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Mendel’s Genetics
Where did that blonde hair come from?
law of segregation and random assortment
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 Intro
notes
GENETICS and EVOLUTION
 The
purpose of this chapter is to show
how genetic traits are passed from one
generation to another. This is called
HEREDITY
 If there was no genetic variation through
mutation or crossing over of genes there
would be no evolution
Gregor Mendel
The Monastery
I. Gregor Mendel- the father of
genetics theory (1822-1884)
A. Background
1. entered monastery at 21
2. studied math and science
at University of Vienna
3. 1857-1865 – investigated
inheritance in pea plants
B. Peas – A Fortunate Choice
(Pisum sagivum)
1. seven distinct characteristics
(flower color, flower position,
seed color, seed shape, pod
shape, pod color, height)
2. easy to grow
3. mature quickly
4. easy to pollinate
D. Mendel’s Experiments- monohybrid
crosses (one purple & one white parent)
1. P1 Generation (Parental)
a. crossed plants pure for a
trait – TRUE- BREEDING
2. F1 Generation (Offsprng of P1)
a. all plants show one form of
the trait
3. F2 Generation (Offsprng of F1)
a. show forms of trait in 3:1
Mendel’s P, F1and F2 Generations
Examples of P1 Cross
Tall X Short
(both are pure)
TT X tt
All offspring are tall (T t) F1 Generation
F1 Generation (all are hybrids)
Purple Flower X White Flower (both
pure) P P X p p
All offpsring are purple (Pp) F1
Generation
F1 Generation (all are hybrids)
Mendel’s F1 Cross (hybrid x hybrid)
Tall X Tall (hybrid cross)
Tt X Tt
3 tall plants : 1 short plant (F2 Generation)
Ratio of 3:1
Purple Flowers X Purple Flowers
(hybrid) P p X P p
3 purple flower plants : 1 white flower (F2)
Ratio of 3:1
II. Vocabulary
A. Dominant (represented by upper
case letter)
1. allele that masks the recessive
allele for the same characteristic
B. Recessive (represented by lower
case letter)
1. allele that is masked by the
dominant allele for the same
characteristic
II. Vocabulary
C. Genotype
1. genetic makeup
2. examples
a. TT, Tt, tt,
b. PP, Pp, pp
D. Phenotype
1. external appearance
2. examples
a. tall, short
E. Homozygous (pure)
1. two alleles code for the same
trait
2. examples
a. TT, tt, PP, pp
F. Heterozygous (hybrid)
1. two alleles do not code for the
same trait
2. examples
a. Tt and Pp
III. Complete Dominance
(Monohybrid Cross)
A. Both parents are pure
1. homozygous x homozygous
2. example T T x t t
B. Both parents are hybrid
1. heterozygous X heterozygous
2. example Tt x Tt
III. Complete Dominance
C. Pure parent X hybrid parent
1.homozygous dominant X heterozygous
a. Example T T x T t
2.homozygous recessive x heterozygous
a. Example tt x Tt
PUNNETT SQUARES
MENDEL”S THEORY
 1.
Each individual has two copies of an
individual trait -these traits controlled
by a pair of factors
a. today factors are called alleles
2. There are alternate versions traits
 TT
= tall tall Tt= tall short tt= short short
MENDEL”S THEORY
3.
One trait may be expressed
and other may not have an
effect.
 dominant and recessive
Analysis of Mendel’s Results
1. Principle of Dominance
a. one factor (gene) can prevent
expression of another
(dominance)
IE: hybrid tall plant – phenotype- tall
genotype- Tt
2. Law of Segregation
a. a pair of factors separate
when gametes form
3. Law of Independent Assortment
a. factors (genes) for different
characteristics separate
independently
Animation: The Inheritance of Single Traits
 WORK
ON Inheritance lab
Where did that blonde hair come from?
law of segregation and random assortment
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incomplete /co dominance/
sex linked
 www.youtube.com/watch?v=fQvER3MyI2c
Incomplete Dominance/
blended traits
 The
phenotype of an individual is the
intermediate trait between the two parents:
 Or-When two heterozygous genotypes
make a third, different phenotype.
 EXAMPLES:


Straight haired mother and curly haired fatherthe child will have an intermediate trait such
as wavy hair
Red snapdragon when crossed with white
snapdragons produce pink snapdragons.
10.5 Do the Mendelian Rules of
Inheritance Apply to All Traits?
 In
incomplete dominance, the phenotype of
the heterozygotes is intermediate between the
phenotypes of the homozygotes



In the genes studied by Mendel, one allele was
dominant over the other, which was recessive
Some alleles, however, are incompletely dominant
over others
When the heterozygous phenotype is intermediate
between the two homozygous phenotypes, the
pattern of inheritance is called incomplete
dominance
10.5 Do the Mendelian Rules of
Inheritance Apply to All
 In incomplete dominance,
the phenotype of
Traits?
the heterozygotes is intermediate between
the phenotypes of the homozygotes
(continued)

