Monohybrid Inheritance - IGCSE Biology Revision

Inheritance: Definitions

• Inheritance: Transmission of genetic information from one generation to the next.
• Gene: A short length of DNA on a chromosome coding for a specific characteristic.
• Alleles: Versions or forms of a gene.
  • Individuals have two alleles for each gene (one from each parent).
• Phenotype: Observable characteristics of an organism.
• Genotype: Combination of alleles controlling each characteristic.
• Dominant Allele: Only needs to be inherited from one parent to show in phenotype.
• Recessive Allele: Needs to be inherited from both parents to show in phenotype.

Homozygous and Heterozygous

• Homozygous: Two identical alleles for a gene.
  • Homozygous Dominant: Two dominant alleles.
  • Homozygous Recessive: Two recessive alleles.
• Heterozygous: Two different alleles for a gene.

Monohybrid Inheritance

• Involves inheritance of characteristics controlled by a single gene.
• Determined using a Punnett square.

Punnett Square Basics

• Visual tool to show possible allele combinations in offspring.
• Used to determine ratios and probabilities of genetic combinations.

Example: Pea Plant Height

• Tall allele (dominant): T
• Short allele (recessive): t

Cross between Homozygous Tall and Short Plants

• Parents: TT (tall), tt (short).
• All offspring: Tt (tall).
• Probability of offspring being tall: 100%.

Cross between Heterozygous and Short Plant

• Parents: Tt (heterozygous), tt (short).
• Offspring Genotypes: Tt, Tt, tt, tt.
• Offspring Phenotypes: 2 tall, 2 short.
• Probability of short offspring: 50%.

Constructing Punnett Squares

1. Determine allele letters.
2. Identify parental genotypes.
3. Determine gametes and fill square.
4. Indicate phenotype outcomes.

Pedigree Diagrams

• Trace inheritance patterns through family generations.
• Males: squares; Females: circles.
• Affected individuals: shaded.

Identifying Unknown Genotypes

• Use a test cross with a recessive phenotype individual.
• Analyze offspring phenotypes to determine unknown genotype.

Tips

• Choose distinguishable letters for alleles (e.g., A vs. a).
• Use pedigree charts to understand family inheritance patterns.