Genetic Inheritance (AQA GCSE Biology)

Key Terms

Table of key terms & definitions for genetic inheritance

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)

• As we have two copies of each chromosome, we have two copies of each gene and therefore two alleles for each gene

• One of the alleles is inherited from the mother and the other from the father

  This means that the alleles do not have to ‘say’ the same thing

• 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

Alleles of a gene can carry the same instructions or different instructions. You can only inherit two alleles for each gene, and they can be the same or different

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

• Remember 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

A pure-breeding genetic cross in pea plants

• 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’

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: 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:

A cross between a heterozygous plant with a short plant

• 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

A family tree diagram

• 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

• 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 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%