Genetics
Genetics is the scientific
study of heredity and
variation.
Genetics
In asexual reproduction:
•Single-celled organisms reproduce by simple cell division
•There is no fertilization of an egg by a sperm
LM
FUNCTIONS OF CELL DIVISION
Asexual Reproduction
Amoeba
Sea stars
African Violet
Genetics
Sexual reproduction requires fertilization of an egg by a
sperm using a special type of cell division called meiosis.
Genetics
Gregor Mendel
•Worked in the 1860s
•Was the first person to analyze patterns of
inheritance
•Deduced the fundamental principles of genetics
Genetics
Removed
stamens
from purple
flower.
Mendel studied garden
peas because they:
White
Stamens
•Easy to grow
Parents
(P)
Carpel
• Come
in many readily
distinguishable varieties
• Easily
• Can
Pollinated carpel
matured into pod.
manipulated
self-fertilize
Transferred pollen from
stamens of white flower
Purple to carpel of purple
flower.
Planted seeds
from pod.
Offspring
(F1)
Genetics
• A character is a heritable
feature that varies among
individuals.
• A trait is a variant of a
character.
• Each of the characters
Mendel studied occurred in
two distinct forms.
Genetics
*P - parental generation
*F1 – first filial generation
*F2 – second filial generation
-Example:
F2
P
X
Tall
Dwarf
F1 – all Tall
Tall
Genetics
*genes and alleles
Mendel’s hypotheses (to explain his results)
1. Alternative
versions of genes
(alleles) account for
variation in inherited
characters.
2. For each character,
an organism inherits
two alleles, one from
each parent.
Genetics
3. If two alleles differ,
one is dominant, the
other recessive
4. The two alleles for each
character segregate (separate)
during gamete production.
P:
X
DD
Tall
dd
Dwarf
F1 – all Tall
Tall
Dd
Mendel’s 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
Dwarf
X
dd
d
d
d
d
D
*genotype
d
4/4 are Dd
4/4 are Tall
D
D
d
D
d
*phenotype
*Homozygous
*Heterozygous
Genetics
Genetics
Dihybrid cross- A genetic cross between two individuals
involving two characters
Example:
P1
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
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
*1909 Thomas Hunt Morgan
II
III
XY
IV
or
XX
*Sex chromosomes
*Autosomes
Example: In Drosophila and all mammals
sex chromosomes designated as X and Y
XX=female
XY=male
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.