B12: Inheritance

A gene is an inherited DNA sequence for a characteristic, while alleles are alternative forms of that gene. Continuous variation spans a range, such as height; discontinuous variation falls into distinct categories, such as blood group.

A dominant allele affects the phenotype in a heterozygote, while a recessive allele is expressed when no dominant allele is present. Homozygous means two identical alleles, heterozygous two different alleles, genotype the allele combination, and phenotype the observed characteristic.

Use parental genotypes to list gametes, combine them in a genetic diagram, and state offspring genotypes and phenotypes. The F1 generation is the first offspring generation and an F1 cross can produce an F2 generation. A cross between two heterozygotes can give an expected $3:1$ phenotype ratio; a test cross with a recessive homozygote can give $1:1$.

Expected ratios describe probabilities, not a guaranteed sequence. Small offspring numbers often differ markedly by chance; larger samples usually approach the expected proportion more closely.

Human females have XX chromosomes and males XY. Every ovum carries X, while sperm carry X or Y, so the sperm's chromosome determines the zygote's chromosomal sex.

Sickle cell anaemia illustrates a mutation within a gene sequence. Down syndrome illustrates a chromosome-number change, with 47 chromosomes. Ionising radiation such as X-rays and chemical mutagens can increase mutation rate.