Chapter 10: Patterns of Inheritance

Advantages of pea plants (Pisum sativum) in genetics

Easy to cultivate, short generation time, distinct and easily observable traits, and the ability to strictly control pollination (self-pollination vs. cross-pollination).

Gene

A unit of heredity located on a chromosome that codes for a specific trait.

Allele

Alternative forms of a gene occupying the same locus (e.g., dominant $P$ for purple flowers vs. recessive $p$ for white flowers).

Homozygous

Having two identical alleles for a specific gene (e.g., homozygous dominant $PP$ or homozygous recessive $pp$).

Heterozygous

Having two different alleles for a specific gene (e.g., $Pp$).

Genotype

The specific allelic combination or genetic makeup of an organism (e.g., $PP$, $Pp$, or $pp$).

Phenotype

The observable physical or physiological expression of an organism's genotype (e.g., purple or white flowers).

Mendel's Law of Segregation

Each organism carries two alleles for each trait, which separate during meiosis I so that each gamete receives only one allele.

Monohybrid cross ($Pp \times Pp$) ratios

Genotypic ratio: $1\,PP : 2\,Pp : 1\,pp$; Phenotypic ratio: $3$ dominant : $1$ recessive.

Testcross

Breeding an individual of dominant phenotype but unknown genotype (P_ ) with a homozygous recessive individual ($pp$) to deduce the unknown genotype based on offspring phenotypes.

Mendel's Law of Independent Assortment

Alleles of different genes assort independently of one another into gametes during meiosis, applicable to genes on separate chromosomes.

Dihybrid cross ($PpYy \times PpYy$) phenotypic ratio

$9 : 3 : 3 : 1$ ($9$ Dominant-Dominant : $3$ Dominant-Recessive : $3$ Recessive-Dominant : $1$ Recessive-Recessive).

Pedigree features of autosomal recessive disorders

Trait typically skips generations; affected individuals ($aa$) frequently have unaffected carrier parents ($Aa \times Aa$); and males and females are affected equally (e.g., Cystic Fibrosis, PKU).

Pedigree features of autosomal dominant disorders

Trait appears in every generation; affected individuals must have at least one affected parent; unaffected parents ($aa \times aa$) cannot have affected offspring (e.g., Huntington disease).

Incomplete dominance

A pattern where neither allele is completely dominant, resulting in a heterozygous phenotype that is an intermediate blend between homozygous parents (e.g., pink $C^R C^W$ snapdragons; $1:2:1$ phenotypic ratio).

Codominance

A condition in which both alleles are distinctly and equally expressed in the heterozygote without blending (e.g., type AB blood expressing both A and B antigens, $I^A I^B$).

Multiple alleles

The presence of three or more alternative forms of a gene within a population, although any individual inherits only two (e.g., $I^A$, $I^B$, and $i$ in ABO blood typing).

Polygenic inheritance

A single phenotypic trait controlled by two or more independent genes exerting an additive effect, generating continuous variation along a bell-shaped curve (e.g., height, skin color).

Pleiotropy

A phenomenon where a single gene impacts multiple, seemingly unrelated phenotypic characteristics (e.g., Marfan syndrome affecting eyes, skeleton, and the cardiovascular system).

Epistasis

An interaction where the expression of a gene at one locus masks or alters the phenotypic expression of a gene at a second distinct locus (e.g., coat color in Labrador retrievers).

Hemizygous

Having only one copy of a gene or chromosome, characteristic of males for X-linked genes ($X^r Y$) where a single recessive allele determines phenotype.

Inheritance pattern of X-linked recessive traits

Expressed far more frequently in males ($X^r Y$); fathers pass the allele only to daughters and never to sons; carrier mothers ($X^R X^r$) transmit the allele to $50\%$ of sons (affected) and $50\%$ of daughters (carriers).