Human hair texture is influenced by a gene with
two incompletely dominant alleles, H1 and H2
• A person with two copies of the H1 allele has curly hair
• Someone with two copies of the H2 allele has straight
hair
• Heterozygotes (with the H1H2 genotype) have wavy
hair
10.5 Do the Mendelian Rules of
Inheritance Apply to All Traits?
 In
incomplete dominance, the phenotype of
the heterozygotes is intermediate between
the phenotypes of the homozygotes
(continued)

If two wavy-haired people marry, their children
could have any of the three hair types: curly
(H1H1), wavy (H1H2), or straight (H2H2)
10.5 Do the Mendelian Rules of
Inheritance Apply to All Traits?
A


single gene may have multiple alleles
An individual may have at most two different
gene alleles
A species may have multiple alleles for a given
characteristic
• However, each individual still carries two alleles for this
characteristic
IV. Incomplete Dominance
(both alleles influence the trait)
A. Pure X Pure = all hybrids
1.example (red flower and white
flower) a. RR x WW
B. Hybrid X Hybrid
1. e (pink x pink flower)a. RW x RW
C. Pure X hybrid
1.ex (red x pink or white x pink)
a. RR x RW or WW x RW
Incomplete Dominance Four O’clock
Flowers
Pink (RW)
White (WW)
Red (RR)
Codominant traits
both are shown- zebra
traits are shown – for instance the
person with AB blood type is a child with
one parents that was A blood type and
one parent with B blood type.
 Neither trait is dominant. Both are in the
genotype and phenotype.
 Both
V. Codominance
(both alleles are expressed)
A. Pure X Pure
1. example (white horse x red
horse) a. WW x RR
B. Hybrid x Hybrid
1. example (roan horse x roan
horse) a. RW x RW
C. Pure X Hybrid
1. example(redxroan /white xroan)
a. RR x RW or WW x RW
VI. Multiple Allele Problems (Blood
Types)
A. PHENOTYPE
Type A
B. GENOTYPE
AA, AO ( IA IA , IA i )
Type B
BB, BO ( IB IB , IB i )
Type AB
AB
( IA IB )
Type O
OO
( ii )
Table 10-1
Blood Donors and Recipients
VII. Sex-linked Inheritance
(X linked-carried on X
chromosome)
A. Examples of sex-linked traits
1. color blindness
2. hemophilia
3. muscular dystrophy
4. Icthyosis
Individual
Chromosomes
Normal Male
Male with Disease
XY
X* Y
Normal Female
Female Carrier
Female - Disease
X X
X* X
X* X*
Problem Solving- Sex Linked Diseases
A. A man is colorblind and his wife is a
carrier for colorblindness. What is the
probability that they will have a child who
is colorblind? (A son? A daughter?)
B. A man and woman are both colorblind.
Can they have a child who is not
colorblind?
(A son? A daughter?)
 Website
–sex linked traits
 www.edc./weblabs
 http://www.biology.arizona.edu/mendelian
_genetics/problem_sets/sex_linked_inherit
ance/sex_linked_inheritance.html
Pedigree-a genetic family tree
www.genetics.gsk.com/graphics/autosomal_recessive.gif
h
tt
:

Pedigree chart tells us two things



1. WHETHER IT IS AN AUTOSOMAL(22 BODY
PAIRS) OR SEX-LINKED (1PAIR OF SEX TRAITS
XX OR XY) If male and female is close to equal it is
autosomal
2. WHETHER IT IS DOM. OR RECESS. TRAIT- IF
THE TRAIT IS PASSED TO NEXT GENERATION –
BUT SKIPPED A GENERATION IT IS RECESSIVEIF THE PARENTS WERE NORMAL(WERE
CARRIERS) AND HAD A CHILD WITH THE TRAIT IT
IS RECESSIVE
Autosomal pedigree chart
Sex-linked pedigree
 ROYAL
FAMILY PEDIGREE
Dihybrid Cross
A
cross that involves two traits.
 Example:
 Predict
the results of two pea plants that
are heterozygous for
 seed shape – R-round r-wrinkled
 Seed color- Y- yellow y- green
10.4 How Are Multiple Traits
Inherited?



Mendel concluded the origination of single traits
inheritance
He then pursued more complex questions relative to
the inheritance of multiple traits
Initial experiments included crossbreeding plants
that differed in two traits
• Seed color (yellow or green)
• Seed shape (smooth or wrinkled)
Figure 10-10 Traits of pea plants studied by Gregor Mendel
Trait
Seed
shape
Seed
color
Dominant form
Recessive form
smooth
wrinkled
yellow
green
inflated
constricted
green
yellow
purple
white
at leaf
junctions
at tips of
branches
tall
(about
6 feet)
dwarf
(about 8 to
16 inches)
Pod
shape
Pod
color
Flower
color
Flower
location
Plant
size
10.4 How Are Multiple Traits
Inherited?

