Inheritance and Selection

This topic brings together genetics, evolution, and natural selection — connecting how traits are passed from one generation to the next with how populations change over time. A thorough understanding of genetic crosses, statistical testing, and evolutionary mechanisms is essential for A-Level Biology.

Key Genetic Terminology

Term	Definition
Gene	A length of DNA that codes for a polypeptide (or functional RNA)
Allele	An alternative form of a gene, arising by mutation
Locus	The specific position of a gene on a chromosome
Genotype	The combination of alleles an organism possesses for a particular gene (or set of genes)
Phenotype	The observable characteristics of an organism, resulting from the interaction of genotype and environment
Homozygous	Two identical alleles for a gene (e.g., AA or aa)
Heterozygous	Two different alleles for a gene (e.g., Aa)
Dominant	An allele that is expressed in the phenotype of a heterozygote
Recessive	An allele that is only expressed in the phenotype when homozygous
Codominant	Both alleles are expressed equally in the phenotype of a heterozygote

Key Definition: The phenotype of an organism is determined by its genotype, the interaction between its alleles, and the influence of the environment. For example, identical twins (same genotype) raised in different environments may differ in height or weight.

Formatting Genetic Diagrams

Exam Tip: Examiners award marks for the correct layout of genetic diagrams. Always follow this structure:

State the parental phenotypes (e.g., "Tall × Dwarf")

State the parental genotypes (e.g., Tt × tt)

State the gametes produced by each parent (circle them if handwriting)

Draw a Punnett square showing all possible combinations

State the offspring genotypes and phenotypic ratio Missing any of these steps will lose you marks, even if the final ratio is correct.

Described diagram — Standard genetic diagram layout. The diagram is arranged vertically in five clearly labelled rows. Row 1 (top): the parental phenotypes are written on the left and right (e.g., "Tall" on the left, "Dwarf" on the right), connected by a multiplication sign (×) in the centre. Row 2: directly beneath each phenotype, the parental genotypes are written (e.g., Tt on the left, tt on the right). Row 3: beneath each genotype, the possible gametes are shown inside circles — two circles beneath each parent (e.g., T and t from the Tt parent; t and t from the tt parent). Row 4: a Punnett square is drawn as a 2×2 grid. The gametes from one parent label the columns across the top and the gametes from the other parent label the rows down the left side. Each cell of the grid contains the offspring genotype produced by combining the column gamete with the row gamete (e.g., Tt, Tt, tt, tt). Row 5 (bottom): the offspring genotypic ratio is stated (e.g., 2 Tt : 2 tt, simplified to 1 Tt : 1 tt) and the phenotypic ratio is stated (e.g., 1 tall : 1 dwarf). The entire layout proceeds logically from top to bottom, making it clear how parental alleles are separated into gametes and then recombined in offspring.

Monohybrid Inheritance

A monohybrid cross considers the inheritance of a single gene.

Example: In pea plants, tall (T) is dominant over dwarf (t).

Parental phenotypes: Tall × Tall Parental genotypes: Tt × Tt Gametes: T or t from each parent

	T	t
T	TT	Tt
t	Tt	tt

Phenotypic ratio: 3 tall : 1 dwarf Genotypic ratio: 1 TT : 2 Tt : 1 tt

Test Cross

Crossing an individual with the dominant phenotype with a homozygous recessive individual to determine whether it is homozygous dominant (TT) or heterozygous (Tt).
If all offspring show the dominant phenotype → the unknown parent is TT.
If offspring show a 1:1 ratio of dominant to recessive → the unknown parent is Tt.

Dihybrid Inheritance

A dihybrid cross considers the inheritance of two genes on different chromosomes (independently assorted, as per Mendel's second law).

Example: In pea plants, round (R) is dominant over wrinkled (r); yellow (Y) is dominant over green (y).

Cross: RrYy × RrYy Gametes from each parent: RY, Ry, rY, ry

Expected phenotypic ratio: 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green

This 9:3:3:1 ratio is the hallmark of a dihybrid cross with complete dominance and independent assortment. Deviations from this ratio may indicate linkage, epistasis, or other gene interactions.

