Genetic Inheritance (AQA GCSE Combined Science: Trilogy: Biology): Revision Note

Inheritance key terms

Monohybrid inheritance

• Some characteristics are controlled by a single gene, such as fur colour in mice; and 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, one inherited from teach parent, there are two copies of each gene and therefore two alleles for each gene

  • For example, an individual has two copies of the gene for eye colour but 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), or 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

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Multiple gene 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

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

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

‘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

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• This shows that all the offspring will be tall

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

Use this image

• 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 this 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

‘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:

Use this image

• In this cross, there is a 1:1 ratio of tall to 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 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

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 the genetic disorder

Use this image

• 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

  • 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

Predicting probability

Higher Tier Only

• 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

• Offspring characteristics can be predicted 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%