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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