Genetics: The Study of Heredity
I.
Review
A.
B.
C.
D.
E.
F.
Why do you look like your parents?
They gave you your DNA
How is this information transferred?
Through sexual reproduction
(gametes and fertilization)
What do we call a segment of DNA that codes
for a protein?
Gene
Where are genes located?
On chromosomes
How many chromosomes do humans have?
46 (23 from Mom, 23 from Dad)
How many genes are on human
chromosomes?
Estimated at about 20,000 - 25,000 genes in
the human genome, containing 3 million
II.
General Info
A.
Genetics is the study of heredity
1. Heredity – passing of traits from parent
to offspring (baby)
2. Another word for “trait” is
characteristic. A trait is something that
can be scientifically observed, such as:
 What determines our traits?
Our genes!
Another genetic trait we can
investigate is called
hitchhikers thumb:
B.
Gene - a segment of DNA that codes for a
protein. It controls an organism’s form and
function.
1. Each gene can control one or more trait (ex.
human eye color is controlled by at least 5
different genes!)
2. Human DNA is made of ~ 20,000 - 25,000
genes
3. A gene can have different forms or physical
expressions of a trait known as an allele
(ex. You have a gene for little finger type with
two alleles: bent and straight)
C. Organisms get half of their genes from each
parent when sperm and egg meet
Our genes
influence how
we look
And in many
cases genes
influence how
we act
III.
Gregor Mendel: “Father of Modern Genetics”
(1824-1884)
A. Mendel’s Life
1. Austrian monk born in current Czech Republic
2. Joined a monastery, then studied math and
science at the U. of Vienna
3. As a monk he taught high school and cared
for the monastery garden, which is where he
developed his ideas on genetics
4. Significance of his work was not recognized
until the early 1900’s
B. Mendel’s Experiment
1.
He cross-bred pea plants with different
characteristics, then studied 7 plant traits
(seed/pea shape, color, height, etc.) of the
hybrid offspring
Seed
Shape
Seed
Color
Round
Yellow
Seed Coat
Color
Gray
Pod
Shape
Pod
Color
Smooth
Green
Flower
Position
Plant
Height
Axial
Tall
Short
Wrinkled
Green
White
Constricted
Yellow
Terminal
Round
Yellow
Gray
Smooth
Green
Axial
Tall
2.
All of the first generation offspring (F1) had
the characteristics of only one of the parent
plants (P)
P Generation
Tall

Short
F1 Generation
Tall
Tall
F2 Generation
Tall
Tall
Tall
Short
DRAW THIS
Why did the “short” trait appear in the F2
generation but not in the F1 generation?
Although
Mendel did
most of his
genetics work
on pea plants,
his ideas about
genetics also
apply to
humans…
IV.
Mendel’s Basic Principles of Genetics
A. Every trait is determined by a pair of “units”
B. Each parent provides one unit (allele)
C. Each gene has forms called an allele.
1. If an organism has 2 of the same alleles for
a trait, it is purebred.
2. If the organism has different alleles for a
trait, it is hybrid
3. Example in pea plants: Height
a. T = tall allele
t = short allele
b. Purebred plant is TT or tt
c. Hybrid plant is Tt
D.
Some alleles can cover-up the
expression of the other
a. Dominant allele always show
b. Recessive allele is hidden when it is
with a dominant allele
E.
In the formation of sperm and eggs (gametes),
one member of each allele separates into
different gametes. This is called segregation
F.
How the alleles segregate during gamete
formation is determined by chance. This is
called Independent Assortment
V.
Important Terms
A. Genotypes are the letters used to
describe the two alleles of a gene (ex. TT,
Tt, tt)
1.
Homozygous genotype means both
alleles are the same (purebred) therefore
we use the same letters
a. Dominant alleles are represented by
Capital letters (ex. a plant that has two
dominant alleles is homozygous dominant
= TT)
b. Recessive alleles are represented by the
same letter but lower case (ex. a plant that
has two recessive alleles is homozygous
recessive = tt)
2.
Heterozygous genotype (hybrid) means
B.
Phenotype
1. The physical expression of the genotype
(Ex. Eye, hair, skin color. Flower color,
shape and location)
VI. Probability and Punnett Squares
A.
Genetics & Probability
1. Probability is the likelihood that a
particular event will occur (e.g. coin toss)
2. Can be used to predict the outcomes of
genetic crosses
3. Probability does not predict precise
outcomes
4. The larger the number of events, the more
accurate the probability
5. In genetics, the larger the number of
offspring, the closer the resulting offspring
will be to what was predicted
B.
Punnett squares
1. Punnett square is a diagram used to
determine the gene combinations that might
result from a cross
2. Used to predict genetic variations that will
result from a cross
Monohybrid Cross
(cross for only one trait)
Draw