7-1-2-predicting-inheritance-monohybrid-crosses

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Monohybrid crosses

Monohybrid inheritance involves tracking the inheritance of one gene with two alleles
- It uses known genotypes, phenotypes, and the process of meiosis to predict offspring outcomes
During gamete formation, each allele from a homologous pair has an equal chance of being passed on
- This means the zygote has an equal probability of inheriting either parental allele
Genetic diagrams (especially Punnett squares) are used to present this information clearly
Predicted genotypes are based on probability, not certainty
Observed ratios in offspring may differ from predictions due to random fertilisation of gametes

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 homozygous individual
In this example, a heterozygous male is crossed with a heterozygous female

Parental phenotype: black coat x black coat

Parental genotype: Bb Bb

Parental gametes: B or b B or b

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

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 → produces 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

Predicted ratio of phenotypes in offspring – 1 Blood type AB : 1 Blood type A : 1 Blood type B : 1 Blood type O
Predicted ratio of genotypes in offspring: 1 I^AI^B: 1 I^AI^O : 1 I^BI^O : 1 I^OI^O

Sex-linked genes are located on the X chromosome (rarely on Y)
Males (XY) have one copy of X-linked genes, so are more likely to express recessive traits
For example,
- XᴬY = unaffected
- XᵃY = affected
Females (XX) have two copies, so can be unaffected, carriers, or affected
For example,
- XᴬXᴬ = unaffected
- XᴬXᵃ = carrier
- XᵃXᵃ = affected
Males cannot be carriers and do not pass X-linked traits to sons, as they only pass on the Y chromosome

Haemophilia is a sex-linked disorder caused by a gene on the X chromosome
The gene codes for Factor VIII, a protein essential for blood clotting
- F = dominant allele → produces normal Factor VIII
- f = recessive allele → results in no Factor VIII, leading to haemophilia
Males only need one copy of f (XᶠY) to be affected, as they have only one X chromosome
Females must inherit two copies (XᶠXᶠ) to be affected; heterozygous females (XᶠX^F) are carriers
Two parents with normal clotting (e.g. X^FXᶠ mother and X^FY father) can produce a child 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

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

Tips for drawing Punnet squares

Always show your working:
- Include the possible gametes produced by each parent
- Do not skip straight to the Punnett square
State both the genotype and phenotype of offspring
For sex-linked inheritance questions:
- Read carefully to see if the question refers to all children or a specific sex (e.g. only boys or girls)