BI226 Lecture Notes - Lecture 2: Lethal Allele, Hh Blood Group, Allele Frequency

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11 Feb 2017
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Some phenotypic variation poses a challenge to Mendelian analysis
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William Bateson
Crosses of pure-breeding lines can result in progeny phenotypes that don't appear to follow
Mendel's rules
o No definitively dominant or recessive allele
o More than two alleles exist
o Multiple genes involved
o Gene-environment interactions
3.1- Extensions to Mendel for single-gene inheritance
Dominance is not always complete
o Incomplete dominance
o Codominance
A gene may have >2 alleles (multiple alleles0
Pleiotropy- one gene may contribute to several characteristics
o Recessive lethal alleles
o Delayed lethality
Summary of different dominance relationships
The phenotype of the heterozygote defines the dominance relationship of two alleles
Complete dominance: hybrid resembles one of the two parents
Incomplete dominance: hybrid resembles neither parent
Codominance: hybrid shows traits from both parents
Flower color in snapdragons is an example of incomplete dominance
Crosses of pure-breeding red with pure-breeding white results in all pink F1 progeny
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Pink flowers in snapdragons are the result of incomplete dominance
Molecular explanation for incomplete dominance in snapdragon flower color
The normal allele produces an enzyme involved in pigment production
o AA=2 normal alleles==red color
o Aa=1 normal allele=pink color (less enzyme)
o aa=0 normal alleles= white colour (no functional enzyme)
Palamino Horses: Incomplete Dominance
Palaminos do not breed true when bred together:
Progeny are: 1/4 light chestnuts (C/C)
1/2 Palomino (C/Ccr)
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1/4 Cremellos (Ccr/Ccr)
1:2:! Ratio is characteristic of incomplete dominance
Is is thought that there are up to 7 different loci that contribute to the coat color of horses
In codominance, the F1 hybrids display traits of both parents
Spotted (CSCS) X dotted (CDCD)
All F1 progeny are spotted and dotted (CSCD)
F2 progeny ratios:
o 1 spotted (CSCS)
o 2 spotted and dotted (CSCD)
o 1 dotted (CDCD)
Phenotype ratios reflect the genotype ratios
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