Codominance and Multiple Alleles

Codominance

Both alleles are fully expressed in the heterozygote (neither is dominant over the other).
Example: ABO blood groups — the gene (locus I) has three alleles: Iᴬ, Iᴮ, and i.
- Iᴬ and Iᴮ are codominant with each other (both expressed in the heterozygote IᴬIᴮ → blood group AB).
- Both Iᴬ and Iᴮ are dominant over i.

Genotype	Phenotype (Blood Group)	Antigens on Red Blood Cells	Antibodies in Plasma
IᴬIᴬ or Iᴬi	A	A antigen	Anti-B
IᴮIᴮ or Iᴮi	B	B antigen	Anti-A
IᴬIᴮ	AB	Both A and B antigens	Neither
ii	O	Neither antigen	Anti-A and Anti-B

Key Definition: Multiple alleles means that a gene has more than two alleles in the population (though any individual still has only two alleles — one on each homologous chromosome). The ABO blood group system has three alleles: Iᴬ, Iᴮ, and i.

Sex Linkage

Sex-linked genes are located on the sex chromosomes (usually the X chromosome, since the Y chromosome is much smaller and carries fewer genes).
Males (XY) have only one copy of X-linked genes and are hemizygous — they will express any allele on the X chromosome, whether dominant or recessive.
Females (XX) can be carriers of X-linked recessive conditions (heterozygous) without showing symptoms.
Examples: haemophilia A, red-green colour blindness, Duchenne muscular dystrophy.
In genetic diagrams, the alleles are written as superscripts on the X chromosome (e.g., X^H X^h for a carrier female; X^h Y for an affected male).

Worked Example 1 — Sex-Linked Inheritance (Colour Blindness):

A carrier female (X^B X^b) × a normal male (X^B Y):

Parental phenotypes: Carrier female (normal vision) × Normal male Parental genotypes: X^B X^b × X^B Y Gametes: X^B or X^b (female); X^B or Y (male)

	X^B	Y
X^B	X^B X^B (normal female)	X^B Y (normal male)
X^b	X^B X^b (carrier female)	X^b Y (colour-blind male)

Offspring ratio: 1 normal female : 1 carrier female : 1 normal male : 1 colour-blind male

Probability of a child being colour-blind = 1/4 (25%), but this is only males — so 50% of sons will be colour-blind, and 0% of daughters will be colour-blind (though 50% of daughters will be carriers).

Epistasis

Key Definition: Epistasis occurs when one gene (the epistatic gene) affects the expression of another gene at a different locus. The epistatic gene may mask or modify the phenotypic expression of the other gene (the hypostatic gene).

Epistasis produces modified dihybrid ratios (not the standard 9:3:3:1).

Worked Example 2 — Epistasis in Flower Colour:

In a plant species, flower colour is controlled by two genes:

Gene 1: allele C (produces a colourless precursor) is dominant over c (no precursor produced).
Gene 2: allele P (converts precursor to purple pigment) is dominant over p (no conversion).

The biosynthetic pathway is: No colour → (Gene 1: C needed) → Colourless precursor → (Gene 2: P needed) → Purple pigment

flowchart LR
    A["No colour"] -->|"Gene 1: C allele
(functional enzyme)"| B["Colourless precursor"]
    B -->|"Gene 2: P allele
(functional enzyme)"| C["Purple pigment"]
    A -.-|"cc genotype:
no enzyme → blocked"| X1["WHITE"]
    B -.-|"pp genotype:
no enzyme → blocked"| X2["WHITE"]

Parental genotypes: CcPp × CcPp

A standard dihybrid cross gives 16 offspring combinations. The phenotypic classes are:

Genotype	Phenotype	Explanation
C_P_ (9/16)	Purple	Both functional enzymes present; precursor made and converted to purple pigment
C_pp (3/16)	White	Precursor is made but NOT converted to purple (no functional enzyme from gene 2)
ccP_ (3/16)	White	No precursor made (gene 1 non-functional), so gene 2 has no substrate to act on
ccpp (1/16)	White	Neither enzyme functional

Modified ratio: 9 purple : 7 white (the 3:3:1 categories all produce white, combining to give 7 white).

