Mendelian Genetics

Mendelian genetics forms the foundation of heredity. Gregor Mendel's work with pea plants in the 1860s established the fundamental laws of inheritance that underpin modern genetics. A thorough understanding of monohybrid crosses, genotype and phenotype relationships, and the behaviour of dominant and recessive alleles is essential for A-Level Biology.

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Key Terminology

Term	Definition
Gene	A length of DNA that codes for one or more polypeptides (or functional RNA molecules)
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
Phenotype	The observable characteristics of an organism, determined by genotype and environmental influences
Homozygous	Having two identical alleles for a given gene (e.g., AA or aa)
Heterozygous	Having two different alleles for a given gene (e.g., Aa)
Dominant	An allele whose effect on the phenotype is expressed in both homozygous and heterozygous individuals
Recessive	An allele whose effect on the phenotype is only expressed when homozygous (aa)

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 is influenced by soil pH despite having the same genetic makeup.

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Mendel's First Law — The Law of Segregation

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

• 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 separated into different gametes.
• Fertilisation restores the diploid number, bringing together one allele from each parent.

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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

In codominance, both alleles are expressed equally in the heterozygote — neither is dominant over the other. The heterozygote displays a phenotype that is distinct from both homozygotes, showing features of both alleles simultaneously.

Example — Snapdragon Flower Colour (Antirrhinum)

• Allele C^R codes for red pigment.
• Allele C^W codes for white pigment (no pigment production).
• The heterozygote C^R C^W produces pink flowers (both alleles contribute; less red pigment than the homozygous red).

Note: Codominance is NOT the same as incomplete dominance. In codominance, both alleles are fully expressed (e.g., in the AB blood group, both A and B antigens are present on red blood cells). In incomplete dominance, the heterozygote shows a blended intermediate phenotype.

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Multiple Alleles — The ABO Blood Group System

The ABO blood group system is an important example involving both codominance and multiple alleles. Although any individual has only two alleles, the gene has three alleles in the population: I^A, I^B, and i.

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) and a father with blood group B (genotype I^B 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 can produce a wide variety of phenotypes from a single gene.

Exam Tip: When writing alleles for ABO blood groups, use superscript notation (I^A, I^B, i). In exam questions, you may be asked to work backwards — given the blood groups of parents and children, determine the parental genotypes.

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Punnett Squares — Best Practice

When constructing Punnett squares:

1. Identify the alleles for each parent.
2. Determine the gametes each parent can produce. Remember: gametes are haploid and contain one allele from each gene.
3. Arrange gametes along the top and side of the grid.
4. Fill in each cell by combining the column allele with the row allele.
5. Summarise the genotypic and phenotypic ratios.

Common Mistake: Students often forget that ratios from Punnett squares represent probabilities, not certainties. A 3:1 ratio means that each individual offspring has a 3/4 probability of showing the dominant phenotype — it does not mean that exactly 3 out of every 4 offspring will show it.

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Summary

• Mendel's law of segregation states that two alleles separate during gamete formation.
• Monohybrid crosses involving one gene yield predictable ratios (3:1 for heterozygous × heterozygous).
• Test crosses reveal whether an individual with the dominant phenotype is homozygous or heterozygous.
• Codominance occurs when both alleles are expressed equally in the heterozygote.
• Multiple alleles (e.g., ABO blood groups) increase the number of possible phenotypes.
• Always present genetic diagrams in the standard layout to gain full marks in examinations.