Genetics and Heredity
Core concepts
DNA is a double helix of nucleotides; adenine pairs with thymine and guanine with cytosine. DNA replicates semiconservatively, and genes are transcribed into mRNA, which ribosomes translate into proteins using codons. A mutation is a change in DNA sequence; it may be silent, missense, or nonsense. Meiosis produces haploid gametes with genetic variation through crossing over and independent assortment. Mendel's laws describe segregation of alleles and independent assortment of traits. Genotype is the allele combination; phenotype is the observable trait. Dominant alleles mask recessive ones in heterozygotes. A Punnett square predicts offspring ratios. Sex-linked traits sit on sex chromosomes, so inheritance patterns differ by sex. Pedigrees trace traits across generations.
Common traps
Do not confuse mitosis (growth, identical cells) with meiosis (gametes, halved chromosome number). Remember that a recessive trait appears only when both alleles are recessive. Codominance and incomplete dominance are not the same: codominance shows both traits, incomplete dominance blends them. Do not assume a dominant allele is more common in a population.
Worked example
Two heterozygous parents (Aa x Aa) are crossed. Each parent contributes A or a with equal probability. Offspring genotypes are AA, Aa, Aa, and aa, a 1:2:1 ratio; phenotypes are 3 dominant to 1 recessive. Rule of thumb: multiply probabilities of independent events to find combined outcomes.