Patterns of Inheritance¶

Part of Module 6: Genetics, evolution and ecosystems.

Genetics at A level extends well beyond simple Mendelian ratios. This topic covers the full range of inheritance patterns — from monogenic to dihybrid crosses, including sex linkage, autosomal linkage, codominance and epistasis. It also introduces the statistical tools (chi-squared test and Hardy-Weinberg principle) used to analyse whether observed allele and genotype frequencies match theoretical predictions.

Learning Objectives¶

Phenotypic Variation¶

Variation in a population can be described as:

Sources of variation: - Genetic variation: differences in allele combinations (from mutation, meiosis, independent assortment, crossing over — see 2.1.6) - Environmental variation: diet, temperature, sunlight (e.g. etiolation — pale elongated growth in low light; chlorosis — yellowing due to mineral deficiency) - Most phenotypic traits are influenced by both genetic and environmental factors (multifactorial)

Key Genetics Terminology¶

Meiosis and the Origin of Genetic Variation¶

Genetic variation in sexually reproducing organisms arises through meiosis:

1. Crossing over: Non-sister chromatids of homologous chromosome pairs exchange segments at chiasmata during prophase I; new combinations of alleles are created on each chromatid
2. Independent assortment: Homologous chromosome pairs align randomly at the metaphase I plate; maternal and paternal chromosomes are distributed independently into daughter cells → 2²³ possible chromosome combinations in human gametes

Sexual reproduction then combines gametes from two parents, further increasing variation.

Monogenic Inheritance¶

Monogenic inheritance is the inheritance of a trait controlled by a single gene.

Autosomal recessive example — cystic fibrosis: - Let F = functional allele (dominant); f = non-functional allele (recessive) - Carriers are Ff (phenotypically unaffected); affected individuals are ff - Two carrier parents (Ff × Ff) → expected ratio 3 unaffected : 1 affected

Worked example — carrier × carrier cross:

Expected phenotype ratio: 3 unaffected : 1 affected

Dihybrid Inheritance¶

A dihybrid cross considers the simultaneous inheritance of two unlinked genes (on different chromosomes), following Mendel's law of independent assortment.

Worked example: Seed colour (Y yellow dominant over y green) and seed shape (R round dominant over r wrinkled).

Parental cross: YYRR × yyrr → all YyRr (yellow round)

F₂ cross: YyRr × YyRr → 9 yellow round : 3 yellow wrinkled : 3 green round : 1 green wrinkled (9:3:3:1 ratio)

This 9:3:3:1 ratio only holds when the two genes assort independently (are on different chromosomes, or far apart on the same chromosome).

Sex Linkage¶

Sex linkage occurs when a gene is located on a sex chromosome. In mammals: - Males are XY: one X chromosome and one Y chromosome - Females are XX: two X chromosomes - The Y chromosome is much smaller than the X and carries few functional genes

A gene on the X chromosome is said to be X-linked. Males have only one copy of X-linked genes (hemizygous), so they will express a recessive allele that a female heterozygote would not.

Example — haemophilia A (X-linked recessive): - H = normal clotting factor (dominant); h = defective clotting factor (recessive) - Males: X^H Y (unaffected) or X^h Y (haemophiliac) - Females: X^H X^H (unaffected), X^H X^h (carrier), X^h X^h (haemophiliac — rare)

Worked example — carrier female × unaffected male:

Expected offspring: 25% normal female : 25% carrier female : 25% normal male : 25% haemophiliac male

Autosomal Linkage¶

Autosomal linkage occurs when two genes are located on the same autosome (non-sex chromosome). Linked genes do not assort independently — they tend to be inherited together.

If the two genes are completely linked (no crossing over between them), the offspring ratios differ from the 9:3:3:1 expected for independent assortment. Crossing over during meiosis can separate linked alleles, producing recombinant offspring. The further apart two genes are on a chromosome, the more likely crossing over is between them.

Codominance¶

In codominance, both alleles in a heterozygote are fully expressed, producing a phenotype distinct from either homozygote.

Example — ABO blood groups: ABO blood type is controlled by a single gene with three alleles: Iᴬ, Iᴮ and Iᴼ. - Iᴬ and Iᴮ are codominant; Iᴼ is recessive to both

Another example — snapdragon flower colour: - C^R = red pigment allele; C^W = white pigment allele - C^R C^R = red; C^W C^W = white; C^R C^W = pink (both pigments produced but diluted)

Epistasis¶

Epistasis is the interaction between two different gene loci where the alleles at one locus mask or modify the expression of alleles at a second locus. It produces offspring ratios that deviate from the expected 9:3:3:1 dihybrid ratio.

Recessive Epistasis (9:3:4 ratio)¶

A homozygous recessive genotype at one locus prevents expression of the other locus.

