Patterns of Inheritance

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Patterns of Inheritance
By observing how traits are passed to the next
generation, how can the inheritance patterns be
used to understand the principles of heredity?
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Use of Garden Pea for Genetics Experiments
Stamens(male)producepollen
Carpel(female)produceseggs
Flower dissected to showreproductive structures
Intact pea flower
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Mendels Experiment With Peas Differing in a
Single Trait
F1 smooth plants x F1 smooth plants
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Patterns of Inheritance

• Mendel needed to explain
• Why one trait seemed to disappear in the first
  generation.
• 2. Why the same trait reappeared in the second
  generation in one-fourth of the offspring.

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Mendels Proposal

• Each trait is governed by two factors now
  called genes.
• 2. Genes are found in alternative forms called
  alleles.
• 3. Some alleles are dominant and mask alleles
  that are recessive.

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Mendels Experiment With Peas Differing in a
Single Trait
SS
ss
HomozygousRecessive
HomozygousDominant
Ss
Heterozygous
F2
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Homozygous parents can only pass one form of an
allele to their offspring.
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Heterozygous parents can pass either of two forms
of an allele to their offspring.
S
s
S
s
Locus Area on the chromosome where a gene is
located. For a heterozygote, homologous
chromosomes will have different alleles at the
same locus.
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Additional Genetic Terms
Term Definition Example
Genotype Alleles carried by an individual SS, Ss, ss
Phenotype Physical characteristic or appearance of an individual smooth or wrinkled
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Mendels Principle of Genetic Segregation

• In the formation of gametes, the
• members of a pair of alleles separate (or
  segregate) cleanly from each other so that only
  one member is included in each gamete.
• Each gamete has an equal probability of
  containing either member of the allele pair.

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Genetic Segregation
Parentals SS x ss
F1 x F1 Ss x Ss
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Traits Studied by Mendel







Seed shape
Seed color
Pod shape
Pod color
Flower color
Flower location
Plant size
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Mendels Experiment With Peas Differing in Two
Traits
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Patterns of Inheritance

• Mendel needed to explain
• Why non-parental combinations appeared in the F2
  offspring.
• 2. Why the ratio of phenotypes in the F2
  generation was 9331.

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Mendels Principle of Independent Assortment

• When gametes are formed, the alleles of one
  gene segregate independently of the alleles of
  another gene producing equal proportions of all
  possible gamete types.

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Genetic Segregation Independent Assortment
Parentals SSYY x s s y y
F1
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Genetic Segregation Independent Assortment
F1 x F1 S s Y y x S s Y y
Four different types of gametes are formed in
equal proportions.
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F1 x F1 SsYy X SsYy
Eggs
SY
Sy
sY
sy
14
14
14
14
SY
14
Sy
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Pollen
sY
14
sy
14
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F2 Genotypes and Phenotypes
Phenotypes Genotypes
Smooth Yellow
Smooth Green
Wrinkled Yellow
Wrinkled Green
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Meiotic Segregation Explains Independent
Assortment
Two possibleorientations
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Additional Genetic Patterns
Mendels peas Alternative Pattern
Complete Dominance Incomplete Dominance
Incomplete dominance neither allele masks the
other and both are observed as a blending in the
heterozygote
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Incomplete Dominance
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Incomplete Dominance
F1 x F1Pink x Pink RR x RR
Genotypic Ratio Phenotypic Ratio
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Additional Genetic Patterns
Mendels peas Alternative Patterns
Complete Dominance Codominance
Two alleles per gene Multiple Alleles
Codominance Neither allele masks the other so
that effects of both alleles are observed in
heterozygotes without blending
Multiple Alleles Three or more alleles exist for
one trait Note A
diploid individual can only carry any two
of these alleles at once.
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Multiple Alleles and Codominance

• ABO Blood Type in Humans

Blood Type Allele
Type A A
Type B B
Type O o
A B gt o
A and B are codominant. A and B are completely
dominant over o.
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Human ABO Blood Types

• Freq

• Donates

• Re-ceives

• Anti-bodies

• Antigen on RBCs

• Genotype

• Type

• 40

• A or AB

• A or O

• B

• AA or Ao

• A

• 10

• B or AB

• B or O

• A

• BB or Bo

• B

• 4

• AB(universal)

• AB, A,B, O(universal)

• Neither

• AB

• AB

• 46

• O,AB,A,B(universal)

• O

• Both

• oo

• O

Codominance is observed for Type AB Blood since
the products of both the A and B alleles are
found on the cells.
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Inheritance of Rh Factor
Phenotype Genotype Gene Product Antibodies Present
Rh Positive RR or Rr Rhesus Protein None
Rh Negative rr None None unless exposed
Although there are multiple R alleles, R1, R2,
R3, etc. all are completely dominant over all of
the r alleles, r1, r2, r3, etc.
ABO Blood Type and Rh Factor are controlled by
separate genes. They show independent
assortment.
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Multiple Alleles and Codominance
Type A, Rh positive x Type B, Rh negative
Phenotypic Ratio of Offspring
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Additional Genetic Patterns
Mendels peas Alternative Patterns
One gene affects one trait Polygenic Inheritance
Polygenic Inheritance Many genes affect
one trait
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Example of Polygenic Inheritance

• Two genes affecting one trait

Number of Dominant Alleles Skin Color (Phenotype) Genotypes Pigmentation
0 White aabb 0-11
1 Light Black Aabb or aaBb 12-25
2 Medium Black AAbb or AaBb or aaBB 26-40
3 Dark Black AABb or AaBB 41-55
4 Darkest Black AABB 56-78
Based on a study conducted in Jamaica.
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Example of Polygenic Inheritance
Medium Black Woman X Darkest Black Man
(her mother is white)
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Additional Genetic Patterns
Mendels peas Alternative Patterns
One gene affects one trait Pleiotropy
Pleiotropy One gene affects many traits
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Sickle-Cell Anemia

• One gene affects many phenotypic
  characteristics

Gene Product Cell Shape Disease Conditions
SS Hemoglobin A Spherical, slightly concave No anemia
SS Hemoglobin A Hemoglobin S Some sickling under extreme conditions Sickle Cell Trait Resistance to Malaria
SS Hemoglobin S Sickled under low O2 tension Sickle Cell Anemia