Gregor Mendel

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Gregor Mendel

• Pea Plants and Inheritance Patterns

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At the Monastery
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Mendels Hypothesis
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Mendels Hypothesis

• There are alternative forms for genes, the units
  that determine inheritable characteristics (AA or
  Aa or aa)
• For each inherited characteristic, an organism
  has two alleles, one inherited from each parent.
• A sperm or egg carries only one allele (A or
  a)for each inherited characteristic, because
  allele pairs separate from each other during
  meiosis.
• When the two alleles of a pair are different, one
  is fully expressed (dominant) and the other is
  completely masked (recessive).
• The members of the pair may be identical
  (homozygous) or non-identical (heterozygous).

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Revisiting Meiosis
Principle of Independent Assortment The
assortment of one pair of genes into gametes is
independent of the assortment of another pair of
genes.
The alignment of one pair of homologs is
independent of any other.
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Making crosses

• Can represent alleles as letters
• flower color alleles ? P or p
• true-breeding purple-flower peas ? PP
• true-breeding white-flower peas ? pp

Pp
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Punnett squares
Aaaaah, phenotype genotypecan have different
ratios

• Pp x Pp

1st generation (hybrids)
genotype
phenotype


PP
Pp
Pp
pp
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121
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Extending Mendelian genetics

• Mendel worked with a simple system
• peas are genetically simple
• most traits are controlled by single gene
• each gene has only 2 version
• 1 completely dominant (A)
• 1 recessive (a)
• But its usually not that simple!

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Incomplete dominance

• Hybrids have in-between appearance
• RR red flowers
• rr white flowers
• Rr pink flowers
• make 50 less color

?RR
?WW
?RW
RR
Rr
rr
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Incomplete dominance
X
true-breeding red flowers
true-breeding white flowers
P
self-pollinate
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Incomplete dominance

• RW x RW

genotype
phenotype


RR
RW
RW
WW
121
121
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Codominance

• Equal dominance
• human ABO blood groups
• 3 version
• A, B, i
• A B alleles are codominant
• both A B alleles are dominant over i allele
• the genes code for different sugars on the
  surface of red blood cells
• name tag of red blood cell

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Genetics of Blood Types
Phenotype Genotype Antigenon RBC Antibodiesin Blood DonationStatus
A IA IA or IA i type A antigenson surface of RBC anti-B antibodies Type A
B IB IB or IB i type B antigenson surface of RBC anti-A antibodies Type B
AB IA IB both type A type B antigens on surface of RBC no antibodies Universal Recipient
O i i no antigens on surface of RBC anti-A anti-B antibodies Universal Donor
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Blood donation
clotting
clotting
clotting
clotting
clotting
clotting
clotting
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One gene many effects

• The genes that we have covered so far affect
  only one trait
• But most genes are affect many traits
• 1 gene affects more than 1 trait
• dwarfism (achondroplasia)
• gigantism (acromegaly)

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Inheritance pattern of Achondroplasia
Aa x aa
Aa x Aa
a
a
A
a




?
A
A
Aa
Aa
AA
Aa
a
a
aa
aa
aa
Aa
50 dwarf50 normal or 11
67 dwarf33 normal or 21
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Many genes one trait

• Polygenic inheritance
• additive effects of many genes
• humans
• skin color
• height
• weight
• eye color
• intelligence
• behaviors

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Human skin color

• AaBbCc x AaBbCc
• can produce a wide range of shades
• most children intermediate skin color
• some can be very light very dark

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Albinios
Johnny Edgar Winter
albinoAfricans
melanin universal brown color
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Coat color in other animals

• 2 genes E,e and B,b
• color (E) or no color (e)
• how dark color will be black (B) or brown (b)

EB
Ebb
eeB
eebb
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Environment effect on genes

• Phenotype is controlled by both environment
  genes

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

• Women men are very different, but just a few
  genes create that difference
• In mammals 2 sex chromosomes
• X Y
• 2 X chromosomes female XX
• X Y chromosome male XY

X
X
X
Y
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Sex chromosomes
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Sex-Linked Traits
2 normal parents, but mother is carrier
x


XX
XXh
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Sex-Linked Traits

• Sex chromosomes have other genes on them,
  especially the X chromosome
• Hemophilia in humans
• Blood doesnt clot
• Duchenne muscular dystrophy in humans
• Loss of muscle control
• Red-green color blindness
• See green red as shades of grey

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Polydactyly

• Individuals are born with extra fingers and toes
• The allele for 6 fingers and toes is
  dominant,while the allele for 5 digits is
  recessive
• Recessive is far more common! (1500 have
  polydactyly)