Mendelian Foundations

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Heredity is the passage of traits from parents to offspring through genes, which are segments of DNA wound into chromosomes. Genotype refers to the alleles an organism carries (e.g., Aa), while phenotype is its observable trait. A monohybrid cross yields a 3:1 phenotype ratio and 1:2:1 genotype ratio; a dihybrid cross gives 9:3:3:1 when genes assort independently. The Central Dogma states DNA → mRNA (transcription) → Protein (translation), with complementary base pairing A=T and G≡C. Natural selection acts on heritable variation, producing adaptation and, over time, new species. Know Hardy-Weinberg: p² + 2pq + q² = 1, where p and q are allele frequencies summing to 1.

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Mendelian Foundations

Gregor Mendel’s two laws govern single-gene crosses through the cell division process of meiosis, which separates homologous chromosomes so each gamete receives one allele per gene.

Law of Segregation: paired alleles separate so each gamete carries only one.
Law of Independent Assortment: alleles of different genes sort independently when on separate chromosomes.

A homozygous individual has identical alleles (AA or aa); heterozygous has two different alleles (Aa). A dominant allele masks a recessive one in the phenotype, though recessive alleles persist in the population.

DNA, RNA, and Protein Synthesis

DNA is a double helix (Watson–Crick, 1953) with antiparallel strands. Replication uses complementary base pairing: adenine pairs with thymine (A=T) by two hydrogen bonds, and guanine pairs with cytosine (G≡C) by three. Transcription copies a gene’s information into messenger RNA (mRNA) in the nucleus, replacing T with U (uracil). Translation at the ribosome reads mRNA codons (three bases) and links amino acids via transfer RNA (tRNA) anticodons to build a polypeptide.

Hardy-Weinberg Equilibrium

Under the five assumptions (no mutation, no migration, infinitely large population, random mating, no selection), allele and genotype frequencies remain constant across generations.

Key UPCAT Question Patterns

Format	What to compute
Punnett square	offspring genotype/phenotype ratios
Pedigree	dominance type and carrier probability
DNA/mRNA sequence	complementary strand and codon translation
Hardy-Weinberg	allele frequencies from phenotype %

Trap to avoid: in dihybrid problems, if the problem states independent assortment, do not treat genes as linked.

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Common Mistakes and Edge Cases

Genotype vs phenotype: a 3:1 phenotype ratio in a monohybrid F2 hides a 1:2:1 genotype split, because heterozygous Aa shows the dominant phenotype.
Recessive alleles never disappear unless selection, drift, or non-random mating acts on them.
Lamarckian error: acquired traits (e.g., a bodybuilder’s muscles) are not heritable because they do not alter germline DNA.
Mitosis vs meiosis: mitosis yields two diploid clones (somatic division); meiosis yields four genetically unique haploid gametes, the only division that reduces chromosome number by half.

Evolutionary Mechanisms

Natural selection has three requisites: heritable variation (often from mutation), differential survival/reproduction, and adaptation to environment. Genetic drift changes allele frequencies randomly in small populations (founder effect, bottleneck). Gene flow moves alleles between populations. Speciation occurs via allopatry (geographic barrier) or sympatry (reproductive isolation without physical separation, e.g., polyploidy in plants).

Evidence for Evolution

Fossils document transitional forms; homologous structures (mammalian forelimbs) reveal common ancestry despite differing function; embryology shows shared pharyngeal slits; molecular biology finds conserved genes such as cytochrome c across species.

Worked Example

In a population, 16% show a recessive phenotype (q² = 0.16), so q = 0.4 and p = 0.6. Carrier frequency 2pq = 2(0.6)(0.4) = 0.48, or 48% — a typical UPCAT-style calculation.

Practice Prompts

Two heterozygous parents for a recessive disease have a child. What is the probability the child is both unaffected and not a carrier?
Given mRNA codon AUG, name the tRNA anticodon and the amino acid it carries (methionine, the start codon).

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Mendelian Foundations