Download Genetics Basics - Peoria Public Schools

When we study genetics, we examine the following
genetic entities (from most complex to least complex)
 Chromosome: A long,
continuous thread of DNA
that consists of numerous
genes and regulatory
information
 Gene: A piece of DNA that
provides a set of instructions
to a cell to make a certain
protein.
 Allele: Variation of a gene for
a particular trait
Remember Mendel’s Findings
 Mendel found that even though two traits can
be present in an organism, some traits
“masked” others.
 A dominant trait is one that “masks” another
and is expressed
 A recessive trait is one can be “masked” be
another trait and is only expressed when a
copy of the trait is present
Genotypes and Phenotypes
Alleles are represented using letters. We usually choose a letter
corresponding to the dominant trait. For example, if we are looking at eye
color, we can compare both green and brown eyes. If brown is the
dominant trait, we will represent the brown allele with an uppercase B. If
green is recessive, we will represent the green allele with a lowercase b.
We call the combination of alleles an organism’s Genotype (the letters).
Using the above example, the here are the possible genotypes for eye color:
BB, Bb, bb
We call an organism’s observable characteristic due to its allele pairs it’s
Phenotype. Using the above example, here are the possible phenotypes
for eye color: Brown eyes, Green eyes.
A closer look at Dominance and
Recessiveness
Each organism has two alleles for a trait. One allele comes from the
organism’s mother and the other allele comes from an organism’s
father. Organisms can be homozygous (have two of the same
alleles), also known as purebred, for a trait or be heterozygous
(have two different alleles) for a trait.
Father  B B  Mother
Example: Brown Eyes (B) and Green eyes (b)
 Homozygous Dominant: BB
 Homozygous Recessive: bb
 Heterozygous: Bb
Lets Practice Allele Notation!
A certain species of parrot is known to have two body color
variations. The dominant variation is red. The recessive variation is
yellow.
 Write all possible genotypes for the parrot. State what phenotype
will go with which genotype.
African elephants are known for their large ears, while Indian
elephants are known for their small ears. The dominant condition in
elephants is small ears and the recessive condition is large ears.
 Write all possible genotypes for the elephant. State what
phenotype will go with which genotype.
Different ways that genes can
affect organisms
Epistasis
 When one gene interferes
Polygenic Traits:
Traits produced by two or more
traits.
with the expression of
another gene.
Examples:
Example:
 Eye color is controlled by 3 genes
 Albinism in mice is caused  Skin color is controlled by 4
genes
by one gene blocking four
other genes
Can the environment affect
genotypes and phenotypes?
Example of Genotype
Sea turtles lay their eggs in the
sand. Eggs that mature in
warmer temperatures develop
into female turtles. Eggs that
mature in cooler temperatures
develop into male turtles.
Example of Phenotype
Identical twin studies have
shown that twins raised in
the same environment are
more similar than those
raised in separate
environments. Also, twins
raised with different
treatments may show
different physical features.