Genetics & Evolution

Genes, alleles and Mendel’s laws

A gene is a functional unit of heredity; alleles are alternative versions at a locus. A diploid individual normally has two alleles at an autosomal locus. A homozygote carries two copies of the same allele, while a heterozygote has different alleles. Genotype describes genetic constitution, and phenotype describes observable characteristics.

The law of segregation follows the separation of alleles into different gametes. Under complete dominance, a monohybrid cross between heterozygotes can produce a 3:1 phenotypic ratio and a 1:2:1 genotypic ratio. The result assumes the stated dominance relation, equal viability and the usual segregation pattern.

Worked cross: Aa × Aa

Each parent produces A and a gametes with probability 1/2. The offspring probabilities are 1/4 AA, 1/2 Aa and 1/4 aa. If A is completely dominant, AA and Aa share a phenotype. For a test cross Aa × aa, the two expected genotypes occur in a 1:1 ratio.

Beyond complete dominance

In incomplete dominance, the heterozygote has a phenotype distinguishable from both homozygotes. In codominance, both allelic products are expressed in the heterozygote. Multiple alleles describe more than two allelic forms within a population, although one diploid individual still normally carries only two at a given locus.

ABO blood groups illustrate multiple alleles and codominance: Iᴬ and Iᴮ are codominant, and each is dominant to i. Polygenic inheritance involves contributions from several genes, often producing continuous variation. Pleiotropy occurs when one gene influences multiple characteristics.

• Dominant does not mean more common, stronger, beneficial or evolutionarily superior.
• A 1:2:1 phenotypic ratio can arise from incomplete dominance when all three genotypes are distinguishable.
• Environmental conditions can influence phenotype without changing the inherited DNA sequence.

Independent assortment, linkage and sex linkage

Independent assortment applies to genes that assort independently, such as genes on different chromosomes. Linked genes lie on the same chromosome and can be inherited together more often than expected under independent assortment. Crossing over produces recombinant combinations.

Recombination frequency can estimate short genetic distances, but multiple crossovers can conceal some recombination. A recombination frequency approaching 50% is consistent with independent assortment and cannot by itself distinguish genes on different chromosomes from widely separated genes on one chromosome.

For X-linked recessive inheritance, a typical XY individual expresses a recessive allele present on the single X chromosome. In the usual pattern, a father passes his X chromosome to daughters and his Y to sons.

Use probability with unlinked genes

In AaBb × AaBb with independent assortment, the probability of aa is 1/4 and of bb is 1/4. Therefore P(aabb) = 1/16. Do not use this multiplication as an independent-events shortcut when linkage changes the gamete frequencies.

DNA replication and gene expression

DNA strands are antiparallel and complementary. Replication is semiconservative: each daughter DNA molecule contains one parental strand and one newly synthesised strand. DNA polymerases extend from a primer in the 5′ to 3′ direction. The leading strand is synthesised continuously relative to fork movement, while the lagging strand forms fragments that are joined.

Transcription produces RNA from a DNA template. In eukaryotes, protein-coding precursor RNA is processed by capping, splicing and addition of a poly-A tail. Translation reads mRNA codons at the ribosome; tRNA molecules link codons to the corresponding amino acids. The genetic code is degenerate because several codons can specify one amino acid.

• A codon is a triplet on mRNA; an anticodon is the complementary sequence on tRNA.
• AUG commonly initiates translation and specifies methionine. Stop codons signal termination rather than an amino acid.
• Regulation of transcription lets cells change expression without changing which genes they contain.

Mutation and inherited variation

Mutations alter DNA sequence or chromosome structure/number. A base substitution can be silent, missense or nonsense depending on its effect on the encoded product. Insertions or deletions within a coding sequence can shift the reading frame if their size is not a multiple of three.

Meiotic nondisjunction can produce gametes with abnormal chromosome numbers. Structural changes include deletions, duplications, inversions and translocations. The consequences depend on the location, size and biological context; mutations are not automatically harmful.

• Somatic mutations affect cell lineages within a body; germline mutations can be transmitted through gametes.
• Mutations create new sequence variants; recombination reshuffles existing variants.
• A mutation does not arise because an organism needs a particular adaptation.

Evolutionary forces and the Hardy–Weinberg model

Evolution in population genetics involves changes in allele frequencies across generations. Natural selection acts through differences in survival and reproductive success associated with heritable variation. Genetic drift is random change, with especially strong effects in small populations. Gene flow moves alleles among populations.

For a two-allele locus, let p and q represent allele frequencies, with p + q = 1. Under Hardy–Weinberg assumptions, genotype frequencies are p², 2pq and q². The model assumes random mating, a very large population and no selection, migration or mutation affecting the locus.

Worked example: recessive phenotype frequency

If a fully penetrant recessive phenotype occurs in 9% of a population under Hardy–Weinberg conditions, q² = 0.09, so q = 0.3 and p = 0.7. The expected heterozygote frequency is 2pq = 0.42, or 42%. The 9% is the recessive genotype frequency, not the allele frequency.

Evidence, adaptation and speciation

Homologous structures can have different functions while reflecting common ancestry. Analogous structures perform similar functions but need not have the same evolutionary origin. Fossils, comparative anatomy, biogeography and molecular sequence comparisons provide complementary evidence for evolutionary relationships.

Speciation occurs when populations diverge sufficiently to become distinct species. Geographic separation can reduce gene flow, but reproductive isolation is the key connection to the biological species concept. Isolation can act before fertilisation or through reduced hybrid viability or fertility after fertilisation.

• Individuals can acclimatise during life; populations evolve over generations.
• Fitness means reproductive contribution in a particular environment, not simply physical strength.
• Selection acts on existing variation; its direction depends on environmental conditions.