Genetics & Inheritance (no pedigree or environmental)

Allele

An alternative form or version of a specific gene (represented by letters like B or b).

Genotype

The genetic makeup or combination of alleles an organism inherits (e.g., BB, Bb, or bb).

Phenotype

The physical appearance or observable expression of a trait (e.g., purple flowers, white fur, Type A blood).

Homozygous

Having two identical alleles for a trait (e.g., BB is homozygous dominant; bb is homozygous recessive).

Heterozygous

Having two different alleles for a trait (e.g., Bb); also known as a hybrid.

Complete Dominance

A pattern of inheritance where the dominant allele completely masks or hides the recessive allele in a heterozygote (Bb shows the dominant trait).

Incomplete Dominance

A pattern where neither allele is fully dominant; heterozygous individuals show an intermediate, blended phenotype (e.g., Red RR × White WW = Pink RW).

Codominance

A pattern where both alleles are equally dominant and both are fully expressed at the same time in heterozygotes (e.g., Roan cattle with red and white hairs, speckled chickens).

Multiple Alleles

When a single gene has three or more allele options available in the population (e.g., Human ABO blood types: I^A, I^B, and i).

Type A Blood Genotypes

Can be homozygous (I^AI^A / AA) or heterozygous carrying the recessive allele (I^Ai / AO).

Type B Blood Genotypes

Can be homozygous (I^BI^B / BB) or heterozygous carrying the recessive allele (I^Bi / BO).

Type AB Blood Genotype

I^A I^B (AB); a classic example of codominance because both A and B carbohydrate markers are produced equally on red blood cells.

Type O Blood Genotype

ii (OO); homozygous recessive. Individuals lack both A and B markers on their red blood cells.

Parentage Proof with Blood Types

A parent with Type AB blood can NEVER have a biological child with Type O blood because they only pass an IA or IB allele, never the recessive i allele.

Sex Chromosomes

Chromosomes that determine biological sex.

Sex-Linked (X-Linked) Trait

A trait controlled by a gene located on the X chromosome (e.g., red-green colorblindness and hemophilia).

Carrier Female

A heterozygous female (X^BX^b) who carries one copy of a recessive disease allele on an X chromosome but has a normal phenotype.

Why Males Are Affected More by X-Linked Recessive Traits

Males have only one X chromosome (XY). If they inherit a single recessive allele on their X, they have the condition because there is no second X to mask it.

Fathers Passing X-Linked Traits

Fathers NEVER pass X-linked traits to their sons because fathers contribute a Y chromosome to male offspring, not an X.

Polygenic Inheritance

A trait controlled by the additive effect of two or more separate genes, resulting in continuous variation across a population (e.g., human skin color, height, plant seed pod length).

Continuous Variation (Bell Curve)

A smooth bell-shaped distribution of phenotypes produced by polygenic traits, where intermediate phenotypes are most common and extreme phenotypes are rare.

Multiple Alleles vs. Polygenic (Key Difference)

Multiple Alleles = ONE gene with 3+ allele choices. Polygenic = MANY genes working together to control ONE trait.

Sex Chromosome for Females

XX

Sex Chromosome for Males

XY