Glossary: Inheritance

This chapter's terms in context

Chapter 26 introduces seven interlocking terms that form the working vocabulary of genetics for the rest of the syllabus: gene, allele, dominant allele, recessive allele, genotype, phenotype and monohybrid inheritance. A gene is a segment of DNA, located at a specific position on a chromosome, that codes for a particular characteristic or trait, such as flower colour in a pea plant or a particular blood group in a human. An allele is one of the different possible versions of the same gene, for example, the gene for flower colour might exist as a purple-flower allele or a white-flower allele, and because a diploid organism inherits one chromosome of each homologous pair from each parent, it always carries two alleles of every gene, one on each chromosome of the pair.

A dominant allele is one whose characteristic is expressed in the phenotype whenever it is present, even if only one copy of it is inherited (that is, even in a heterozygous individual carrying one dominant and one recessive allele), and it is conventionally represented by a capital letter. A recessive allele, by contrast, is only expressed in the phenotype when two copies of it are present (that is, only in a homozygous recessive individual, since a single copy is masked by a dominant allele if one is present), and it is conventionally represented by the same letter in lower case. A frequent exam error is assuming that a dominant allele is somehow more common in a population or 'stronger' in some general biological sense, when dominance and recessiveness describe only the pattern of expression in a heterozygote, with no necessary connection to how frequent an allele actually is in a population.

Genotype and phenotype describe two different things about the same organism and must never be used interchangeably: the genotype is the actual genetic makeup of an organism for a particular gene, written using letter symbols (such as Tt for a heterozygous individual), while the phenotype is the observable physical characteristic that results from that genotype (such as 'tall'), influenced by the combination of alleles present and, in many traits, by environmental factors as well. Two organisms with different genotypes can share the same phenotype, a homozygous dominant individual (TT) and a heterozygous individual (Tt) for a simple dominant trait will look identical despite having different genetic makeups, which is exactly why a test cross is sometimes needed to determine an organism's exact genotype from its phenotype alone.

Monohybrid inheritance is the pattern of inheritance for a single gene controlling one characteristic, traced across a cross between two parents, and it is represented using a Punnett square: the possible gametes from each parent are written along two adjacent sides of a grid, and each internal cell of the grid shows one possible genotype of an offspring, formed by combining the gamete from its row with the gamete from its column. A classic monohybrid cross between two heterozygous parents for a simple dominant-recessive trait produces offspring in a 3:1 phenotypic ratio (three showing the dominant characteristic for every one showing the recessive characteristic), and this specific ratio, along with the reasoning that produces it, is one of the most heavily and consistently tested calculations across the whole SPM Biology syllabus.

A frequent structural confusion across all seven terms is failing to keep gene (the segment of DNA), allele (a version of that gene), genotype (the pair of alleles an individual carries) and phenotype (the resulting observable trait) in the correct logical order, a gene has alleles, an individual's genotype is made up of a pair of alleles, and that genotype produces a phenotype, and examiners frequently test this chain directly by asking a candidate to define more than one of these terms in relation to a single stated example, such as a specific gene for seed shape in a pea plant.

Paper 2 questions on this chapter typically supply a genetic cross scenario and require a candidate to construct a full Punnett square, correctly identify parental genotypes and gametes, and state the resulting phenotypic ratio, with marks awarded separately for the genotype symbols used, the construction of the grid itself, and the final ratio stated in its simplest whole-number form.