Download Genetics: Review Variations in Mendel`s Laws Variations in

10/28/2011
Genetics: Review
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
3. If two alleles differ, one is
dominant, the other recessive
4. The two alleles for each
character segregate (separate)
during gamete production.
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
R
Rr
1
Gametes 1
2R 2 r
F2 Generation
Sperm
1
1
2R 2 r
1
R
2
Eggs RR Rr
1 r
Rr rr
2
Variations in Mendel’s Laws
Hypercholesterolemia
PHENOTYPE
GENOT
TYPE
•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
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Variations in Mendel’s Laws
Multiple Alleles
Blood
Group
Genotypes Red Blood Cells
(Phenotype)
Carbohydrate A
IAIA
or
IAi
A
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
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Variations in Mendel’s Laws
*Epistasis
Example:
In Drosophila, gene:eyeless
*Black (B) is dominant to brown (b)
*Second gene responsible for allowing
pigment to be deposited
d
d in hair
h
C = presence, c = absence (colorless)
Variations in Mendel’s Laws
– Polygenic inheritance is the additive effects of two
or more genes on a single phenotype.
Polygenic
inheritance
Single trait
(e.g., skin color)
Multiple genes
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
1
8
AaBbCc
AaBbCc
Sperm
1 1 1 1 1
8 8 8 8 8
6
64
15
64
20
64
15
64
6
64
1
8
1
64
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Variations in Mendel’s Laws
*Gene interactions
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.
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Variations in Mendel’s Laws
*Phenotype depends on environment and genes
How do we account for genetic variation?
*Independent assortment
*Crossing over
*Random fertilization
Cross over:
Independent Assortment:
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