Genetic Crosses



Instructions for Calculating the Percent Chances in Genetics


Step 1 - Choose a letter.

Choose a letter for the characteristic you are calculating. Be careful, because characteristic means "hair color" or "blood type" and not "brown hair" or  "type A blood".


Color of flowers:

R = red
r = white


The capital (or big) letter represents the dominant gene and the lower case (or small) letter represents the recessive gene.  It is important that you use the same letter for one characteristic.  In other words, use R and r, not R and w.  A and a, not A and b.

Be careful to chose a letter which looks very different when it is written as a big or small letter.  Do not use letter such as Cc, Oo, Pp, Ss, Uu, Vv or Ww.

Step 2 - Write the genotypes.

In other words, write the genetic information about the parents concerning their alleles for this particular characteristic.  Remember that an allele is one form of a gene (examples are given in the book).  A red flower could be RR or Rr.  For this first example, we will choose RR.  If white is recessive than a white flower must be rr.  If you do not know why, you need to go back, re-read and re-learn what "dominant" and "recessive" mean.

This flower could be RR or Rr.

In our example, let's suppose that the red flower is the one which will give the male reproductive cells in the pollen.  The white flower will hold the ovule which will be fertilized.  The genotype of the flower which made the pollen is RR.  The genotype of the flower with the ovule to be fertilized is rr.

This flower can only be rr.  (If it was Rr, it would be red because red is dominant in this species of flower.)

When talking about genotypes, we use certain technical terms:  heterozygous or homozygous.  For example, RR is "homozygous dominant" and rr is "homozygous recessive".  The term 'homo' means "same". We have to specify if the genotype is dominant or recessive since just saying "homozygous" only says that they are the same, not what they are.  Since 'hetero' means "different", we use the term "heterozygous" to describe the genotype Rr.  When a genotype is heterozygous, the terms dominant and recessive have no meaning since each allele is represented.

Step 3 - Prepare the gametes.

We need to separate the genes in each pair to prepare for the next generation.  In other words, we need to make the gametes - the sperm and the egg cells.  To remember what happens in meiosis to make the reproductive cells, here is a table and a diagram:


  Parent Cell Gametes
Male RR 4 sperms with only one "R" in each
Female rr 4 eggs with only one "r" in each.





Remember, this only shows one gene.  In fact what happens is that each gene pair (and there are 10's of thousands of them) are split up.  Remember that each chromosome carries thousands of genes.

Step 4 - Prepare a cross diagram.

Draw a table as shown below with three rows and three columns and place the letters from the parent's gametes on the top and on the side:

r r


The rest is as easy as 1, 2, 3.  Simply copy each letter vertically and horizontally to fill in each box.


r r
R Rr  


r r
R Rr Rr


r r
R Rr Rr
R Rr Rr

Note that the big letter is always written first.

Here is a little movie of another possible cross: Rr x Rr:


Step 5 - Calculate the percent chance.

What do the boxes tell us?  Look back at the results obtained in step 3 above.  (not the one in the movie just above)  Each box represents a 25% chance of the offspring having the genotype listed inside the box.  Since there are 4 boxes, and since all of them contain Rr, then this couple of flowers has a 100% chance of having offspring with a genotype of Rr.  Since R (for red flowers) is dominant, all of the offspring from this cross will be red.  It is impossible to have any white flowers.

Chance of having red flowers in the offspring 4 out of 4 squares = 100%
Chance of having white flowers 0 out of 4 squares = 0%




Another example:

If the red flower was Rr instead of RR, what would the chances be for having white flowers?

Here, the cross is Rr x rr:

r r
R Rr Rr
r rr rr


Chance of having red flowers in the offspring 2 out of 4 squares = 50%
Chance of having white flowers 2 out of 4 squares = 50%

Before you try the questions for homework, make sure you understand how this works.  If you have any questions, contact me.

Questions for Homework >>