This is an example of recessive epistasis — the homozygous recessive genotype at one locus (cc) masks the expression of the other gene.

Exam Tip: If a question describes two genes controlling one characteristic and the phenotypic ratio is NOT 9:3:3:1, consider epistasis. Common modified ratios include 9:7 (complementary gene interaction), 9:3:4 (recessive epistasis where one class has a distinct phenotype), 12:3:1 (dominant epistasis), and 13:3 (inhibitory gene interaction).

Autosomal Linkage and Recombination

Key Definition: Autosomal linkage occurs when two or more genes are located on the same autosome (non-sex chromosome). Linked genes tend to be inherited together because they do not assort independently during meiosis.

If two genes are on the same chromosome, they do not follow Mendel's law of independent assortment.
Instead of a 1:1:1:1 ratio of gamete types from a double heterozygote (AaBb), linked genes produce mainly parental-type gametes (AB and ab, if coupling; Ab and aB, if repulsion) and fewer recombinant-type gametes.
Recombinant gametes arise due to crossing over during meiosis I (prophase I), when homologous chromosomes exchange segments.

Recombination frequency = (number of recombinant offspring / total offspring) × 100%

A recombination frequency of 50% indicates the genes are on different chromosomes (or so far apart on the same chromosome that they assort independently).
A recombination frequency significantly less than 50% indicates the genes are linked (on the same chromosome). The lower the recombination frequency, the closer together the genes are on the chromosome.
Recombination frequencies can be used to construct genetic maps showing the relative positions of genes on a chromosome (1% recombination = 1 map unit or centiMorgan, cM).

Described diagram — Meiosis showing crossing over and independent assortment. The diagram is divided into two parts. Part A (Crossing Over): A cell is shown entering meiosis I. During prophase I, two pairs of homologous chromosomes are visible — each homologous pair consists of two closely aligned chromosomes (a bivalent), and each chromosome is made up of two sister chromatids joined at a centromere. The homologous chromosomes are drawn in two colours (e.g., red for the maternal chromosome and blue for the paternal chromosome). At several points along the bivalent, non-sister chromatids from the maternal and paternal chromosomes are shown crossing over one another, forming X-shaped structures labelled chiasmata (singular: chiasma). An enlarged inset shows that at each chiasma the chromatids have swapped segments — a section of the red chromatid is now attached to the blue chromosome and vice versa. These swapped sections are labelled recombinant segments. After meiosis I is complete, the two daughter cells each contain one chromosome from the homologous pair, but some chromatids now carry a mixture of maternal and paternal alleles due to the crossover. After meiosis II, four genetically distinct haploid gametes are produced — two with parental-type combinations of alleles and two with recombinant-type combinations. Part B (Independent Assortment): A cell with two pairs of homologous chromosomes (pair 1: long chromosomes; pair 2: short chromosomes) is shown at metaphase I. The bivalents line up along the cell's equator, and two equally possible arrangements are depicted side by side. In arrangement 1, both maternal chromosomes face the same pole; in arrangement 2, the maternal long chromosome and the paternal short chromosome face the same pole. Arrows show that each arrangement produces a different combination of chromosomes in the resulting gametes. With two chromosome pairs, there are 2² = 4 possible gamete combinations; with 23 pairs (as in humans), there are 2²³ = over 8 million combinations. A caption reads: "Independent assortment and crossing over together generate enormous genetic variation in the gametes produced by a single individual."

The Chi-Squared (χ²) Test

The chi-squared test determines whether there is a statistically significant difference between observed and expected results in a genetic cross.

Formula:

χ² = Σ [(O − E)² / E]

Where O = observed frequency, E = expected frequency.

