Phenotypic Variation and Monogenic Inheritance

Spec Mapping — OCR H420 Module 6.1.2 — Patterns of inheritance, content statements covering genetic and environmental causes of phenotypic variation, monogenic (monohybrid) inheritance including dominant, recessive, codominant and multiple-allele patterns (refer to the official OCR H420 specification document for exact wording). This lesson is the foundation for every subsequent inheritance lesson — sex linkage, autosomal linkage, dihybrid crosses, epistasis, chi-squared, Hardy-Weinberg — all build on the Mendelian principles established here.

Variation is the raw material on which natural selection acts. Within any species, individuals differ in countless ways — some because of their genes, some because of their environment, and most because of an interaction between the two. OCR A-Level Biology A specification module 6.1.2(a) and (b) require you to describe genetic and environmental causes of variation, and to apply Mendelian genetics to monohybrid crosses including dominant, recessive, codominant and multiple-allele inheritance.

Key Definitions:

• Gene — a section of DNA that codes for a polypeptide or a functional RNA.
• Allele — a different version of a gene.
• Locus — the position of a gene on a chromosome.
• Genotype — the combination of alleles an individual has.
• Phenotype — the observable characteristics of an individual.
• Homozygous — having two identical alleles at a locus.
• Heterozygous — having two different alleles at a locus.
• Dominant — an allele whose effect is seen in the heterozygote.
• Recessive — an allele whose effect is masked in the heterozygote.
• Codominant — both alleles in the heterozygote are fully expressed.
• Multiple alleles — more than two possible alleles exist at one locus in the population.

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Sources of Variation

Phenotypic variation has three broad sources:

1. Genetic variation — arising from mutation, independent assortment, crossing over and random fertilisation.
2. Environmental variation — differences in diet, temperature, light, stress, disease, etc.
3. The interaction between the two — the reaction norm: the same genotype produces different phenotypes in different environments.

Source	Example
Genetic only	Human blood group — determined entirely by alleles of ABO
Environmental only	Scars or piercings — not encoded in DNA
Interaction	Human height — ~80% heritable but depends on nutrition
Interaction	Hydrangea flower colour — determined by soil pH acting on the same plant

Most traits are polygenic (controlled by many genes) and continuous (showing a range of values — e.g. human height, skin colour, milk yield in cattle). A small number of traits are monogenic (controlled by one gene) and discontinuous (a few distinct categories — e.g. blood group, cystic fibrosis status).

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Monogenic Inheritance and the Monohybrid Cross

A monohybrid cross follows the inheritance of a single gene with two alleles. Gregor Mendel's famous pea-plant experiments established the rules:

1. Each individual has two alleles of each gene (one on each homologous chromosome).
2. When gametes form, the two alleles segregate so that each gamete carries only one.
3. Fertilisation brings two gametes together, restoring the pair.

If we let T be the dominant allele for tall and t the recessive allele for short:

Cross	Offspring genotypes	Offspring phenotypes
TT × tt	All Tt	All tall
Tt × Tt	1 TT : 2 Tt : 1 tt	3 tall : 1 short
Tt × tt	1 Tt : 1 tt	1 tall : 1 short

Punnett square for Tt × Tt

	T	t
T	TT	Tt
t	Tt	tt

The classic 3:1 ratio emerges.

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Dominant and Recessive Alleles

• Dominant alleles produce a functional protein whose presence is enough to produce the phenotype.
• Recessive alleles usually produce a non-functional protein. Two copies are needed before the loss is visible.
• Dominant human disorders include Huntington's disease and achondroplasia (one faulty copy is enough).
• Recessive disorders include cystic fibrosis, sickle-cell anaemia and phenylketonuria (two faulty copies are needed).

Note that "dominant" does not mean "more common". Most human genetic diseases involve recessive, rare alleles.

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Codominance

When both alleles are fully expressed in the heterozygote, they are said to be codominant. Neither is masked. The classic example is coat colour in shorthorn cattle:

• CᴿCᴿ — red hair.
• CᵂCᵂ — white hair.
• CᴿCᵂ — roan (a mixture of red and white hairs, not pink).

A roan cow has both red and white hairs visible side by side — the alleles are not blending, they are both expressed. Similarly, in human ABO blood groups, the Iᴬ and Iᴮ alleles are codominant: genotype IᴬIᴮ gives blood group AB, where both A and B antigens are present on red blood cell surfaces.

Incomplete dominance (a related idea — producing an intermediate blend, as in pink snapdragons from red × white) is not the same as codominance, but both are examples of heterozygotes looking different from either homozygote.

