Simple Mendelian
Genetics
History of Genetics
 Gregor Mendel was an Austrian monk
who is essentially the founder of modern
genetics.
 Heredity had been known about for a
very long time, but he was the first
scientist to be able to explain how it
worked.
 Mendel discovered two laws, Law of
Segregation and the Law of Independent
Assortment.
 Law of Segregation tells us that each offspring
has two alleles for a trait. One comes from the
mother and one comes from the father.
 Law of Independent Assortment tells us that
each trait is independent of other traits.
Meaning that traits can be inherited separately.
Genetics
 Allele= Different forms of a gene. Each
gene is represented by a letter.
 Dominant allele= An allele that expresses
its phenotype over other alleles for that
trait. Represented by a capital letter.
 Recessive allele= An allele that
expresses its phenotype only when in a
homozygous situation. Represented by a
lower case letter.
 Homozygous= Two identical alleles for a trait.
 Heterozygous= Two different alleles for a trait.
 Dominant alleles phenotypes are expressed in
homozygous or heterozygous genotypes.
 Recessive alleles phenotypes are expressed
only in homozygous genotypes.
 Phenotype= The observable traits of an
organism.
 Genotype= The complete genetic makeup of
an individual.
Punnett Squares
 Punnett squares are useful tools for
predicting genetic outcomes.
 Look at board…this is an empty Punnett
square.
 The following are rules for using Punnett
Squares:
 1) Write out the parents genotypes.
 2) Draw your Punnett Square.
 3) Place Parent 1’s genotype on the top
of your square. One gene (letter) for
each column.
 4) Place Parent 2’s genotype on the side
of your square. One gene (letter) for
each row.
 5) Fill in the Punnett Square.
 6) Write a phenotypic ratio for the
predicted outcomes.
 7) Write a genotypic ratio for the
predicted outcomes.
 8) Convert ratio’s to percentages if asked
to do so.
 Punnett squares are useful when doctors
are trying to predict the probability that an
offspring may inherit a genetic condition.
EXAMPLE
 In hamsters having long teeth is
dominant to having short teeth. One
parent is heterozygous for long teeth and
the other parent is homozygous for short
teeth. Construct a Punnett square and
calculate the percentage of offspring that
will have long teeth and the percentage
that will have short teeth.