Unit 2 - Construct Genetic Diagrams (H)
'Tall' is an example of a phenotype.

Unit 2 - Construct Genetic Diagrams (H)

In GCSE Biology students will spend some time studying genetics. Learning how to construct and interpret genetic diagrams, such as the Punnet square, will help them to predict the outcomes of genetic crosses.

Scientists in the 19th Century were fascinated by the living world, but there were many things that they still didn't understand. It took Charles Darwin many years to work out and publish his book about evolution by natural selection (titled The Origin of Species) but had he known about genetics, it would have been much easier for him. At the time when Darwin published his book, an Augustinian friar called Gregor Mendel was carrying out a series of experiments with pea plants. He cross-pollinated plants with different characteristics and observed which of these were passed on to subsequent generations. He eventually noticed patterns appearing that could only be explained if some of the 'factors' were dominant and others were recessive. His work was re-discovered in the early part of the 20th Century and from that the science of genetics was born.

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Mendel's observations can be represented using genetic diagrams. Genetics is the study of inheritance and when scientists try to predict the outcomes of crosses, they construct genetic diagrams. There are two types - the Punnett square and the ball and stick diagram. The Punnett square is where 4 boxes are completed, joining alleles in all possible combinations. The ball and stick diagram uses circles and lines to achieve the same. The Punnett square is the easiest way to predict outcomes from genetic crosses, as you are far less likely to make an error. In both types of diagram, the dominant allele is represented using an upper case letter and the recessive allele by a lower case letter

Constructing genetic diagrams allows you to predict ratios and probabilities of the outcomes of genetic crosses in terms of the genotypes and phenotypes. Depending on which tier of the exam you are taking, you need to be able to interpret these diagrams and to be able to draw them for any combination of alleles.

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  1. 'Tall' is an example of which of the following?
    Phenotypes are descriptions of the effects of alleles
  2. What does homozygous mean?
    The prefix homo always means the same e.g. homologous, homogenous
  3. Phenotype is the expression of what?
    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. Which word describes TT?
    Since the alleles are the same, it could only be one of the last two alternatives
  5. Genotype describes which of the following?
    It can also refer to an individual's collection of genes
  6. Alleles are different forms of the same...
    In some inherited diseases, like cystic fibrosis or Huntingtons disease, one of the alleles is faulty
  7. Which of the following is an example of a heterozygous genotype?
    The same letter of the alphabet is used for each feature (gene) - T in this case - so Tc is incorrect
  8. 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) and 1 short (tt), hence a 3:1 ratio Tall:short
  9. Which word describes Tt?
    Since the alleles are different, you can immediately rule out the two options beginning with 'homo'
  10. What is the ratio of Tall:short if Tt is crossed with tt? Use a genetic diagram to predict the outcome.
    Using a Punnett square:
              T          t
    t        Tt         Tt
    t        tt           tt      2 Tall Tt and 2 short tt cancels to 1:1, Equal numbers of Tall to short in the offspring

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