Genetics
A study of inheritance
Gregor Mendel
• Father of modern
genetics
• Conducted
research with pea
plants
• Developed ideas of
dominance and
trait segregation
Mendelian Genetics
Mendel studied a number of characteristics in pea plants
including:
•Height - short or TALL
•Seed color - green or YELLOW
•Seed shape - wrinkled or ROUND
•Seed coat color - white or GRAY
•Pod shape - constricted or SMOOTH
•Pod color - yellow or GREEN
•Flower position - terminal or AXIAL
Seed Coat
Color
Pod
Shape
Pod
Color
Smooth
Green
Seed
Shape
Seed
Color
Round
Yellow
Gray
Wrinkled
Green
White
Constricted
Round
Yellow
Gray
Smooth
Flower
Position
Plant
Height
Axial
Tall
Yellow
Terminal
Short
Green
Axial
Tall
Some vocabulary:
1. Generations:
• P or P1 = Parent generation (1st in a cross)
• F1 = First filial (offspring of P1)
• F2 = Second filial (offspring of F1)
2. Phenotype = description of physical appearance
of an organism (which trait is expressed)
3. Genotype = Actual genetic makeup of an
organism (expressed in letters,ex. “B”= Brown, “b”
= blue”). BB, Bb, or bb are possible genotypes.
4. Gene = segment of DNA on a particular
chromosome that controls a trait
More vocabulary:
5. Gametes = sex cells (egg or sperm)
6. Allele = form of a gene (B or b)
7. Homozygous = two of the same alleles for a trait
(BB or bb)
8. Heterozygous = two different alleles for a trait
(Bb)
9. Locus = the location of a gene on a chromosome
Phenotype
• Phenotype
– Physical
characteristics
Phenotype
Notice the similarities:
–
–
–
–
–
–
Facial structure
Eyes
Smile
Ears
Nose
Neck
Predicting Inheritance
To determine the chances of inheriting a given
Punnett
trait, scientists use _________
squares and
symbols to represent the alleles given to children
by their parents.
UPPERCASE letters
DOMINANT alleles.
__________
letters
are
recessive
___________
alleles.
lowercase
are
used
used
to
to
represent
represent
Predicting Inheritance
For example:
T = represents the allele for TALL in pea plants
t = represents the allele for short in pea plants
So:
TT & Tt both result in a TALL plant, because T is
dominant over t. t is recessive.
tt will result in a short plant.
Remember there are TWO
two alleles for every trait!
Mendel’s Principle of Dominance
alleles
Some ____________
are dominant and others are
recessive. The phenotype (_____)
traits of a dominant allele
will be seen when it is paired with a recessive allele.
TT & Tt both result in a TALL plant, because T is
dominant over t. t is recessive.
tt will result in a short plant.
Remember there are two alleles for every trait!
Predicting Inheritance
Let’s cross a totally dominant tall plant (TT) with a
short plant (tt).
Each plant will give only one of its’ two alleles to
the offspring or F1 generation.
This is referred to as Mendel’s Law of
Segregation.
TT x tt
T
T
t
t
Punnett Squares
The alleles from one parent
go here.
The alleles from the other
parent go here.
Punnett Squares
TT x tt
T
t
t
T
Punnett Squares
t
t
T
T
Tt
Tt
Tt
Tt
F1 generation
Interpreting the Results
The genotype for all the offspring is ____.
Tt
The genotype ratio is:
4 Tt or All Tt
The phenotype for all the offspring is Tall.
The phenotype ratio is:
All Tall or 4 Tall
Punnett Squares
T
t
??
??
Your
Turn!!
T
??
??
t
Punnett Squares
T
t
T
t
TT
Tt
Tt
tt
F1 generation
Punnett Squares
This is a monohybrid T
cross. We worked
with
t
only one trait. The height of the plant.
T
TT
Tt
t
Tt
tt
Punnett Squares
This is a monohybrid T
cross. We worked
with
t
only one trait. The height of the plant.
two
T
TT
Tt
Later we will work with a dihybrid cross, using
the height of the plant and either seed color
or seed shape.
t
Tt
tt
3) Principle of Independent Assortment:
• “Factors” aka: genes for different traits are
distributed to reproductive cells (gametes)
independently of each other
(for example, plant height is not dependent on
seed color)
• Mendel proved this by crossing pea plants with
different characteristics to see if one trait
controlled another trait. They didn’t (but it turned
out that all of “his” traits were found on different
chromosomes).