Predicting Genetic Inheritance

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

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

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

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%

Exam 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

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