MGSLG. (2020). Module 1 Unit 5

Dihybrid crosses and how they are solved

  • A dihybrid cross involves the inheritance of two characteristics
  • According to the Law of Independent Assortment, alleles of a gene for one characteristic segregate independently of the alleles of a gene for another characteristic. The alleles for the two genes will therefore come together randomly during gamete formation
  • This means that the two characteristics are transmitted to the offspring independently of one another
  • The above law only applies if the genes for the two characteristics are not on the same chromosome.


Steps you should follow in working out a dihybrid cross:

Example: In hamsters, the allele for black coat colour (B) is dominant over the allele for white coat colour (b). The allele for rough coat (R) is dominant over the allele for smooth coat (r).  If you cross a hamster that is heterozygous black and homozygous rough, with one that is heterozygous black and heterozygous rough, what will be the phenotypes and genotypes of the offspring?

STEP

What to do generally

What to do in this problem

Step 1

Identify the phenotypes of the two hamsters for each of the two characteristics.

According to the statement of the problem, both parents are black and have rough coats.

Step 2

Choose letters to represent the alleles for the gene responsible for each characteristic.

Use the letters, e.g. B for black, b for white, R for rough, and r for smooth as provided in the question.

Step 3

Write the genotypes of each parent.

According to the statement of the problem, both parents are heterozygous black, while the one is homozygous rough and the other one heterozygous rough for coat texture. Their genotype will therefore be BbRR and BbRr  

Step 4

  • Determine the possible gametes that each parent can produce.
  • Remember that each parent will have two alleles for each gene.
  • The gametes of each parent will have only one allele for each gene because of segregation during meiosis.
  • Remember that because of the principle of independent assortment an allele for one gene could appear in the same gamete with any of the alleles for the other gene.

 

  • The genotype of the parents are: BbRR and BbRr

 

  • If we represent the alleles for each gene in the following format, then we can see how these alleles could come together randomly (principle of independent assortment) to form the different types of gametes:

 

BbRR:                    BbRr

Alleles

B

b

 

Alleles

B

b

R

BR

bR

 

R

BR

bR

R

BR

bR

 

r

Br

br

Step 5

Enter the possible gametes at the top and side of a Punnett square.

Please refer to the solution that follows.

Step 6

  • Because of random fertilisation, gametes from both parents could fuse in different combinations to form the offspring.
  • In the punnet square, write down the genotypes of the offspring that will result from each possible combination of gametes

Please refer to the solution that follows.

Step 7

Determine the phenotypes of the offspring from the genotypes obtained in the punnet square.

Please refer to the solution that follows.

Solution to the problem:

Step 1: P1  Phenotype: Black,Rough x Black, Rough
Step 2 - 3: Genotype: BbRR  x BbRr

Meiosis and Fertilisation : Steps 4 - 6

Step 7: 

F1  Genotype: 6 different genotypes, as in the table above
Phenotype: 12 Black, rough; 4 White, rough







Last modified: Wednesday, 17 March 2021, 1:19 PM