exists in both gene pairs, but one gene, when present in dominant condition, masks the effect of the other gene.

Dominant Epistasis Case II

A gene interaction where complete dominance exists in both gene pairs; one dominant gene is epistatic to the second, and the second gene when homozygous recessive is epistatic to the first.

Complementary Genes (Duplicate Recessive Epistasis)

A gene interaction where complete dominance exists in both gene pairs, but either recessive homozygote is epistatic to the effects of the other gene.

Duplicate Genes (Duplicate Dominant Epistasis)

A gene interaction where complete dominance exists in both gene pairs, but either gene, when dominant, is epistatic to the other.

Environmental Influence on Gene Expression

The concept that an organism's phenotype is a product of the interaction between its genes and both internal and external environmental factors.

Temperature (Environmental Factor)

An external factor that can influence gene expression, for example, pigment development in Himalayan rabbits.

Light (Environmental Factor)

An external factor affecting gene expression, seen in corn plant pigmentation or caterpillar wing color.

Nutrition (Environmental Factor)

The presence of specific chemicals or drugs in the environment or diet that can affect gene expression, such as thalidomide effects or rabbit fat color.

Age (Internal Factor)

An internal factor where genes are expressed at different developmental stages, e.g., pattern baldness or diabetes mellitus manifesting later in life.

Sex (Internal Factor)

An internal factor where the individual's sex can affect gene expression, leading to sex-influenced or sex-limited traits.

Sex-Influenced Traits

Autosomal traits expressed differently in males and females, where an allele is dominant in one sex but recessive in the other, e.g., pattern baldness.

Sex-Limited Traits

Autosomal traits whose expression is restricted to only one sex due to physiological or anatomical reasons, e.g., feather plumage in chickens or milk production in cattle.

Modifier Gene

A gene that can modify the expression of another gene, affecting traits like the intensity of mouse coat color.

Expressivity

The degree to which a genotype is expressed phenotypically in individuals, which can vary from partial to full expression, e.g., retinoblastoma.

Penetrance

The proportion of individuals with a given genotype that actually shows the expected phenotype, expressed as a percentage.

Pleiotropy

A condition in which a single gene has multiple, seemingly unrelated phenotypic effects, e.g., Marfan's syndrome.

Phenocopy

An environmentally caused trait that mimics an inherited or genetic condition, e.g., thalidomide-induced limb deformities.

Genetic Linkage

A measure of the tendency of genes to be inherited as a group because their loci are in close proximity on the same chromosome.

Complete Linkage

A condition where genes are so closely associated that they are always inherited together.

Incomplete Linkage

A condition where gene pairs assort at least partially of each other, allowing for some recombination.

Recombination

The creation of new combinations of alleles at two or more loci through independent segregation or crossing-over between genes on the same chromosome.

Coupling Gametes

Gametes that carry either two wild type alleles or two mutant alleles for linked genes.

Repulsion Gametes

Gametes that carry one wild type allele and one mutant allele for linked genes.

Recombination Rate

The proportion of recombinant offspring observed in a test cross or other crosses, used to measure genetic distance.

Three-Point Test Cross

An efficient genetic mapping method used to determine the order and distance of three linked genes.

Coefficient of Coincidence (CC)

A measure of interference in the formation of chromosomal crossovers during meiosis, calculated as (actual DCO / expected DCO).

Interference (I)

The degree to which one crossover event inhibits the occurrence of another nearby crossover event, calculated as 1 - Coefficient of Coincidence.

Map Unit (mu) / Centimorgan (cM)

Units of genetic distance on a chromosome, where 1% recombination frequency is equal to one map unit or centimorgan.

Sex Linkage

The inheritance pattern of traits controlled by genes located on the sex chromosomes, primarily the X chromosome.

Y-linked Traits (Holandric Traits)

Traits controlled by genes located specifically on the Y chromosome.

Polygene

A type of inheritance where a single characteristic trait is determined by two or more pairs of genes located at several loci, with the phenotype influenced by their interaction.

Gene

The fundamental unit of heredity; a segment of DNA that codes for a specific protein or RNA molecule.

Allele

One of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome.

Genotype

The genetic constitution of an individual organism.

Phenotype

The set of observable characteristics of an individual resulting from the interaction of its genotype with the environment.

Locus

The specific physical location of a gene or other DNA sequence on a chromosome.

Homozygous

Having two identical alleles of a particular gene or genes.

Heterozygous

Having two different alleles of a particular gene or genes.

Dominant Allele

An allele that produces its phenotype regardless of whether its paired allele is identical or different.

Recessive Allele

An allele that produces its phenotype only when homozygous; its effect is masked by a dominant allele.

Law of Segregation

Mendel's First Law, stating that during the formation of gametes, the two alleles for a heritable character separate (segregate) from each other and end up in different gametes.

Law of Independent Assortment

Mendel's Second Law, stating that