Law of Segregation alleles separate monohybrid cross

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• Law of Segregation alleles separate
  monohybrid cross
• Law of Independent assortment allele pairs
• inherited independently dihybrid cross

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Rules of Multiplication Additions

• Multiplication likely that all events coincide
• Addition sum of all possibilities

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Co-dominance

• 2 alleles affect the phenotype in separate,
  distinguishable ways
• ABO blood groups
• 3 alleles (MULTIPLE ALLELES)
• IA, IB, i
• both IA IB are dominant to i allele
• IA IB alleles are co-dominant to each other
• determines presences of oligosaccharides on the
  surface of red blood cells

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Polygenic inheritance

• Some phenotypes determined by additive effects of
  2 or more genes on a single character
• phenotypes on a continuum
• human traits
• skin color
• height
• weight
• eye color
• intelligence
• behaviors

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Albinism
Johnny Edgar Winter
albinoAfricans
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Nature vs. nurture

• Phenotype is controlled by both environment
  genes

Human skin color is influenced by both genetics
environmental conditions
Coat color in arctic fox influenced by heat
sensitive alleles
Color of Hydrangea flowers is influenced by soil
pH
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Pleiotropy

• Most genes are pleiotropic
• one gene affects more than one phenotypic
  character
• wide-ranging effects due to a single gene
• dwarfism (achondroplasia)
• gigantism (acromegaly)

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Acromegaly André the Giant
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Epistasis in Labrador retrievers

• 2 genes E B
• pigment (E) or no pigment (e)
• how dark pigment will be black (B) to brown (b)

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Incomplete dominance

• Heterozygotes show an intermediate phenotype
• RR red flowers
• rr white flowers
• Rr pink flowers
• make 50 less color

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Dihybrid heterozygous cross
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It all started with a fly

• Chromosome theory of inheritance
• experimental evidence from improved microscopy
  animal breeding led us to a better understanding
  of chromosomes genes beyond Mendel
• Drosophila studies

A. H. Sturtevant in the Drosophila stockroom at
Columbia University
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Thomas Hunt Morgan
1910 1933

• embryologist at Columbia University
• 1st to associate a specific gene with a specific
  chromosome
• Drosophila breeding
• prolific
• 2 week generations
• 4 pairs of chromosomes
• XXfemale, XYmale

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Morgans first mutant

• Wild type fly red eyes
• Morgan discovered a mutant white-eyed male
• traced the gene for eye color to a specific
  chromosome

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Discovery of sex linkage
allred eye offspring
How is this possible?
Sex-linked trait!
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Genes on sex chromosomes

• Y chromosome
• SRY sex-determining region
• master regulator for maleness
• turns on genes for production of male hormones
• pleiotropy!
• X chromosome
• other traits beyond sex determination
• hemophilia
• Duchenne muscular dystrophy
• color-blind

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Sex-linked traits
sex-linked recessive
Hh x HH


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Sex-linked traits summary

• X-linked
• follow the X chromosomes
• males get their X from their mother
• trait is never passed from father to son
• Y-linked
• very few traits
• only 26 genes
• trait is only passed from father to son
• females cannot inherit trait

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Gene Recombination in Linked Genes

• In contrast, linked genes, genes located on the
  same chromosome, tend to move together through
  meiosis and fertilization.
• Under normal Mendelian genetic rules, we would
  not expect linked genes to recombine into
  assortments of alleles not found in the parents.
• If the seed color and seed coat genes were
  linked, we would expect the F1 offspring to
  produce only two types of gametes, YR and yr when
  the tetrads separate.
• One homologous chromosome from a P generation
  parent carries the Y and R alleles on the same
  chromosome and the other homologous chromosome
  from the other P parent carries the y and r
  alleles.

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Recombination Frequency RecombinantsTotal
Recombinants Offspring that vary from parent
phenotype.
Conclusions Genes are linked, but not entirely
due to crossing over during meiosis.
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Mechanisms of inheritance

• What causes dominance vs. recessive?
• genes code for polypeptides
• polypeptides are processed into proteins
• proteins function as
• enzymes
• structural proteins
• hormones

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Prevalence of dominance

• Because an allele is dominant does not mean
• it is better
• it is more common

Polydactyly dominant allele
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Polydactyly
individuals are born with extra fingers or
toes dominant to the recessive allele for 5
digits

• recessive allele far more common than dominant
• ? 399 individuals out of 400 have only 5 digits
  
• ? most people are homozygous recessive (aa)

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Hound Dog Taylor
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Other Genetic Diseases of Interest

• Blue People! Progeria

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Chromosomal errors, I

• Nondisjunction members of a pair of homologous
  chromosomes do not separate properly during
  meiosis I or sister chromatids fail to separate
  during meiosis II
• Aneuploidy chromosome number is abnormal
  Monosomy missing chromosome Trisomy extra
  chromosome (Down syndrome) Polyploidy extra
  sets of chromosomes

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Chromosomal Abnormalities

• Turners (XO) Kleinfelters (XXY) Down
  Syndrome
• Cri du Chat