Example — Labrador coat colour: - Gene B: B = black pigment (dominant), b = brown (recessive) - Gene E: E = pigment expressed (dominant), e = pigment not expressed (recessive) — epistatic gene - If ee (homozygous recessive at E locus): no pigment expressed regardless of B genotype → yellow coat - Dihybrid cross (BbEe × BbEe) expected ratio: 9 black : 3 brown : 4 yellow (the 3 bbE_ + 1 bbee collapse to 4 yellow)

Dominant Epistasis (12:3:1 ratio)¶

A dominant allele at one locus prevents expression of the other locus.

Example — fruit colour in some plants: - One dominant allele at locus A produces a white pigment that inhibits colour expression at locus B - If A_ is present: colour from locus B is masked → white - Dihybrid cross expected ratio: 12 white : 3 coloured : 1 other

Why Epistasis Ratios Differ¶

Epistasis ratios are derived by identifying which phenotypic categories collapse. Always: 1. Work out the genotypic ratio from the 16-box Punnett square (9:3:3:1) 2. Identify which genotypes produce which phenotype based on the specific epistatic interaction 3. Group the ratios accordingly

The Chi-Squared (χ²) Test¶

The chi-squared test is a statistical test used to determine whether the difference between observed and expected results is due to chance.

When to use it: - Comparing observed and expected frequencies of phenotypes in a genetics cross - Data must be in raw counts (not percentages or ratios) - Minimum expected frequency in any class ≥ 5

Formula:

χ² = Σ [(O − E)² / E]

Where O = observed frequency, E = expected frequency, Σ = sum over all categories.

Worked example: A cross expected to give 3:1 ratio of tall:short plants. 120 plants observed: 84 tall, 36 short.

Degrees of freedom = number of classes − 1 = 2 − 1 = 1

At p = 0.05, critical value for 1 degree of freedom = 3.84

Since 1.60 < 3.84, the difference is not significant at the 5% level: the null hypothesis (no difference between observed and expected) is accepted. The deviation from 3:1 is consistent with chance.

Interpretation rules: - χ² < critical value → deviation is not significant → data is consistent with the expected ratio → accept null hypothesis - χ² > critical value → deviation is significant → data is NOT consistent with the expected ratio → reject null hypothesis → some other factor (e.g. linkage, selection) is causing the deviation

The Hardy-Weinberg Principle¶

The Hardy-Weinberg principle states that allele frequencies in a large, randomly mating population remain constant from generation to generation, provided certain conditions are met: - No mutations - No natural selection (all genotypes equally fit) - Random mating - No migration into or out of the population - Large population size (no genetic drift)

In practice, deviations from Hardy-Weinberg equilibrium indicate that one of these conditions is violated — i.e., evolution is occurring.

Hardy-Weinberg Equations¶

Let: - p = frequency of the dominant allele (A) - q = frequency of the recessive allele (a)

Since there are only two alleles: p + q = 1

The frequency of each genotype in the population: - p² = frequency of homozygous dominant genotype (AA) - 2pq = frequency of heterozygous genotype (Aa) - q² = frequency of homozygous recessive genotype (aa) - And: p² + 2pq + q² = 1

Worked Example¶

In a population, 9% of individuals are affected by a recessive condition (aa). Estimate allele frequencies and the proportion of carriers.

1. q² = 0.09 → q = √0.09 = 0.3
2. p = 1 − q = 1 − 0.3 = 0.7
3. Frequency of carriers (Aa) = 2pq = 2 × 0.7 × 0.3 = 0.42 (42%)
4. Frequency of unaffected homozygous dominant = p² = 0.7² = 0.49 (49%)

Using Hardy-Weinberg to Detect Evolution¶

If allele frequencies measured in one generation differ significantly from those calculated using Hardy-Weinberg, natural selection (or another evolutionary force) is likely operating. Comparing q² (observed frequency of affected individuals) over time directly indicates whether a recessive allele is changing in frequency.

Factors Affecting Evolution¶

Evolution is the change in inherited characteristics of a population over generations, driven by changes in allele frequencies in the gene pool.

The gene pool is the complete set of all alleles of all genes in all individuals of a population at a given time.

Several forces can change allele frequencies:

Genetic Drift, the Bottleneck Effect and the Founder Effect¶

Genetic drift is random variation in allele frequencies that is not caused by selection. Its effects are greatest in small populations: - Rare alleles may be lost entirely by chance - Harmful alleles may increase in frequency - Beneficial alleles may be lost

Bottleneck effect: a sudden, drastic reduction in population size (e.g. due to disease, natural disaster, hunting). The surviving population has a reduced and potentially unrepresentative gene pool. Genetic diversity is reduced and inbreeding depression may follow.

Founder effect: a small number of individuals colonise a new habitat, establishing a new population. Only the alleles present in that small founding group are represented; rare alleles from the original population may become common, and the genetic diversity of the new population is limited.

Types of Natural Selection¶

Natural selection can act in three different ways on a normally distributed phenotypic trait:

Speciation¶

Speciation is the evolution of new species from an existing population. A species is defined as a group of organisms capable of interbreeding to produce fertile offspring.

