Topic 7—A: Genetics- 3. Genetic diagrams- multiple allele and dihybrid cross Flashcards
What do multiple allele crosses still only involve?
- one gene
- its just the gene can have more than 2 alleles
When is inheritance more complicated?
- when there are more than two alleles of the same gene (multiple alleles)
How many alleles are in the ABO blood group?
3 alleles
What are the 3 alleles for blood type?
- IO is the allele for blood group O
- IA is the allele for blood group A
- IB is the allele for blood group B
What is IA dominant to?
IO
What is IB dominant to?
IO
What is IA codominant to?
IB
What blood group will people with genotype IA IB have ?
Blood group AB
Example of multiple allele crosses
Parents genotypes- IA IO X IB IO
Gametes alleles- IA IO IB IO
Possible genotypes of offspring- IAIB, IAIO, IBIO, IOIO
Phenotypes of offspring- Group AB, Group A, group B, Group o
Expected phenotypic ratio is 1:1:1:1
Definition of dihybrid inheritance
- The inheritance of two characteristics which are controlled by different genes
- each of the two genes will have different alleles
What can dihybrid crosses be used to show?
The likelihood of offspring inheriting certain combinations of the two characteristics from particular parents
Example of dihybrid cross
A dihybrid cross showing how seed shape and colour are inherited in pea plants:
- Gene for seed shape has 2 alleles
- Allele for round seeds- R (dominant)
- Allele for wrinkled seeds- r (recessive)
- Gene for seed colour has 2 alleles
- Allele for a yellow seed- Y (dominant)
- Allele for a green seed- y (recessive)
Genetic diagram shows a cross between two heterozygous parents- both have round and yellow seeds (RrYy)
Parents genotypes: RrYy x RrYy
Gametes alleles: RY, Ry, rY, ry, RY, Ry, rY, ry
RY. Ry. rY. ry RY RRYY RRYy RrYY RrYy Ry RRYy RRyy RrYy Rryy rY RrYY RrYy rrYY rrYy ry RrYy Rryy rrYy rryy
Round and yellow seeds- 9
Round and green seeds- 3
Wrinkled and yellow seeds- 3
Wrinkled and green seeds-1
Phenotypic ratio- 9:3:3:1
When you do a dihybrid cross with two heterozygous parents what phenotypic ratio do you get?
9:3:3:1
What can you also do dihybrid crosses involving?
Codominant alleles
- they work in exactly the same way as the example above but the phenotypic ratios produced are quite different (there are more than 4 possible phenotypes in the offspring)
What could be the reason why neither of the genes involved in the dihybrid cross is codominant and haven’t got the expected 9:3:3:1 phenotypic ratio in the offspring of two heterozygous parents?
- This could be because of linkage or epistasis
