Chapter 10:
Patterns of
Inheritance
Mendel’s Experimental
Procedure
Gregor Mendel is an Austrian Monk
Observed and work with 22 varieties
of garden pea plants, Pisum sativum
True-breeding: when the flower selfpollinated
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o
The offspring will be like the
parent plants and like each other.
Particulate theory of inheritance:
characteristics are passed from one
generation to the next through
discrete particles (genes) that retain
their ability to be expressed, even
though they may not appear in every
generation
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Inheritance involves the
reshuffling of the same genes from
generation to generation
http://www.webref.org/anthropology/p/particulate_theory_of_inheritanc.htm
Trait
One-Trait Inheritance
One trait was chosen to be observed
In this case, height
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P generation or the original parents are either tall
(TT) or short (tt)
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F1 generation is the first-generation offspring
o
F2 generation is the second-generation offspring
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Punnet Square: shows every possible combination
of gametes the offspring may inherit
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A testcross between F1 generation tall plants with
true-breeding short plants resulted in a 1:1 ratio
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Indicating that the factors segregate when
gametes are formed
He arrived at the law of segregation, which
states:
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Each individual has 2 factors for each trait
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The factors segregate during the formation
of the gametes
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Each gamete contains only one factor from
each pair of factors
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Fertilization gives each new individual 2
factors for each trait
Modern Genetics View
Traits are controlled by alleles (alternate
forms of a gene) that are found on the
chromosome at the gene locus
The dominant allele has the ability to
mask the expression of the recessive
allele
What process that we have learned so far
helps explain Mendel’s law of
segregation?
Genotype v phenotype
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Genotype refers to the alleles of an
organism for a particular trait or traits
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Homozygous refers to inheritance of 2
identical alleles
Heterozygous refers to inheritance of
2 different alleles
Phenotype refers to the physical
appearance of the individual
Two-Trait Inheritance
Testcrossing of true-breeding plants that
differed in two traits
F1 plants were crossed pollinated and F2
generation was observed
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If the dominant factors (TG) always
go together into the F1 gametes and
the recessive factors (tg) always stay
together, then only 2 phenotypes will
be observed in the F2 plants
If the 4 factors segregate into the F1
gametes independently, then 4
phenotypes will be observed in the F2
plants
Law of independent assortment:
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Each pair of factors segregates
(assorts) independently of the other
pairs
All possible combinations of factors
can occur in the gametes
Two-Trait Testcross
Fruit flies Drosophila melanogaster are usually the
favorite subject in studying genetics
The wild type fly are crossed with a mutant fly
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An individual with dominate phenotype for
both traits is cross with one having the
recessive phenotype for both traits
A dihybrid is an individual that is
heterozygous for 2 traits (LlGg)
When crossed with a homozygous recessive,
it will give a phenotypic ratio of 1:1:1:1
What are the chances of an offspring with
long wings and gray body?
What are the chances of an offspring with
short wings and gray body?
o
What will happen if the L_G_ fly is homozygous
for both traits?
o
What will happen if the L_G_ fly is
heterozygous for one trait but not the other?
o
The rule of multiplication: the chance of 2 (or
more independent events occurring together is
the product of their chances of occurring
separately
EX: coin toss
Mendel’s Laws and Meiosis
Homologues separate independently so it does not
matter which member of each pair goes into which
gamete
Incomplete Dominance
Incomplete dominance:
inheritance pattern in which
the offspring has an
intermediate phenotype
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Ex: when a red-flowered
plant and a white-flowered
plant produce pink-flowered
offspring
When pink-flowered plants
self-pollinate, ¼ is red, ¼ is
white, and ½ pink
We can say that since only the
dominant allele codes for a
gene product, one allele of the
product gives the intermediate
result
Multiple-Allele Traits
In ABO blood group, 3 alleles determine the
presence or absence of antigens on RBCs
Every individual has only 2 of the 3 alleles
IA = A antigen on RBCs
IB = B antigen on RBCs
I = Neither A nor B antigen
on RBCs
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IA and IB are dominant over I
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Type A blood (IAIA, Iai)
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Type B blood (IBIB, IBi)
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Type O blood (ii)
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Codominance: Inheritance pattern in which
both alleles of a gene are equally expressed in a
heterozygote
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IA and IB are fully expressed in the presence of
each other
Type AB blood (IAIB)
Polygenic Inheritance
A trait is governed by two or more sets of
alleles
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Copies of allelic pairs are located on
many different pairs of chromosomes
Each dominant allele has a
quantitative effect on the phenotype,
and the effects are additive
Multifactorial traits are those that are
controlled by polygenes subject to
environmental influences
1.
2.
3.
white primroses above 32°C and red
primroses at 24°C
Water buttercups have different
appearance above and below water
Siamese and Himalayan rabbits have
black fur only on the extremities
where body heat is lost to the
environment
Pleiotrophy
A single gene has more than one effect
Syndrome: a group of symptoms that
appear together caused by a particular
genetic mutation
Marfan Syndrome
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Due to production of abnormal
connective tissue
Mutated gene (FBN1) on
chromosome 15 that make
functional protein, fibrillin
Essential in the formation of
elastic fibers in connective tissue
Symptoms:
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Disproportionately long arms,
legs, hands, feet
Weakened aorta
Poor eyesight
Porphyria
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Caused by a chemical insufficiency
in the production of hemoglobin
Hemoglobin is necessary for
functional RBCs
Symptoms:
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Photosensitivity
Strong abdominal pain
Port-wine-colored urine
Paralysis
Members of the British royal family
suffered from this disorder
Sex-Linked Inheritance
Y chromosome contains only 80 genes (most are
concerned with sex difference)
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SRY is not present on the X chromosome
o
X chromosome contains nearly 2,000 genes
(most not involving gender)
o
X-linked are genes that are carried on the X
chromosome
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The Y chromosome is blank for these alleles
Inheritance of a Y chromosome cannot
offset the inheritance of an X-linked
recessive allele
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Males always receive an X-linked recessive
mutant allele from their female parent
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Heterozygous females are carriers (they do
not exhibit a recessive trait, but can pass on
the recessive allele)
Males cannot be carriers
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What can you deduce from this?
Inheritance of Linked Genes
Alleles that occur on the same
chromosome form a linkage group and
they are usually inherited together
However, the daughter chromosomes
would have recombined alleles if
crossing-over occurs between 2 alleles
Recombination occurs infrequently
The closer the 2 genes are together,
the less likely they are to cross over
Scientist use linkage data to construct
a chromosome map, which shows the
relative distance between the gene loci
on a chromosome
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1% of crossing-over equals one
map unit