Mendel's second law

You are also going to need to know about Mendel's second law....so here is a heads up:

Mendel's Second Law: The Law of Independent Assortment

This law is about the inheritance of more than one trait i.e. more than one set of gene pairs/alleles...

During gamete formation, 

alleles for one trait will separate into the gametes 

independently of alleles for another trait.

In other words, the way one pair of genes segregates into the gametes is not affected by the way a different pair of genes segregates into the gametes. This means that the inheritance of one trait is not dependent on the inheritance of another.

Take a look at this Punnett Square:

image taken from: http://course1.winona.edu/sberg/241f07/Lec-note/Mendel.htm

The Punnett Square above shows the possible genotype and phenotype of pea plants with respect to their pea colour and pod colour. Here is a description of what this Punnett Square shows:

• The pea colour is determined by the Y gene, where big Y is the dominant allele and little y is the recessive allele. The pod colour is determined by the G gene, where big G is the dominant allele and little g is the recessive allele. 
• Both of the plants that are being crossed have genotype GgYy -i.e. they are heterozygous for pod colour and pea colour. Since they have green pods and yellow peas we know that the G dominant allele codes for a green pod and the Y dominant allele codes for a yellow pea.
• Along the top and left side of the square are the four different possible combinations of G/g and Y/y there could be in gametes from each parent plant. So, a parent plant with a genotype GgYy can have four different combinations of these genes in its gametes - GY, Gy, gY and gy.
• The middle of the square shows all the possible combinations of genes you could get as a result of all the different possible gametes coming together.
• The middle of the square also gives a nice pictorial representation of the phenotypes that would arise for all of the different gene combinations. I.e. where-ever there is a GG or Gg you get a green pod, a gg gives a white pod, a YY or Yy gives a yellow pea, a yy gives a green pea.
• If you count the different possible phenotypes up you will notice there is a 9:3:3:1 ratio between them:

Green Pod and Yellow Pea    -->    9 squares have this combination

 Green Pod and Green Pea     -->    3 squares have this combination

White Pod and Yellow Pea     -->    3 squares have this combination

 White Pod and Green Pea     -->    1 square has this combination

• The 9:3:3:1 ratio means that it you performed this cross 160 times, you would expect to get 90 plants with green pods and yellow peas, 30 plants with green pods and green peas, 30 plants with white pods and yellow peas and 10 plants with white pods and green peas.