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Biology Biology
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14–1 Human Heredity14-1 Human Heredity
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14–1 Human Heredity
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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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Human Chromosomes
Human Karyotype
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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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Human Chromosomes
How is sex determined?
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Human Chromosomes
All human egg cells carry a single X
chromosome (23,X).
Half of all sperm cells carry an X
chromosome (23,X) and half carry a Y
chromosome (23,Y).
About half of the zygotes will be 46,XX
(female) and half will be 46,XY (male).
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Human Chromosomes
Males and females are
born in a roughly 50 : 50
ratio because of the
way in which sex
chromosomes
segregate during
meiosis.
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Human Traits
Human Traits
In order to apply Mendelian genetics to humans,
biologists must identify an inherited trait controlled
by a single gene.
They must establish that the trait is inherited and
not the result of environmental influences.
They have to study how the trait is passed from
one generation to the next.
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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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Human Traits
A circle
represents
a female.
A horizontal line
connecting a male and
a female represents a
marriage.
A shaded circle or square
indicates that a person
expresses the trait.
A square
represents
a male.
A vertical line and a
bracket connect the
parents to their
children.
A circle or square
that is not
shaded indicates
that a person
does not express
the trait.
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Human Traits
Genes and the Environment
Some obvious human traits are almost impossible
to associate with single genes.
Traits, such as the shape of your eyes or ears, are
polygenic, meaning they are controlled by many
genes.
Many of your personal traits are only partly
governed by genetics.
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Human Genes
Human Genes
The human genome includes tens of thousands of
genes.
In 2003, the DNA sequence of the human genome
was published.
In a few cases, biologists were able to identify
genes that directly control a single human trait
such as blood type.
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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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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 Rh-
positive.
Individuals with two Rh- alleles are said to be Rh-
negative.
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Human Genes
ABO blood group
• There are three alleles for this gene, IA, IB, and i.
• Alleles IA and IB are codominant.
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Human Genes
Individuals with alleles IA and IB produce both A and B
antigens, making them blood type AB.
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Human Genes
The i allele is recessive.
Individuals with alleles IAIA or IAi produce only the A
antigen, making them blood type A.
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Human Genes
Individuals with IBIB or IBi alleles are type B.
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Human Genes
Individuals who are homozygous for the i allele (ii)
produce no antigen and are said to have blood
type O.
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Human Genes
Recessive Alleles
The presence of a normal, functioning gene is
revealed only when an abnormal or nonfunctioning
allele affects the phenotype.
Many disorders are caused by autosomal
recessive alleles.
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Human Genes
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Human Genes
Dominant Alleles
The effects of a dominant allele are expressed
even when the recessive allele is present.
Two examples of genetic disorders caused by
autosomal dominant alleles are achondroplasia
and Huntington disease.
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Human Genes
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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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Human Genes
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From Gene to Molecule
From Gene to Molecule
How do small changes in DNA cause
genetic disorders?
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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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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.
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From Gene to Molecule
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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From Gene to Molecule
Normal CFTR is a
chloride ion channel in
cell membranes.
Abnormal CFTR
cannot be transported
to the cell membrane.
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From Gene to Molecule
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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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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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.
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From Gene to Molecule
People who are heterozygous for the sickle cell allele
are generally healthy and they are resistant to
malaria.
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From Gene to Molecule
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
commonRegions where the sickle
cell allele is common
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14–1
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14–1
A chromosome that is not a sex chromosome is
know as a(an)
a. autosome.
b. karyotype.
c. pedigree.
d. chromatid.
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14–1
Whether a human will be a male or a female is
determined by which
a. sex chromosome is in the egg cell.
b. autosomes are in the egg cell.
c. sex chromosome is in the sperm cell.
d. autosomes are in the sperm cell.
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Mendelian inheritance in humans is typically studied by
a. making inferences from family pedigrees.
b. carrying out carefully controlled crosses.
c. observing the phenotypes of individual humans.
d. observing inheritance patterns in other animals.
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An individual with a blood type phenotype of O
can receive blood from an individual with the
phenotype
a. O.
b. A.
c. AB.
d. B.
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The ABO blood group is made up of
a. two alleles.
b. three alleles.
c. identical alleles.
d. dominant alleles.
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