Inheritance (OCR GCSE Combined Science A (Gateway))

Single Gene Inheritance

• Some characteristics are controlled by a single gene, this 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 it is possible to have two different alleles for a gene

  • For example, two copies of a particular gene could contribute to eye colour but one allele could code for brown eyes and one allele could code for blue eyes

• The observable characteristics of an organism is called the phenotype

  • For example: eye colour or blood type

• The combination of alleles that control each characteristic is called the genotype and is usually represented using letters

  • For example: Bb might be a genotype for brown eyes

Allele types

• Alleles can be

  • Dominant: it only needs to be inherited from one parent in order for the characteristic to show up in the phenotype

  • Recessive: It needs to be inherited from both parents (have 2 copies of in the genotype) 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

  • The recessive allele is given the same letter, but lower case

Alleles can be dominant or recessive

Single Gene Crosses

• Monohybrid inheritance 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 Gregor 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%

Polygenic 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 phenotype that can show a wide range of combinations in features

  • 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

• 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

Sex Chromosomes

• Sex is determined by the 23rd pair of chromosomes

  • In females, the sex chromosomes are the same (XX)

  • In males, the sex chromosomes are different (XY)

• The inheritance of sex can be shown using a genetic diagram (Punnett square), with the X and Y chromosomes taking the place of the alleles usually written in the boxes

Inheritance of sex