Chapter 11
Introduction to Genetics
Who was Gregor Mendel?
 1st person to study the transmission of traits from parent to
offspring
 He applied
 In 1851 he would have had no concept of
 He used rules of
 He used _________ _________ as his subject of study.
Why did he use Pea Plants?




Animation http://www.dnaftb.org/dnaftb/1/bio/index.html
What is a Hybrid?
What is a “Pure breed”?
What did Mendel Learn from his Studies of Pea Plants?
 Mendel chose seven different pairs of pea traits to study
 In the 1st generation only ____ of the two traits appeared and there
was no blending or mixing of traits
 The traits that appeared in the 1st generation are called
____________ traits
 Those that did not appear are called _____________ traits

Seed
Shape
Seed
Color
Seed Coat Pod
Shape
Color
Pod
Color
Flower
Position
Plant
Height
 Mendel established three important ideas about how traits are
transmitted from parent (P) to the first generation offspring(F1):
o
o
o
The Principle of Dominance:
Mendel noticed when crossing a “pure breed” tall with a “pure breed” short
that all the F1 (first generation offspring) were of tall trait.
But when the F1 were cross pollinated the short trait reappeared in the F2.
 Mendel termed the trait that appeared in the F1 generation
the_________________ trait.
 The trait that failed to appear in the F1 generation he termed the
____________________trait.
 For the remaining 6 traits Mendel found the following:
Note: D=dominant trait and R=recessive trait.
a.
Seed form: round (D) or wrinkled (R).
b.
Color of seed: yellow (D) or green(R).
c.
Color of flower: Purple (D) or white (R).
d.
Color of unripe seed pods: green (D) or yellow (R).
e.
Shape of ripe seed pods: inflated (D) or constricted between seeds
(R).
f.
Position of flower: axial (in axial of leaves) (D) or terminal (at the
end of the stem) (R).
The Rule of Unit Factors
 each organism has _______ factors “_________” (genes) that can
be transmitted from parent to offspring through gametes (sex
cells)
 These are ____________ of a trait
 For every trait there are ______alleles that influence the outcome
of that trait
 -one comes from the ____________
 -one comes from the ____________
The Law of Segregation
 The two alleles for a trait separate (or segregate) when gametes are
formed.
 When a heterozgote reproduces, its gametes will be of _____ types in
equal proportions.
 alleles will be ____________ distributed in egg and sperm
 fertilization which is also a random event, will result in new pairings of
alleles
 ________ allele from each parent will join to form a pair, but only one
allele will be expressed in the offspring
 Mendel used algebraic symbols to represent what was happening.
He let upper case letters (T) represent the dominant factor and lower
case letters (t) represent the recessive factors.
 A hybrid, like the F1 would then be represented as (Tt) showing that
a factor was inherited from each parent but one overshadowed the
other.
Tt
Gametes
T
TT
F1
Tt
t
T
Tt
Tt
S
E
G
R
E
G
A
T
I
O
N
t
tt
F
E
R
T
I
L
I
Z
A
T
I
O
N
F2
Mendel’s conclusions:
The seven characteristics were controlled by transferable factors we call
_____________.
The factors came in two forms:_______________ and _____________.
Today, we call these transferable factors _______________.
Every heterozygote (hybrid) had 2 different copies of the factor controlling each
character – one from each parent.
The dominant factor determined the appearance of the plant (phenotype).
 The two alleles for a trait separate (or segregate) when gametes are
formed.
 When a heterozgote reproduces, its gametes will be of two types in
equal proportions.
 Either the gamete will have ‘T’ or ‘t’. Tt  1/2 T or 1/2 t
Sample Problem:
Look at the trait for Flower Position in the chart on the previous page.
When Mendel crossed a plant with Axial flower position with one that
was Terminal for flower position the resulting F1 were all Axial. Show
the symbols you would use for each.
Axial Flower position (
)
Terminal Flower position (
F1 plants (
)
)
If two of the F1 plants were cross pollinated (allowed to mate) then how
many would be
(AA)=
(Aa)=
(aa)=
Early in the 20th century, these relationships were put into graphic form by
Reginald Punnett.
The forms are called punnett squares.
Each little square represents a possible offspring.
Above the squares are the parents’ gametes.
Parental Gametes 
Possible offspring
Genotypess 
If Mom Aa and Dad Aa decided to have a child, what are the possible genotype of
the offspring and their probabilities?
Mom’s gametes:
_______ or _______
Dad’s gametes:
_______ or _______
Parental Gametes
?
?
?
?
Solution:
P1 = AA x aa
The F1 (hybrids) would be heterozygous plants ‘Aa’, the plant received ‘A’ from one parent and
‘a’ from the other parent. If one parent was ‘AA’ axial and the other parent was ‘aa’ terminal,
Mendel reasoned that the female could only give ‘A’ to the offspring and that the male could only
give ‘a’ to the offspring. All offspring would be ‘Aa’
Crossing of the F1:
F1 = Aa x Aa
If the ‘Aa’ offspring were allowed to mate, then 1/4 of their offspring would be ‘AA’, 1/2 would
be ‘Aa’ and 1/4 would be ‘aa’. So the F2 = AA, 2Aa, aa
Gregor
Mendel
experimented
with
concluded that
which is
called the
Law of
Dominance
which is
called the
Law of
Segregation
What About Crosses Involving Two Traits?
 Crosses that involve two traits are called ______________ crosses.
 Segregation of alleles for different traits is random.
 During gamete formation only one allele for each trait will be passed
from parent to offspring.
 Mendel discovered that when crossing for two traits, alleles for
different traits segregated independent of each other and that even
greater variations and more complex ratios would occur.
 The Law of ___________________ __________________ states
that alleles for different traits are located on different genes/different
chromosomes and therefore segregate independently of one another
during gamete formation (meiosis).
Chromosomes can line-up in two ways during meiosis, both dominant
may end up in the same gamete or in different gametes.
Sample problem:
Let the trait Round shaped peas (R) be dominant to wrinkled peas (r),
and a second trait, Yellow color (Y) is dominant to green color (y). If a
Homozygous Dominant plant for both shape and color is crossed with a
Homozygous Recessive plant, what would be the resulting genotypes
and phenotypes of the F1?
Homozygous Dominant for seed
shape and color
X
Homozygous recessive for seed
shape and color
Allele pairings:
Allele pairings:
___________________
_____________________
Gametes:
F1 Zygote
What Would the F2 Look Like?
If we take two F1 and cross them,
RrYy
X RrYy
Law of Independent Assortment states that alleles for different traits
segregate independently of one another therefore all gamete combinations
must be considered.
The homolog of one chromosome can be inherited
with either homolog of a second chromosome.
So we must consider all possible allele alignments in meioisis
r
R
y
Y
_______
______
r
R
y
Y
___
___
To simplify the Independent Assortment of Alleles we can use an algebraic
expression:
FOIL
Firsts _____
Outsides _____
Insides _____
Lasts
_____
Each heterozygous parent will
produce 4 different gametes
with unique allele pairings
The resulting F2 will be in
a (9:3:3:1) phenotypic
ratio.
9
_
3
_
_______ _________
Round Green
_______ _________
Wrinkled ________
Summary:
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In gamete formation each pair of factors segregates independently of
other pairs of factors.
In chromosome terms, each pair of homologs segregate independently of
every other pair in Meiosis I.
Pairs of alternative traits behaved independently.
This is because maternal and paternal chromosome pairs line up and
separate during meiosis.
If an organism is heterozygous at two unlinked loci (two genes for two
traits are on different pairs of chromosomes), each locus will assort
independently of any other.