Genetics and Inheritance: Key Concepts and Terminology

Gene

A segment of DNA that codes for a specific protein or trait. Example: gene for eye color.

Allele

A version of a gene producing variations of a trait. Example: brown-eye allele (B).

Dominant allele

Expressed even if only one copy is present (B).

Recessive allele

Expressed only if two copies (b).

Genotype

Genetic makeup (BB, Bb, bb).

Phenotype

Observable trait (brown eyes).

Homozygous dominant

AA

Homozygous recessive

aa

Heterozygous

Aa

Punnett Squares

Letters outside: parental gametes. Inside: offspring genotypes.

Genotypic ratio

1:2:1

Phenotypic ratio

3:1 (long:short)

Law of Segregation

Alleles separate during gamete formation (Bb → B or b).

Law of Independent Assortment

Alleles of different genes assort independently (seed color vs. shape).

Gametes for Two Traits

AABB → AB; aabb → ab; AaBb → AB, Ab, aB, ab.

Autosomal Dominant

Requires one allele (AA/Aa). Examples: Huntington's disease, Achondroplasia.

Autosomal Recessive

Requires two alleles (aa). Examples: Cystic fibrosis, Tay-Sachs.

Carriers

Aa.

Complete dominance

One allele masks another (Rr → red).

Incomplete dominance

Blended phenotype (RR red × rr white → Rr pink).

Codominance

Both expressed (RR red × WW white → RW spotted).

Blood type

IA, IB, i. IAIB → AB (codominance).

Multiple Alleles

More than two possible alleles for one gene (e.g., IA, IB, i).

Polygenic Inheritance

Trait controlled by multiple genes. Example: skin color, height. Shows continuous variation.

Nondisjunction

Chromosomes fail to separate (anaphase).

Down Syndrome

Trisomy 21.

Structural changes

Deletion - missing segment, Duplication - repeated segment, Inversion - segment reversed, Translocation - segment moved.

Natural Selection

Process by which organisms with advantageous traits survive and reproduce more.

Adaptation

Trait increasing fitness in an environment (e.g., polar bears' fur).

Sickle Cell allele

Originated from a mutation in the hemoglobin gene.

Heterozygotes

HbA HbS resist malaria - selective advantage in affected regions.