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Genetics
P Generation
(true-breading
parents)
Purple flowers
F1 Generation
White flowers
All plants have
purple flowers
Fertilization
among F1 plants
(F1  F1)
F2 Generation
3
4 of plants
have purple flowers
1
4 of plants
have white flowers
Genetics
Mendel developed four hypotheses from the monohybrid cross:
1. There are alternative versions of genes, called alleles.
2. For each character, an organism inherits two alleles, one from
each parent.
– An organism is homozygous for that gene if both alleles
are identical.
– An organism is heterozygous for that gene if the alleles
are different.
Genetics
3. If two alleles of an inherited pair differ:
The allele that determines the organism’s appearance is the
dominant allele
The other allele, which has no noticeable effect on the
appearance, is the recessive allele
4. Gametes carry only one allele for each inherited character.
– The two members of an allele pair segregate (separate)
from each other during the production of gametes.
– This statement is the law of segregation.
Genetics
A Punnett Square predicts the results of a genetic cross
between individuals of known genotype
Tall
P:
DD
Gamete
formation:
D
D
D
D
D
d
Dwarf
X
dd
d
d
d
d
D
d
All (4/4) are Dd
All (4/4) are Tall
D
d
D
d
Genetics
Geneticists distinguish between an organism’s physical
traits and its genetic makeup.
• An organism’s physical traits are its phenotype.
• An organism’s genetic makeup is its genotype.
Genetics
Recall: Independent Assortment:
Genetics
Genetics
Dihybrid cross- A genetic cross between two individuals
involving two characters
Example:
P
GW
X
yellow, round
green, wrinkled
GGWW
ggww
gw
gw
gw
F1
All
yellow, round
GgWw
gw
GW
GW
GW
GW
F1
F1
X
Gw
gW
gw
GW
Gw
All
yellow, round
All
yellow, round
GgWw
GgWw
F2
9/16 yellow, round
gW
gw
9:3:3:1 Phenotypic ratio; Genotypic ratio as follows:
1/16 GGWW, 2/16 GGWw, 2/16 GgWW, 4/16 GgWw
3/16 yellow, wrinkled
1/16 GGww, 2/16 Ggww
3/16 green, round
1/16 ggWw, 2/16 ggWw
1/16 green, wrinkled
1/16 ggww
Genetics
Mendelian inheritance is based on probability
F1 Genotypes
Example- coin toss
Bb female
*1/2 chance landing heads
Bb male
Formation of sperm
Formation of eggs
F2 Genotypes
*Each toss is an independent event
*Coin toss, just like the
distribution of alleles into gametes
1
2
Female gametes
*The rule of multiplication –
determines the chance that two or
more independent events will occur
together
½x½=¼
Male gametes
1
2
1
2
B
B
b
1
2
B
B
1
4
( 12  12
B
b
1
4
b
B
b
1
4
)
b
b
1
4
Genetics: Pedigrees
First generation
(grandparents)
Ff
Second generation
(parents, aunts, and FF ff
uncles)
or
Ff
Third generation
(brother and
sister)
Female Male
Attached
Free
ff
Ff
ff Ff
Ff
ff
ff
FF
or
Ff
Ff
Human Disorders
Variations in Mendel’s Laws
In incomplete dominance,
F1 hybrids have an
appearance in between the
phenotypes of the two
parents.
P Generation
White
rr
Red
RR
Gametes R
r
F1 Generation
Pink
Rr
1
Gametes 1
R
2 2 r
F2 Generation
Sperm
1
1
R
2 r
2
1
2R
Eggs RR Rr
1 r
Rr rr
2
Variations in Mendel’s Laws
Hypercholesterolemia
PHENOTYPE
GENOTYPE
•Dangerously high levels of cholesterol in the blood.
•Is a human trait that is incompletely dominant.
•Heterozygotes have blood cholesterol levels about 2X normal.
•Homozygotes have blood cholesterol levels about 5X normal.
HH
Homozygous
for ability to make
LDL receptors
Hh
Heterozygous
hh
Homozygous
for inability to make
LDL receptors
LDL
LDL
receptor
Cell
Normal
Mild disease
Severe disease
Variations in Mendel’s Laws
Multiple Alleles and Codominance
Blood
Group
Genotypes Red Blood Cells
(Phenotype)
Carbohydrate A
IAIA
A
or
IAi
B
IBIB
or
IBi
AB
IAIB
O
ii
Carbohydrate B
Variations in Mendel’s Laws
Pleiotropy is the impact of a single gene on more than one
character.
Single
gene
Pleiotropy
Multiple traits
(e.g., sickle-cell
disease)
Variations in Mendel’s Laws
Variations in Mendel’s Laws
Polygenic inheritance is the additive effects of two or
more genes on a single phenotype.
Polygenic
inheritance
Multiple genes
Single trait
(e.g., skin color)
Variations in Mendel’s Laws
P Generation
aabbcc
AABBCC
(very light) (very dark)
F1 Generation
F2 Generation
1
8
1
8
1
8
1
8
1
Eggs 8
1
8
1
8
1
8
1
8
1
64
AaBbCc
AaBbCc
Sperm
1 1 1 1 1 1
8 8 8 8 8 8
1
8
20
64
1
64
6
64
15
64
15
64
6
64
Sex Linkage
• Autosome

Chromosome that is not directly involved in determining the
sex of an organism
• Sex chromosome
A chromosome that determines whether an
individual is male or female

Male
44

XY
Female
Somatic
cells
44

XX
Sex Linkage
Any gene located on a sex chromosome is called a
sex-linked gene.
• Most sex-linked genes are found on the X chromosome.
Genetics and the Environment
*Phenotype depends on environment and genes