Punnett Square Review

Today in class we learned about Gregor Mendel and his manipulation of genetic traits using pea plants.  These traits are called alleles and occur in sets of two; receiving one from mom and one from dad.  A plant is homozygous when it receives the same alleles for a particular trait from each parent.  This is also called a purebred. A heterozygous plant receives a different allele from each parent for a single particular trait.  Alleles are considered dominant or recessive based upon the physical appearance of the plant (phenotype) as well as what their genes dictate (genotype).

  

 A dominant allele will always show up in the phenotype regardless if the plant is homozygous or heterozygous.  The dominant allele is represented by using an uppercase letter ( i.e T= tall, Y= yellow, R=round). The recessive allele will only alter the plant’s appearance if the plant receives the recessive trait from both parents.  For recessive alleles a lowercase letter of the dominant feature is used (i.e t=short, y=green, r=wrinkled).  We can assume these traits are dominant and recessive based Gregor Mendel’s study of selective cross breeding.  By breeding his pea plants for particular traits Gregor Mendel was able to discover that traits of offspring are not purely a mix or an intermediate of the parental traits. 

In order to predict the offspring of a particular mating cross, us scientists like to use what is called a Punnett Square and it looks like this:

This example is a cross of two heterozygous yellow pea plants.  These alleles are representing the color of the pea plant pods; green or yellow.  We know that the allele for yellow pea pods is dominant over the allele for green pea pods, therefore, YY and Yy genotypes indicate a yellow phenotype and yy indicates a green phenotype.

For your review answer questions 1 and 2 above in addition to these practice Punnett Square crosses.

1.Does genotype determine phenotype? Why?

2. Sometimes the traits show up in the phenotype of offspring that did not occur in the phenotype of either parent! How can this happen?

3. Heterozygous yellow x Homozygous green

4. Homozygous yellow x Homozygous green

5. Heterozygous yellow x Homozygous yellow