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Cambridge IGCSE Biology · 0610

Chapter 17: Inheritance (Part 2)

Key definitions

Inheritance
The transmission of genetic information from generation to generation.
Genotype
The genetic make-up of an organism in terms of the alleles present.
Phenotype
The observable features of an organism resulting from the expression of its alleles.
Homozygous
Having two identical alleles of a particular gene (e.g., GG or gg). These individuals are pure-breeding.
Heterozygous
Having two different alleles of a particular gene (e.g., Gg). These individuals are not pure-breeding.
Dominant allele
An allele that is always expressed if it is present in the genotype.
Recessive allele
An allele that is only expressed when there is no dominant allele present.

Exam Traps

  • Do not use genotype and phenotype interchangeably — genotype is genetic; phenotype is what you observe.
  • Avoid saying a recessive allele is never expressed — it is expressed when homozygous recessive.

Inheritance tools

Pedigree diagrams
Used to track the inheritance of a trait across generations.
  • Squares represent males; circles represent females.
  • Affected individuals are filled in; unaffected are unfilled.
  • Traits appearing in every generation are usually dominant; traits that skip generations are usually recessive (carried by carriers).
Punnett squares
Genetic diagrams used to predict the results of monohybrid crosses.
  • 3:1 ratio: Typically results from crossing two heterozygous parents.
  • 1:1 ratio: Typically results from crossing a heterozygous parent with a homozygous recessive parent.
Punnett square for Gg x Gg cross showing 3:1 green to yellow phenotypic ratio
Diagram 1: A Punnett square. A cross between two heterozygous green pea plants (Gg × Gg) with parent genotypes labelled and a resulting 3:1 phenotypic ratio of green to yellow offspring.

Exam Traps

  • Do not confuse 3:1 (two heterozygotes) with 1:1 (heterozygote × homozygous recessive).
  • Do not assume a 3:1 ratio means 3 green and 1 yellow genotype — state phenotypic ratio unless asked for genotypes.

Advanced inheritance

Test cross
A breeding experiment used to identify an unknown genotype (homozygous dominant vs. heterozygous). The individual is bred with a homozygous recessive individual.
Codominance
A situation where both alleles in a heterozygous organism contribute to the phenotype.
  • Example: ABO blood groups. Alleles are IA, IB and IO. IA and IB are codominant, while IO is recessive.
Sex-linked characteristics
A feature where the gene responsible is located on a sex chromosome (usually the X), making it more common in one sex.
  • Example: Red-green colour blindness. A recessive trait found on the X chromosome (Xr). Because males only have one X chromosome, they are more likely to express the trait if they inherit the recessive allele.
Sex-linkage cross for colour blindness showing carrier female XRXr crossed with normal male XRY
Diagram 2: Sex-linkage cross. A genetic diagram for colour blindness showing a carrier female (XRXr) crossed with a normal vision male (XRY), resulting in a 25% chance of a colour-blind male (XrY).

Exam Traps

  • Avoid treating codominance like simple dominance — both alleles contribute (e.g. blood group AB, not a blend).

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