Inheritance

Edexcel GCSE Biology: Combined Science 3.2 Inheritance

Contents Overview

  • 3.2.1 Key Definitions

  • 3.2.2 Predicting Genetic Inheritance

  • 3.2.3 Inheritance of Sex

  • 3.2.4 Polygenic Inheritance

  • 3.2.5 Variation

  • 3.2.6 Mutations

3.2.1 Key Definitions

  • Gene: Short length of DNA on a chromosome coding for a specific characteristic through proteins.

  • Alleles: Variations of the same gene, two copies (one from each parent) per gene.

  • Phenotype: Observable traits of an organism (e.g., eye color).

  • Genotype: Combination of alleles that dictate phenotypic traits.

Alleles

  • Dominant Alleles: Need to be inherited from one parent for expression.

  • Recessive Alleles: Require contributions from both parents to be expressed.

  • Homozygous: Identical alleles (dominant or recessive).

  • Heterozygous: Different alleles.

Genetic Diagrams

  • Alleles represented by letters; dominant = capital, recessive = lowercase.

3.2.2 Predicting Genetic Inheritance

Monohybrid Inheritance

  • Definition: Inheritance of traits controlled by a single gene.

  • Punnett Square: Tool for visualizing allele combinations and ratios in offspring.

Example: Pea Plants

  • Alleles for Height: Tall (T, dominant) and Short (t, recessive).

  • Pure Breeding: Refers to homozygous individuals (both alleles the same).

Crosses

  • Pure short (tt) x pure tall (TT) = all offspring (Tt) are tall.

  • F2 Generation Ratio: 3 tall (TT and Tt) : 1 short (tt).

3.2.3 Inheritance of Sex

  • Chromosome Determination: XX for females, XY for males; sex is determined by the father.

  • Use of Punnett Squares for sex determination, showing potential combinations of X and Y chromosomes.

3.2.4 Polygenic Inheritance

  • Definition: Traits controlled by multiple genes.

  • Phenotype Variety: Complex traits like eye color can exhibit a wide range of variations, not limited to simple dominant/recessive alleles.

3.2.5 Variation

Types of Variation

  • Continuous Variation: Gradual differences (e.g., height); results in bell curve when graphed.

  • Discontinuous Variation: Clear categories (e.g., blood type); results in step-like graph when plotted.

Causes of Variation

  • Genetic: Differences controlled by genes (meiosis creates genetic variation).

  • Environmental: Influenced by factors outside the organism (e.g., diet, climate).

3.2.6 Mutations

Definition and Types

  • Mutations: Random changes in DNA sequences; can be beneficial, neutral, or harmful.

  • Types of Mutations:

    • Insertions: Addition of a base, shifting the sequence.

    • Deletions: Removal of a base, also shifting the sequence.

    • Substitutions: Replacement of one base with another without shifting the sequence.

Effects of Mutations

  • Mostly neutral; some can alter protein function.

  • Example: Sickle Cell Anaemia

    • Mutation alters hemoglobin, leading to altered red blood cell shape.

    • Symptoms include joint pain, blood flow issues, and potential strokes.

    • Many affected may not live beyond 20, yet treatments can extend life.

Causes of Increased Mutation Rates

  • Environmental factors: ionizing radiation and certain chemicals.

  • Higher mutation rates can lead to cancerous changes.