Edexcel IGCSE Biology: Double Science 复习笔记 3.2.3 Predicting Genetic Inheritance

Edexcel IGCSE Biology: Double Science 复习笔记 3.2.3 Predicting Genetic Inheritance

Family Pedigrees


  • Family pedigree diagrams are usually used to trace the pattern of inheritance of a specific characteristic (usually a disease) through generations of a family
  • This can be used to work out the probability that someone in the family will inherit the genetic disorder






A family pedigree chart




  • Males are indicated by the square shape and females are represented by circles
  • Affected individuals are red and unaffected are blue
  • Horizontal lines between males and females show that they have produced children (which are shown underneath each couple)
  • The family pedigree above shows:
    • both males and females are affected
    • every generation has affected individuals
    • That there is one family group that has no affected parents or children
    • the other two families have one affected parent and affected children as well



Monohybrid Crosses


  • Monohybrid inheritance is the inheritance of characteristics controlled by a single gene
  • This can be determined using a genetic diagram known as a Punnett square
  • A Punnett square diagram shows the possible combinations of alleles that could be produced in the offspring
  • From this, the ratio of these combinations can be worked out
  • Remember the dominant allele is shown using a capital letter and the recessive allele is shown using the same letter but lower case



Pea plants

  • The height of pea plants is controlled by a single gene that has two alleles: tall and short
  • The tall allele is dominant and is shown as T
  • The small allele is recessive and is shown as t




A pure breeding short plant is bred with a pure breeding tall plant

  • The term ‘pure breeding’ indicates that the individual is homozygous for that characteristic






A pure-breeding genetic cross in pea plants. It shows that all offspring will be have the tall phenotype.




Crossing the offspring from the first cross


A genetic cross diagram (F2 generation). It shows a ratio of 3 tall : 1 short  for any offspring.





  • All of the offspring of the first cross have the same genotype, Tt (heterozygous), so the possible combinations of offspring bred from these are: TT (tall), Tt (tall), tt (short)
  • There is more variation in the second cross, with a 3:1 ratio of tall : short
  • The F2 generation is produced when the offspring of the F1 generation (pure-breeding parents) are allowed to interbreed



Crossing a heterozygous plant with a short plant

  • The heterozygous plant will be tall with the genotype Tt
  • The short plant is showing the recessive phenotype and so must be homozygous recessive – tt
  • The results of this cross are as follows:






A cross between a heterozygous plant with a short plant




Constructing Punnett squares

  • Determine the parental genotypes
  • Select a letter that has a clearly different lower case, for example, Aa, Bb, Dd
  • Split the alleles for each parent and add them to the Punnett square around the outside
  • Fill in the middle four squares of the Punnett square to work out the possible genetic combinations in the offspring
  • You may be asked to comment on the ratio of different allele combinations in the offspring, calculate percentage chances of offspring showing a specific characteristic or just determine the phenotypes of the offspring
  • Completing a Punnett square allows you to predict the probability of different outcomes from monohybrid crosses



Calculating probabilites from Punnett squares

  • A Punnett square diagram shows the possible combinations of alleles that could be produced in the offspring
  • From this, the ratio of these combinations can be worked out
  • However, you can also make predictions of the offsprings’ characteristics by calculating the probabilities of the different phenotypes that could occur
    • For example, in the second genetic cross (F2 generation) that was given earlier (see above), two plants with the genotype Tt (heterozygous) were bred together
    • The possible combinations of offspring bred from these two parent plants are: TT (tall), Tt (tall), tt (short
    • The offspring genotypes showed a 3:1 ratio of tall : short
    • Using this ratio, we can calculate the probabilities of the offspring phenotypes
    • The probability of an offspring being tall is 75%
    • The probability of an offspring being short is 25%




Polygenic inheritance

  • Most characteristics are a result of multiple genes interacting, rather than a single gene
  • Characteristics that are controlled by more than one gene are described as being polygenic
  • Polygenic characteristics have phenotypes that can show a wide range of combinations in features
  • The inheritance of these polygenic characteristics is called polygenic inheritance (poly = many/more than one)
  • Polygenic inheritance is difficult to show using genetic diagrams because of the wide range of combinations
  • An example of polygenic inheritance is eye colour – while it is true that brown eyes are dominant to blue eyes, it is not as simple as this as eye colour is controlled by several genes
  • This means that there are several different phenotypes beyond brown and blue; green and hazel being two examples



Exam Tip

You will NOT be expected to explain the polygenic inheritance of characteristics using a genetic diagram, you just need to be aware that many characteristics are controlled by groups of genes and that this is known as polygenic inheritance.If you are asked to use your own letters to represent the alleles in a Punnett square, try to choose a letter that is obviously different as a capital than the lower case so the examiner is not left in any doubt as to which is dominant and which is recessive.