Extensions of Mendelism

1
Extensions of Mendelism
2
William Bateson

• 1902 Mendelism, the principles of dominance,
  segregation, and independent assortment
• 1909 Mendels principles of heredity
• Questions the nature of unit-characters today
  known as genes
• Study of heredity becomes a scientific
  discipline

3
Alleles

• Alternative expressions of genes
• Dominant vs. Recessive
• Genes can exist in more than two allelic states
  multiple alleles
• Each allele has a different affect on the
  phenotype

4
Incomplete or partial dominance

• Heterozygotes for a given allele have a different
  phenotype than the homozygotes
• A dominant allele is not absolutely dominant in
  heterozygotes
• Semidominant a partially dominant allele whose
  phenotype in the heterzygote falls between the
  phenotypes of the homozygotes
• Snapdragons

5
Codominance

• Two alleles are expressed equally in the
  heterozygote
• MN blood type

6
Codominance and multiple alleles

• More than two alleles for a trait with the
  dominant alleles expressed equally in the
  heterozygote in which they occur
• ABO blood type

7
Multiple alleles

• Many alleles expressed at the same loci giving
  many variations of the trait
• Coat color in rabbits
• Wild Type ( superscript)
• Mutants
• Polymorphic having many forms or alleles
• Null not functional, recessive alleles
  amorphic
• Hypomorphic partially functional, but recessive
  to dominant alleles

8
Testing for Allelism

• Mutation existing allele changes to a new
  genetic state. Same trait, different properties
• Mutations generally have a detectable phenotypic
  effect
• How do we detect truly mutant alleles
• Observe the phenotypic effect of both alleles in
  the same individual.
• If the hybrid is a mutant, the alleles are
  mutations
• If the hybrid expresses the wild-type, the
  mutants are not alleles

9
Effects of mutations

• Mutations conspicuous change in the phenotype
• Change in eye color
• Change in eye shape
• Visible mutations easily observable change in
  morphology
• Most visible mutations are recessive
• Sterile mutations prevent or limit the ability
  to reproduce

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Effects of mutations

• Lethal mutations alteration in some essential
  function, resulting in death
• Most mutations are lethal, indicating that the
  majority of genes are necessary for life
• Early dominant lethals offspring who inherit
  them die early
• Late dominant lethals offspring live to
  transmit them, but die later
• Gene function
• One gene/one product one gene/one polypeptidie
• Mutations which change or alter the polypeptide,
  change the phenotype

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Effects of mutations

• Dominant vs. Recessive
• Dominant mutations have an affect in both the
  homozygote and heterozygote
• Recessive mutations have an affect only in the
  homozygote
• Recessive mutations are reflected via loss of
  function- no gene product
• Eye color in fruit flies Drosophilia
• Occasionally a dominant mutation may reflect a
  loss-of functionespecially when the phenotype is
  sensitive to the amount of gene product
• Dominant-negative mutations are dominant
  mutations in which the product of the mutant gene
  inhibits, limits, or antagonizes the polypeptides
  of the wild-type gene
• Gain-of-Function mutations dominant mutation
  which creates a new or expanded role for genes at
  a certain locus.

12
Environment and gene expression

• Genes do not act in isolation they are affected
  by the environment and by other genes
• Temperature effect on phenotype in Drosophilia
• Diet effect on phenotype
• Phenylketonuria
• Pleiotropic
• Horomones effect on phenotype
• Pattern-baldness
• Sex-influenced phenotype

13
Penetrance and Expressivity

• Incomplete penetrance
• A trait or phenotype is not expressed even though
  the genotype is present
• Polydactyly
• Dominant mutation expressed in some carriers
• Pedigree(4.11)
• Often leads to incorrect genotypic analysis
• Expressivity
• A trait or phenotype is not expressed uniformly
  in individuals with the genotype
• Variable expressivity many expressions of the
  same genotype
• Drosophilia lobed eyes
• Polydactyly also shows variable expressivity
  because the size, shape and location of extra
  fingers varies considerably

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Gene interactions

• Some phenotypes or traits can be influenced by
  more than one gene
• The pathway to expression of the genotype is
  modulated by other genetic factors
• Comb development in chickens Bateson and
  Punnett
• Epistasis
• Two independently assorting genes can affect the
  same trait
• Two or more genes influencing a given trait, but
  the allele of one overrides or
• negates the manifestation of the other genes
• A mutant allele is considered epistatic when is
  conceals the presence of another mutant allele in
  the gentype
• Eye color in Drosophilia
• Color of flowers in sweet peas Lathyrus
  odoratus
• Mechanism pathway to anthocyanin
• Seed shape in shepherd purse
• Mechanism two genetically governed pathways
  affecting the same product

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Pleiotropy

• Property of some genes which may affect the
  expression of many phenotypes
• Phenylketonuria
• In addition the metabolism of phenyalanine, other
  pathways are also affected
• Bristles in Drosophilia a singedgene
  pleiotropically affects bristles and egg
  formation in females only affects only bristles
  in males