One way to calculate the odds that parents with a particular gene in their genotype
will pass it on to their children is to draw a table with the father’s two genes for that genotype on the top and the mother’s two genes on the left.
Father’s Two Genes
F1 F2
Mother’s M1
Two
Genes
M2
The box is filled in very simply. The letters at the top of each column are written into the boxes reading down and the letters at the side are written into the boxes reading across.
Father’s Two Genes
F1 F2
F1M1
F2M1
F1M2
F2M2
Mother’s M1
Two
Genes
M2
As you can see from examining the boxes, a child could inherit any one of four possible combinations from the parents’ two pairs of genes. The chances of each child inheriting one of these combinations is one in four, or 25 percent.
Consider how this works in practice. When both parents have two brown-eye
genes, all the children will have brown eyes as well as two brown-eye genes.
Parents’ genotype: Father: BB; Mother: BB
Parents’ phenotype: brown eyes.
Using B to represent the gene for brown eyes, fill in the table to show the possible
genotypes of the children.
Father’s Two Genes
B B
Mother’s B
Two
Genes
B
What is the children’s genotype?
What is the children’s phenotype?
The same pattern holds true if both parents have two blue-eye genes. Here
we use b to represent the blue-eye gene.
Parents’ genotype: Father: bb; Mother: bb.
Parents’ phenotype: blue eyes.
Using b to represent the gene for blue eyes, fill in the table to show the children’s
genotype.
Father’s Two Genes
b b
Mother’s b
Two
Genes
b
It gets more complicated if one parent—for example, the father—has two
brown-eye genes (BB) and the other, the mother, has two blue-eye genes (bb).
Father’s Two Genes
B B
Mother’s b
Two
Genes
b
What is the children’s genotype?
What is the children’s phenotype?
Now, what if one parent—let’s say the father—has one brown-eye gene and
one blue-eye gene (Bb), while the mother has two blue-eye genes (bb)?
Father’s Two Genes
B b
Mother’s b
Two
Genes
b
What are all the possible genotypes?
What are all thepossible phenotypes?
The following possibility is the most surprising: Two brown-eyed parents can
produce blue-eyed children:
Father’s Two Genes
B b
Mother’s B
Two
Genes
b
What is the probability of a blue-eyed child?
Now, imagine both parents were carriers for a recessive genetic disorder called tay sachs disease (where T is the ‘normal’ gene, and t is the gene for the disorder).
Father’s Two Genes
T t
Mother’s T
Two
Genes
t
What is the probability of having a child with tay sachs?
And finally, what if the father was color-blind, and the mother was a carrier for color-blindness. (Note: color-blindness is a sex-linked recessive genetic disease, it is carried on the X chromosome- the X denotes the ‘diseased’ chromosome).
Father’s Two Genes
X Y
Mother’s X
Two
Genes
X
How many daughters will be color-blind?
How many daughters will be carriers for color-blindness?
How many daughters will be ‘normal’ (meaning they are not color-blind, nor are they carriers)?
How many sons will be color-blind?
How many sons will be carriers for color-blindness?
How many sons will be ‘normal’?
