Genetics
I. Genetics
A. genetics: scientific study of heredity
1. we have known for centuries that traits are
passed from parents to offspring
2. we didn’t know how the traits were
determined
B. recall chromosomes and cell division
1. chromosomes are replicated and distributed
to daughter cells
2. reproduction requires cell division and
chromosome replication
3. we now know that traits are passed from
parents to offspring in these chromosomes
 But the relationship between chromosomes
and traits was not always understood
II. Gregor Mendel (1822 – 1884) “Father of Genetics”
A. Facts:
1. Austrian monk
2. Began his work in the 1860’s
3. Used mathematics in his study of the garden pea
plant
 During Mendel’s time, most people thought that
traits were a result of a blending of the parents’
traits. Mendel showed something different.
I. Mendel’s Work
A. worked for more than 8 years on his pea plant
experiments, using more than 20,000 plants!
1. chose the pea plant for 3 reasons:
a. structure of the pea flower (more later)
b. presence of distinctive traits
c. rapid reproductive cycle
2. these characteristics allowed Mendel to isolate and
control variables, and to produce observable
results that he could duplicate
B. Structure of the Pea Flower
1. easy to self-fertilize (pollen from the anther
fertilizes the pistil of the same plant)
2. through self-fertilization, able to get purebred
plants
a. purebred: any organism that receives the
same genetic traits from both of its parents
example: if you get the gene for attached
earlobes from your mom AND from your
dad, you are purebred for attached earlobes
(both copies of the gene are the same)
3. the pea flower structure was also good for crossfertilization (fertilization using 2 different plants)
a. produced hybrids: organisms that receive
different forms of a genetic trait from each
parent
example: if you get the gene for attached
earlobes from your dad and the gene for free
earlobes from your mom, you are a hybrid
for earlobe shape
4. Mendel studied 7 different traits in the garden pea
plant, all of which were easy to observe
a. each of these traits has only 2 distinct forms
(rather unusual)
examples:
pea pods are either yellow or green
pea plant stems are either tall or short
 never an “inbetween” like medium height
The Experiments
I.
Mendel began by using 2 different groups of purebred
plants  examining pea color
A. he called this generation (the 2 purebred plants) the
parental generation , or P generation
 the P generation was either purebred yellow or
purebred green
1. He crossed these 2 parental plants to produce
offspring, called the F1 generation (“F” stands for
filial, which means offspring)
P
purebred green X
purebred yellow
F1
**2. The F1 generation all had yellow peas
3. Next, Mendel allowed the F1 generation to selffertilize, producing the F2 generation
4. In the F2 , ¾ of the plants had yellow peas and ¼ of
the plants had green peas
B. What had Mendel expected to happen?
1. The blending hypothesis would predict greenishyellow (or yellowish-green) peas
2. Instead, the F1 generation plants all had yellow
peas  no green peas, even though one of the
parents had green peas
 But when the F1 self-fertilized, the green peas
came back in the F2 generation!
The F2 generation had 75% plants with yellow
peas and 25% of plants had green peas
**** Ratio – 3 plants w/ yellow peas : 1 plant w/ green peas
C. Mendel repeated the experiment for the other 6 traits
1. for each trait, he observed the same results in the
F1 generation:
 all of the F1 plants showed only one form of the
trait
ex.
P
purebred tall
X
purebred short
F1
all tall plants
____________________________________________________
P
F1
purebred purple flowered X purebred white flowered
all purple flowered plants
2. Mendel defined each form of the trait as either
dominant or recessive.
 The dominant form appeared in the F1
generation, the recessive form did not appear
in the F1
3. In the F2 generation of his experiments, Mendel
always got 2 types of plants – 75% were the
dominant form, 25% showed the recessive form
*see diagram of Mendel’s experiments through the F2
II. Mendel’s Conclusions
A. Mendel’s experiments proved that the blending
hypothesis was wrong
1. never observed pea plants with mixtures of the 2
forms of the trait
2. Mendel reasoned that forms of a trait must remain
separate in offspring
B. Mendel hypothesized that each trait is controlled by a
distinct “factor”
1. since there were 2 forms of each trait, Mendel
realized that there must be at least 2 forms of each
factor
2. He reasoned that for every trait, a pea plant must
carry a PAIR of factors which could affect each
other; When a trait is inherited, the offspring
receives one factor from each parent.
