Predicting Genetic Inheritance (Edexcel GCSE Biology: Combined Science)

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Monohybrid Inheritance

  • Monohybrid inheritance is the inheritance of characteristics controlled by a single gene
  • It can be investigated 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

  • Pea plants were used by the scientist Mendel to investigate monohybrid inheritance
  • 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

F1 genetic cross, IGCSE & GCSE Biology revision notes

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

Crossing the offspring from the first cross

F2 genetic cross, IGCSE & GCSE Biology revision notes

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:

Heterozygous pure recessive cross, IGCSE & GCSE Biology revision notes

A cross between a heterozygous plant with a short plant

How to construct 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 to determine the phenotypes of the offspring
  • Completing a Punnett square allows you to predict the probability of different outcomes from monohybrid crosses

Calculating probabilities 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 penotypes 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%

Examiner Tip

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.

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

Family pedigree chart, IGCSE & GCSE Biology revision notes

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

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Lára

Author: Lára

Expertise: Biology Lead

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.