Download Ch 14 In a Nutshell

1414-1Human
Heredity
(In a nutshell)
Human Heredity
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14–1 Human Heredity
Human Chromosomes
Human Chromosomes
Cell biologists analyze
chromosomes by looking at
karyotypes.
Cells are photographed during
mitosis. Scientists then cut out
the chromosomes from the
photographs and group them
together in pairs.
A picture of chromosomes
arranged in this way is known as
a karyotype.
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14–1 Human Heredity
Human Chromosomes
Two of the 46 human chromosomes are known as
sex chromosomes, because they determine an
individual's sex.
• Females have two copies of an X chromosome.
• Males have one X chromosome and one Y
chromosome.
The remaining 44 chromosomes are known as
autosomal chromosomes, or autosomes.
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14–1 Human Heredity
Human Traits
Pedigree Charts
A pedigree chart shows the relationships within a
family.
Genetic counselors analyze pedigree charts to
infer the genotypes of family members.
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14–1 Human Heredity
A horizontal line
connecting a male and
a female represents a
marriage.
A circle
represents
a female.
Human Traits
A square
represents A vertical line and a
a male.
bracket connect the
parents to their
children.
A circle or square
that is not
shaded indicates
that a person
does not express
the trait.
A shaded circle or square
indicates that a person
expresses the trait.
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14–1 Human Heredity
Human Genes
Blood Group Genes
Human blood comes in a variety of genetically
determined blood groups.
A number of genes are responsible for human
blood groups.
The best known are the ABO blood groups and the
Rh blood groups.
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14–1 Human Heredity
Human Genes
The Rh blood group is determined by a single gene
with two alleles—positive and negative.
The positive (Rh+) allele is dominant, so individuals
who are Rh+/Rh+ or Rh+/Rh are said to be Rhpositive.
Individuals with two Rh- alleles are said to be Rhnegative.
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14–1 Human Heredity
Human Genes
ABO blood group (Codominance)
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14–1 Human Heredity
Human Genes
Recessive Alleles
The presence of a normal, functioning gene is revealed
only when an abnormal or nonfunctioning allele affects the
phenotype.
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14–1 Human Heredity
Human Genes
Dominant Alleles
The effects of a dominant allele are expressed
even when the recessive allele is present.
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14–1 Human Heredity
Human Genes
Codominant Alleles
Sickle cell disease is a serious disorder caused by
a codominant allele.
Sickle cell is found in about 1 out of 500 African
Americans.
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14–1 Human Heredity
From Gene to Molecule
From Gene to Molecule
How do small changes in DNA cause
genetic disorders?
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14–1 Human Heredity
From Gene to Molecule
In both cystic fibrosis and sickle cell
disease, a small change in the DNA of a
single gene affects the structure of a
protein, causing a serious genetic
disorder.
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14–1 Human Heredity
From Gene to Molecule
Cystic Fibrosis
Cystic fibrosis is caused by a
recessive allele.
Sufferers of cystic fibrosis
produce a thick, heavy mucus
that clogs their lungs and
breathing passageways.
The most common allele that
causes cystic fibrosis is missing 3
DNA bases.
As a result, the amino acid
phenylalanine is missing from the
CFTR protein.
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14–1 Human Heredity
From Gene to Molecule
Normal CFTR is a chloride
ion channel in cell
membranes.
Abnormal CFTR cannot
be transported to the cell
membrane.
The cells in the person’s
airways are unable to
transport chloride ions.
As a result, the airways
become clogged with a
thick mucus.
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14–1 Human Heredity
From Gene to Molecule
Sickle Cell Disease
Sickle cell disease is a
common genetic
disorder found in
African Americans.
It is characterized by
the bent and twisted
shape of the red blood
cells.
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14–1 Human Heredity
From Gene to Molecule
Hemoglobin is the protein in red blood cells that carries oxygen.
In the sickle cell allele, just one DNA base is changed.
As a result, the abnormal hemoglobin is less soluble than normal
hemoglobin. Low oxygen levels cause some red blood cells to
become sickle shaped.
People who are heterozygous for the sickle cell allele are
generally healthy and they are resistant to malaria.
There are three phenotypes associated with the sickle cell gene.
An individual with both normal and sickle cell alleles has a
different phenotype—resistance to malaria—from someone with
only normal alleles.
Sickle cell alleles are thought to be codominant.
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From Gene to Molecule
Malaria and the Sickle Cell Allele
Regions where malaria is
common
Regions where the sickle
cell allele is common
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14–1 Human Heredity
Sex-Linked Genes
• The X chromosome and the Y chromosomes
determine sex.
• Genes located on these chromosomes are called
sex-linked genes.
• More than 100 sex-linked genetic disorders have
now been mapped to the X chromosome.
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14–1 Human Heredity
X Chromosome
• The Y chromosome is
much smaller than the
X chromosome and
appears to contain only
a few genes.
Duchenne muscular
dystrophy
• ONE X- chromosome in
each female cell is
randomly switched off
to keep gene dosage
correct
Colorblindness
– Called a Barr body.
Melanoma
X-inactivation center
X-linked severe combined
immunodeficiency (SCID)
Hemophilia
Y Chromosome
Testis-determining
factor
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14–1 Human Heredity
Why are sex-linked disorders more common in
males than in females?
Males have just one X chromosome. Thus, all Xlinked alleles are expressed in males, even if they
are recessive.
X-linked Disorder
Major symptoms
Colorblindness
Inability to see some or all colors due to a lack of
photoreceptors
Hemophilia
Blood clotting deficiency due to defective gene
coding for clotting factors
Duchenne Muscular
Dystrophy
Weakening and loss of muscle mass due to a
defective gene that codes for muscle protein
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14–1 Human Heredity
• Chromosomal Disorders
• The most common error
in meiosis occurs when
homologous
chromosomes fail to
separate.
• This is known as
Homologous
chromosomes
fail to
separate.
Meiosis I:
Nondisjunction
Meiosis II
nondisjunction, which
means, “not coming
apart.”
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– Down Syndrome
• If two copies of an
autosomal chromosome fail
to separate during meiosis,
an individual may be born
with three copies of a
chromosome.
• Down syndrome involves
three copies of chromosome
21.
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14–1 Human Heredity
Sex Chromosome Disorders
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