Chapter 11 Notes

Dihybrid Crosses

 

Mendel’s Principle of Segregation: the two alleles that control a trait separate during the formation of gametes.

·    Half of the gametes receive one of the alleles and the other half of the gametes receive the second allele.

 

Ex. F=Freckles          f= no freckles

 

Parent Genotype

Ff

 

 

F                  f

                                                                                                 50% of gamete cells              50% of gamete cells

       

 

Mendel’s Principle of Independent Assortment: alleles for different traits can separate independently from each other during the formation of gametes.

·    Exceptions would be if the two traits are on the same chromosome.

 

Dihybrid crosses: looks at the inheritance of two traits at the same time.

Ex: In pea plants, round (smooth) peas are dominant to wrinkled peas. AND Yellow peas are dominant to green peas. What would be the phenotypic and genotypic ratios if two plants heterozygous for both traits were crossed?

 

Use the “foil” method to determine the possible gametes!!!!

·      Sometimes called the Charlie Brown method

 

  


 

Parent Genotype: RrYy

 

 

Possible gametes:  RY   Ry   rY    ry

 

Parent Genotype     

RrYy

 

 

 

RY       Ry       rY       ry

                                                                                       25% of gametes   25% of gametes   25% of gametes  25% of gametes

 

Dihybrid Punnett Square:

 

 

RY

Ry

rY

ry

RY

 

RRYY

RRYy

RrYY

RrYy

Ry

 

RRYy

RRyy

RrYy

Rryy

rY

 

RrYY

RrYy

rrYY

rrYy

ry

 

RrYy

Rryy

rrYy

rryy

 

Genotypes, frequencies and phenotypes:

RRYY    1              round, yellow

RRYy           2              round, yellow

RrYY      2              round, yellow

RrYy       4              round, yellow

RRyy      1              round, green

Rryy        2              round, green

rrYY       1              wrinkled, yellow

rrYy        2              wrinkled, yellow

rryy         1              wrinkled, green

 

Phenotypic Ratio:

9 round, yellow: 3 round, green: 3 wrinkled, yellow: 1 wrinkled, green