Mendelian Genetics
I. A Historical Perspective
A. Genetics has always been suspected.
B. There are records of trait selection in almost all early civilizations
C. Early genetic debates:
1. Who contributed more to offspring? Male? Female?
2. What is actually passed down? Fluid? Life Forces?
Pre-formation?
D. Pangenesis: Many sperm with many eggs form one organism
E. Theory of Blending: tall x short  medium
Offspring traits are always a blending of parental traits
Combined traits lost their “identities” thus were not discrete units
Uterus
A Homunculus
II. Gregor Mendel: His Background
A. Austrian Monk
B. Educated at U of Vienna
C. High school teacher
D. Worked with common “garden pea”
E. Published his work but
died an “Unknown”
III. Why pea plants were a good choice
A.
B.
C.
D.
Control reproduction
Large # of offspring to provide good ratios
Short generation span
Easy traits to distinguish from one another
IV. Mendel’s Experimental Methods
A. Established “Pure Lines”
Tall X Tall
(self-pollination)
Short X Short (self-pollination)
B. Cross the pure lines of the contrasting traits
Pure Tall X Pure Short (P1 X P1)
Results : F1 (First Family) 100% Tall
C. Cross the F1 offspring
Hybrid Tall (P2) X Hybrid Tall (P2)
Results: 75% Tall
25% Short (F2)
Ratio
3 Tall : 1 Short
Results from Mendel Crossing 7 Different “Characters” in Pea Plants
V. Mendel’s Conclusions (Laws)
Known as Law of:
Mendel’s Conclusions
Alternate versions of factors account for
inherited characteristics
Unit Characters
For each character an organism inherits
2 factors
Dominance
One member of the 2 inherited factors may
mask over the expression of the other factor
Segregation
(Meiosis)
The 2 factors segregate (separate) in the
formation of gametes
Independent
Assortment
When two or more paired factors form gametes by
segregation, one member will segregate independently
with any other member of a different factor
VI. Modern Genetic Terminology
MENDEL
Factor
Character
Paired
Factors
Pure
Hybrid
MODERN
DEFINITION
Gene
The “stuff” that produces a trait
Allele
One member of the gene pair
Phenotype
EXAMPLES
The trait produced by a gene
Brown hair
The visible expression
Blue eyes
Genotype
The interacting gene pair
that produces a trait
Homozygous
Genes of a pair are identical
TT = (tall)
tt = (short)
TT
tt
Genes of a pair are different
Heterozygous The dominant gene is expressed, recessive is hidden
Tt
VII. Punnett Squares
A. Definition: A probability “tool” used to predict possible offspring using
Mendel’s laws
B. Example: In pea plants, purple flowers are dominant to white
flowers. A heterozygous purple flower plant is crossed
with another heterozygous purple flower plant. What %
of the offspring would be expected to be purple? What
% would be white? What would be the expected ratio of
purple offspring to white offspring?
Punnett Square
Symbols:
P = Purple
p = white
Phenotype:
Purple
X
Purple
X
Pp
P p
Genotype:
Gametes:
Pp
P p
Answer
P
p
% Purple = 75%
P
PP
Pp
% White = 25%
p
Pp
pp
Ratio = 3:1
VII. Mendel’s Law Independent Assortment
A. Definition:
When genes of different gene pairs make gametes (meiosis), the
gene pairs separate and assort independently of other gene pairs
B. Application
1. Only applies when keeping track of more than one gene pair
2. Must follow the patterns of meiosis
C. Example of law
2 Chromosome (Haploid)
4 Chromosomes (Diploid)
Meiosis
A
B
A
b
Aa B b
a
B
a
b
D. Forming gametes following Independent Assortment
AaBB
aaBB
AaBb
AaBbCc
AB, aB
aB
AB, Ab, aB, ab
ABC, ABc, AbC, Abc
aBC, aBc, abC, abc
E. Mendel’s Laws of Independent Assortment and Punnett Squares
All possible gametes
must be used to
make an accurate
punnett square
F. Punnett Squares that Require the Law of Independent Assortment
In pea plants purple flowers are dominant to white and yellow seeds are dominant to green.
