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Patterns of Heredity &
Human Genetics
Chapter 12
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Pedigree

A pedigree chart can help trace the phenotypes and
genotypes in a family to determine the chance that a child
might have a certain genetic disorder.

A pedigree is made up of sets of symbols that identify males
and females, individuals affected by the trait being studied,
and family relationships.
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Pedigree Key
Female (unaffected)
Affected Female
Female Carrier
Male (unaffected)
Affected Male
Male Carrier
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Pedigree Key
Mating (Parents)
Siblings
F1 Generation
Each horizontal row of circles and squares in a pedigree designates a generation,
with the most recent generation shown at the bottom.
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Analyzing a Pedigree

See handout
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Why are pedigrees important?

Most genetic disorders are caused by recessive alleles and
can be identified easily using a pedigree.

Cystic Fibrosis (CF) – Common among white Americans,
there is a defective protein in the plasma membrane that
causes thick mucus to build in the lungs and digestive tract. It
is treated with physical therapy, special diets, and drug
therapy.

Tay-Sachs disease – A recessive disorder of the central
nervous system. Lipids fail to break down properly and
accumulate in the cells of the CNS.

Phenylketonuria (PKU) – results from the absence of an
enzyme that breaks down substances. It also causes a build
up of by-products in the Central Nervous System.
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Why are pedigrees important?

Unlike the inheritance of recessive traits in which a recessive
trait must be inherited from both parents for a person to show
the recessive phenotype, many traits are inherited because
of the dominant allele.

Tongue rolling, free-hanging earlobes, hitch-hiker’s thumb,
etc. are all examples of dominant traits.

Huntington’s disease – a genetic disorder caused by a rare
dominant allele. There are no effective treatments available
and always results in death. The disease does not occur until
the individual is between 30 and 50 years of age, and has
almost always passed on the trait already.
Complete section 12.1 Review – Write the Questions & the Answers!
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Complex Patterns of Inheritance

Mendel explained simple patterns of inheritance – if you
have the dominant trait it shows, and if you have two
recessive traits, it shows! However, there are more complex
patterns of inheritance than those studied by Mendel.
1.
Incomplete Dominance
2.
Codominance
3.
Multiple Alleles
4.
Sex-linked traits
5.
Polygenic Inheritance
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Incomplete Dominance

Incomplete dominance is demonstrated when the
heterozygous form of a gene is an intermediate between the
homozygous dominant & the homozygous recessive.

Example RR (red snapdragon) and R’R’ (white snapdragon)
crossed together result in RR’ (pink snapdragon)
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Codominance

Codominant alleles cause the phenotypes of both
homozyotes to be produced in heterozygous individuals.

In codominance, both alleles are expressed equally.

For example, both flowers would be homozygotes with two
different uppercase letters (R and W). The resulting offspring
would show both red and white.
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Multiple Alleles

Although all of the traits you have studied thus far has only two
alleles, it is common for more than two alleles to control a trait.

Traits controlled by more than two alleles is said to have multiple
alleles.

For example, blood typing is controlled by three alleles: IA, IB, and i.

Both IA and IB produce a protein called an antigen on the surface of
red blood cells. IA and IB are codominant.

Allele i is recessive and does not produce an antigen.

The four different phenotypes possible are shown on the next
page.
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Blood Type [Multiple Alleles &
Codominance]
Phenotype (Blood Type)
A
Antigen A
Possible Genotypes
B
Antigen B
IBIB or IBi
AB
Antigen A & Antigen B
IAIB
O
No antigens
ii
IAIA or IAi
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The Importance of Blood Typing

Determining blood type is necessary before a person can
receive a blood transfusion because the red blood cells of
incompatible blood types clump together, causing death.
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Blood typing can also be helpful in solving issues
parenthood – knowing the child’s blood type.

The father has type-O blood and the mother has type-A
blood. Is it possible for their child to be born with type-AB?
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Sex-linked Traits

We all know that a male’s sex chromosomes are XY and a
female’s sex chromosomes are XX.

When people talk about sex-linked genes, they are usually
talking about genes on the X chromosome.

Because female’s have two X chromosomes, they have the
ability to mask whatever recessive trait that is linked to one
of their X chromosomes.

However, male’s only have one X and one Y and the Y is so
much smaller than the X and carries far fewer genes. Males
are more likely to display sex-linked traits because they are
unable to mask it.
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Examples: color-blindness and hemophelia (p. 333)
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Polygenic Inheritance
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Traits produced by two or more genes are called polygenic
traits. (Poly – Many // Genic – Genes)
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For example: eye color, skin color

There are at least three genes that produce a range of eye
color

All heterozygotes are intermediate in phenotype.
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Environmental Influences

Phenotype is not determined only by genes. The
environment – the conditions surrounding an organism – also
affect phenotype.

For example, the sex of sea turtles depends on the
temperature of the environment in which the egg develops.
Eggs in warmer nests result in females, and eggs in cooler
nests result in males.

Likewise, genes and environment interact with human traits.
Genes influence height, but do not completely control it.
Take two identical twins and raise them in separate
environments, one that has good nutrition and health care
and the other that does not. Their height and size can vary
greatly.
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Karyotype
A
picture of all of the chromosomes located
in the nucleus of a eukaryotic cell
 How
can we see all the chromosomes?
 Chemicals
are used to stain the chromosomes.
The stains produce a pattern of bands on the
chromosomes. These patterns can be used to tell
the different chromosomes apart.
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Why is it important to see all of the
chromosomes?

Karyotypes allow scientists and doctors to see if there are any
extra chromosomes or missing parts of chromosomes.
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Each of these mutations leads to a genetic disorder.
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Down syndrome
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Klinefelter Syndrome
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Turner Syndrome
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Down Syndrome

A genetic disorder, Trisomy 21, in which a person has the
presence of all or part of a third copy of chromosome 21.

It is typically associated with physical growth delays,
characteristic facial features, and mild to moderate
intellectual disability.
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Klinefelter Syndrome

A genetic disorder characterized by an extra X chromosome
in males (genotype XXY, instead of XY)
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It occurs in 1 in every 500-1000 male births, but the rate is up
to 20 times higher in newborns with mental retardation
rather than healthy birth

Klinefelter can result in behavioral problems, learning
difficulties and speech disorder. If often goes undetected
until the male is investigated for infertility.

Around 95-99% of KS men are infertile but sperm is found in
50% of them and can be injected directly into their female
partner’s ovum.
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Turner Syndrome

A genetic disorder that results when one normal X
chromosome is present in a female’s cells and the other sex
chromosome is missing or structurally altered.
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Turner syndrome is not genetically inherited, but occurs at
random in 1 in every 2,000 women.
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Many affected girls have an early loss of ovarian function.
They do not usually go through puberty, unless they receive
hormone therapy, and most are infertile.
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Most women with TS have normal intelligence.
Developmental delays, nonverbal learning disabilities, and
behavioral problems are possible, although these
characteristics vary among affected individuals.