5.1.5 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

• 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%

• 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 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