A heterozygous purple flowered, heterozygous yellow seeded plant was crossed with
another heterozygous purple, heterozygous yellow plant. What would be the phenotypic
ratio of the traits found in the offspring?
Symbols
P = Purple, p = white, Y = Yellow seeds, y = green seeds
Phenotypes
Purple flowers, yellow seeds X Purple flowers yellow seeds
Genotype
Gametes
PpYy
PY
Punnett Square
PY
X
Py pY
PpYy
py
PY
PY
Py
pY
PPYY
PPYy
PpYY
Py pY
py
PpYy
py
Ratio:
9:3:3:1
Possible Traits of Offspring
Purple and Yellow
9
P_Y_ = ________
Py
pY
py
PPYy
PpYY
PpYy
PPyy
PpYy
Ppyy
PpYy
Ppyy
ppYY
ppYy
ppYy
ppyy
Purple and Green
3
P_yy = ________
White and Yellow
3
ppY_ = ________
White and Green
1
ppyy = ________
VIII. Human Genes that follow Mendialian Patterns
A. Recessive genes
1. Examples
a. Cystic Fibrosis
-overproduction of mucus resulting in breathing and digestive
complications. Missing a membrane bound chloride ion pump
b. Tay-Sachs
-lethal nervous system disorder. Lethal by the age of 4. Missing a
lipid metabolizing enzyme.
c. Sickle Cell Anemia
-deformed red blood cells clog
capillaries. Oxygen deprived cells
result in a severe form of anemia.
Gene produces faulty hemoglobin.
d. PKU
- lacks an enzyme to metabolize phenylalanine.
Increasing levels of this amino acid
becomes toxic to brain cells resulting in mental retardation.
Special diet can control the problem.
2. Characteristics
a. Requires 2 carriers (heterozygous) as parents to have a child
expressing the trait
N = Normal gene
Normal (carrier)
Nn
n = Bad gene
X
X
Normal (carrier)
Nn
N
N
n
NN
Nn
n
1 chance out of 4
to have an
affected child
Nn
nn
b. Many have ‘high risk” ethnic groups
c. Some bad genes may have become established to give a carrier a
“heterozygous advantage”
3. Typical Recessive Pedigree
“Married”
Male
Female
“out of nowhere”
“males and females”
“marriage hides the gene”
Shows trait
a) Tend to “appear out of nowhere”
-parents do not show trait but a child does
b) Does not show a sex bias - Found in both males and females
c) Marriage into family tends to hide the gene
B. Dominant
1. Examples
a. Huntington’s disease – Neurological degeneration that begins
later on in life (mid 20s-45)
b. Achondroplasia – Dwarfism; “Little People” long bones do not
grow properly
c. Polydactyly – Extra fingers and toes
2. Characteristics
Only one bad gene is required to express the trait
B = bad gene
b = good gene
Affected parent X
Bb
X
Normal Parent
bb
B
b
b
Bb
bb
Children have a 50% chance
to express the trait
b
Bb
bb
3. Typical Dominant Pedigree
“every generation”
“marriage does not
hide the trait”
a) Tends to be expressed in every generation
b) Marriage into the family does not hide the trait
VIII. Genetics Since Mendel
“There is no such thing as a dominant or recessive gene”
A. Since proteins have many different functions, the gene that makes the
proteins will show a wide range of expression
Protein Functions: Structural, Enzymes, Transport, Hormones
B. Gene expression is due to the type of protein made by the gene
1. A dominant gene:
Produces a protein that is phenotypically observable when only one of
the genes is present (Heterozygous)
Usually the type of protein produced is a structural protein
2. A recessive gene:
Results of gene is only observable in the homozygous condition
Usually the type of protein produced is an enzyme OR
no protein product at all
IX. Non-Mendelian Gene Patterns
A. Incomplete Dominance: One gene produces an observable protein while
the other gene does not make any protein, a
THIRD phenotype results
Example: Snap Dragons
RR
WW
R gene = Makes Red Pigment
W gene = Makes no pigment
R
W
RR = Red Flower (Red Pigment)
RW
WW = White Flower (no pigment)
R
R
W
RW = Pink Flower (1/2 as much red)
W
RR
RW
R
Ratio: 1 red : 2 pinks: 1 white
RW
W
Does this support the theory of Blending?
