Genotype

In the syllabus and the exam

Genotype is the specific combination of alleles an organism carries for a given gene, written using letters such as TT, Tt or tt, and candidates are expected to classify a genotype as homozygous dominant (two identical dominant alleles, such as TT), homozygous recessive (two identical recessive alleles, such as tt) or heterozygous (one of each, such as Tt) before predicting the resulting phenotype. A frequent source of lost marks is failing to distinguish TT from Tt when both produce the same tall phenotype; candidates must remember that genotype cannot always be determined from phenotype alone, and that a test cross (crossing an individual of unknown genotype with a homozygous recessive individual) is the standard method taught for determining whether a dominant phenotype hides a homozygous or heterozygous genotype, based on whether any recessive offspring appear. Genetic diagrams and Punnett squares are built entirely around tracking genotype through a cross, so candidates should be comfortable writing out the possible gametes produced by each parent's genotype and combining them systematically to predict the genotype ratio of the offspring. Candidates should also avoid writing an impossible genotype, such as combining three letters or mismatching the case of paired letters, which examiners treat as a clear conceptual error. Candidates should further note that a genotype is always written for a specific gene under discussion; a full description of an organism's overall genetic make-up would require listing the genotype for every relevant gene separately, not a single combined term.

Example

A pea plant with the genotype Tt is heterozygous for height, carrying one dominant allele for tall (T) and one recessive allele for short (t), even though its phenotype shows only the tall characteristic.

Related terms

PhenotypeAlleleMonohybrid inheritance

Source:SRC-DSKP-EN