DIHYBRID CROSSES

Sometimes more than 1 allele can determine a trait. For example you can follow 2 traits of peas. R = Round r = wrinkled Y = Yellow y = green Round is dominant Yellow is dominant

When you follow two traits that means you will use 2 letters instead of 1 in a punnet square. However, you no longer use the 4 square punnet square. Now you will use a 16 square… For each parent, you have 2 alleles you follow 1.Round or wrinkled 2. Yellow or Green Each Parent will have a total of 4 letters… R or r Y or y RrYy

A few basic rules when figuring out the alleles… 1.Put the letters alphabetically 2. Dominant (big letters go first always) So if each parent has 4 alleles and you cross 4 alleles x 4 alleles = 16 (why the 16 square box is needed)

heterozygous Let’s take a one parent pea plant that is heterozygous for both traits. R = Round r = wrinkled Y = Yellow y = green Rr Yy How do you figure out the possible combinations? Each gamete (sperm or egg) from this pea must give: 1 – “R or r” from this parent 1 – “ Y or y ” from this parent

R r Y y How to find the right combo of alleles. To produce sperm or egg RYRY 2 RyRy 3 rYrY 4 ryry 3 4

Remember, the alleles we just figured out are for just one parent. Every offspring needs two letter “R or r” and two letter “Y or y” So let’s cross it with another heterozygous plant… RrYy x RrYy

So let’s cross two heterozygous RrYy x RrYy It is set up in a 16 square block.

R r Y y Now take your alleles from the first parent and set them on the box this way.. 1 RYRY 2 RyRy 3 rYrY 4 ryry

RYRY RyRy rYrY ryry RYRY RyRy rYrY ryry Remember, every square will have 4 letters plus the rules are in effect!

Next you just fill in the boxes…. RYRY RyRy rYrY ryry RYRY RyRy rYrY ryry RR YY RR Yy Rr YY Rr Yy RR Yy RR yy Rr Yy Rr yy Rr YY Rr Yy rr YY rr Yy Rr Yy Rr yy rr Yy rr yy

RYRY RyRy rYrY ryry RYRY RyRy rYrY ryry RR YY RR Yy Rr YY Rr Yy RR Yy RR yy Rr Yy Rr yy Rr YY Rr Yy rr YY rr Yy Rr Yy Rr yy rr Yy rr yy Phenotype Round Yellow Round Green 9 3 Wrinkled Yellow 3 Wrinkled Green 1 R = Round r = wrinkled Y = Yellow y = Green

Anytime you are working a Dihybrid Punnet square… And you are crossing two hybrids exactly like our example. You will always get a 9:3:3:1 Phenotypic ratio. Now the hard Part. What is the Genotypic ratio? Yes, I want you to do this now…

RRYY = RRYy = RRyy = (1) = 6.25% (2) = 12.5% (1) = 6.25% RrYY = (2) = 12.5% RrYy =(4) = 25% Rryy =(2) = 12.5% rrYY = rrYy = rryy = (1) = 6.25% (2) = 12.5% (1) = 6.25% %

What if you wanted to follow 3 traits at one time? How big would your punnett square be? Each parent has 6 alleles - RrYyXx So: 6 from mom x 6 from dad = 64 squares

This is a sample question from the Old TAKS tests.

This is an example of the questions you will get for the Test. Now I will show you how to solve it without having to do the large punnet square. G g B b Both dominant traits G = Gold B = Banded

Since all they are asking for is both dominant traits, Count how many Capital letters are in cross Which would give him the least amount of Golden banded hampsters?

VOCABULARY ! Test Cross Dihybrid cross 9:3:3:1 ratio