Inheritance and Genetics

🧬 Key Genetic Terminology

Precise terminology is essential in genetics exam questions.

• Gene — a section of DNA coding for a specific protein/characteristic
• Allele — alternative forms of a gene (e.g. B = brown eyes; b = blue eyes)
• Dominant — allele expressed when one OR two copies present (written as capital: B)
• Recessive — allele only expressed when TWO copies present (written as lowercase: b)
• Homozygous — two identical alleles (BB or bb)
• Heterozygous — two different alleles (Bb)
• Genotype — the allele combination (BB, Bb, bb)
• Phenotype — the observable characteristic (brown eyes)

🧬 Monohybrid Crosses

Use Punnett squares to work out the expected ratios from genetic crosses.

• Each parent provides one allele per gene (gametes formed by meiosis are haploid)
• Ratios give probabilities — not guaranteed exact numbers in a small family
• Codominance — both alleles fully expressed in heterozygote (e.g. ABO blood groups, sickle cell trait)
• Sex-linked genes — on X chromosome; more common in males (e.g. haemophilia, colour blindness: X^H X^h carrier female × X^H Y → 50% sons affected)

🧬 Chromosomal Abnormalities

• Down syndrome (trisomy 21) — non-disjunction in meiosis → extra chromosome 21 → 47 chromosomes
• Turner syndrome (45,X0) — missing second sex chromosome in females
• Karyotyping (chromosome analysis from amniocentesis/chorionic villus sampling) can detect these prenatally