Reproductive Isolation¶

New species form when populations become reproductively isolated — unable to interbreed. Isolation can be:

Prezygotic barriers (prevent fertilisation): - Habitat isolation (populations use different habitats in the same area) - Temporal isolation (different breeding seasons) - Behavioural isolation (different mating rituals or signals) - Mechanical isolation (incompatible reproductive structures)

Postzygotic barriers (reduce success of hybrids): - Hybrid offspring are infertile (e.g. mules: horse × donkey) - Hybrid offspring are non-viable - Reduced hybrid fitness

Types of Speciation¶

Allopatric speciation (most common): 1. A physical barrier (mountain range, river, sea) separates a population into isolated sub-populations 2. Each sub-population experiences different selection pressures and accumulates different mutations 3. Genetic divergence occurs through natural selection and genetic drift 4. Eventually the populations become so different that even if the barrier is removed, they cannot interbreed → new species

Sympatric speciation (within the same geographic area): - Ecological, behavioural or chromosomal barriers lead to reproductive isolation without physical separation - More common in plants (polyploidy can produce instant reproductive isolation)

Adaptive Radiation¶

Adaptive radiation is the rapid diversification of one ancestral species into many new species, each adapted to a different ecological niche. It is more likely when new resources or habitats become available (e.g. after a mass extinction).

Artificial Selection¶

Artificial selection (selective breeding) is the human-directed process of selectively breeding organisms with desired traits over many generations. It works on the same principles as natural selection but with humans choosing which individuals reproduce.

Process: 1. Select a population showing variation in the desired trait 2. Choose individuals displaying the desired characteristic 3. Breed selected individuals together 4. Assess offspring for the desired trait 5. Repeat over many generations

Examples: high-yielding wheat varieties; disease-resistant crops; cattle breeds with high milk production.

Problems with Inbreeding¶

Repeated selective breeding often involves inbreeding (breeding closely related individuals): - Reduces genetic diversity and heterozygosity - Increases expression of harmful recessive alleles (inbreeding depression) - Reduces adaptability to environmental change

Outbreeding (breeding less closely related individuals) can counter inbreeding depression: - Increases heterozygosity - May produce hybrid vigour — increased health, yield and fitness

Gene Banks and Seed Banks¶

To preserve genetic diversity after artificial selection: - Seed banks store seeds from wild and domestic plant varieties - Gene banks store reproductive cells (sperm, eggs, embryos) from animals

Stored samples represent a broad gene pool including wild-type alleles. They can be used to reintroduce genetic diversity through outbreeding, reducing the frequency of harmful recessive alleles and improving population adaptability.

Key Terms¶

• Phenotype: the observable characteristics of an organism.
• Genotype: the alleles present in an organism for a particular characteristic.
• Allele: an alternative form of a gene at a given locus.
• Monogenic inheritance: inheritance pattern controlled by a single gene.
• Dihybrid cross: cross considering inheritance of two genes simultaneously.
• Sex linkage: inheritance of a gene located on a sex chromosome.
• Autosomal linkage: inheritance pattern caused by genes being located on the same autosome.
• Codominance: pattern in which both alleles are fully expressed in the heterozygote.
• Epistasis: interaction in which one gene masks or modifies expression of another gene at a different locus.
• Chi-squared test: statistical test used to compare observed and expected frequencies.
• Hardy-Weinberg principle: model stating that allele frequencies remain constant in a large randomly mating population if no evolutionary forces act.
• Allele frequency: proportion of a given allele in a population’s gene pool.
• Gene pool: the complete set of all alleles present in a population at a given time.
• Genetic drift: random change in allele frequency due to chance events; most significant in small populations.
• Bottleneck effect: reduction in genetic diversity resulting from a sudden dramatic decrease in population size.
• Founder effect: reduced genetic diversity in a new population established by a small number of founding individuals.
• Directional selection: natural selection that favours one extreme phenotype, shifting the mean of the distribution.
• Stabilising selection: natural selection that favours intermediate phenotypes and acts against extremes.
• Disruptive selection: natural selection that favours both extreme phenotypes and acts against the intermediate.
• Allopatric speciation: formation of a new species by the geographic separation of populations.
• Sympatric speciation: formation of a new species within the same geographic area, through ecological or behavioural isolation.
• Reproductive isolation: the inability of two populations to interbreed successfully.
• Adaptive radiation: rapid diversification of one ancestral species into multiple new species occupying different niches.
• Artificial selection: selective breeding carried out by humans to favour desired traits in a population.
• Inbreeding depression: loss of fitness and vigour caused by repeated breeding between closely related individuals.
• Hybrid vigour: increased fitness and productivity in offspring produced by outbreeding (crossing unrelated individuals).

Connected Pages¶

• 6.1.1 Cellular control (mutations as source of new alleles)
• 4.2.2 Classification and evolution (natural selection, variation)
• 2.1.6 Cell division, cell diversity and cellular organisation (meiosis and variation)
• Mutation, selection and speciation
• Module 6: Genetics, evolution and ecosystems