Gregor Mendel

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Gregor Mendel
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Gregor Mendel

• Austrian monk
• Studied science and mathematics at University of
  Vienna
• Conducted breeding experiments with the garden
  pea Pisum sativum
• Carefully gathered and documented mathematical
  data from his experiments
• Formulated fundamental laws of heredity in early
  1860s
• Had no knowledge of cells or chromosomes
• Did not have a microscope

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Fruit and Flower of theGarden Pea
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Garden Pea TraitsStudied by Mendel
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Blending Inheritance

• Theories of inheritance in Mendels time
• Based on blending
• Parents of contrasting appearance produce
  offspring of intermediate appearance
• Mendels findings were in contrast with this
• He formulated the particulate theory of
  inheritance
• Inheritance involves reshuffling of genes from
  generation to generation

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Mendels Monohybrid CrossesAn Example
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One-Trait Inheritance

• Mendel performed cross-breeding experiments
• Used true-breeding (homozygous) plants
• Chose varieties that differed in only one trait
  (monohybrid cross)
• Performed reciprocal crosses
• Parental generation P
• First filial generation offspring F1
• Second filial generation offspring F2
• Formulated the Law of Segregation

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Mendels Monohybrid CrossesAn Example
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Law of Segregation

• Each individual has a pair of factors (alleles)
  for each trait
• The factors (alleles) segregate (separate) during
  gamete (sperm egg) formation
• Each gamete contains only one factor (allele)
  from each pair
• Fertilization gives the offspring two factors for
  each trait

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Modern Genetics View

• Each trait in a pea plant is controlled by two
  alleles (alternate forms of a gene)
• Dominant allele (capital letter) masks the
  expression of the recessive allele (lower-case)
• Alleles occur on a homologous pair of chromosomes
  at a particular gene locus
• Homozygous identical alleles
• Heterozygous different alleles

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Homologous Chromosomes
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Genotype Versus Phenotype

• Genotype
• Refers to the two alleles an individual has for a
  specific trait
• If identical, genotype is homozygous
• If different, genotype is heterozygous
• Phenotype
• Refers to the physical appearance of the
  individual

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Punnett Square

• Table listing all possible genotypes resulting
  from a cross
• All possible sperm genotypes are lined up on one
  side
• All possible egg genotypes are lined up on the
  other side
• Every possible zygote genotypes are placed within
  the squares

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Punnett Square ShowingEarlobe Inheritance
Patterns
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Try this one

• MONOHYBRID CROSS
• Cross a heterozygous tall plant with a
  heterozygous tall plant (use T tall and t
  short) Determine expected genotype and phenotype
  ratios.

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Try these!Show work

• Cross a heterozygous red flower with a white
  flower. What is the genotype and phenotype ratio
  for the offspring? Key R red r white

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Two-Trait Inheritance

• Dihybrid cross uses true-breeding plants
  differing in two traits
• Observed phenotypes among F2 plants
• Formulated Law of Independent Assortment
• The pair of factors for one trait segregate
  independently of the factors for other traits
• All possible combinations of factors can occur in
  the gametes

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Try Mendels Classic Dihybrid Cross

• Cross two heterozygous tall, heterozygous green
  pod producing plants. Use a punnett square to
  show expected offspring and complete a phenotype
  ratio.
• Key
• T tall G green pods
• t short g yellow pods

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Two-Trait (Dihybrid) Cross
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Must Know Your Vocab!

• Homozygous?
• Heterozygous?
• Genotype?
• Phenotype?

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Try this one DIHYBRID CROSS2 traits

• Key T tall R red
• t short r white
• Cross two heterozygous tall, heterozygous red
  flowered plants.
• What is the phenotypic ratio of the offspring?

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Two-Trait (Dihybrid) Cross
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P-square practice

• Practice crosses on a separate sheet of paper.
• Show parental cross
• Show p-square
• Show phenotype ratio

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• WHATs IN YOUR GENES?
• Mom 22 autosomes plus X sex chromosome
• Dad 22 autosomes plus X or Y chromosome

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Boy or Girl?

• Dad determines this sperm carries 22 autosomes
  and either an X or Y sex chromosome
• BOYS your mom gave you the X and dad gave you
  the Y so what?

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Sex-linked disorders carried on the X
chromosome

• Colorblindness
• Hemophilia
• Baldness (?)

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Analyze Sex-linked traits

• Due tom!

