Phenotypic Variation and Monogenic Inheritance

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).