Observing Patterns in Inherited Traits

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Observing Patterns in Inherited Traits

• Unit 9

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Talking Genetics

• Genes units of information about specific
  traits that are transmitted from parent to
  offspring
• Locus location of a gene on a chromosome
• Alleles different forms of the same gene
• Hybrids offspring having nonidentical alleles
  for a trait

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Talking Genetics

• Homologous chromosomes pairs of chromosomes
  identical in size, shape and gene sequence. They
  exist in diploid (2N) cells and separate during
  meiosis.
• Homozygous having a pair of identical alleles
  on homologous chromosomes
• Heterozygous having a pair of nonidentical
  alleles on homologous chromosomes

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Talking Genetics

• Dominant an allele or trait is dominant when it
  is observable even if it is only present on one
  homologous chromosome. It is also expressed when
  present on both homologous chromosomes. Dominant
  alleles are signified by capital letters.

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Talking Genetics

• Recessive an allele or trait is recessive when
  it is observable only if present on both
  homologous chromosomes. Recessive alleles are
  signified by lower case letters
• AA is homozygous dominant
• aa is homozygous recessive
• Aa is heterozygous

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Talking Genetics

• Genotype refers to the alleles an individual
  carries
• Phenotype refers to the individuals observable
  traits
• P stands for parents
• F1 stands for first generation offspring
• F2 stands for second generation offspring

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Pea Plants
Gregor Mendel, a scholarly monk interested in
math and plant breeding.
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Mendels Experimental DesignA Monohybrid Cross

• P AA (purple) and aa (white)
• Mendel knew the parents were pure breeds from
  crossing them many times and always producing
  offspring identical to the parent
• AA x aa produced F1 all Aa
• Aa x Aa produced F2 in a ratio of 3 purple to 1
  white
• He crossed many plants and tracked thousands of
  offspring
• This led him to the theory of segregation

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Monohybrid CrossIllustrated
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Theory of Segregation

• An individual inherits a unit of information (a
  gene) from each parent.
• During gamete formation, the genes segregate from
  one another.

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Simple Dominance

• Some alleles are dominant. They are expressed if
  present on one gene. They are also expressed if
  present on both genes.
• Some alleles are recessive. They are only
  expressed if present on both genes.

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Another of Mendels ExperimentsA Dihybrid Cross

• P AABB (purple and tall) x aabb (white and
  small) Mendel knew the parents were pure breeds
  from crossing them many times and always
  producing offspring identical to the parent
• AABB x aabb produced an F1 all AaBb
• AaBb x AaBb produced an F2 of 9 tall purple
  plants 3 dwarf purple plants 3 tall white
  plants 1 dwarf white plant
• This led Mendel to the theory of independent
  assortment

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Dihybrid Cross

• AaBb X AaBb
• Experimental cross between individuals that are
  heterozygous for different versions of two traits
  (trait A and trait B)

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Dihybrid Cross F1 Results
purple flowers, tall
white flowers, dwarf
TRUE- BREEDING PARENTS
AABB
aabb
x
GAMETES
AB
AB
ab
ab
AaBb
F1 HYBRID OFFSPRING
all purple-flowered, tall
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Independent Assortment

• During meiosis, genes on pairs of homologous
  chromosomes have been sorted out for distribution
  into one gamete or another independently of gene
  pairs of other chromosomes.
• Remember this from meiosis?
• Mendel figured this out without seeing
  chromosomes!

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Independent Assortment
Metaphase I
OR
A
A
A
A
a
a
a
a
B
B
B
B
b
b
b
b
Metaphase II
A
A
A
A
a
a
a
a
B
B
B
B
b
b
b
b
Gametes
B
B
B
B
b
b
b
b
A
A
A
A
a
a
a
a
1/4 AB
1/4 ab
1/4 Ab
1/4 aB
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Summary of Mendels Work

• The factors that control heredity are individual
  units known as genes. In organisms that reproduce
  sexually, genes are inherited from each parent.
  (GENES COME FROM EACH PARENT)
• In cases in which two or more forms for a single
  trait exist, some forms may be dominant and
  others may be recessive. (SOME GENES ARE
  EXPRESSED OVER OTHERS)
• The two forms of each gene are separated during
  meiosis when gametes are formed. (THEORY OF
  SEGREGATION)
• Homologous chromosomes for different traits
  assort independently of one another. (THEORY OF
  INDEPENDENT ASSORTMENT)

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How to work genetics problems

• First goal correctly write genotypes and
  phenotypes
• Second goal show the gametes of the mother and
  father on a punnett square
• Third goal predict the genotypes of the
  offspring using a punnett square
• Fourth goal predict the phenotypes of the
  offspring using a punnett square
• Fifth goal use a punnett square to solve
  genetics problems involving simple dominance

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To predict the outcome of a cross, we use a
special tool called a punnett square.
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Punnett Square
So, a cross between two parents with the
genotypes Aa x Aa would look like the following
Lets look at the anatomy of a punnett square
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Anatomy of a punnett square
Lets see how it looks on a real problem
Possible gametes from mother
Possible gametes from father
Possible offspring
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Setting up the punnett Square for a dihybridhow
would you do it?
If the F1 Parent genotypes are AaBb, how many
different gametes can they make? (Remember, each
sperm/egg must have one allele from each gene)
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AB
Ab
aB
ab
What are they?
AB Ab aB ab
AB
Just like small punnett squares
Now lets put the mothers gametes in the punnett
square ?
And now the fathers gametes ?
Ab
aB
ab
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Dihybrid Cross F2 Results
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Not So-Straight-Forward Phenotypes

• Incomplete dominance
• Codominance
• Continuous variation

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Not-so-straight-forward phenotypes

• First goal understand the inheritance patterns
  of incomplete dominance, codominance and
  continuous variation.
• Second goal use a punnett square to solve
  genetics problems involving incomplete dominance
  and codominance.

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Codominance ABO Blood Types

• The gene that controls ABO type codes for the
  enzyme that dictates structure of a glycolipid on
  blood cells
• Two alleles (IA and IB) are codominant when
  paired
• Third allele (i) is recessive to others
• This is an example of codominance!

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ABO Blood TypeA Multiple Allele System
Range of genotypes
IA IA
IB IB
or
or
IA i
IA IB
IB i
ii
Blood types
A
AB
B
O
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Incomplete Dominance

• All allele of a pair is not fully dominant.
  Heterzygotes phenotype is between the phenotype
  of the two homozygotes.
• An example is petal color in snap dragons. Red
  has incomplete dominance and white is recessive.

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Incomplete Dominance
X
homozygous parent
homozygous parent
All F1 are heterozygous
Incomplete Dominance
X
F2 shows three phenotypes in 121 ratio
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Continuous Variation

• This is a more or less continuous range of small
  differences in a given trait among the
  individuals of a population.
• It results from the effects of many genes and
  environmental effects on a trait.
• Example is eye color

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The End