GCSE Biology Quiz
Unit 2 - Construct Genetic Diagrams (H)
'Tall' is an example of a phenotype. (Photo courtesy of Neal Dench at Flickr.)

Unit 2 - Construct Genetic Diagrams (H)

The Punnett square is one type of genetic diagram. Genetics is the study of inheritance and when scientists try to predict the outcomes of crosses, they use genetic diagrams. There are two types. The Punnett square where 4 boxes are completed or stick diagrams joining alleles in all possible combinations. The Punnett square is the easiest way to predict outcomes from genetic crosses, as you are far less likely to make an error.

Genetic diagrams allow us to predict ratios and probabilities of the outcomes of genetic crosses in terms of the genotypes and phenotypes.

  1. Genotype describes...
  2. Alleles are different forms of the same...
  3. Phenotype is the expression of the...
    Phenotype is usually described as 'what you look like'! Blue eyes or brown hair, tall or short. The genes (alleles) in the genotype dictate what we look like
  4. Using a Punnett square, predict the ratio of tall to short when Tt is crossed with Tt. The allele for tall is T and the allele for short is t.
    The gametes are T or t. The Punnett Square looks like this:
              T          t
    T       TT      Tt
    t        Tt         tt        3 Tall (either TT or Tt
                                    1 short (tt) hence 3:1 ratio Tall:short
  5. An example of a heterozygous genotype is...
    The same letter of the alphabet is used for each feature (gene). T in this case. So Tc is incorrect
  6. Homozygous means that the alleles are...
  7. Use a genetic diagram to predict the outcome if Tt is crossed with tt. The ratio of Tall:short is...
    Using a Punnett square:

              T          t
    t        Tt         Tt
    t        tt           tt      2 Tall Tt
                                  2 short tt cancels to 1:1 Equal numbers of Tall to short in the offspring
  8. TT is described as...
  9. Tt is...
  10. 'Tall' is an example of...

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