Steps

State the null hypothesis (H₀): there is no significant difference between observed and expected results (i.e., the data fit the predicted ratio).
Calculate expected values from the predicted ratio (expected = total × predicted proportion for each category).
Calculate χ² using the formula for each category and sum the values.
Determine degrees of freedom (df) = number of categories − 1.
Compare the calculated χ² with the critical value at the p = 0.05 (5%) significance level from the chi-squared table.
- If χ² < critical value → accept H₀ (results are consistent with the expected ratio; any deviation is due to chance).
- If χ² ≥ critical value → reject H₀ (there is a statistically significant difference; the results do not fit the expected ratio — another factor may be involved, such as linkage or epistasis).

Critical Values Table (p = 0.05)

Degrees of freedom (df)	Critical value (p = 0.05)
1	3.84
2	5.99
3	7.82
4	9.49
5	11.07

Worked Example 3 — Chi-Squared Calculation

A student crosses two heterozygous tall pea plants (Tt × Tt) and expects a 3:1 ratio of tall to dwarf. From 120 offspring, she observes 82 tall and 38 dwarf.

Step 1: H₀ = There is no significant difference between the observed and expected 3:1 ratio.

Step 2: Calculate expected values.

Expected tall = 120 × 3/4 = 90
Expected dwarf = 120 × 1/4 = 30

Step 3: Calculate χ².

Category	Observed (O)	Expected (E)	O − E	(O − E)²	(O − E)² / E
Tall	82	90	−8	64	0.711
Dwarf	38	30	+8	64	2.133
Total	120	120			χ² = 2.844

Step 4: Degrees of freedom = 2 categories − 1 = 1

Step 5: The critical value at p = 0.05 with 1 df is 3.84.

Since χ² (2.844) < critical value (3.84) → accept H₀. The deviation from the expected 3:1 ratio is not statistically significant. The results are consistent with a monohybrid cross with complete dominance.

Exam Tip: Always show your working in chi-squared calculations. Draw the table with all columns clearly labelled. State your null hypothesis before calculating, and state your conclusion clearly: "The calculated χ² value (X) is less than / greater than the critical value (Y) at p = 0.05 with Z degrees of freedom, so we accept / reject the null hypothesis."

The Hardy-Weinberg Principle

The Hardy-Weinberg principle states that allele frequencies in a population remain constant from generation to generation, provided certain conditions are met:

No mutation
No natural selection (all genotypes are equally fit)
Random mating
No migration (no immigration or emigration)
Large population size (no genetic drift)

Hardy-Weinberg Equations

For a gene with two alleles (A and a):

p + q = 1 (where p = frequency of dominant allele A; q = frequency of recessive allele a)

p² + 2pq + q² = 1 (where p² = frequency of AA; 2pq = frequency of Aa; q² = frequency of aa)

Worked Example 4 — Hardy-Weinberg Calculation

In a population of 10,000 people, 100 individuals have the autosomal recessive condition phenylketonuria (PKU).

Step 1: Frequency of affected individuals (aa) = q² = 100 / 10,000 = 0.01

Step 2: q = √0.01 = 0.1 (frequency of the recessive allele a)

Step 3: p = 1 − q = 1 − 0.1 = 0.9 (frequency of the dominant allele A)

Step 4: Frequency of carriers (Aa) = 2pq = 2 × 0.9 × 0.1 = 0.18 (18% of the population, or 1,800 individuals)

Step 5: Frequency of homozygous dominant (AA) = p² = 0.9² = 0.81 (81%, or 8,100 individuals)

Check: 0.81 + 0.18 + 0.01 = 1.00 ✓

Exam Tip: The Hardy-Weinberg equation is almost always entered via q². Since only homozygous recessive individuals can be identified by phenotype (they express the recessive trait), start by calculating q² from the data, then find q, then p, then 2pq. Always check that p² + 2pq + q² = 1.

Natural Selection

Natural selection is the mechanism of evolution proposed by Charles Darwin and Alfred Russel Wallace.

