Predicting Inheritance: Monohybrid Crosses | AQA A Level Biology Revision Notes 2017 (2024)

Monohybrid Crosses

Monohybrid inheritance looks at how the alleles for a single gene are passed on from one generation to the next
Known information about the genotypes, phenotypes and the process of meiosis are used to make predictions about the phenotypes of offspring that would result from specific breeding pairs
When two individuals sexually reproduce there is an equal chance of either allele from their hom*ologous pair making it into their gametes and subsequently the nucleus of the zygote
- This means there is an equal chance of the zygote inheriting either allele from their parent
Genetic diagrams are often used to present this information in a clear and precise manner so that predictions can be made
- These diagrams include a characteristic table called a Punnett square
The predicted genotypes that genetic diagrams produce are all based on chance
- There is no way to predict which gametes will fuse so sometimes the observed or real-life results can differ from the predictions

Worked example

Worked example: Genetic diagram

One of the genes for the coat colour of horses has the following two alleles:
- B, a dominant allele produces a black coat when present
- b, a recessive allele produces a chestnut coat when present in a hom*ozygous individual
In this example a heterozygous male is crossed with heterozygous female

Parental phenotype: black coat x black coat

Parental genotype: Bb Bb

Parental gametes: B or b B or b

Monohybrid punnett square with heterozygotes table

Predicted ratio of phenotypes in offspring – 3 black coat : 1 chestnut coat
Predicted ratio of genotypes in offspring – 1 BB : 2 Bb : 1 bb

Codominance

When working with codominant alleles the genetic diagrams can be constructed in a similar way, however the genotypes are represented using a capital letter for the gene and superscript letters for the alleles (eg. I^AI^A)
There will be more possible phenotypes and so the predicted ratios will be different

Worked example

Worked example: Codominance

The gene for blood type has three alleles:
- A, a dominant allele produces blood type A
- B, a dominant allele produces blood type B
- O, two recessive alleles will produce blood type O
In this example a blood type A person is crossed with a blood type B person

Parental phenotype: Blood type A x Blood type B

Parental genotype: I^AI^O I^BI^O

Parental gametes: I^A or I^O I^B or I^O

Sex-linkage

Sex-linked genes are located on the sex chromosome
This means the sex of an individual affects which alleles they pass on to their offspring through their gametes
If the gene is on the X chromosome,males (XY), will only have one copyof the gene, whereas females (XX) will have two
- The X chromosome has many more genes on it than the Y chromosome, so sex-linkage that involves the Y chromosome is very rare
Sex linkage is notated using a capital letter to represent the chromosome X or Y and a superscript letter to represent the allele
There arethree genotypesforfemales,e.g. for a genetic trait caused by a recessive allele
- X^AX^A = unaffected
- X^AX^a = carrier
- X^aX^a = affected
Maleshave onlytwo genotypes, e.g.
- X^AY = unaffected
- X^aY = affected
It is not possible for males to be carriers of x-linked traits, nor for them to pass such traits on to their sons; males only pass y chromosomes on to their sons

Worked example

Worked example: Sex-linkage

Haemophilia is a well known sex-linked disease
There is a gene found on the X chromosome that codes for a protein called factor VIII. Factor VIII is needed to make blood clot
There are two alleles for factor VIII, the dominant F allele which codes for normal factor VIII and the recessive f allele which results in a lack of factor VIII
When a person possesses only the recessive allele f, they don’t produce factor VIII and their blood can't clot normally
The genetic diagram below shows how two parents with normal factor VIII can have offspring with haemophilia

Parental phenotypes: carrier female x normal male

Parental genotypes: X^FX^f X^FY

Parental gametes: X^F or X^f X^F or Y

Monohybrid punnett square with sex-linkage table

Predicted ratio of phenotypes in offspring - 1 female with normal blood clotting : 1 carrier female : 1 male with haemophilia : 1 male with normal blood clotting

Predicted ratio of genotypes in offspring: 1 X^FX^F : 1 X^FX^f : 1 X^FY : 1 X^fY

Exam Tip

Make sure to include all of your working out when constructing genetic diagrams. It is not enough just to complete a punnett square, you need to show that you have thought about the possible gametes that can be produced by each parent.Also, remember to state the phenotype as well as the genotype of the offspring that result from the cross. Read the questions carefully when answering sex-linked inheritance questions – is the question asking for a probability for all children or is it asking about a specific gender (boys or girls).