Lesson 5 of 9 / Inheritance and phenotype
More alleles in the population, two in a person
How do multiple alleles fit a two-gene diagram?
I can solve a dihybrid problem involving multiple alleles and codominance.
I can use locus, allele, dominance, genotype, phenotype, homo-/heterozygosity and sex-linkage terms.Syllabus
Syllabus 8876, Core 2(l). Use locus, allele, dominance, genotype, phenotype, homo-/heterozygosity and sex-linkage terms.
I can solve dihybrid crosses including codominance, incomplete dominance, multiple alleles and sex linkage.Syllabus
Syllabus 8876, Core 2(o). Solve dihybrid crosses including codominance, incomplete dominance, multiple alleles and sex linkage.
About 6 min
Make a guess. You are not marked.
The ABO gene has three alleles. How many does a person with blood group AB carry?
Show the answer
Two: IA and IB.
A diploid person carries two alleles at a locus. The third exists in the population.
The key idea
A population can have more than two alleles at a locus, but an ordinary diploid individual has two. Solve that locus before combining it with a second independent locus.
Explore the idea
Four ABO groups, two marker classes
IAiMm x IBimm
IA and IB are codominant, both dominant to i. M/m is an invented independent marker, not a clinical trait. Row and column labels are gametes, not parent genotypes.
| Gamete | IBm | im |
|---|---|---|
| IAM | IAIBMm | IAiMm |
| IAm | IAIBmm | IAimm |
| iM | IBiMm | iiMm |
| im | IBimm | iimm |
Explanation
The ABO blood-group model has three alleles: IA, IB and i. IA and IB are codominant with each other, and each is dominant to i. IAIA or IAi gives group A; IBIB or IBi gives B; IAIB gives AB; ii gives O.
Multiple alleles describes the range in the population, not three copies in one ordinary diploid individual. For IAi x IBi, the ABO offspring genotypes are IAIB, IAi, IBi and ii, each expected at 1/4.
Now add an invented, independently assorting marker locus: M is dominant to m. In the exercise IAiMm x IBimm, the first parent makes IAM, IAm, iM and im gametes; the second makes IBm and im. A four-by-two genetic diagram has eight equally likely combinations.
The marker cross Mm x mm gives 1/2 M_ and 1/2 mm. Thus AB with mm has probability 1/4 x 1/2 = 1/8, as does each specified ABO group with one marker class. The marker is a teaching model, not a real disease or clinical interpretation of blood group.
Step by step
- 1
Define all alleles
State codominance and recessiveness at the multiple-allele locus.
- 2
List complete gametes
Include one ABO allele and one marker allele in every gamete.
- 3
Combine the loci
Use independence to calculate the required joint probability.
Worked example
Blood group O with a dominant marker
For IAiMm x IBimm under the stated model, calculate P(group O and M_).
One way to explain it
Group O requires ii, with probability 1/4. M_ occurs with probability 1/2 from Mm x mm. The joint probability is 1/8.
Why this answer works
- Use the genotype required for O.
- Apply the second-locus cross, not an assumed 3:1 ratio.
- Multiply under the stated independence.
Watch out for this
A student says: "An AB person carries IA, IB and i because ABO has three alleles." What is wrong with this?
Show the answer
An AB individual carries IA and IB. The third allele exists in the population but need not be present in that person.
Check your understanding
Which is a possible gamete from IAiMm?
Show the answer
IAm.
It contains one ABO allele and one marker allele.
Quick recall
How do multiple alleles fit a two-gene diagram?