Monohybrid Inheritance and Genetic Diagrams

Method

1. Define the key terms precisely, since they are marked individually: allele (a different version of a gene); genotype (the combination of alleles an organism has for a gene, written with letters, for example Bb); phenotype (the observable characteristic resulting from the genotype, for example brown eyes); homozygous (having two identical alleles for a gene, for example BB or bb); heterozygous (having two different alleles for a gene, for example Bb); dominant allele (an allele whose characteristic appears in the phenotype even when only one copy is present, conventionally shown with a capital letter); recessive allele (an allele whose characteristic only appears in the phenotype when two copies are present, conventionally shown with a lower-case letter).
2. Set out a full genetic diagram in this exact order every time: state the parents' phenotypes and genotypes; state the gametes each parent can produce (each gamete carries one allele, since gametes are haploid); draw a Punnett square with one parent's gametes along the top and the other's down the side, and fill in every offspring genotype in the grid; read off the genotype ratio from the grid, then convert to a phenotype ratio using which genotypes give which phenotype; state the final ratio in simplest form (for example, 3:1) and, if asked, as a percentage or fraction.
3. For a monohybrid cross between two heterozygotes (Bb x Bb), show that the offspring genotype ratio is 1 BB : 2 Bb : 1 bb, giving a phenotype ratio of 3 dominant : 1 recessive.
4. For a test cross (an individual of unknown genotype, showing the dominant phenotype, crossed with a known homozygous recessive), explain the logic: if any recessive-phenotype offspring appear, the unknown parent must be heterozygous (since it must carry a recessive allele to pass on); if all offspring show the dominant phenotype, the unknown parent is more likely to be homozygous dominant.
5. For codominance, define it precisely (both alleles are expressed in the phenotype of a heterozygote, with neither masking the other, giving a distinct third phenotype rather than a blend) and apply it to a named example such as human ABO blood groups, where allele I_A and allele I_B are codominant and both are dominant to i, so genotype I_A I_B gives phenotype AB.For codominance, define it precisely (both alleles are expressed in the phenotype of a heterozygote, with neither masking the other, giving a distinct third phenotype rather than a blend) and apply it to a named example such as human ABO blood groups, where allele I_A and allele I_B are codominant and both are dominant to i, so genotype I_A I_B gives phenotype AB.
6. For sex linkage, explain that a sex-linked gene is carried on a sex chromosome (usually the X chromosome), so its inheritance pattern differs between males (XY, only one X, so a single recessive allele is always expressed) and females (XX, need two recessive alleles to show the recessive phenotype), which is why sex-linked recessive conditions such as red-green colour blindness or haemophilia are much more common in males.
7. Always show every step of the diagram in an exam answer, even when the final ratio seems obvious, since IGCSE mark schemes award marks for stating gametes and completing the Punnett square correctly, not only for the final ratio.

Practice questions

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