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.