Biology (Holt)

Heads

Heads

1 _

4

Tails

Tails

1 _

4

Heads

1 _

2

Heads^1 _ 2

Tails^1 _ 2

Tails

1 _

2

Heads

Tails

1 _

4

Tails

Heads

1 _

4

The green boxes have the same

combination (heads, tails), so

the probabilities are added

together.

1 _

4 +

1 _

4 =

1 _

2

Probability of each

coin landing on

heads or tails

The possible results of tossing a nickel and a

penny at the same time and the probability of

each outcome are shown in Figure 11.Since the

combination of heads and tails can occur in two

possible ways, those two probabilities are added

together.

^14  ^14  ^24  or^12 

Consider the possible results that can occur

in a cross between two pea plants that are het-

erozygous for seed shape (Rr). The Rallele for

round seed shape is dominant over the rallele

for wrinkled seed shape. The probability of each

parent carrying gametes with Ror ralleles is ^12 .

The probability of offspring with RRalleles is

^12  ^12  ^14 

Similarly, the probability of offspring with rr

alleles is

^12  ^12  ^14 

The combination of Rralleles can occur in two

possible ways. One parent can contribute the R

allele, and the second parent the rallele, or vice

versa. Thus, the probability of offspring with Rr

alleles is

^14  ^14  ^12 

174 CHAPTER 8Mendel and Heredity

Analysis

1. Calculatethe probability of
   homozygous dominant (BB)
   offspring resulting from a
   cross between two heterozy-
   gous (Bb) parents.


2. Calculatethe probability of
   heterozygous offspring result-
   ing from a cross between a

heterozygous parent and a

homozygous recessive (bb)

parent.

3. Calculatethe probability of
   heterozygous offspring result-
   ing from a cross between a
   homozygous dominant parent
   and a homozygous recessive
   parent.
   4. Calculatethe probability of
   homozygous dominant off-
   spring resulting from a cross
   between a heterozygous
   parent and a homozygous
   recessive parent.

Predicting the Results of

Crosses Using Probabilities

Background

In rabbits, the allele Bfor black hair is dominant

over the allele bfor brown hair. You can practice

using probabilities to predict the outcome of genetic

crosses by completing the genetic problems below.

Draw Punnett squares for each problem.

x + 6x - 7 - 0^2 <

8

493

0

5

2

The probability of the results of flipping two coins

is easy to compute.

Figure 11 Probability with two coins

2C 6D