How to Set Up &Solve Genetic Problems


Step 1.  Read the problem.

Gregor Mendel carried out experiments with pea plants.  He determined that in peas having yellow seeds (Y) is dominant to having green seeds (y).  If Mendel crossed a homozygous yellow pea plant with a green pea plant, what will be the genotypic and phenotypic ratios for the F1 and F2 generations.

Step 2: Figure out the genotypes of the parents. (P generation) Since the yellow pea plant is homozygous its genotype must be YY.  The green plant can only be yy if it is to be green since green is recessive.     P                         YY          x            yy

 

Step 3: Figure out what kinds of sex cells the parents can produce. Remember that sex cells have a haploid number and will only put 1 allele from this pair into a sex cell.  To keep the sex cells straight in your mind it is a good idea to circle them.  

 

 

 

Step 4: Set up a Punnett Square for your mating.  This allows you to look at possible combinations of sex cells for the offspring.  The little boxes on the inside of the table are the kinds of offspring this pair can produce.  Each of the boxes inside should represent an equal proportion of potential offspring.
Dad                Mom y y
Y Yy Yy
Y Yy Yy

We can see from our table that in the F1

all are Yy (genotypic ratio)

all are Yellow (phenotypic ratio)

Step 5: Take 2 of the F1 and cross them.    F1                   Yy          x            Yy
Step 6: Figure out what kinds of sex cells the F1 can produce.

 

 

 

 

 

Step 7: Set up a Punnett Square for your mating.
  Y y
Y YY Yy
y Yy yy
Step 8: Write down the ratios for the F2.

F2     1YY : 2Yy: 1yy (genotypic ratio)

3 Yellow: 1 green (phenotypic ratio)

Following the above procedure will help insure that fewer careless errors are made.

Not all problems begin with a pure parental line.  In these cases you will not be asked to determine the F1 and F2, but just the children from the cross.

In pea plants, tall (T) plants are dominant to short (t) plants.  Cross a heterozygous tall pea plant with a short pea plant.  What are the expected genotype and phenotype ratios for their offspring?

                                                          

  t t
T Tt Tt
t tt tt

The offspring can be expected in a ratio of

2 Tt : 2 tt        or       1 Tt : 1 tt  (genotypic ratio)

2 Tall to 2 short     or     1 Tall to 1 Short (phenotypic ratio)