Genetics: Variation, Inheritance and Life

💡 NOTE

Big question: Why do offspring resemble their parents without being identical to them?

Variation means differences in characteristics among individuals of the same species. Continuous variation forms a measurable range with intermediate values, such as height or mass. Discontinuous variation falls into distinct categories, such as ABO blood groups. Observed characteristics may reflect genetic factors, environmental factors, or interactions between them.

📋 ACTIVITY

Vibe Lab — Measure the heights of a suitable sample of learners using one consistent method. Group the measurements into intervals, construct a frequency distribution and graph, then use the pattern of values as evidence to identify the type of variation. Record method, results, interpretation and conclusion.

Chromosomes occur in the nucleus and contain DNA. A gene is a section of DNA carrying hereditary information; an allele is an alternative form of a gene. Human body cells normally contain 46 chromosomes while normal gametes contain 23. Meiosis reduces chromosome number before fertilisation and separates alleles into gametes.

📋 ACTIVITY

Model meiosis with coloured threads or cards representing homologous chromosomes. Follow the alleles as homologous chromosomes separate and explain why each gamete receives one allele of a pair.

Genotype is the genetic constitution for a characteristic; phenotype is its observable expression. Homozygous individuals carry identical alleles and heterozygous individuals carry different alleles. During gamete formation paired alleles segregate, and fertilisation combines alleles from two parents.

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Vibe Cross Method: define symbols → write parental genotypes → identify gametes → construct the Punnett square → state genotype outcome → state phenotype outcome → answer the biological question.

For Tt × Tt, where T is dominant to t, the expected genotype ratio is 1 TT : 2 Tt : 1 tt and the phenotype ratio is 3 dominant : 1 recessive. These are probabilities, not a guarantee that every four offspring will appear in exactly that ratio.

A test cross with a homozygous recessive organism can provide evidence about an unknown dominant phenotype genotype. In incomplete dominance the heterozygote has an intermediate phenotype, so a heterozygote cross can produce a 1:2:1 phenotype ratio.

The ABO system illustrates multiple alleles and codominance. Iᴬ and Iᴮ are codominant to one another and each is dominant over i. Group A may be IᴬIᴬ or Iᴬi; B may be IᴮIᴮ or Iᴮi; AB is IᴬIᴮ; O is ii. The Rhesus factor is another inherited blood characteristic with important medical applications.

Typical females have XX sex chromosomes and typical males XY. Ova normally carry X while sperm may carry X or Y. Some genes carried on sex chromosomes show sex-linked inheritance. X-linked recessive characteristics such as common forms of red-green colour blindness and haemophilia are expressed more readily in males because males normally have only one X chromosome.

A mutation is a change in genetic material. Mutations can involve genes or chromosomes and may be harmful, neutral or occasionally advantageous depending on context. Genetics is applied in selective breeding, genetic counselling, blood-transfusion reasoning and genetic engineering. Applications must be considered together with evidence, safety and ethical implications.

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Misconception clinic: dominant does not mean common; continuous variation is not caused only by environment; antibiotics do not teach bacteria to mutate; a probability of 25% does not mean every fourth child must show a characteristic.