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Why are my eyes green?
She has her mother’s eyes!
She’s a chip of the old block!
Who does he look like?
Mother or Father? Or both?
Genetics
Why are we learning this?
To understand Genetics and
Gregor Mendel’s Contribution to
Heredity.
To understand how traits are
passed from one generation to the
next.
Gregor Mendel
An Austrian monk who worked
with pea plants to develop our
basic understanding of Genetics
Genetics – the scientific study of
how traits are passed down from
parents to offspring (Heredity)
Heredity—The passing of traits
from parents to offspring
Mendel’s Experiments
Mendel studied 7 different traits in
pea plants
Trait – a specific characteristic that
varies from one individual to another
Examples:
Green seeds vs. Yellow seeds
Tall plants vs. Short plants
Mendel crossed plants with
different traits and studied their
offspring
The offspring are called hybrids
Mendel saw that the hybrids had
the character of only one of the
parents. The character of the
other parent disappeared
Mendel’s Experiments cont’d
Mendel’s Experiments cont’d
Mendel then made a cross of
the two hybrids and saw that
the characteristic that
disappeared in the first cross
came back
Mendel’s Experiments cont’d
Results of Mendel’s Experiments
Genes are passed from parent to
offspring
Genes – the genetic information
that controls one trait
There are different forms of a
gene that create specific traits.
These are called alleles
Example: One allele of the gene for
plant height makes it tall, the other
allele makes it short
Describing Genes and Traits
Most organisms have 2 copies
of a gene (1 from mom, 1 from
dad)
Genotype – shows what type of
alleles the 2 genes are
Phenotype – what the organism
looks like (what trait is being
expressed)
Principle of Dominance
Some alleles are dominant,
some are recessive
An organism with a:
Dominant allele will always
exhibit that form of a trait
Recessive – only shows that form
when there is no dominant
form present
Dominance

Represented with Upper and lower case
letters:
 T – dominant trait for height (tall)
 t – recessive trait for height (short)
TT = tall plant
Tt = tall plant
tt = short plant
genotype
Homozygous Dominant
Plant
Heterozygous Plant
Homozygous
Recessive Plant
phenotype
What were those words again?
Homozygous Dominant – 2
dominant alleles (Capital
Letters TT)
Homozygous Recessive – 2
recessive alleles (lower case
letters tt)
Heterozygous – 1 dominant and 1
recessive (one capital and one
lower case Tt)
Results of Mendel’s Experiments
From his observations of the pea plants, Mendel derived
his first theory, the
Theory of Segregation.
There are four parts to the Theory of
Segregation:
1.Genes come in multiple forms,
called alleles.
2.Individuals will inherit two alleles
for a given trait.
3.Gametes, produced through
meiosis, will have one allele per
cell, per trait.
4.When there are two alleles
present, one will be dominant and
one will be recessive.
Theory of Segregation in Action
When a trait is dominant, its effects will mask those of its recessive
partner. In this example, the dominant R allele produces a red
pigment, while the recessive r allele produces no pigment at all.
Theory of Segregation in Action
Therefore, when the plant is
Rr or heterozygous, it will show a
red color, but when it is homozygous
will produce a white color.
recessive (rr), it
If a homozygous dominant and homozygous recessive plant are
crossed, all the first generation (F1) offspring will be
Heterozygous (Rr).
If two of the F1 offspring are crossed, their offspring (second
generation – F2) will be one-quarter Homozygous Dominant
(RR), one-quarter Homozygous recessive (rr), and one-half
Heterozygous (Rr).
Further Results of Mendel’s Experiments
Mendel’s second theory was the Theory of Independent
Assortment.
• Independent Assortment means that the traits in question
aren’t linked; meaning that just because an individual is
dominant for one trait, does not mean that they are
dominant in another trait.
• This is how we can have individuals with brown hair and
blue eyes, while another individual has blonde hair and
brown eyes. The traits are not inherited together, meaning
that the traits are on different chromosomes.
• “Life is like a box of chocolates………….” Forrest Gump
• Chromosomes are separated during meiosis.
Theory of Independent Assortment
Homologous chromosomes are chromosomes that
carry the same genes; one of the chromosomes will
come from the mother, and one will have come from
the father.
During meiosis, the homologous chromosomes will
go separately into each gamete. This is the process of
Independent Assortment.
Since each set of homologous chromosomes can be
arranged into a gamete individually, when dealing with
multiple chromosomes, they can all also assort
independently, leading to a wide variety of potential
Incomplete dominance
Heterozygous phenotype is
intermediate between the
two parental homozygotes.
Incomplete dominance
If you cross true breeding
red and white snapdragons
F1 are all pink flowers
F2 - 1:2:1 red:pink:white
It takes 2 doses of the red allele to get red
RR- red
Rr – pink rr - white
Vocabulary
Heredity—The passing of traits from parents
to offspring
Trait—a characteristic that an organism can
pass on to its offspring through its genes
Genes—factors that control traits
Alleles—different forms of genes
Dominant Allele—is one whose trait always
shows up in the organism when present
Recessive Allele—is one that is masked,
whenever the dominant allele is present
Genotype—the specific set of alleles forming
the genome of an individual
Phenotype—visible characteristics of an
individual
Tools of the Trade –Punnett Square
Parent Genotype AA crossed with Aa