Inheritance (OCR Gateway GCSE Biology: Combined Science)

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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

F1 genetic cross

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

F2 genetic 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:

Heterozygous pure recessive cross_1

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%

Exam Tip

If you are asked to use your own letters to represent the alleles in a Punnett square, try to choose a letter that is obviously different as a capital than the lower case so the examiner is not left in any doubt as to which is dominant and which is recessive.

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

Exam Tip

You will NOT be expected to explain the polygenic inheritance of characteristics using a genetic diagram, you just need to be aware that many characteristics are controlled by groups of genes and that this is known as polygenic inheritance.

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

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