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Patterns of Inheritance
Chapter 10
Blending Hypothesis of Inheritance
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Trait
 A variation of a particular characteristic
Blending hypothesis (1800s)
 Early explanation of how offspring
inherit trait from both parents
 Example: if a red flower plant crossed
with a yellow flower, the offspring
would be orange
 Later discarded
Gregor Mendel
Austrian monk
 Father of Genetics (study of
heredity)
 Said parents pass on to their
offspring separate and distinct genes
 Studied 7 characteristics in pea plants
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True breeding plants
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A true plant will show the same physical
appearance generation after generation after
self-fertilization
Cross fertilization
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The sperm from the pollen of one true flower
fertilizes the eggs in the flower of a different
plant
Mendel's Experiments
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Cross-fertilized 2 true-breeding plants
each with contrasting traits (i.e. white and
purple flowers)
What color of flowers do you think the
offspring plants were?
Principle of Segregation
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P generation
 Parental plants (purebred and true
breeding)
F1 generation ( F for filial “son”)
 Hybrid offspring
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Hybrids
 The offspring of 2 different true-breeding
varieties
F2 generation
 When F1 self-fertilize or fertilize each other
Monohybrid Cross
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Monohybrid cross
 Cross fertilization in which only one
physical characteristic is considered
 In Mendel's cross, all F1 were purple but
¼ of F2 were white
Gene Hypotheses #1
•
There are alternative forms of genes
which determine physical appearances
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Allele is the term
Example: Flower color can be white or
purple
Gene Hypotheses #2
For each characteristic, an organism has
2 alleles for genes controlling the
physical appearances (one from each
parent)
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•
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If 2 alleles are the same= homozygous
If 2 alleles are different =heterozygous
Gene Hypotheses #3
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Dominant alleles determine the physical
appearance in a heterozygous individual.
Recessive allele is the other allele that
does not affect the physical appearance
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Capital letter represents dominant allele : P
Lower case letter represents recessive allele: p
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Phenotype is the physical appearance
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purple
purple
white
Genotype is the genetic makeup
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Possible genotype are PP, Pp, pp.
Gene Hypotheses #4
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The two alleles for a character segregate
(separate) during meiosis so that each
gamete carries only one allele for each
character, known as principle of
segregation.
Punnett Square
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The alignment of combination of gametes
to form zygotes with pairs of alleles is
random
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Like tossing a coin.
Punnett Square is a diagram that shows all
possible outcomes of a genetic cross.

Used to predict probabilities of outcomes if
you know the genotypes of the parents
The Testcross
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Breeds an individual of unknown
genotype, but dominant phenotype
(purple) with a homozygous recessive
individual (white)
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Appearance of F1 will reveal the genotype of
the mystery parent
If white flowers are produced, the unknown
parent must be heterozygous (Pp) and have a
recessive trait
Intermediate Dominance
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Heterozygotes have a phenotype
intermediate between the phenotypes of the
two homozygote
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This is referred to as INCOMPLETE DOMINANCE
Rules: (example: snapdragon flowers)
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Capital/lower case letters not used
Instead, a C for “color” is paired with a
superscript R for “red” and W for “white”
CR CR is red and CW CW is white
CR CW is pink
There is a breed of chicken called
Andalusians, black and white parents
produce F1 hybrid offspring, called
"blues," with grayish-blue feathers.
Because neither the black nor white
allele is dominant, capital and
lowercase letters are not used to
represent them.
Instead, a C for "color" is paired
with a superscript B for "black" or
W for "white" to represent the two
alleles.
A heterozygote chicken has one of
each allele, CBCW, and is grayishblue in color
Although the F1 phenotypes are
intermediate, this inheritance
pattern does not support the
blending hypothesis.
This is because the parent
phenotypes can reappear in the F2
generation.
Multiple alleles
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Heterozygote express the distinct traits of
both alleles
Example: Human blood system
 A, B, AB, or o
 The letters are antigens found on the
surface of red blood cells
 Red blood cells may be coated with one
protein (A), the other (B), both (AB), or
neither (O)
 There are six possible genotype
combinations
ABO blood type is a genetic example of
multiple alleles.
There are three alleles in the gene pool for
ABO blood type.
IA
IB
i
IA codes for protein A
IB codes for protein B
i codes for neither protein A nor protein B.
Within this multiple allele pool the gene
interactions illustrate both simple
dominance as well as co-dominance.
Remember each individual has only two
alleles for each trait even if there are
multiple alleles in the gene pool.
IAIA
both code for A type blood
IAi
Phenotype
Genotype
Protein on
RBC
(antigen)
Antibodies
in the blood
plasma
Type A
IA IA and
IA i
A
b
Type B
IB IB and
IB i
B
a
Type AB
IA IB
A and B
-------------
Type O
ii
-----------
a and b
ABO Blood System
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Antibodies (proteins) also found in the
blood serum that attacks foreign antigens
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Blood
Blood
Blood
Blood
A has antibody Anti-B
B has antibody Anti-A
AB has no antibody
O has Antibody Anti A and B
Blood O is the universal donor
Blood AB can receive any blood type
Rh Factor
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Rh positive (Rh +) has protein in blood
Rh negative (Rh -) has no protein in blood
Rh+ is dominant
Blood Typing
Blood Type
Anti-a Sera
Anti-b Sera
A
Clumping
No clumping
B
No clumping
Clumping
AB
Clumping
Clumping
O
No clumping
No clumping
Sex-linked genes
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The eggs contain a single X chromosome
and sperm contain either an X or a Y
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Sex of the offspring depends on whether the
sperm that fertilizes the egg has an X or a Y
Any gene located on a sex chromosome
(X) is called a sex-linked gene
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Most are found on the X (2,000) and few on
the Y (24)
Sex-linked traits
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Written as a XRXr for heterozygous.
Y chromosome carries no allele and the
phenotype is dependant upon the
woman’s allele
Therefore, males carry one allele for a
sex-linked trait.
Sex-linked disorders
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Red-green blindness
Hemophilia (inability of blood to clot)