C. We now know that Mendel’s “factors” are genes
1. recall: most organisms have 2 copies of every gene
and chromosome (homologous chromosomes) 
one copy from each parent
2. We refer to the different forms of Mendel’s factors
as alleles
Allele: distinct form of a gene
• If an organism has 2 different alleles for a trait, only one is
expressed or visible
Example: A person has an allele for free earlobes and an
allele for attached earlobes, yet that person’s earlobe shape
is just free. (only one of the alleles is expressed)
• Dominant allele: form of a gene that is fully expressed
when 2 different alleles are present
• Recessive allele: form of a gene that is NOT expressed
when paired with a dominant allele
Mendel published an account of his experiments in 1866. His
work was unrecognized for 37 years.
Rediscovering Mendel
Late 1880’s : a new staining technique allows chromosomes
to be viewed for the first time
Early 1900’s : Mendel’s work is rediscovered
I.
Walter S. Sutton – 1903
A. Sutton was observing stained cells through a
microscope and realized that chromosomes behaved
like Mendel’s factors
B. Developed the Chromosomal Theory of Heredity
which states that the material of heredity is carried
by the genes in chromosomes (remember – didn’t know
that this “material of heredity” was DNA until the late
1940’s-early 1950’s, and didn’t know the structure of
DNA until 1953)
C. To explain and use the theory, a system of terms and
symbols was developed.
WE USE LETTERS TO REPRESENT ALLELES.
But, how do you know which letters to use?
• Use the same letter for both forms of the trait because
both dominant and recessive alleles are for the same
trait. We just use capital and lowercase forms of the
letters to show the difference.
• The dominant allele always determines the letter – the
first letter of the dominant form of the trait is chosen and
the dominant allele always uses an capital letter
• The recessive allele always gets a lowercase letter.
Ex. pea color
Yellow is dominant, green is recessive.
We use the letter “Y” because it is the first letter in
the dominant form (Yellow).
 The allele for yellow peas is represented by an
capital letter “Y”
 The allele for green peas is represented by a
lowercase letter “y”
D. Genotype and Phenotype
1. genotype: the genetic make-up of an organism
a. includes both genes in a homologous pair of
chromosomes
examples:
a plant purebred for yellow peas: __YY__
a hybrid plant for pea color:
_ Yy___
a plant purebred for green peas: __ yy___
2. phenotype: outward expression or appearance of a
trait
example:
the hybrid with genotype Yy has the phenotype
yellow peas (recall dominance)
3. Describing genotypes
a. homozygous: the 2 alleles in a gene pair are the
same
examples: YY and yy
“Mendel’s purebred plants were homozygous for pea color.”
b. heterozygous: the 2 alleles for a trait are
different
examples: Gg, Yy, Pp
“Mendel’s hybrid plants were heterozygous for pea color.”
Mendel’s Laws
I.
Mendel’s Laws
A. basic rules of inheritance
1. there are exceptions (more on that later)
2. many traits do follow Mendel’s laws (called
Mendelian traits)
B. The Law of Segregation
“Each pair of genes segregates, or separates, during meiosis.”
1. Because of segregation, half of an organism’s
gametes contain one gene from a homologous pair
and half of the gametes contain the other gene.
C. The Law of Independent Assortment
“Gene pairs segregate into gametes randomly and independently
of each other.”
1. Mendel studied the inheritance of 2 traits at the
same time
Example: pea color and pea shape
P
peas
purebred yellow round peas
Genotype: ____YYRR__
X
purebred green wrinkled
Genotype: __yyrr____
F1
all plants had yellow round peas
(self fertilized to get F2)
F2
All possible combinations were produced:
Yellow round peas
Yellow wrinkled peas
Green round peas
Green wrinkled peas
• The association of traits in a parent didn’t seem to
matter.
 yellow wasn’t always with round
 green wasn’t always with wrinkled
We could get combinations that were different from the
parents.
D. Law of Dominance
“The dominant allele is expressed and the recessive allele can
be hidden.”
1. a recessive allele is expressed only when an
organism has NO COPY of the matching
dominant allele
Example: plant heterozygous for flower color
Recall: purple is dominant over white
Allele for purple flowers: ___P____
Allele for white flowers: ___p____
Genotype: ____Pp___
Phenotype: ___purple flowers____