From the many pea plant phenotypes, he chose
seed color (yellow vs. green peas) and seed shape
(smooth vs. wrinkled peas)
• Yellow color is dominant to green color
• Smooth shape is dominant to wrinkled

The allele symbols were assigned, as follows:
• Y = yellow (dominant), y = green (recessive)
• S = smooth (dominant), s = wrinkled (recessive)
10.4 How Are Multiple Traits
Inherited?

The two-trait cross was between two true-breeding
varieties for each characteristic, one dominant for
both traits, the other recessive for both traits
• P: SSYY (smooth, yellow)  ssyy (wrinkled, green)
• The SSYY plant produced only SY gametes, and the
ssyy plant produced only sy gametes
• Therefore, the F1 consisted solely of SsYy individuals,
with smooth skins and yellow coloring
10.4 How Are Multiple Traits
Inherited?


Mendel next allowed the F1 individuals to selffertilize: SsYy  SsYy
Crossing the F1 plants yielded 315 plants with
smooth, yellow seeds; 101 with wrinkled, yellow
seeds; 108 with smooth, green seeds; and 32 with
wrinkled, green seeds
• This is a ratio of approximately 9:3:3:1

Two-trait crosses of other traits produced similar
proportions of phenotype combinations
Figure 10-11 Predicting genotypes and phenotypes for a cross between parents that are heterozygous for two traits
Ss Yy
self-fertilize
eggs
SY
Sy
sY
sy
SSYY
SSYy
SsYY
SsYy
SSyY
SSyy
SsyY
Ssyy
SY
sperm
Sy
seed shape
seed color
phenotypic ratio
(9:3:3:1)
sY
sSYY
sSYy
ssYY
ssYy
sy
sSyY
sSyy
ssyY
Punnett square of a two-trait cross
ssyy
smooth 
yellow

smooth yellow
smooth 
green

smooth green
wrinkled 
yellow

wrinkled yellow
wrinkled 
green

wrinkled green
Using probabilities to determine the offspring
of a two-trait cross
10.4 How Are Multiple Traits
Inherited?
 Mendel
hypothesized that traits are inherited
independently

Mendel predicted that if the two traits were
inherited independently, then for each trait, threequarters of the offspring should show the
dominant phenotype and one-quarter should
show the recessive phenotype
• a 3:1 ratio, as he had found for the single trait flower
color
10.4 How Are Multiple Traits
Inherited?
 Mendel
hypothesized that traits are inherited
independently (continued)


He found 423 plants with smooth seeds
(of either color) and 133 with wrinkled seeds
(a ratio of about 3:1)
He found 416 plants produced yellow seeds
(of either shape) and 140 produced green seeds
(also about 3:1)
10.4 How Are Multiple Traits
Inherited?
 Mendel
hypothesized that traits are inherited
independently (continued)



The independent inheritance of two or more traits
is called the law of independent assortment
Multiple traits are inherited independently
because the alleles of one gene are distributed to
gametes independently of the alleles for other
genes
Independent assortment will occur when the traits
being studied are controlled by genes on different
pairs of homologous chromosomes
10.4 How Are Multiple Traits
Inherited?
 Mendel
hypothesized that traits are inherited
independently (continued)


The physical basis of independent assortment
has to do with the way homologous pairs line up
during meiosis
Which of the two homologues is “on top” occurs
randomly for all pairs, so the homologues assort
randomly and independently of one another at
anaphase I
Animation: The Inheritance of Multiple Traits
Figure 10-12 Independent assortment of alleles
S
s
pairs of alleles on homologous
chromosomes in diploid cells
Y
y
chromosomes replicate
S
Y
s
y
replicated homologous
pair during metaphase of
meiosis I, orienting
like this
or like this
S
s
y
Y
meiosis I
S
Y
s
y
S
y
s
Y
S
Y
s
y
S
y
s
Y
meiosis II
S
S
Y
s
Y
SY
S
s
y
y
sy
s
S
y
y
Sy
independent assortment produces four equally
likely allele combinations during meiosis
s
Y
Y
sY
10.4 How Are Multiple Traits
Inherited?.
 In
an unprepared world, genius may go
unrecognized



Mendel’s work was published in 1865 but went
unnoticed
Three biologists—Carl Correns, Hugo de Vries,
and Erich Tschermak—independently (of Mendel
and each other) rediscovered Mendel’s principles
of inheritance in 1900
Mendel was credited in new papers as laying the
groundwork of genetics 30 years previously
10.5 Do the Mendelian Rules of
Inheritance Apply to All
 Each trait is completelyTraits?
controlled by a single gene
 Only two possible alleles of each gene exist
 One allele is completely dominant to the other,
recessive, allele
 Most traits are influenced in more varied and subtle
ways
Dihybrid cross
 Parent-
round and yellow
RrYy
Dihybrid Cross
 Fill
in all the genotypes for the previous
slide.