Monohybrid InheritanceCambridge IGCSE Biology: Revision notes

Inheritance is 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
• Homozygous: having two identical alleles of a particular gene
• Heterozygous: having two different alleles of a particular gene
• Dominant allele: an allele that is expressed if it is present in the genotype
• Recessive allele: an allele that is only expressed when there is no dominant allele of the gene present

Genetic diagrams, including Punnett squares, are used to predict the results of monohybrid crosses (crosses involving a single gene) and calculate expected phenotypic ratios.

• Crossing two heterozygous individuals (e.g. Bb × Bb) typically gives offspring in a 3:1 ratio (3 showing the dominant phenotype : 1 showing the recessive)
• Crossing a heterozygous individual with a homozygous recessive individual (e.g. Bb × bb) typically gives a 1:1 ratio

Punnett squares are especially useful for crosses that can result in more than one genotype, since they lay out every possible combination of gametes systematically.

Pedigree diagrams show the inheritance of a characteristic through a family tree; they can be interpreted to work out whether an allele is dominant or recessive, and to identify the genotypes of individuals shown.

A test cross is used to identify an unknown genotype. An individual showing the dominant phenotype (who could be homozygous dominant or heterozygous) is crossed with a homozygous recessive individual:

• If any offspring show the recessive phenotype, the unknown parent must have been heterozygous
• If all offspring show the dominant phenotype, the unknown parent was most likely homozygous dominant