Key Definition: Natural selection is the process by which organisms with heritable traits that are better adapted to their environment are more likely to survive and reproduce, passing those advantageous alleles to the next generation. Over time, the frequency of advantageous alleles increases in the population.

Process

Variation exists within a population (due to mutation, meiosis, and sexual reproduction).
Individuals with characteristics better suited to the environment have a selective advantage — they are more likely to survive and reproduce (differential reproductive success).
These individuals pass on their advantageous alleles to the next generation.
Over many generations, the frequency of advantageous alleles increases in the population, while disadvantageous alleles decrease.
This leads to evolution — a change in allele frequencies in a population over time.

flowchart TD
    V["Variation exists in population
(mutation, meiosis, sexual reproduction)"] --> SA["Some individuals have traits
better suited to environment"]
    SA --> SR["Selective advantage:
more likely to survive & reproduce"]
    SR --> Pass["Advantageous alleles passed
to next generation"]
    Pass --> Inc["Frequency of advantageous alleles
increases over generations"]
    Inc --> Evo["Evolution:
change in allele frequencies
in population over time"]

Types of Selection

Type	Description	Effect on Phenotype Distribution	Example
Stabilising	Individuals with the mean phenotype are favoured; extreme phenotypes are selected against	Distribution narrows (reduced variation); mean stays the same	Human birth weight (~3.5 kg optimal); very heavy or very light babies have lower survival
Directional	Individuals at one extreme are favoured; the mean shifts in one direction	Distribution shifts towards the favoured extreme	Antibiotic resistance in bacteria; peppered moth (Biston betularia) during the industrial revolution; increasing beak depth in Darwin's finches during drought
Disruptive	Individuals at both extremes are favoured over intermediates	Distribution becomes bimodal; may lead to speciation	Beak size in African seedcrackers (Pyrenestes ostrinus); large beaks crack hard seeds, small beaks handle soft seeds, intermediate beaks are less efficient at both

Described diagram — Three types of natural selection shown as bell-curve graphs. Each of the three graphs has the same axes: the horizontal axis represents the range of a continuous phenotypic trait (e.g., body size, from small on the left to large on the right), and the vertical axis represents the number of individuals (frequency). On every graph, a dashed bell curve shows the original population distribution before selection, and a solid curve shows the distribution after selection. Shaded regions on the original curve indicate which individuals are selected against (have lower fitness). Graph 1 — Stabilising selection: The shaded (selected-against) regions are at both tails of the original distribution — individuals with extreme phenotypes at either end have reduced fitness. The solid curve after selection is narrower and taller than the original, centred on the same mean. The peak is higher because individuals near the mean survive and reproduce preferentially, reducing variation while the mean stays unchanged. Example label: "Human birth weight — very light and very heavy babies have lower survival; ~3.5 kg is optimal." Graph 2 — Directional selection: The shaded region covers one tail of the original distribution (e.g., the left tail — smaller individuals are selected against). The solid curve after selection has shifted its peak to the right, towards the favoured extreme. The mean of the population has moved in one direction. Example label: "Antibiotic resistance — bacteria with higher resistance are favoured, shifting the population mean towards greater resistance." Graph 3 — Disruptive selection: The shaded region is in the centre of the original distribution — individuals with intermediate phenotypes are selected against. The solid curve after selection is bimodal, with two separate peaks at either extreme and a trough in the middle. This can eventually lead to the population splitting into two distinct groups (and potentially speciation). Example label: "African seedcracker finch beak size — large beaks crack hard seeds, small beaks handle soft seeds, but intermediate beaks are less efficient at both."

