1. Chapter 3
Genetics: The science of
heredity
Miss Luzma Fabre

2. Introduction
Toss a coin and describe the probability
2 possibilities: heads or tails
Each of these events is equally likely to
occur (50%)

3. Section 2: Probability and
heredity
Probability is a number that describes
how likely it is that an event will occur
It can be described as a fraction or
percentage
Example: ½ or 50%

4. Independence of events
Suppose you toss a coin 5 times and it
lands with heads up each time. What is
the probability that it will land with
heads up on the next toss?
1 in 2, or 50%
Each event occurs independently

5. Probability and genetics
Mendel recognized that the principles of
probability can be used to predict the
results of genetic crosses
A Punnett square is a chart that shows
all the possible combinations of alleles
that can result from a genetic cross

6. How to make a Punnett
square
Male parent´s alleles
Female parent´s alleles

7. Using a Punnett square
You can use a Punnett square to
calculate the probability that offspring
with a certain combination of alleles will
result
In a genetic cross, the allele that
each parent will pass on to its
offspring is based on probability

8. What tool can be used to predict the
results of a cross?

9. Phenotypes and genotypes
An organism´s phenotype is its
physical appearance, or visible traits.
Ex: blue eyes
An organism´s genotype is its
genetic makeup, or allele
combinations. Ex: bb

10. An organism that has 2 identical
alleles for a trait is said to be
homozygous. Ex: SS, ss
An organism that has 2 different
alleles for a trait is heterozygous
(hybrid). Ex: Ss

11. Describe the genotypes given as:
Homozygous recessive, homozygous
dominant or heterozygous
Dd
dd
DD

12. In humans, brown eyes (B) are dominant over
blue (b)*. A brown-eyed man marries a blue-
eyed woman and they have three children,
two of whom are brown-eyed and one of
whom is blue-eyed. Draw the Punnett square
that illustrates this marriage.
What is the man’s genotype? What are the
genotypes of the children?

13. Bb
bb
b
Bb
Genotype of male parent: Bb
Phenotype: brown eyes
Genotype of children: Bb and bb
Phenotype of children: brown and blue

14. What are the phenotypes of the
following genotypes? In other words,
what eye color will they have?
BB
Bb
bb

15. Codominance
For some alleles, an inheritance pattern
called codominance exists
In codominance, the alleles are
neither dominant nor recessive. As a
result, both alleles are expressed in
the offspring.

16. Codominance