2. The Blending Hypothesis
A Trait is a variation of a particular characteristic
such as tall and short
In the early 1800’s many biologists believed in the
Blending Hypothesis
They believed offspring were a blend of their parents
traits
BLACK SQUIRRELL + WHITE SQIRRELL = GREY
SQUIRRELL!!
3. Thomas Hunt
Morgan. “Father
of Modern
Genetics”
Gregor Mendel
“Father of
Genetics”
4. The Father of Genetics- Gregor Mendel
Gregor Mendel (1822-1884)
Austrian Monk
Pea Plant Experiments
Mendel studied the inheritance of
traits (factors) from one
generation to the next
These factors are now called genes
Genes are sections of DNA that
code for a specific trait or a
characteristic
5. Mendel’s Breeding Experiments
Mendel bred plants for years to ensure that they
were true-breeding
True-breeding means that offspring always had the
same trait as the parent
Mendel Cross-fertilized all his pea plants by hand to
control which traits he wanted to test
6. Mendel’s Breeding Experiments
Mendel expected a blending of the traits
IE: Tall pea x Short pea resulting in Medium height plants
What actually occurred:
7. Some Mendelian Conclusions:
Mendel and Alleles:
Mendel developed the hypothesis that
genes came in different forms that he
called Alleles
Tall vs. Short
Yellow vs. Green
We use letters to represent alleles
Tall = T
Short = t
8. Some Mendelian Conclusions:
Law of segregation
When gametes are made the
alleles for a trait separate,
so each gamete carries only
one allele for each trait
Law of dominance:
“of the two factors
controlling a trait, one may
dominate the other”
9. Punnett Squares
Punnett Squares are tools used to determine
offspring probabilities from a genetic cross
10. Punnett Squares
Genotype – the Allele combination of an individual
(like their I.D. number)
Phenotype – the physical characteristic that the
individual shows (tall, short, eye color, etc…)
11. Sample Problem
Let’s say you are Mendel. You take a pure-
breeding pea plant that is tall (TT) and you
combine it with a pure-breeding pea plant that is
short (tt). What would their possible offspring
be?
What would you do first?
T
T
t t
Tt
Tt
Tt
Tt
12. Let’s look a little more closely at the plants
Homozygous – An individual that has two copies of the same
allele
Which of the plants in the
sample are homozygous?
Heterozygous – An individual
that has two different copies
of an allele
Which of the plants in the
sample are heterozygous?
Tt Tt
Tt Tt
T
T
t t
Sample Problem
13. The offspring would look
something like this
Why are all the offspring
tall?
Dominant – an allele that
“masked” or covers up another
allele
Recessive – an allele that does
not show up in a heterozygous
individual, it is “masked”
Sample Problem
14. Dominant Genes
If an individual is Homozygous
Dominant the dominant gene
will be seen
Example: TT = tall
If an individual is Heterozygous
the dominant gene will be seen
Example: Tt = tall
If an individual is Homozygous
Recessive the recessive gene
will be seen
Example: tt = short
15. Some More Sample Problems As a Class:
Brown fur = B ; Grey fur = b
What is the genotype of a Heterozygous
individual?
What is the genotype of a Homozygous dominant
individual?
What phenotype would someone who is bb be
(haha)?
Complete a Punnett square for Bb x BB
Bb
BB
Grey fur
B B
B
b
16. Key Terms
Genotype – Allele combination of an individual, two letters
Phenotype – Physical characteristics based on genotype (tall,
brown hair, blue eyes, etc…)
Dominant – Covers up the other allele, will always be seen in
the phenotype (UPPER CASE)
Recessive – Gets covered by the dominant, only seen if the
individual is homozygous recessive (lower case)
Homozygous – Individual has two of the same allele (TT, bb,
GG, hh)
Heterozygous – Individual has two different alleles (Tt, Bb, Gg,
Hh)
