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Extensions to Mendel
Complexities in relating
genotype to phenotype
Outline of extensions to Mendel’s analysis
Single-gene inheritance In which pairs of alleles show deviations from
complete dominance and recessiveness In which different forms of the gene are not
limited to two alleles Where one gene may determine more than one
trait Multifactorial inheritance in which the
phenotype arises from the interaction of one or more genes with the environment, chance, and each other
Dominance is not always complete
Crosses between true-breeding strains can produce hybrids with phenotypes different from both parents Incomplete dominance
F1 hybrids that differ from both parents express an intermediate phenotype. Neither allele is dominant or recessive to the other
Phenotypic ratios are same as genotypic ratios
Codominance F1hybrids express phenotype of both
parents equally Phenotypic ratios are same as genotypic
ratios
Summary of dominance relationships
Fig. 3.2
Incomplete dominance in snapdragons
Fig. 3.3
Codominant lentil coat patterns
Fig. 3.4a
Codominant blood group alleles
Fig. 3.4b
Do variations on dominance relations negate Mendel’s
law of segregation? Dominance relations affect phenotype and
have no bearing on the segregation of alleles
Alleles still segregate randomly Gene products control expression of
phenotypes differently Mendel’s law of segregation still applies Interpretation of phenotype/genotype
relation is more complex
Human blood type is an example one trait that is determined by multiple
alleles
O
no
Biochemical basis of ABO blood group
Individual crosses between pure-breeding lines for a trait controlled by
multiple alleles can be used to establish a dominance relationship
How do multiple alleles arise? Mutation
Pleiotropy: a single gene influencing
more than one characteristic
Sickle-cell anemia as a comprehensive
example
Pleiotropy
Multiple allelesDifferent
dominance relationships
Recessive lethality
Fig. 3.10
The interaction of two genes to effect one trait
Fig. 3.11
Epistasis: effects of a gene mask the effects of another
The homozygous recessive bb will cause the color gene A to be blockedDominant: Color1(enzyme A)Color 2(enzyme B) PurpleRecessive: Color 1(NO enzyme A) Color 1(enzyme B cant act) colorless9:7
Homozygous dom gives different color than heterozygous domYellowEnzyme (E_) BrownEnzyme (B_) blackYellNo enzyme (ee)yellow nothing for enzyme to act on yellowYellowEnzyme (E_) brown no enzyme (bb) brownNo enzymes cant change the yellow
9:3:4
The Bombay phenotype another example of recessive epistasis
Fig. 3.14
How can two parents of blood type OHave a child that is blood type A?
Parents genotype: ii H_ x IA_ hh
• Substance H for sugars to bind to
• With hh there is no substance h(substrate) for the sugars from A or B types to bind to
Biochemical basis of ABO blood group
12:3:1 or 13:3 ratio typify dominant epistasis
Fig. 3.15
Heterogenious traits: many genesgive rise to a phenotype
Fig. 3.16
Genetic cross can be used to determine the mechanism of inheritance for a trait
df
Summary of multifactorial traits
Genes can interact to yield novel phenotypes
Gene interactions can display epistasis, where an allele of a gene can mask the effects of another gene
One trait can be influenced by many different genes
Penetrance: the percentage of a population with a particular genotype that show the expected phenotype.
Expressivity: degree with which a genotype is expressed in a phenotype. Phenotype shows more than another individual with the same genotype
The same genotype does not always yield the same
phenotype
Dominant inheritanceV-2 incomplete penetrance
Environment can affect phenotypic expression of a
genotype Enzyme is temperature sensitive and is functional at extremities
Continuous traits vary within a population over a range
There are multiple genes(more than 3) that affect the resulting phenotype
Causes broad variation (EX: height, skin color)
Each gene that contributes to a continuous or quantitative trait are referred to as quantitative trail loci or QTL’s
Mendelian explanation of continuous variation
Fig. 3.22
CrossRatio Type
Heterozygous (Aa x Aa)
3:1 Normal Heterozygous cross
1:2:1
Incomplete Dominance: heterozygote resembles neither homozygote (blending)Codominance: both parental phenotypes expressed
2:1 Lethality: homozygosity for either dominant or recessive causes death
Dihybrid (AaBb x AaBb)
9:3:3:1
Normal dihybrid cross
9:7Complementary: recessiveness for either of the two genes disrupts enzyme path and prevents expression
9:3:4 Recessive epistasis: homozygous recessive of one gene masks both alleles of another gene
12:3:1
Dominant epistasis I: dominant allele of one gene hides effects of both alleles of another gene. Dominance on one gene prevents expression of other gene.
13:3Dominant epistasis II: dominant allele of one gene hides effects of dominant allele of another gene.