Pedigree

P generation

The parental generation in a genetic cross.

F_{1} generation

The first filial generation; the offspring of the P generation.

F_{2} generation

The second filial generation; offspring produced by crossing F_{1} individuals.

Mendel’s Law of Segregation

Two alleles for a gene separate during gamete formation, resulting in each gamete carrying only one allele for each gene.

Mendel’s Law of Independent Assortment

Alleles of different genes assort independently of one another during gamete formation; this applies to genes located on different chromosomes.

Laws of Probability

Mathematical rules, such as the product and sum rules, used to predict the numerical outcomes of genetic crosses.

Gene vs. Allele

A gene is a unit of heredity (DNA sequence) coding for a product, while alleles are alternative versions of that same gene.

Locus

The specific physical location of a gene on a chromosome.

Dominant vs. Recessive

A dominant allele determines the phenotype in a heterozygote; a recessive allele is expressed only when two copies are present (aa).

Homozygous vs. Heterozygous

Homozygous means having two identical alleles (AA or aa); heterozygous means having two different alleles (Aa).

Genotype vs. Phenotype

Genotype refers to the genetic makeup of an organism, while phenotype refers to its observable physical or physiological traits.

Punnett square

A diagram utilized to predict the possible genotypes and phenotypes of offspring from a specific cross.

Test cross

A cross between an individual with a dominant phenotype but unknown genotype and a homozygous recessive individual to determine the unknown genotype.

Incomplete dominance

A pattern where the heterozygote exhibits an intermediate phenotype between the two homozygous phenotypes.

Codominance

A pattern where both alleles are fully and separately expressed in the phenotype of a heterozygote.

Pleiotropy

A genetic phenomenon where a single gene affects multiple, seemingly unrelated phenotypic traits.

Epistasis

A type of gene interaction where the phenotypic expression of one gene masks or modifies the expression of another gene at a different locus.

Polygenic inheritance

An additive effect of two or more genes on a single phenotypic character (e.g., human skin color).

Pedigree

A diagram representing a family tree used to track the inheritance of specific traits across multiple generations.

Sex-linked genes

Genes located on the sex chromosomes, most commonly the X chromosome.

X chromosome inactivation

A process in female mammals where one of the two X chromosomes is randomly and permanently silenced in each cell.

Barr body

The inactive, highly condensed X chromosome found in the nuclei of female cells.

Nondisjunction

An error in meiosis or mitosis in which homologous chromosomes or sister chromatids fail to separate properly.

Aneuploidy

A chromosomal aberration in which one or more chromosomes are present in extra copies or are deficient in number (2n \pm 1).

Down syndrome

A human genetic disorder resulting from the presence of an extra chromosome 21, also known as Trisomy 21.

Chi-square (\chi^{2}) test

A statistical test used to compare observed experimental data with expected data to determine if the results are due to chance.