Genetic Inheritance (AQA GCSE Biology): Revision Note

Inheritance key terms

Monohybrid inheritance

• Some characteristics are controlled by a single gene, such as:

  • fur colour in mice

  • red-green colour blindness in humans

• The inheritance of these single genes is called monohybrid inheritance

  • Mono = one

• Because there are two copies of each chromosome in each body cell (one inherited from each parent), there are two alleles of each gene present

  • For example, an individual has two copies of the gene for eye colour; one allele could code for brown eyes and one allele could code for blue eyes

• The observable characteristics of an organism (seen just by looking – like eye colour; or found – like blood type) is called the phenotype

• The combination of alleles that control each characteristic is called the genotype

• Alleles can be dominant or recessive

  • A dominant allele only needs to be inherited from one parent in order for the characteristic to show up in the phenotype

  • A recessive allele needs to be inherited from both parents in order for the characteristic to show up in the phenotype

    • If there is only one recessive allele, it will remain hidden and the dominant characteristic will show

• If the two alleles of a gene are the same, we describe the individual as being homozygous

  • Homo = same

• An individual could be:

  • homozygous dominant: having two copies of the dominant allele

  • homozygous recessive: having two copies of the recessive allele

• If the two alleles of a gene are different, we describe the individual as being heterozygous

  • Hetero = different

• When completing genetic diagrams, alleles are abbreviated to single letters

  • The dominant allele is given a capital letter and the recessive allele is given the same letter, but lower case

Use this image

Multiple gene inheritance

• Most characteristics are determined by multiple genes interacting, rather than by a single gene

• An example of multiple gene inheritance is eye colour

  • While it is true that brown eye alleles are dominant to blue eye alleles, there are other genes involved

  • This is why eye colour has many more phenotypes than just brown and blue

Predicting inheritance

• 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

• The dominant allele is shown using a capital letter and the recessive allele is shown using the same letter but lower case

• It is a good idea to always follow the same process when constructing a Punnett square:

  • Determine the parent genotypes

  • Determine the parent gametes

  • Construct and fill in the Punnett square

  • Use the results to answer the question, e.g.:

    • determine the number of offspring with each genotype

    • determine the probability of offspring having a particular genotype or phenotype

    • determine the phenotype ratio in the offspring

  • Indicate visually where in the Punnett square your answer has come from, e.g. by labelling each genotype

Family trees

• Family tree 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 a genetic disorder

Family tree example

• Horizontal lines between males and females indicate that two individuals are the parents of the vertically linked offspring

• In this family tree:

  • Males are indicated by squares and females are represented by circles

  • Affected individuals are red outline shapes and unaffected are blue solid shapes

• The family tree shows:

  • Both males and females are affected

  • Every generation has affected individuals

  • There is one immediate family group that has no affected parents or children

  • The other two families have one affected parent and at least one affected child

Use this image

Predicting probability

Higher Tier Only

• Offspring characteristics can be predicted by calculating the probabilities of the different phenotypes that could occur

• For example, in the second genetic cross that was given in part (b) above, two plants with the genotype Tt 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 offspring being tall is 75%

  • The probability of offspring being short is 25%