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Monohybrid inheritance and genetic diagrams

Reproduction and inheritance · Inheritance · note 4 of 6

Spec 3.23Download all Inheritance notes (PDF)Download all notes (PDF)6 pages

Monohybrid inheritance and genetic diagramsSpec 3.23

Monohybrid inheritance is the inheritance of a characteristic controlled by a single gene. You show it using a genetic diagram, such as a Punnett square.

  1. Choose letters for the alleles: a capital letter for dominant, lower case for recessive.
  2. Write the genotypes of the parents.
  3. Write the possible gametes from each parent. Each gamete has one allele.
  4. Combine the gametes in a Punnett square to show the possible genotypes of the offspring.
  5. Write the phenotype for each genotype.

Crossing two heterozygous pea plants

Tall (T) is dominant to short (t). Two heterozygous tall plants (Tt) are crossed. Show the possible offspring.

  1. Parent genotypes: Tt and Tt.
  2. Gametes: T or t from each parent.
  3. Punnett square: the four boxes are TT, Tt, Tt and tt.
  4. Phenotypes: TT, Tt and Tt are tall; tt is short.

Answer: Offspring genotypes: 1 TT : 2 Tt : 1 tt. Phenotypes: 3 tall : 1 short.

Punnett square for Tt × Tt: gametes T and t from each parent give offspring TT, Tt, Tt (tall) and tt (short). (opens full size in a new tab)
Tt × Tt gives 3 tall : 1 short (1 TT : 2 Tt : 1 tt).
Exam tip:

Always write the gametes in circles or in a labelled row and column, and say which letter is dominant. Marks are given for gametes, offspring genotypes and phenotypes.