Punnett Square
• Demonstrates how
alleles can be
combined when the F1
plants are self-fertilized
to produce an F2
generation.
• Shows that 1/4 of the
F2 plants should be
homozygous dominant
(TT), 1/2 heterozygous
(Tt), and the remaining
1/4 to be homozygous
recessive (tt).
Genotype
• The genetic makeup of an individual.
• Genotype can refer to an organism’s
entire genetic makeup or to the alleles
at a particular locus.
The Genotype of
the pea pods here
would be either
“RR” “Rr” “Rr”
or “rr”
Phenotypes
• The observable or
detectable
physical
characteristics of
an organism; the
detectable
expressions of
genotypes,
frequently
influenced by
environment.
Codominance
• The expression of two alleles in
heterozygotes.
• In this situation, neither allele is
dominant or recessive; thus,
both influence the phenotype.
(AB bloodtype)
Exercise…
Microevolution
• Small genetic changes that occur
within a species.
• A human example is the variation seen
in the different ABO blood types.
Principle of
Independent Assortment
The distribution of one
pair of alleles into
gametes does not
influence the
distribution of another
pair.
The genes controlling
different traits are
inherited independently
of one another.
Mendelian Traits
• Characteristics that are
influenced by alleles at only one
genetic locus.
• Examples include many blood
types, such as ABO.
• Many genetic disorders,
including Achondroplasia
(dominant), Huntington disease
(dominant), and Sickle-cell
anemia (recessive) are also
Mendelian traits.
Huntington Disease
• Huntington disease affects about 1 out of
every 100,000 people and is caused by a
dominant mutation on chromosome 4.
• A person with the allele has a 50% chance of
passing it their offspring.
• There is no cure and symptoms most often
occur between ages 35 and 45.
• By this time, most people who want children
have had them and may have passed the
mutant allele on to their offspring.
Huntington’s Disease…
• Suppose one of your parents was
diagnosed with Huntington disease.
• Would you be tested to determine
whether you carried the allele for the
disease?
• If you have the test, you will either be
relieved by the results or know that
you’ll develop a neurological
disease that will ultimately kill you.
Non-Mendellian traits:
Polygenic Inheritance
• Polygenic traits, or
continuous traits, are
governed by alleles at
two or more loci, and
each locus has some
influence on the
phenotype.
• Hair, eye and skin
color are polygenic
traits
Polygenic traits cont….
• Stature, shape of face, fingerprint
patterns are polygenic traits
• Most can be measured, i.e. height
in feet and inches or meters and
centimeters
Mitochondrial Inheritance
All cells contain mitochondria that convert energy
into a form that can be used by the cell.
Each mitochondrion contains copies of a ring-shaped
DNA molecule, or chromosome.
Animals of both sexes inherit their mtDNA, and all
mitochondrial traits, from their mothers.
All the variation in mtDNA is caused by mutation,
which makes it very useful for studying genetic
change over time.
Gene Flow
• The exchange of
genes between
populations.
• If individuals move
temporarily and
mate in the new
population (leaving
a genetic
contribution), they
don’t necessarily
stay there.
• Example: “Sex
Tourism”
Genetic Drift
•
•
•
In each generation, some individuals may, just by chance,
leave behind a few more descendents (and genes, of
course!) than other individuals.
The genes of the next generation will be the genes of the
“lucky” individuals, not necessarily the healthier or “better”
individuals. That, in a nutshell, is genetic drift. It happens to
ALL populations—there’s no avoiding the vagaries of
chance.
Genetic drift affects the genetic makeup of the population
but, unlike natural selection, through an entirely random
process. So although genetic drift is a mechanism of
evolution, it doesn’t work to produce adaptations.
Founder Effect
• Occurs when a small band of
“founders” leaves its parent
group and forms a colony
elsewhere.
• A new population is established
and as long as mates are
chosen within this population, all
the members will be descended
from the founders.
• A once rare allele that was
carried by even one of the
founders can eventually
become common.
Bottleneck
• Cheetahs, like
many other
species, have
passed through
a genetic
bottleneck.
• As a species
they have little
genetic
variation.