WW
Protein produced by both alleles are observable, both alleles
B. Codominance : are expressed’ co means “together”
1. Example: Cattle fur color
W gene = White fur
WW = white fur
Cross a white cow and red bull
R gene = Red fur
RR = red fur
WR = roan fur (red and white)
Cross roan cow and roan bull
X
X
Results?
1. Both the red and white
genes are expressed
2. Does this support the
theory of Blending?
Does this support the theory of Blending?
C. Multiple Alleles:
More than two genes of a pair are found in a
population of which an organism can only have two
Example: Blood Types:
Proteins found (or not found) on red blood cells
Blood Type Genes
A
Genotypes
A
or
I
B
or
I B
O
or
i
Codominant
Recessive
Phenotypes
AA, AO
A
BB, BO
B
AB
AB
OO
O
Example Problems
1. Cross a person with AB blood with a person with O blood
2. A parent with A blood and a parent with B blood have a child with O blood?
Note: Other blood proteins are found on blood cells as well.
Examples:
1. M and N group (M and N genes are codominant)
2. Rh factor group Having the factor(+) is dominant to not having the factor (-)
Example
D. Epistatic genes: One gene pair influencing the expression of other gene pairs
Usually due to genes “upstream” in a shared metabolic
pathway
Example: Fur color in mice
B = gene for black fur color (Dominant to brown)
b = gene for brown fur color (Recessive to black)
C = gene to make color in fur (Dominant and epistatic to black or brown)
c = gene can not make color in fur (Recessive)
Cross 2 mice heterozygous for both genes What ratio would you expect?
(9:3:3:1)
Important Points:
1. A capital “C” must be present in
order to show color
2. Unique ratios result from epistasis
9 black : 3 brown : 4 white
E. Polygenic Traits:
Many gene pairs work together to produce a trait
1. Characteristics
a. Show a wide distribution of traits over a gradient
b. Most human traits are polygenic
c. Examples: height, hair color, hair texture, eye color, skin color
Example: Skin Color
Capital letters Produces Pigment
Lower case produces little pigment
# of Capitals
aabbcc Aabbcc
AaBbcc
AaBbCc AABbCc
AABBCc AABBCC
aaBbcc
AAbbcc
AABbcc AaBBCc
AaBBCC
aabbCc
aaBBcc
AaBBcc AaBbCC
AABbCC
aabbCC
AAbbCc AABBcc
AabbCc
aaBBCc AAbbCC
aaBbCc
aaBbCC aaBBCC
AabbCC
Phenotype
6
Darkest
5
Darker
4
Dark
3
Medium
2
Light
1
Lighter
0
Lightest
Multifactorial Traits
• Several genes AND environmental factors
influence the phenotype
• Examples in humans – height, heart
disease, alcoholism
Slide 15
Example Blood Problem
A man with heterozygous A and heterozygous “+” blood has a child with
woman with O “–” Blood. What could be the possible blood types of their
children?
Symbols
R=+
r=-
A = A gene
B = B gene
Phenotypes
A+
X
O-
Genotypes
AoRr
X
oorr
Gametes
AR, Ar, oR, or
Square
or
Answer Question
o = O gene
or
AR
Ar
oR
or
AoRr
Aorr
ooRr
oorr
A+
A-
o+
o-
Slide 15
Slide 13
Alleles
Slide 13
Slide 8
Make own pathway to show
epistasis, search human
examples