Chapter 3 Genetics: The science of

heredityMiss Luzma Fabre

Introduction

Toss a coin and describe the probability

2 possibilities: heads or tails

Each of these events is equally likely to occur (50%)

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%

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

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

How to make a Punnett square

Male parent´s alleles

Female parent´s alleles

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

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

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

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

Describe the genotypes given as:

Homozygous recessive, homozygous dominant or heterozygous

Dd

dd

DD

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?

Genotype of male parent: Bb

Phenotype: brown eyes

Genotype of children: Bb and bb

Phenotype of children: brown and blue

If they have children: BbbBbbbBbbbb

What are the phenotypes of the following genotypes? In other words, what eye color will they have?BBBbbb

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.

Codominance