Chapter 4: Modification of Mendelian Ratios
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Transcript of Chapter 4: Modification of Mendelian Ratios
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Chapter 4: Modification of Mendelian RatiosAllele
*Wild-type allele*Mutant allele
Conventional symbols for alleles:
recessive allele - initial letter of the name of the recessive trait, lowercased and italicized
dominant allele - same letter in uppercase
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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
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In incomplete dominance, F1 hybrids have an appearance in between the phenotypes of the two parents.
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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.
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Codominance:
Condition in which the phenotypic effects of a genes alleles are fully and simultaneously expressed in the heterozygote.
Example:
MN Blood group - red blood cells contain a transmembrane glycoprotein (glycophorin); two different forms of this protein exist, M and N
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BloodGroup
(Phenotype)Genotypes Red Blood Cells
O
A
B
AB
ii
IAIB
IBIB
orIBi
IAIA
or IAi
Carbohydrate A
Carbohydrate B
Multiple Alleles
Examples:
*Table 4.1: over 100 alleles at a given locus in Drosophila
*ABO Blood group in humans
*Characterized by the presence of glycoprotein antigens on the surface of red blood cells
*Distinct from the M and N antigens
*Also exhibits codomiance
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Lethal Alleles: Example: Coat color in mice*A = agouti = wild-type allele*AY = yellow = mutant allele
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EpistasisExample: In Drosophila, gene:eyeless
*Black (B) is dominant to brown (b)
*Second gene responsible for allowing pigment to be deposited in hair,
C = presencec = absence (colorless)
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*Gene product interactions
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– Pleiotropy is the impact of a single gene on more than one character.
Pleiotropy Multiple traits(e.g., sickle-celldisease)
Singlegene
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Multiple genes
Polygenicinheritance Single trait
(e.g., skin color)
– Polygenic inheritance is the additive effects of two or more genes on a single phenotype.
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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
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