International A Level Biology / Gene expression and genetics
Codominance and pedigree diagrams revision notes
Edexcel International A Level Biology (YBI11), chapter 6: Gene expression and genetics, specification point 2.15. Free lesson.
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Codominance is when both alleles are expressed in a heterozygote, so both characteristics appear. Pedigree diagrams show family relationships and can help you identify whether a condition is dominant or recessive and work out each person’s genotype.
What codominance means
Codominance happens when neither allele masks the other. In a heterozygote, both alleles are expressed in the phenotype because both are transcribed and translated, so both proteins are made. For a definition, write that both alleles are expressed in the phenotype of a heterozygote. Do not write that both alleles are dominant.
How to set out a codominant cross
In Shorthorn cattle, red and white coat alleles are codominant. Use capital C letters with superscripts to show them: Cᴿ for red and Cᵂ for white. Both alleles use capitals because neither is dominant. A red cow or bull has genotype CᴿCᴿ. Every hair is red. A white cow or bull has genotype CᵂCᵂ. Every hair is white. A roan animal has genotype CᴿCᵂ. Both alleles are expressed, so its coat has red and white hairs mixed together. The hairs are not pink.
How to read a pedigree
A pedigree is a family tree for one characteristic. Squares represent males, circles represent females, shaded symbols show affected people, and each person has a number. If two unaffected parents have an affected child, the condition is recessive: the parents can carry the allele without showing the condition. For a recessive condition, use A for the unaffected allele and a for the affected allele. An affected person has genotype aa. An unaffected carrier has genotype Aa. If two affected parents have an unaffected child, the condition is dominant and both parents are heterozygous.
Give each person’s genotype with a reason
In the pedigree, individuals 1 and 2 are unaffected, but their daughter 4 is affected. This makes the condition recessive, so 4 is aa. Both parents must be carriers because each passed an a allele to her. Individuals 1 and 2 are both Aa: each is unaffected but passed an a allele to 4. Individual 6 is Aa: he is unaffected, and his affected mother 4 could only pass him a. Individuals 3 and 5 could each be AA or Aa. Being unaffected does not show whether either carries a. For individual 5, one unaffected child does not settle the genotype. Say that the evidence is uncertain.
Calculate the chance of an affected child
If a test shows that individual 5 is a carrier, the cross with individual 4 has a one-in-two chance of producing an affected child. Individual 4 can only pass on a, while individual 5 can pass on A or a.