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Multiple Alleles: ABO Blood Groups

In a population a gene can exist in more than two forms. The ABO blood group locus has three alleles: Iᴬ, Iᴮ and Iᴼ. Each person still has only two of them, but the combinations give four phenotypes:

Genotype	Phenotype	Antigens on RBC	Antibodies in plasma
IᴬIᴬ or IᴬIᴼ	Group A	A	anti-B
IᴮIᴮ or IᴮIᴼ	Group B	B	anti-A
IᴬIᴮ	Group AB	A and B	none
IᴼIᴼ	Group O	none	anti-A and anti-B

• Iᴬ and Iᴮ are both dominant to Iᴼ.
• Iᴬ and Iᴮ are codominant to each other.
• Iᴼ is recessive.

This single locus therefore shows dominance, recessiveness, codominance and multiple alleles all at once — which is why OCR loves to use it as an example.

Worked example

A Group A father and a Group B mother have a Group O child. What are the parents' genotypes?

Group O is only produced by IᴼIᴼ, so both parents must carry Iᴼ. For the father to be Group A, his genotype must be IᴬIᴼ. For the mother to be Group B, her genotype must be IᴮIᴼ. Their possible children are:

	Iᴬ	Iᴼ
Iᴮ	IᴬIᴮ (AB)	IᴮIᴼ (B)
Iᴼ	IᴬIᴼ (A)	IᴼIᴼ (O)

A 1 : 1 : 1 : 1 ratio of AB : B : A : O among their potential children.

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Test Crosses

A test cross is used to determine whether an individual showing a dominant phenotype is homozygous (TT) or heterozygous (Tt). Cross the unknown with a homozygous recessive (tt):

• If the unknown is TT, all offspring will be tall (Tt).
• If the unknown is Tt, offspring will be 1 tall : 1 short.

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Exam Tip

Always write allele symbols in the conventional way: capital for dominant, lowercase for recessive, and superscripts for codominant alleles (Iᴬ, Iᴮ, Iᴼ or Cᴿ, Cᵂ). Draw Punnett squares carefully — show the gametes first, then fill in the squares, then read off the ratio. If the question asks for the probability of a particular outcome, give it as a fraction (e.g. 1/4) or a percentage, not just a ratio.

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Common Exam Mistakes

• Saying "dominant alleles are more common". They need not be — they simply mask recessive alleles in heterozygotes.
• Confusing codominance and incomplete dominance. Codominance: both alleles fully expressed side by side (roan cattle). Incomplete dominance: blended intermediate (pink snapdragons).
• Writing genotypes without superscripts for codominant alleles. IAIB is fine handwritten but should be IᴬIᴮ in proper notation.
• Forgetting that Iᴼ is recessive. A Group O child means both parents must carry Iᴼ.
• Confusing phenotype and genotype. A tall pea plant could be TT or Tt.
• Ignoring environmental influences on continuous traits. Height, skin colour and body mass are polygenic and environmentally influenced.

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Quick Recap

• Genetic variation arises from mutation, meiosis and random fertilisation; environmental variation from external factors; most traits reflect both.
• Monohybrid crosses follow one gene at a time: use a Punnett square.
• Dominant alleles mask recessive ones in heterozygotes (3:1 ratio from Aa × Aa).
• Codominant alleles are both expressed (e.g. roan cattle, human AB blood group).
• Multiple alleles: the ABO locus has three (Iᴬ, Iᴮ, Iᴼ) giving four blood groups.
• Test crosses reveal whether a dominant-phenotype individual is homozygous or heterozygous.

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Mendel's Monohybrid Cross — Inline SVG and Worked KaTeX

The cleanest illustration of Mendelian segregation is the classical monohybrid cross TT × tt → F1 all Tt → F2 in 3:1 phenotypic ratio. Gregor Mendel (1866, paraphrased) proposed that each individual carries a pair of "factors" (we now call them alleles), one inherited from each parent, that segregate independently into gametes. His particulate-inheritance model overturned blending-inheritance theory by showing that traits are inherited as discrete units. William Bateson and Edith Saunders (1900s, paraphrased) rediscovered Mendel's work at the turn of the century; Reginald Punnett (1905, paraphrased) introduced the diagrammatic square that bears his name to predict cross outcomes systematically.

P generation: TT (tall) × tt (short)

tt: T T Tt Tt t: Tt Tt All F1: Tt (tall)

F1 × F1: Tt × Tt

T: T t TT Tt t: Tt tt Phenotypic ratio: 3 tall : 1 short Genotypic ratio: 1 TT : 2 Tt : 1 tt

In KaTeX, the probability of a tall F2 plant is the sum of the probabilities of TT and Tt:

$P(\text{tall}) = P(TT) + P(Tt) = \frac{1}{4} + \frac{1}{2} = \frac{3}{4}$P(tall)=P(TT)+P(Tt)=41​+21​=43​

And the probability of a short F2 plant is:

$P(\text{short}) = P(tt) = \frac{1}{4}$P(short)=P(tt)=41​

Hence the 3:1 phenotypic ratio.

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ABO Blood Group — Inline SVG

The ABO locus illustrates codominance and multiple alleles simultaneously.