1. Alternative versions of genes (alleles) account for variation in inherited characters
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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 recessive4. The two alleles for each character segregate (separate) during gamete production.
Genetics: Review
F1 Generation
RR rr
Gametes
P Generation
F2 Generation Sperm
Gametes
Red White
R r
RrPink
R r
R rR
r
RR Rr
rrRr
Eggs12
12
12
12
12
12
Variations in Mendel’s LawsIn incomplete dominance, F1 hybrids have an appearance in between the phenotypes of the two parents.
Homozygousfor ability to make
LDL receptors
Severe diseaseMild diseaseCell
Normal
LDLreceptor
LDL
Homozygousfor inability to make
LDL receptors
HeterozygousHH Hh hh
GEN
OTY
PEPH
ENO
TYPE
Hypercholesterolemia•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.
Variations in Mendel’s Laws
BloodGroup
(Phenotype)Genotypes Red Blood Cells
O
A
B
AB
ii
IAIB
IBIB
orIBi
IAIA
or IAi
Carbohydrate A
Carbohydrate B
Variations in Mendel’s LawsMultiple Alleles
Variations in Mendel’s Laws– Pleiotropy is the impact of a single gene on more than one
character.
Pleiotropy Multiple traits(e.g., sickle-celldisease)
Singlegene
Variations in Mendel’s Laws
*EpistasisExample: In Drosophila, gene:eyeless
*Black (B) is dominant to brown (b)*Second gene responsible for allowing pigment to be deposited in hairC = presence, c = absence (colorless)
Variations in Mendel’s Laws
Multiple genes
Polygenicinheritance Single trait
(e.g., skin color)
– Polygenic inheritance is the additive effects of two or more genes on a single phenotype.
Variations in Mendel’s Laws
Variations in Mendel’s Laws
F1 Generation
P Generation
F2 Generation Sperm
AABBCC(very dark)
Eggs
aabbcc(very light)
AaBbCcAaBbCc
18
164 64
66415
6420
6415
646 1
64
18181818181818
18
18
18
18
18
18
18
18
*Gene interactions
Variations in Mendel’s Laws
Sex Linkage
*Sex chromosomes*Autosomes
Example: In Drosophila and all mammalssex chromosomes designated as X and YXX=femaleXY=male
*1909 Thomas Hunt Morgan
II III IV
XX
XY
or
Any gene located on a sex chromosome is called a sex-linked gene.•Most sex-linked genes are found on the X chromosome.
Sex Linkage
*Phenotype depends on environment and genes
Variations in Mendel’s Laws
How do we account for genetic variation?*Independent assortment *Crossing over *Random fertilization
Independent Assortment:
Cross over: