Genetics and Heredity
What this domain tests
Genetics and Heredity covers how genetic information is stored, copied, expressed, and passed to offspring. Questions target DNA structure and replication, protein synthesis, meiosis, Mendelian and non-Mendelian inheritance, pedigrees, mutations, and biotechnology. You should be able to predict offspring ratios and explain why observed ratios may differ.
Core rules
- DNA is a double helix; bases pair A–T and C–G. A gene is a DNA segment that codes for a protein or functional RNA.
- Transcription makes mRNA in the nucleus; translation makes protein at the ribosome.
- Meiosis halves chromosome number and creates gametes. Crossing over and independent assortment increase variation.
- In complete dominance, dominant alleles mask recessive alleles. Codominance and incomplete dominance produce different phenotypes.
- A mutation is a change in DNA sequence; it may be harmful, neutral, or beneficial.
- Pedigrees track phenotypes; carriers have a recessive allele but may not show the trait.
Worked example
Two parents are heterozygous for an autosomal recessive condition (Bb × Bb). What is the probability that a child is affected, and if the child is unaffected, what is the probability the child is a carrier? Reasoning: A Punnett square gives BB, Bb, Bb, bb. Only bb is affected, so P(affected) = 1/4 = 25%. Among unaffected children (BB, Bb, Bb), two of three are carriers: P(carrier | unaffected) = 2/3 ≈ 67%. The condition is not sex-linked, so male and female children have the same risk.
Traps
- Reporting 3:1 as “three carriers to one affected”; the ratio describes phenotypes, not genotypes.
- Assuming a dominant trait is more common; dominance describes expression, not frequency.
- Forgetting that each fertilization event is independent.
- Confusing meiosis (gametes) with mitosis (body cells).