Genetic Diversity and Variation

Genetic diversity refers to the total number of different alleles in a population. Variation — differences between individuals of the same species — arises from both genetic and environmental factors. Understanding the sources and significance of genetic diversity, how it is measured, and how populations change over time is crucial for A-Level Biology.

Key Definition: Genetic diversity is the number of different alleles of genes in a population. A population with many different alleles has high genetic diversity.

---

Gene Mutations

Key Definition: A mutation is a random, spontaneous change in the nucleotide sequence of DNA. Mutations are the ultimate source of new alleles and therefore of genetic variation.

Gene mutations (point mutations) affect individual genes.

Types of Point Mutation

Type	Description	Effect
Substitution	One base is replaced by another	May be silent (same amino acid due to degenerate code), missense (different amino acid), or nonsense (creates a premature stop codon)
Insertion	An extra base is added into the sequence	Causes a frameshift — all downstream codons are altered, usually producing a non-functional protein
Deletion	A base is removed from the sequence	Causes a frameshift — usually has a major effect on the protein

• Silent mutations have no effect on the amino acid sequence (often in the third position of a codon, due to degeneracy of the genetic code).
• Missense mutations may or may not alter protein function, depending on the amino acid change and its location in the protein. A conservative missense mutation (replacing one amino acid with a chemically similar one) may have little effect.
• Nonsense mutations produce a premature stop codon, resulting in a truncated, usually non-functional protein.
• Sickle cell anaemia results from a single substitution mutation in the gene for the β-globin chain of haemoglobin: the triplet GAG is changed to GTG on the DNA, changing the mRNA codon from GAG to GUG. This causes glutamic acid (a hydrophilic amino acid) to be replaced by valine (a hydrophobic amino acid) at position 6. The altered haemoglobin (HbS) polymerises under low oxygen conditions, distorting red blood cells into a sickle shape.

Chromosome Mutations

Key Definition: A chromosome mutation is a change in the structure or number of whole chromosomes, rather than a change in individual genes.

Aneuploidy:

• A condition where an organism has one or more extra or missing chromosomes (but not a complete extra set).
• Caused by non-disjunction — the failure of homologous chromosomes (in meiosis I) or sister chromatids (in meiosis II) to separate properly during meiosis.
• If a gamete with an extra chromosome is fertilised, the resulting zygote has trisomy (2n + 1). If a gamete is missing a chromosome, the zygote has monosomy (2n − 1).
• Example: Down syndrome (trisomy 21) — three copies of chromosome 21 instead of two, caused by non-disjunction of chromosome 21 during meiosis.

A diagram of non-disjunction would show a cell undergoing meiosis I. During anaphase I, one pair of homologous chromosomes fails to separate — both homologues move to the same pole. The result is two cells with an extra chromosome (n + 1) and two cells missing a chromosome (n − 1). When the n + 1 gamete is fertilised by a normal (n) gamete, the resulting zygote is 2n + 1.

Polyploidy:

• A condition where an organism has more than two complete sets of chromosomes (e.g., triploid = 3n, tetraploid = 4n).
• Common in plants; rare in animals (usually lethal).
• Can arise from failure of cytokinesis after DNA replication, or from hybridisation between two different species followed by chromosome doubling (allopolyploidy).
• Polyploid organisms are often reproductively isolated from the parent population (they cannot produce viable offspring with diploid organisms), which can lead to sympatric speciation.
• Many important crop plants are polyploid (e.g., bread wheat is hexaploid, 6n = 42).

Exam Tip: Do not confuse aneuploidy (one or a few extra/missing chromosomes, e.g., 2n + 1) with polyploidy (one or more extra complete sets of chromosomes, e.g., 3n, 4n). This is a very common student error.

---

Meiosis as a Source of Genetic Variation

Meiosis introduces variation through several mechanisms:

Crossing Over (Prophase I)

• Homologous chromosomes pair up to form bivalents.
• Non-sister chromatids exchange genetic material at points called chiasmata (singular: chiasma).
• This produces recombinant chromosomes with new combinations of alleles that did not exist in either parent chromosome.

Independent Assortment (Metaphase I)

• The orientation of each bivalent at the metaphase plate is random and independent of all other bivalents.
• For n pairs of homologous chromosomes, there are 2ⁿ possible combinations of chromosomes in the gametes.
• In humans (n = 23), this gives over 8 million possible gamete combinations from independent assortment alone.

Random Fertilisation

• Any male gamete can fuse with any female gamete, further multiplying the possible genetic combinations in offspring.
• Combined with independent assortment: 2²³ × 2²³ = over 70 trillion possible genetic combinations (before considering crossing over).

---

Continuous vs Discontinuous Variation

Feature	Continuous Variation	Discontinuous Variation
Distribution	Normal distribution (bell curve)	Distinct, separate categories
Genetic basis	Controlled by many genes (polygenic)	Usually controlled by one or two genes
Environmental influence	Significant — environment influences phenotype	Little or no environmental influence
Examples	Height, mass, skin colour, foot length	ABO blood group, ability to roll tongue, sex, cystic fibrosis carrier status
Graphical representation	Histogram with a normal curve	Bar chart with distinct categories

Key Definition: Continuous variation is variation in which there is a complete range of phenotypes between two extremes, with no distinct categories. Discontinuous variation is variation in which there are distinct, non-overlapping phenotypic categories.

Polygenic Inheritance

Key Definition: Polygenic inheritance is the inheritance of a characteristic that is controlled by two or more genes, each contributing a small additive effect to the phenotype.

• In polygenic inheritance, many genes each have a small effect, and these effects are cumulative (additive).
• The resulting phenotype shows continuous variation and a normal distribution when plotted.
• Example: human skin colour is controlled by at least three genes, each with two alleles. More "dark" alleles = darker skin.
• The environment also influences the final phenotype — e.g., height is polygenic but is also affected by nutrition.
• Because many genes are involved, traditional Mendelian ratios (3:1, 9:3:3:1) are not observed. Instead, a large number of phenotypic classes arise.

Exam Tip: If asked to explain why a trait shows continuous variation, you must mention both polygenic inheritance (many genes, each with a small additive effect) and environmental influence. Stating only one of these will not gain full marks.

---

Genetic Diversity Within and Between Populations

Allele Frequency

Key Definition: The allele frequency is the proportion of a particular allele at a given locus in a population's gene pool.

• Genetic diversity can be measured using the formula:

  Genetic diversity = 1 − Σp² (where p is the frequency of each allele)

Factors That Reduce Genetic Diversity