Artificial Selection vs Natural Selection

Feature	Natural Selection	Artificial Selection
Selecting agent	The environment (biotic and abiotic factors)	Humans
Purpose	Survival and reproductive fitness in a given environment	Desired traits chosen by humans (e.g., higher yield, specific appearance, disease resistance)
Speed	Generally slow (over many generations)	Can be rapid (selective breeding over a few generations)
Genetic diversity	Tends to maintain or increase diversity (variation is the raw material)	Tends to reduce genetic diversity (inbreeding, selecting for a narrow set of traits)
Examples	Antibiotic resistance, camouflage, beak shape	Domestic dog breeds, high-yield crop varieties (e.g., modern wheat), dairy cattle with high milk yield

Key Definition: Artificial selection (selective breeding) is the process by which humans choose organisms with desirable phenotypic traits to breed, increasing the frequency of the alleles responsible for those traits in the population over successive generations.

Risks of artificial selection:

Reduced genetic diversity — inbreeding reduces heterozygosity, making populations more vulnerable to disease and environmental change.
Inbreeding depression — increased homozygosity can bring together harmful recessive alleles, reducing fitness.
Loss of alleles that might be useful in the future.

Speciation

Key Definition: Speciation is the formation of new, distinct species through the process of evolution. A species is a group of organisms that can interbreed to produce fertile offspring and are reproductively isolated from other such groups.

Allopatric Speciation

A population is geographically separated by a physical barrier (e.g., mountain range, river, ocean, glacier).
Different selection pressures and genetic drift act on each isolated population independently.
Mutations occur independently in each population, introducing new alleles.
Over many generations, the populations accumulate sufficient genetic differences that they can no longer interbreed to produce fertile offspring — they have become reproductively isolated.
They are now separate species.

flowchart TD
    Pop["Original population"] -->|"Geographic barrier
(river, mountain, ocean)"| PopA["Population A"]
    Pop -->|"Geographic barrier"| PopB["Population B"]
    PopA -->|"Different selection pressures
+ genetic drift
+ independent mutations"| DiffA["Accumulates genetic
differences over time"]
    PopB -->|"Different selection pressures
+ genetic drift
+ independent mutations"| DiffB["Accumulates genetic
differences over time"]
    DiffA --> RI["Reproductive isolation:
cannot interbreed to produce
fertile offspring"]
    DiffB --> RI
    RI --> Sp["Two separate species"]

Sympatric Speciation

Speciation occurs without geographic separation — within the same area.
Often involves polyploidy (common in plants) — a mutation during cell division (e.g., failure of spindle fibres) causes an organism to have extra complete sets of chromosomes (e.g., 4n instead of 2n), making it instantly reproductively isolated from the diploid parent population because the polyploid cannot produce fertile offspring with diploid individuals.
Reproductive isolation can also arise through:
- Behavioural isolation — differences in mating calls, courtship displays.
- Temporal isolation — differences in breeding season or time of day.
- Ecological/habitat isolation — populations use different resources or habitats within the same area.

Evolution and the Evidence

Key evidence for evolution includes:

Fossil record — shows changes in organisms over geological time; transitional fossils (e.g., Archaeopteryx — features of both dinosaurs and birds) link major groups.
Comparative anatomy — homologous structures suggest common ancestry (e.g., the pentadactyl limb in vertebrates — same basic bone structure in human arms, whale flippers, bat wings, dog legs). Analogous structures (e.g., wings of insects and birds) show convergent evolution, not common ancestry.
Molecular evidence — similarities in DNA base sequences, amino acid sequences, and protein structures. More closely related species share more sequence similarity. Molecular clocks use the rate of DNA mutation to estimate the time since two species diverged.
Biogeography — the distribution of species across continents supports evolutionary patterns (e.g., marsupials in Australia evolved in isolation after continental drift separated Australia from other landmasses).
Observed evolution — antibiotic resistance in bacteria (e.g., MRSA); pesticide resistance in insects; Darwin's finches' beak changes observed over decades.

Exam Tip: When discussing evidence for evolution, do not simply list the types of evidence. For full marks, explain how each piece of evidence supports the theory. For example: "Homologous structures such as the pentadactyl limb share the same basic bone arrangement despite having different functions in different species. This suggests they were inherited from a common ancestor and have been modified by natural selection to suit different environments — which is strong evidence for evolution by common descent."