Ch.2 Extensions to Mendel's laws

How can crosses of pure breeding results in different progeny phenos? (4)

1. No definitively dominant or recessive allele

2. More than two alleles exist

3. Multiple genes involved

4. Gene-environment interactions

Extensions to Mendel for single-gene inheritance (3)

1. Dominance is not always complete

• Incomplete dominance + Codominance

2. A gene may have >2 alleles

3. Pleiotropy

Pleiotropy

one gene may contribute to several characteristics

• Recessive lethal alleles

• Delayed lethality

Complete dominance

Hybrid resembles oneof the two parents

Incomplete dominance

Hybrid resemblesneither parent

• EX: pink flower color in snapdragons (cross of pure red + white = pink)

Codominance

Hybrid showstraits from both parents

• EX: Spotted (C^SC^S) x dotted (C^DC^D)

  • F1: all spotted (C^SC^D)

  • F2: 1:2:1 ratio

Gene I

Codominance

controls the type of sugar polymer on surface of RBCs

• I^A and I^B (6 genotypes produce 4 blood phenos)

5 alleles for C gene (D to R)

Marbled-1 is the most dominant > Marbled-2 > spotted = dotted > clear

Allele Frequency

the percentage of the total number of gene copies for one allele in a population

• Most common: wild-type (+) allele

  • 1 common = monomorphic vs 1+ = polymorphic

• Rare allele: mutant/variant

Pleiotropy

the phenomenon of a single gene determining several distinct and seemingly unrelated characteristics (pleio=many, trophic=effect)

• EX: Kartagener’s syndrome

recessive lethal alleles

two copies of the mutant allele required for the lethal phenotype

dominant lethal alleles

only one copy of the mutant allele required for the lethal phenotype

Lethality

Before birth: death

Midlife: genetic disorder

• EX: Huntington’s disease (if midlife risk passing to children unknown)

Novel phenotypes

Can result from gene interactions

• EX: Additive gene interaction affects seed coat in lentils

  • tan + gray = brown

  • no tan or gray = green

Epistasis

when an allele at one gene masks the phenotype of alleles at another gene

• Recessive, Reciprocal Recessive, Dominant

Hypostatic

gene that is masked

Recessive Epistasis

9:3:4 ratio F₂

Two copies of one allele needed to mask the other gene

• EX: coat color in labs

  • Recessive allele ee of gene E is epistatic to B and determines yellow (yellow has no eumelanin)

    
    

Reciprocal Recessive

9:7 ratio F₂

Two copies of each gene can mask the dominant allele of the other gene

• EX: Purple F1 progeny are produced by crosses of two pure-breeding white lines

Dominant epistasis

12:3:1 ratio F₂

One copy of an allele masks the other gene

• EX: summer squad, Dominant B allele is epistatic to any A alleles

  • Protein B is a dominant allele that prevents pigment deposition

    
    

Recessive epistasis in blood type

Gene for substance H is epistatic to the ABO gene

• hh = type O

• Parents with type O can have type A/B if child doesnt have hh

Redundant genes

15:1 ratio

EX: Control leaf development in maize

• Must be homozygous recessive for both A and B = SKINNY (aa bb)

  
  

Locus heterogeneity

mutations in 2 or more genes cause the same phenotype

What causes “continous” traits (3)

1. Incomplete dominance, codominance for multiple genes controlling a single trait

2. Various combinations of genetic interactions

3. Environmental effects

Heterogeneous traits

have the same phenotype but are caused by mutations in different genes

• EX: deafnesss can be caused by ~50 diff genes

Complementation testing

Cross two mutant strains with the same phenotype

• Wild-type = complementation = mut in diff genes

• Mutant = no complementation = same gene

Genetic heterogeneity in humans (2)

1. Deaf: 2 parents R mutation = all double hetero (not deaf) BUT if 2 parents R same mut = all deaf

2. OCA

Discontinuous traits

give clear-cut, "either-or" phenotypic differences between alternative alleles

• EX: Mendel’s pea plants

Continuous traits

determined by segregating alleles of many genes that interact together and with the environment

• appear to be blend + “unblend”

• polygenic

• EX: height + skin color

Mendelian explanation of continuous variation

More genes = more phenotypic classes + similarity to continuous variation

• EX: gene E + B = amount of eumelanin in dog coat

• EX: gene S + M = control spotting (M₁ dilute, M₂ norm)

Why Phenotypic variation for some traits occur (4)

1. Penetrance and expressivity

2. Effects of modifier genes

3. Effects of environment

4. Pure chance

Penetrance

the percentage of individuals with a particular genotype that show the expected phenotype

• complete or imcomplete

Expressivity

the degree or intensity with which a particular genotype is expressed in a phenotype

• Can be variable or unvarying

Modifier Genes

alter the phenotypes produced by alleles of other genes

• Can have major effect or more subtle effects

• EX: Mice tail length by T locus

Temperature

common element of the environment that can affect phenotype

• EX: Coat color in Siamese cats