ASVAB Practice

Genetics and Heredity

DNA: Double-helix molecule made of nucleotide base pairs. Carries the genetic code.

Gene: A DNA segment coding for a specific protein or trait.

Chromosome: Tightly coiled DNA + protein structure. Humans have 46 (23 pairs — one from each parent).

Allele: Different versions of the same gene. You inherit one from each parent.

Dominant allele (A): Expressed whenever present (even one copy). Recessive allele (a): Only expressed when two copies are present (aa).

Genotype: Genetic makeup (AA, Aa, aa). Phenotype: Physical expression.

Punnett square: Predicts offspring probability. Aa × Aa gives: 25% AA, 50% Aa, 25% aa → 75% show dominant phenotype.

Y-linked traits: Passed only from father to son (on Y chromosome). X-linked traits: On the X chromosome. Males (XY) have only one X, so X-linked recessives are expressed more often in males.