Chapter 14- Human Genetics

Biology
Copyright Pearson Prentice Hall
Biology

14–1 Human Heredity 14-1 Human Heredity

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 an individual’s
chromosomes, arranged in
homologous pairs, is called a
karyotype.
Human
Karyotype

Human Chromosomes
Humans have 46 chromosomes.
Two are called sex chromosomes,
because they determine an
individual's sex.
•Females have two X
chromosomes.
•Males have one X chromosome
and one Y chromosome.

Human Chromosomes
The other 44 chromosomes are
known as autosomal chromosomes,
or autosomes.

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).

Human Chromosomes
Males and
females are
born in a
roughly 50 : 50
ratio because
of the way in
which sex
chromosomes
segregate
during meiosis.

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.

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.
Active art

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.

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.

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.

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.

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.

Human Genes
ABO blood group
•There are three alleles for this
gene, IA, IB, and i.
•Alleles IA and IB are codominant.

Human Genes
Individuals with alleles IA and IB
produce both A and B antigens,
making them blood type AB.

Human Genes
The i allele is recessive.
Individuals with alleles IAIA or IAi
produce only the A antigen,
making them blood type A.

Human Genes
Individuals with IBIB or IBi alleles
are type B.

Human Genes
Individuals who are homozygous
for the i allele (ii) produce no
antigen and are said to have
blood type O.

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.

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.

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.

Sickle Cell Disease is
characterized by the
bent and twisted
shape of the red
blood cells.

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.

There are three phenotypes associated
with the sickle cell gene.
Homozygous dominant- normal
Heterozygous dominant- healthy
with malaria resistance
Homozygous recessive- sickle cell
Sickle cell alleles are
considered codominant.

Malaria and the Sickle Cell Allele
Regions where malaria is
common
Regions where the sickle
cell allele is common
Because the sickle cell allele gives
resistance to malaria it persists in
areas where malaria is a problem.

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.

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.

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.

These diseases are autosomal
recessive because you only need
one functional gene to make a good
protein.
FF Ff ff
Makes
good
protein
Makes some
good protein
and some that
doesn’t work
Makes protein
that doesn’t
work

14–2 Human Chromosomes
14-2 Human Chromosomes

Sex-Linked Genes
•The X chromosome and the Y
chromosomes determine sex.
•Genes located on sex
chromosomes are called sex-linked
genes.
•More than 100 sex-linked genetic
disorders have now been mapped to the
X chromosome.

Sex-Linked Genes
The Y
chromosome is
much smaller
than the X
chromosome
and appears to
contain only a
few genes.
X Chromosome
Duchenne muscular
dystrophy
Melanoma
X-inactivation center
X-linked severe combined
immunodeficiency (SCID)
Colorblindness
Hemophilia
Y Chromosome
Testis-determining
factor

For a recessive allele to be expressed in
females, there must be two copies of the
allele, one on each of the two X
chromosomes.
Males have just one X
chromosome. Thus, all X-linked
alleles are expressed in
males, even if they are
recessive.

Colorblindness
Three human genes associated with color
vision are located on the X chromosome.
In males, a
defective version
of any one of
these genes
produces
colorblindness.

Possible
Inheritance of
Colorblindness
Allele

Hemophilia
•The X chromosome also carries genes
that help control blood clotting. A
recessive allele in either of these two
genes may produce hemophilia.
• In hemophilia, a protein necessary for
normal blood clotting is missing.
•Hemophiliacs can bleed to death from
cuts and may suffer internal bleeding if
bruised.

Duchenne Muscular Dystrophy
•Duchenne muscular dystrophy is a sex-linked
disorder that results in the
weakening and loss of skeletal muscle.
• It is caused by a defective version of the
gene that codes for a muscle protein.

Recessive traits from genes
on the X-chromosome, like
color blindness and
hemophilia,
are much
more
common
in males.

X-Chromosome Inactivation
•British geneticist Mary Lyon discovered that
in female cells, one X chromosome is
randomly switched off.
•The inactive X chromosome forms a
dense region in the nucleus known as
a Barr body.
• Barr bodies are generally
not found in males because
their single X chromosome
is still active.

GENETIC DISORDERS caused by
NON-DISJUNCTION
•The most common error in meiosis
occurs when homologous
chromosomes fail to separate.

NON-DISJUNCTION

NON-DISJUNCTION
•This is known as nondisjunction,
which means, “not coming apart.”
•nondisjunction causes abnormal
numbers of chromosomes

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.

Chromosomal Disorders
Down syndrome
produces mild to
severe mental
retardation.
It is characterized by:
• increased
susceptibility to
many diseases
•higher frequency
of some birth
defects
Down Syndrome Karyotype

Sex Chromosome Disorders
• In females, nondisjunction can lead
to Turner’s syndrome.
•A female with Turner’s syndrome
usually inherits only one X
chromosome (karyotype 45,X).
•Women with Turner’s syndrome are
sterile.

