Mendelian Genetics

Gregor Mendel's experiments with garden peas, published in 1866, established the fundamental laws of inheritance that underpin all modern genetics. Although his work was overlooked for thirty-four years until its rediscovery in 1900 by de Vries, Correns and von Tschermak, the conceptual framework Mendel developed — discrete heritable units (now called genes), dominant and recessive alleles, segregation of alleles into gametes, independent assortment of unlinked genes — remains the foundation on which the entire edifice of A-Level genetics rests. This lesson establishes the vocabulary and crosses needed to predict and interpret inheritance patterns, with worked examples in monohybrid crosses, codominance, multiple alleles and test crosses.

Spec mapping: This lesson sits in AQA 7402 Section 3.7.1 — Inheritance. The relevant content covers the genetic basis of inheritance through monohybrid crosses, the relationships between genotype and phenotype, the behaviour of dominant, recessive, codominant and multiple alleles, and the use of Punnett squares and genetic diagrams to predict offspring ratios. (Refer to the official AQA specification document for exact wording.)

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Key Terminology

The vocabulary of Mendelian genetics is precise, and exam mark schemes typically reward exact use. The following terms are non-negotiable at A-Level:

Term	Definition
Gene	A length of DNA that codes for one or more polypeptides (or functional RNA molecules), occupying a specific locus on a chromosome
Allele	An alternative form of a gene, arising by mutation, found at the same locus on homologous chromosomes
Locus	The specific fixed 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, determined by genotype together with environmental influences
Homozygous	Having two identical alleles for a given gene (e.g. AA or aa). Sometimes called "pure-breeding" because all gametes carry the same allele
Heterozygous	Having two different alleles for a given gene (e.g. Aa). All-too-often called a "hybrid" — the term is technically correct but ambiguous
Dominant	An allele whose effect on the phenotype is expressed in both homozygous (AA) and heterozygous (Aa) individuals
Recessive	An allele whose effect on the phenotype is only expressed when homozygous (aa); masked in the heterozygote
Carrier	A heterozygous individual for a recessive allele — phenotypically unaffected but capable of passing the recessive allele to offspring
Codominant	Two alleles both expressed equally and independently in the heterozygote, producing a phenotype combining features of both (e.g. AB blood group)
F₁ generation	The first filial generation — the immediate offspring of two parents
F₂ generation	The second filial generation — offspring of F₁ × F₁ crosses

Key Definition: The phenotype of an organism results from the interaction between its genotype and the environment. Two organisms with identical genotypes may display different phenotypes if raised under different environmental conditions — for example, hydrangea flower colour shifts from blue (in acidic soil) to pink (in alkaline soil) without any genetic change in the plant. Identical twins (genetically identical at birth) can show different epigenetic profiles, weight, height and disease incidence in adulthood depending on environment.

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Mendel's First Law — The Law of Segregation

Mendel's first law states that the two alleles of a gene segregate (separate) into different gametes during meiosis, so that each gamete carries only one allele for any given gene.

Although Mendel formulated this law without knowing about chromosomes, the cytological basis is now clear:

• During meiosis I, homologous chromosomes are separated into different daughter cells.
• Since each homologous chromosome carries one allele at each locus, the two alleles for any gene are partitioned into different gametes.
• The probability that a gamete carries one or the other allele is 1/2 — a probability that is directly observable as the ratios of offspring in monohybrid crosses.
• Fertilisation restores the diploid number, bringing together one allele from each parent.

Sutton and Boveri's chromosome theory of inheritance (1902–1903) explicitly identified Mendel's segregation with the meiotic separation of homologues, providing the cytological foundation that Mendel's 1866 paper had lacked. The law of segregation is therefore not just an empirical generalisation but a direct consequence of the mechanics of meiosis I.

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Monohybrid Crosses

A monohybrid cross involves the inheritance of a single gene with two alleles.

Worked Example 1 — Pea Plant Height

In pea plants, the allele for tall stem (T) is dominant over the allele for dwarf stem (t).

Cross: Heterozygous tall × Heterozygous tall (Tt × Tt)

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

	T	t
T	TT	Tt
t	Tt	tt

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

Worked Example 2 — Homozygous Dominant × Homozygous Recessive

Cross: TT × tt

Parental phenotypes: Tall × Dwarf Parental genotypes: TT × tt Gametes: T (from tall parent); t (from dwarf parent)

	t	t
T	Tt	Tt
T	Tt	Tt

All offspring are Tt — heterozygous tall. This is the F1 generation.

If the F1 are then crossed (Tt × Tt), the F2 generation gives the classic 3:1 ratio as shown in Example 1 above.

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Test Crosses

A test cross (or back cross) is used to determine the genotype of an individual showing the dominant phenotype. The individual is crossed with a homozygous recessive individual.

• If all offspring show the dominant phenotype → the unknown parent is homozygous dominant (TT).
• If offspring appear in a 1:1 ratio of dominant to recessive → the unknown parent is heterozygous (Tt).

Worked Example 3 — Test Cross

A tall pea plant (genotype unknown: TT or Tt?) is crossed with a dwarf plant (tt).

Possibility 1: If the tall plant is TT:

	t	t
T	Tt	Tt
T	Tt	Tt

All offspring are tall (Tt). The unknown parent must be TT.

