Monohybrid Inheritance (Cambridge (CIE) IGCSE Biology): Revision Note

Inheritance: definitions

• 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

• Alleles are versions, or forms, of a gene:

  • chromosomes exist in matching pairs, so individuals have two copies of each gene and therefore two alleles of each gene

  • one of each allele is inherited from the mother and the other from the father

    • For example, an individual has two copies of a gene for eye colour; these alleles could be identical, or they could be different

• The observable characteristics of an organism 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, the individual is described as being homozygous ('homo-' means 'the 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, the individual is described as being heterozygous ('hetero-' means 'different')

• When completing genetic diagrams alleles are notated as single letters:

  • the dominant allele is given a capital letter

  • the recessive allele is given the lower case version of the same letter

• It is not always possible to determine the genotype of an individual just by looking at the phenotype

  • A phenotype associated with a dominant allele will be seen in both a dominant homozygous and a heterozygous genotype

• If two individuals who are both identically homozygous for a particular characteristic are bred together, they will produce offspring with exactly the same genotype and phenotype as the parents

  • This is described as pure-breeding as two homozygous individuals will always produce offspring with the same particular characteristic

• A heterozygous individual can pass on different alleles for the same characteristic each time it breeds with any other individual and can therefore produce offspring with a different genotype and phenotype than the parents

  • Heterozygous individuals are not pure-breeding

Genetic diagrams

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

Constructing Punnett squares

• You may be asked to determine 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; this can be done using a Punnett square as follows:

  1. Determine the letter that will be used to notate each allele

     • An exam question may provide this information

     • If not, select a relevant letter, e.g. E and e for eye colour

  2. Determine the parental genotypes

  3. Determine the gametes produced by each parent and add them to the Punnett square headings

  4. Fill in the middle four squares of the Punnett square to work out the possible genetic combinations in the offspring

  5. Indicate clearly on your Punnett square which individual will show each phenotype, e.g. with labels or colour coding

Pedigree diagrams

• 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

• 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

Identifying an unknown genotype: extended

• Breeders can use a test cross to find out the genotype of an organism showing the dominant phenotype

• This involves crossing the unknown individual with an individual showing the recessive phenotype - if the individual is showing the recessive phenotype, then its genotype must be homozygous recessive

• By looking at the ratio of phenotypes in the offspring, it is possible to tell whether the unknown individual is homozygous dominant or heterozygous