Chromosomal Disorders
In males, nondisjunction causes
Klinefelter’s syndrome (karyotype 47,XXY).
The extra X chromosome interferes with
meiosis and usually prevents these
individuals from reproducing.

The X chromosome contains genes
necessary for survival.

14–3 Human Molecular Genetics 14-3 Human Molecular Genetics

Human DNA Analysis
• There are roughly 6 billion base pairs in
your DNA.
•Biologists search the human genome
using sequences of DNA bases.

Genetic tests are available for hundreds of
disorders.
DNA testing can pinpoint the exact
genetic basis of a disorder.

•DNA fingerprinting analyzes the
DNA repeats which make a unique
pattern for each individual.
•Only identical twins are genetically
identical.
•DNA samples can be obtained from blood,
sperm, and hair strands with tissue at the
base.

Human DNA Analysis
Chromosomes
contain large
amounts of
DNA called
repeats that do
not code for
proteins.
This DNA pattern
varies from person
to person.
Active art

Restriction enzymes are used to cut the
DNA into fragments containing genes and
repeats.

DNA fragments
are separated
using gel
electrophoresis.
Fragments
containing repeats
are labeled.
This produces a
series of bands—
the DNA
fingerprint.

DNA Fingerprint

In 1990, scientists in the United States
and other countries began the Human
Genome Project.
The Human Genome Project is
an ongoing effort to analyze the
human DNA sequence.
In June 2000, a working copy of the
human genome was essentially
complete.

Research groups are analyzing the DNA
sequence, looking for genes that may
provide clues to the basic properties of life.
Biotechnology companies are looking for
information that may help develop new
drugs and treatments for diseases.

A Breakthrough for Everyone
•Data from publicly supported research
on the human genome have been
posted on the Internet on a daily basis.
•You can read and analyze the latest
genome data.

In gene therapy, an absent or
faulty gene is replaced by a
normal, working gene.
The body can then make the correct
protein or enzyme, eliminating the cause
of the disorder.

Viruses are often
used because of
their ability to enter
a cell’s DNA.
Virus particles are
modified so that they
cannot cause
disease.
Normal hemoglobin gene
Genetically engineered virus

A DNA fragment containing a replacement
gene is spliced to viral DNA.
Bone marrow cell
Chromosomes
Nucleus
Genetically engineered virus

The patient is then infected with the
modified virus particles, which should
carry the gene into cells to correct genetic
defects.

14–1
A chromosome that is not a sex chromosome is
know as a(an)
a. autosome.
b. karyotype.
c. pedigree.
d. chromatid.

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.

14–1
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.

14–1
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.

14–1
The ABO blood group is made up of
a. two alleles.
b. three alleles.
c. identical alleles.
d. dominant alleles.

14–2
The average human gene consists of how many
base pairs of DNA?
a. 3000
b. 300
c. 20
d. 30,000

14–2
Which of the following genotypes indicates an
individual who is a carrier for colorblindness?
a. XCX
b. XCXc
c. XcY
d. XCY

14–2
Colorblindness is much more common in males
than in females because
a. the recessive gene on the male’s single X
chromosome is expressed.
b. genes on the Y chromosome make genes on
the X chromosome more active.
c. females cannot be colorblind.
d. colorblindness is dominant in males and
recessive in females.

14–2
The presence of a dense region in the nucleus
of a cell can be used to determine the
a. sex of an individual.
b. blood type of an individual.
c. chromosome number of an individual.
d. genotype of an individual.

14–2
Nondisjunction occurs during
a. meiosis I.
b. mitosis.
c. meiosis II.
d. between meiosis I and II.

14–3
DNA fingerprinting analyzes sections of DNA
that have
a. Little or no known function but are identical
from one individual to another.
b. little or no known function but vary widely
from one individual to another.
c. a function and are identical from one
individual to another.

14–3
DNA fingerprinting uses the technique of
a. gene therapy.
b. allele analysis.
c. gel electrophoresis.
d. gene recombination.

14–3
Repeats are areas of DNA that
a. do not code for proteins.
b. code for proteins.
c. are identical from person to person.
d. cause genetic disorders.

14–3
Data from the human genome project is
available
a. only to those who have sequenced the
DNA.
b. to scientists who are able to understand the
data.
c. by permission to anyone who wishes to do
research.
d. to anyone with Internet access.

14–3
Which statement most accurately describes
gene therapy?
a. It repairs the defective gene in all cells of the
body.
b. It destroys the defective gene in cells where
it exists.
c. It replaces absent or defective genes with a
normal gene.
d. It promotes DNA repair through the use of
enzymes.

Chapter 14- Human Genetics

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Chapter 14- Human Genetics