CK12 Life Science

we described that inherited one factor from theTTtall parent and one factor from thett

short parent.

Probability and Punnett Squares

To visualize the results of a genetic cross, aPunnett squareis helpful. An example of a

Punnett square (Figure6.3) that shows the results of a cross between two purple flowers

that each have one dominant factor and one recessive factor (Bb). Notice how the possible

factors in the sperm (Borb) are lined up the side of the square while the possible factors in

the egg (Borb) are lined up across the top. The possible offspring are represented by the

letters in the boxes, with one factor coming from each parent.

Notice how the Punnett square can help you predict the outcome of the crosses. Only one

of the plants out of the four, or 25% of the plants, has white flowers (bb). The other 75%

have purple flowers (BB,Bb) because the color purple is a dominant trait in pea plants.

Now imagine you cross one of the white flowers (bb) with a purple flower that has both a

dominant and recessive factor (Bb). The only possible gamete in the white flower is the

recessive (b), while the purple flower can have gametes with either dominant (B) or recessive

(b). If you write out the Punnett cross, you will see that 50% of the offspring will be purple

and 50% of the offspring will be white.

Keep in mind that the birth of each offspring is an independent event and has the same

probability, so the traits of a previous offspring do not influence the next offspring. In the

cross discussed above with twoBbflowers, each offspring has a 75% chance of being purple

and a 25% chance of being white. For example, even if the first three offspring in the cross

have purple flowers, it does not mean that the next plant must have white flowers. All

probability tells you is that overtime the averages of many, many offspring will work out to

a predicted ratio.

Table 6.1:The Punnett Square of a white flower (bb) crossed with a purple flower
(Bb)

b b

B Bb bb

b Bb bb

Lesson Summary

• Gregor Mendel is considered the father of genetics, the science of studying inheritance.


• According to Mendel’s law of segregation, an organism has two factors for each trait,
  but each gamete only contains one of these factors.