Single Gene Inheritance (WJEC GCSE Biology: Combined Science)

What is monohybrid inheritance?

• Monohybrid inheritance is the inheritance of characteristics controlled by a single gene (mono = one)
• 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 lowercase

Monohybrid inheritance example

• 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

Homozygous dominant X homozygous recessive

• Show the possible allele combinations of the offspring produced when 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 generation monohybrid inheritance Punnett square diagram

A pure-breeding genetic cross in pea plants

• This shows that there is a 100% chance that all the offspring will be tall

Heterozygous X heterozygous

• Show the possible allele combinations of the offspring produced when two of the offspring from the first cross are bred together

F2 generation monohybrid inheritance Punnett square diagram

A genetic cross diagram (F₂ Generation)

• All of the offspring of the first cross have the same genotype, Tt (heterozygous), so the possible combinations of offspring bred from these are:
  • There is more variation in this cross, with a 3:1 ratio of tall : short, meaning each offspring has a 75% chance of being tall and a 25% chance of being short
• The F₂ generation is produced when the offspring of the F₁ generation (pure-breeding parents) are allowed to interbreed

Heterozygous X homozygous recessive 

• Show the results of 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:

Monohybrid inheritance Punnett square diagram

A cross between a heterozygous plant with a short plant

 

• In this cross, there is a 1:1 ratio of tall to short, meaning a 50% chance of the offspring being tall and a 50% chance of the offspring being short

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 a 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