Monohybrid Inheritance (CIE IGCSE Biology: Co-ordinated Sciences (Double Award))

• Inheritance is the transmission of genetic information from one generation to the next generation
• A gene is a short length of DNA found on a chromosome that codes for a particular characteristic (expressed by the formation of different proteins)
• Alleles are variations of the same gene
  • 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

What is monohybrid inheritance?

• Monohybrid inheritance is the inheritance of characteristics controlled by a single gene (mono = one)
• 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 lowercase

Punnett Squares

Monohybrid Inheritance 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 there is a 100% chance 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:
  • There is more variation in this cross, with a 3:1 ratio of tall:short, meaning each offspring has a 75% chance of being tall and a 25% chance of being 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, meaning a 50% chance of the offspring being tall and a 50% chance of the offspring being short

How to construct Punnett squares

• The steps involved are:
• 1. Determine the parental genotypes
  2. Select a letter that has a clearly different lowercase, for example: Aa, Bb, Dd
  3. Split the alleles for each parent and add them to the Punnett square around the outside
  4. 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 a 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

Pedigree Charts

• Family pedigree 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 pedigree chart

• Males are indicated by the square shape and females are represented by circles
• Affected individuals are red and unaffected are blue (in this example)
• 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
  • that 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