2.4Unit 2 — Multicellular Organisms
Variation and Inheritance — SQA National 5 Biology — SQA National 5 Biology Revision Notes & Topical Questions
5 min read 2 parts Lesson 4 of 7
Section 1 of 2Types of variation
| Feature | Discrete Variation | Continuous Variation |
|---|---|---|
| Definition | Characteristics that fall into distinct, separate categories with no intermediates | Characteristics showing a wide spectrum of values across a continuous range between extremes |
| Genetic control | Controlled by alleles of a single gene (single-gene inheritance) | Controlled by the combined interaction of many genes (polygenic inheritance) |
| Environmental effect | Little or no environmental influence | Significantly influenced by environmental factors (e.g. nutrition, climate) |
| Graphical representation | Bar chart | Bell-shaped normal distribution curve / histogram |
| Human examples | Blood group (A, B, AB, O), tongue rolling ability, attached earlobes | Height, body mass, hand span, skin colour |
Section 2 of 2Monohybrid crosses and genetic terminology
1Key genetic terms8 points
:
- GeneA section of DNA on a chromosome that codes for a specific protein / characteristic.
- AlleleDifferent alternative forms of the same gene (e.g. brown eye allele vs blue eye allele).
- PhenotypeThe physical observable characteristics of an organism (e.g. tall or dwarf).
- GenotypeThe genetic makeup and combination of alleles possessed by an organism (e.g. TT, Tt, or tt).
- HomozygousHaving two identical alleles for a gene (e.g. TT = homozygous dominant, tt = homozygous recessive).
- HeterozygousHaving two different alleles for a gene (e.g. Tt).
- Dominant alleleAn allele that is always expressed in the phenotype even if only one copy is present (capital letter).
- Recessive alleleAn allele that is only expressed in the phenotype when homozygous (two recessive copies, lowercase letter).
2Monohybrid cross ratios2 points
:
- Crossing two heterozygous parents (Tt × Tt) yields an expected phenotypic ratio of 3 dominant : 1 recessive.
- Observed vs expected ratiosThe actual observed ratio in offspring often differs from the theoretical expected ratio because fertilisation is a random process involving element of chance.
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