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• - Albinism ppt
• Huntingtons ppt
• Final concepts

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Autosomal Recessive Pedigree Chart
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Autosomal Dominant Pedigree Chart
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INHERITING A GENE - ALBINISM
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This is an albino skunk. The cells are not able
to produce the protein that causes color.
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Cells in the skin produce a black-brown pigment
called melanin.
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The chemical melanin is produced by specialized
cells in the epidermis called melanocytes.
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The melanin leaves the melanocytes and enters
other cells closer to the surface of the skin.
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Different shades of skin colors is determined by
the amount of melanin deposited in these
epidermal cells
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Sunlight causes melanocytes to increase
production of melanin.
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A tan fades because the cells break down the
melanin.
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Some organisms, such as the octopus, can rapidly
change from light to dark.
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They control the color by scattering the melanin
in the cell for a dark color, and concentrating
the melanin in the center for light color.
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Melanin is made by the melanocytes by chemically
changing the amino acid, phenylalanin, into
tyrosine and then into melanin.
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An enzyme is required to change tyrosine into
melanin.
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If the enzyme is not present, then melanin cannot
be produced by the melanocytes.
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The result of no melanin is an albino.
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The eyes of an albino appear pink because there
is no dark melanin in the eye to absorb light.
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The blood in the retina and iris reflects red
light, resulting in pink eyes.
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The gene that produces this enzyme is on
chromo-some 9
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If both the genes produce the enzyme tyrosinase,
there is plenty to convert tyrosine to melanin.
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If neither gene produces tryosinase, no melanin
is produced and
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The crow is an albino rather than the normal black
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What if one gene is normal and one gene does not
produce the enzyme?
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The one normal gene produces enough enzyme to
make normal crow color
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This albino squirrel received one albino gene
from the father and one albino gene from the
mother.
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But what if a squirrel gets a normal gene from
one parent and an albino gene from the other
parent?
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The one functioning gene produces enough enzyme
to make melanin for normal coloration.
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Is it possible for two normal colored cockatiels
to have an albino offspring?
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Yes! Remember the albino has two genes for
albinism. One gene from the father and one gene
from the mother.
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To be albino, both genes must be albino genes
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A normal colored bird could have one albino gene
and one normal gene.
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If the sperm of a normal colored male pigeon has
an albino gene and the ova it fertilizes has an
albino gene than the offspring will be albino.
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The same happens in humans. A normal pigment
father and mother can have an albino offspring.
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We can see this in a genetic family tree called
a pedigree. The circles are females, the squares
are males. The open symbols are normal
coloration, the black symbols are albino.
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The parents in the circle have normal pigment.
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Most of the offspring received at least one
normal gene from a parent.
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But one female offspring received an albino gene
from both the mother and the father.
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A Punnett square is a matrix to show the genetics
of a mating.
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What is the probability of an albino doe giving
birth to a normal fawn if she has mated with a
normal male?
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The female must have two albino genes (use small
a for the albino gene
- aa
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Since the albino gene is relatively rare, the
male probably has two normal genes of color.
(Capital A stands for the normal gene)
- AA
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AA X aa
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Next, add the possible sperm and ova genes.
A A
a a
Aa
Aa
Aa
Aa
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As long as there is one normal gene, none of the
offsprings will be albino
A A
a a
Aa
Aa
Aa
Aa
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Therefore, all offsprings will have a normal and
an albino gene.
A A
a a
Aa
Aa
Aa
Aa
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An albino must get one albino gene from the
father and one albino gene from the mother.
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Then how could an albino female penguin have an
albino chick.
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The normal colored father must have one normal
coloration gene and one albino gene.
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There is only one way for two normal colored
parents to produce an albino offspring.
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Both parents must have one normal gene and one
albino gene.
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Aa X Aa
Both parents have one gene for normal and one
gene for albinism.
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Aa X Aa
The fathers sperm is 50 with normal gene and
50 with albino gene.
A
a
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Aa X Aa
50 of the mothers ova have a normal gene and
50 of the ova have the albino gene
A
a
A
a
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Aa X Aa
The ova and sperm may combine to form an
offspring with two normal genes, a normal gene
and an albino gene, or two albino genes.
A
a
A
AA
Aa
a
aa
Aa
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Aa X Aa
Only the offspring with two albino genes will
lack pigment.
A
a
A
AA
Aa
a
aa
Aa
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Sometimes an albino is born and there is no
history of albinism in the colony.
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The color gene in the cell that produced this
white flower changed to an albino gene.
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A change in a gene is called a mutation.
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