Mendelian Inheritance

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Mendelian Inheritance

• ... the outward physical manifestation of
  internally coded, inheritable, information.

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Genetics expression of genes
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Gregor Mendel

• Father of Genetics
• 1823-1884

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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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One-Trait Inheritance

• Mendel performed cross-breeding experiments with
  pea plants
• 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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Why Peas?

• Either or traits
• Easy to grow
• Many offspring
• Easy to regulate pollination

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Mendels method
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Tall
Short
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Mendels Hypotheses

• Each parent has two factors (alleles)
• Each parent gives one of those factors to the
  offspring
• Tall has TT
• Short has tt
• Tall is dominant
• Short is recessive

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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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TT
tt
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TT
TT
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tt
tt
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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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Law of Segregation and Random Fertilization
genetic variation

• Alleles separate during gamete production
• Gametes have one allele for each trait
• During fertilization gametes combine at random to
  form individuals of the next generation

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Discovery of Chromosomes in 1900 Confirms Law of
Segregation

• Chromosomes are in pairs
• Each chromosome has one of the allele pair

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Homologous Chromosomes

• Chromosomes of the same pair
• Each homologue will have one allele for a paired
  gene
• Homologous chromosomes pair up during meiosis
• Only one of each homologue will be in each gamete

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T
T
t
t
Assume a T allele on each red chromatid and a t
allele on each green chromatid
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t
t
T
T
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Doubled Chromosomes Separate in Second Meiotic
Division
T
T
t
t
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Each gamete will have a T allele or a t allele
T
T
t
t
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Allele

• Member of a paired gene
• One allele comes from each parent
• Represented by a single letter

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Dominant Recessive Alleles

• Dominant alleles are expressed
• Recessive alleles are not expressed in the
  presence of a dominant allele
• Recessive alleles are only expressed if both
  alleles are present

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Homozygous

• Both alleles alike
• AA or aa

or
A A
a a
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Heterozygous

• Alleles are different
• Aa

A
a
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Genotype

• Genetic make up
• Represented by alleles
• TT Tt are genotypes for tall pea plants

This is the "internally coded, inheritable
information" carried by all living organisms.
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Phenotype

• A trait
• Genotype determines the phenotype
• Tall is a phenotype
• Think adjective!
• Descriptive

This is the "outward, physical manifestation" of
the organism.
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• Phenotype red flowers
• Cells contain red granules
• Enzymes help convert colorless pigment into red
  pigment
• Most enzymes are proteins
• Most traits are produced by the action of
  proteins.

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Summary 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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Monohybrid Testcross

• Individuals with recessive phenotype always have
  the homozygous recessive genotype
• However, Individuals with dominant phenotype have
  indeterminate genotype
• May be homozygous dominant, or
• Heterozygous
• Test cross determines genotype of individual
  having dominant phenotype

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One-Trait Test CrossUnknown is Heterozygous
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One-Trait Test CrossUnknown is Homozygous
Dominant
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Human Genetic Disorders

• Autosome - Any chromosome other than a sex
  chromosome
• Genetic disorders caused by genes on autosomes
  are called autosomal disorders
• Some genetic disorders are autosomal dominant
• An individual with AA has the disorder
• An individual with Aa has the disorder
• An individual with aa does NOT have disorder
• Other genetic disorders are autosomal recessive
• An individual with AA does NOT have disorder
• An individual with Aa does NOT have disorder, but
  is a carrier
• An individual with aa DOES have the disorder

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Albinism

• Lack of pigment
• Skin
• Hair
• Eyes

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Melanin Pigment
Enzyme
Amino Acids
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A man woman are both carriers (heterozygous)
for albinism. What is the chance their children
will inherit albinism?
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Man Aa
Woman Aa
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Aa
AA
Aa
aa
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Genotypes
1 AA, 2Aa, 1aa
Phenotypes
3 Normal 1 Abino
Probability
25 for albinism
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Dwarfism
Dwarfism D Normal height d
DD Dwarfism Dd Dwarfism dd Normal height
Dwarf Band
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A man with heterozygous dwarfism marries a woman
who has normal height. What is the chance their
children will inherit dwarfism? Dwarfism is
dominant.
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d
d
D
d
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Genotypes
2 Dd, 2dd
Phenotypes
2 Normal 2 Dwarfs
Probability
50 for Dwarfism
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Polydactyly (more than the normal amount of
fingers or toes) is a dominant trait!
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Law of Independent Assortment

• The inheritance of one gene does not influence
  the inheritance of another gene if they are on
  separate chromosomes.
• The gene for albinism does not affect the gene
  for dwarfism

When in the Course of human events it becomes
necessary for one genome to dissolve the
temporary bonds which have connected them with
another and to assume among the powers of the
earth, the separate and equal station to which
the Laws of Nature and of Nature's God entitle
them, a decent respect to the variation that
mankind requires that they should declare the
causes which impel them to the separation.
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Two Trait Problem

• A heterozygous tall plant that is also
  heterozygous for yellow seeds is crossed with
  another plant with the same genotype
• Tall and yellow seeds are dominant to short and
  green seeds.

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What gametes can each parent produce?
Tall Yellow
Tall Yellow
TtYy
TtYy
TY
Ty
tY
ty
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Match gametes on a Punnent Square
1 Short-Green
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A man with blue eyes and normal height marries a
woman with heterozygous brown eyes and
heterozygous dwarfism. What are the possible
phenotypes of their children? Dwarfism brown
eyes are dominant.
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What gametes can each parent produce?
Dwarf-Brown
Normal height-Blue
ddbb
DdBb
db
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Match gametes on Punnent Square
db
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