DNA, Genes and Principles of Heredity — O/L Science

Genetics is the study of heredity — how traits are passed from parents to offspring. The information is encoded in DNA (deoxyribonucleic acid), a double helix of nucleotide base pairs (A–T, G–C) housed in chromosomes in the cell nucleus. Humans have 46 chromosomes (23 pairs); gametes have 23 (haploid).

Gregor Mendel's experiments with pea plants established two laws: the Law of Segregation (each organism carries two alleles for each trait; they separate during gamete formation) and the Law of Independent Assortment (alleles for different traits are inherited independently). An allele is a variant form of a gene. Dominant alleles mask recessive alleles; if both alleles are recessive, the recessive phenotype appears.

Genetic crosses use Punnett squares to predict offspring ratios. Monohybrid cross (one trait): Aa × Aa gives 3:1 phenotype ratio (dominant : recessive). Sex determination in humans: females XX; males XY. Some conditions are sex-linked (e.g. colour blindness, haemophilia — X-linked recessive).