Possibility 2: If the tall plant is Tt:

	t	t
T	Tt	Tt
t	tt	tt

Offspring: 1 tall (Tt) : 1 dwarf (tt). The unknown parent must be Tt.

Exam Tip: Always lay out genetic diagrams in the standard format: parental phenotypes → parental genotypes → gametes → Punnett square → offspring genotypes and phenotypic ratio. Examiners award marks for each step, even if the final answer is incorrect.

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Codominance and Incomplete Dominance

Many inheritance patterns do not show the simple dominant-recessive relationships of Mendel's pea experiments. Two important departures are codominance and incomplete dominance — often confused, but biologically distinct.

Codominance

In codominance, both alleles are fully and independently expressed in the heterozygote. The heterozygote phenotype shows the products of both alleles simultaneously rather than a blended intermediate.

Classic example — ABO blood group system (see below). The I^A allele directs synthesis of the A antigen; the I^B allele directs synthesis of the B antigen. In I^A I^B heterozygotes, both antigens are present on the red blood cell surface — the heterozygote is blood group AB, displaying both products at full strength.

Incomplete dominance

In incomplete dominance, the heterozygote shows a phenotype intermediate between the two homozygotes. Neither allele is fully expressed; the dose effect produces a blended phenotype.

Example — Snapdragon flower colour (Antirrhinum):

• Allele C^R codes for an enzyme that produces red pigment.
• Allele C^W codes for a non-functional enzyme (no pigment).
• C^R C^R: red flowers (two doses of enzyme → full red pigment).
• C^W C^W: white flowers (no enzyme → no pigment).
• C^R C^W: pink flowers (one dose of enzyme → half the red pigment).

The mechanism is gene-dosage: with only one functional allele, only half the normal pigment is produced, giving an intermediate colour. The contrast with codominance is sharp — in codominance both products are present at full strength; in incomplete dominance one product is present at half strength.

Distinguishing test: in codominance, you can detect both alleles' products by examining the heterozygote (e.g. anti-A and anti-B antibodies both agglutinate AB blood). In incomplete dominance, you can only detect one allele's product (the active enzyme) at a reduced level.

Pleiotropy and the limits of "one gene, one trait"

Many genes affect more than one trait — a phenomenon called pleiotropy. The sickle-cell allele, for example, affects haemoglobin structure (molecular), red blood cell shape (cellular), anaemia, organ damage, malaria resistance, and life expectancy. A single mutation cascades through multiple levels of phenotype. Mendel was fortunate to choose seven traits that did not show obvious pleiotropic interactions.

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Multiple Alleles — The ABO Blood Group System

The ABO blood group system is the canonical A-Level example of inheritance involving codominance and multiple alleles. Although any individual has only two alleles at this locus (one on each chromosome 9), the gene has three alleles in the population: I^A, I^B, and i.

The molecular basis is straightforward: the gene encodes a glycosyltransferase enzyme that adds a terminal sugar to the H antigen on red blood cells.

• The I^A allele encodes an enzyme that adds N-acetylgalactosamine → A antigen.
• The I^B allele encodes a variant enzyme that adds galactose → B antigen.
• The i allele encodes a non-functional enzyme → no terminal sugar added; H antigen alone (no A, no B) → O blood group.

The codominance of I^A and I^B reflects the fact that both functional enzymes operate independently in the heterozygote, decorating different H antigens with their respective sugars. The recessive behaviour of i to both I^A and I^B reflects loss-of-function — one functional copy is enough to produce the antigen.

Genotype	Blood Group (Phenotype)	Antigens on Red Blood Cells	Antibodies in Plasma
I^A I^A or I^A i	A	A antigen	Anti-B
I^B I^B or I^B i	B	B antigen	Anti-A
I^A I^B	AB	Both A and B antigens	Neither anti-A nor anti-B
ii	O	Neither A nor B antigen	Both anti-A and anti-B

• I^A and I^B are codominant — both are expressed in the heterozygote (I^A I^B → blood group AB).
• Both I^A and I^B are dominant over i.

Worked Example 4 — Blood Group Inheritance

A mother with blood group A (genotype I^A i — heterozygous, with one functional I^A allele and one non-functional i allele) and a father with blood group B (genotype I^B i — heterozygous, with one functional I^B and one i):

Gametes: I^A or i (mother); I^B or i (father)

	I^B	i
I^A	I^A I^B (AB)	I^A i (A)
i	I^B i (B)	ii (O)

Offspring phenotypic ratio: 1 A : 1 B : 1 AB : 1 O

All four blood groups are possible in the offspring. This demonstrates how multiple alleles and codominance together can produce a wider variety of phenotypes from a single gene than the simple Mendelian dominant-recessive model would predict.

The ABO system is also clinically critical: transfusion of incompatible blood (e.g. giving group A blood to a group B recipient) triggers a haemolytic transfusion reaction as the recipient's anti-A antibodies attack the donor red blood cells. Group O is the universal donor (no antigens to attack) and group AB the universal recipient (no antibodies to attack incoming antigens). The Rhesus blood group system (the D antigen) is independent of ABO and adds further complexity, particularly in pregnancy when Rh-negative mothers carrying Rh-positive foetuses can develop antibodies.