Genetics and Heredity
Genetics and Heredity covers transmission genetics, probability, pedigrees, linkage, and the molecular basis of inheritance. Expect questions that require you to calculate offspring probabilities, interpret a pedigree, or distinguish molecular processes such as replication, transcription, and translation.
Mendelian rules
- The law of segregation separates alleles during meiosis; the law of independent assortment applies to unlinked genes.
- A monohybrid cross of heterozygotes gives a 3:1 phenotypic ratio; a dihybrid cross gives 9:3:3:1 when genes assort independently.
- Incomplete dominance produces an intermediate phenotype; codominance expresses both alleles; epistasis alters expected ratios.
Probability and pedigrees
- Use the product rule for independent events and the sum rule for mutually exclusive events.
- Autosomal recessive traits can skip generations and affect both sexes; autosomal dominant traits typically appear in every generation; X-linked recessive traits affect mostly males and show no male-to-male transmission.
Linkage and mapping
- Genes on the same chromosome less than 50 map units apart do not assort independently.
- Recombination frequency equals map distance in centimorgans; crossing over occurs in prophase I.
Molecular genetics
- DNA replication is semiconservative. Transcription makes RNA from a DNA template; translation uses mRNA codons and tRNA anticodons.
- Mutations include substitutions, insertions, and deletions; frameshifts usually change more amino acids than substitutions.
Worked example
Two heterozygous tall pea plants (Tt) are crossed. The probability of a tall offspring is 3/4. Given that an offspring is tall, the probability it is heterozygous is 2/3, because the tall class contains TT, Tt, and tT in a 1:2 ratio. If two tall offspring are chosen independently, the probability both are heterozygous is (2/3)² = 4/9.
Traps that cost marks
- Confusing "probability the child is a boy" with "probability a boy is affected."
- Treating recombination frequency as physical distance without the centimorgan relationship.
- Assuming a dominant allele is necessarily common.
- Ignoring that a rare dominant trait can appear in a pedigree without skipping generations.
- Forgetting that linked genes violate independent assortment.