Transcript 14-1 Human Heredity

14–1
Human
Heredity
14-1 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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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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Mendel’s rules of inheritance apply to autosomal
genetic disorders where two alleles interact to
produce a phenotypic trait.
– A heterozygote for a recessive disorder is a carrier.
– Disorders caused by dominant alleles are uncommon.
(dominant)
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Males and females can differ in sex-linked traits.
Genes on sex chromosomes are called sex-linked genes.
•
Y chromosome genes in mammals are responsible
for male characteristics.
•
X chromosome genes in mammals affect many traits.
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14–1 Human Heredity
• Male mammals have an XY genotype.
– All of a male’s sexlinked genes are
expressed because
males have no
second copies of
sex-linked genes.
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• Female mammals have an XX genotype.
– Expression of sex-linked genes is similar to autosomal
genes but one X chromosome in each cell is randomly
“turns off” through X chromosome inactivation.
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Chromosomal Disorders
Chromosomal Disorders
What problems does nondisjunction cause?
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Chromosomal Disorders
• The most common error in meiosis occurs when
homologous chromosomes fail to separate.
• This is known as nondisjunction, which means,
“not coming apart.”
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Chromosomal Disorders
• If nondisjunction occurs, abnormal numbers of
chromosomes may find their way into gametes,
and a disorder of chromosome numbers may
result.
• An individual may be born with three copies of a
chromosome or only 1 copy.
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•Nondisjunction
Chromosomal Disorders
Homologous
chromosomes
fail to
separate.
Meiosis I:
Nondisjunction
Meiosis II
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Down syndrome
produces mild to severe
mental retardation.
Chromosomal Disorders
Down Syndrome Karyotype
It is characterized by
increased susceptibility
to many diseases and
higher frequency of
other birth defects.
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Chromosomal Disorders
Down Syndrome
• Down syndrome involves three copies of
chromosome 21. This is also called Trisomy 21
and occurs in about 1 in 1000 births
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Edward’s Syndrome
•50% of babies who are carried to term will be
born alive, and baby girls will have higher rates
of live birth than baby boys. Significant
developmental delays that do not allow them to
live independently without full time caregiving
•A Trisomy 18 error occurs in about 1 out of
every 2500 pregnancies in the United States
and 1 in 6000 live births
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Patau Syndrome
•Most fetuses with Patau are miscarried spontaneously.
•Trisomy 13 occurs in about 1 in 9,500 births
•Fetal or congenital malformation is usual, with
holoprosencephaly (the failure of the forebrain to develop
into two hemispheres), microphthalmia (small eye), cleft lip
and palate, and polydactyly (extra digits), cardiac
malformations, severe growth, mental retardation and
kidney malformations are also associated with the
condition.
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Chromosomal Disorders
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).
Instead of an addition of a chromosome, there is
a deletion.
• Women with Turner’s syndrome are sterile.
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Chromosomal Disorders
Nondisjunction causes Klinefelter’s syndrome, an
extra X on the 23rd pair. (karyotype 47,XXY for males
or karyotype 47 XXX for females).
The extra X chromosome interferes with meiosis and
usually prevents these individuals from reproducing.
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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
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 horizontal line
connecting a male and a female.
a female represents a
marriage.
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.
A shaded circle or square
indicates that a person
expresses the trait.
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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
Recessive Alleles
• Many disorders are caused by autosomal
recessive alleles. Both alleles must be
present in order to inherit the disease.
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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
In codominant inheritance, two different versions
(alleles) of a gene can be expressed, and each
version makes a slightly different protein. Both
alleles influence the genetic trait or determine the
characteristics of the genetic condition.
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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Interpreting a Pedigree Chart
To determine whether the disorder is dominant or
recessive
• If the disorder is dominant, one of the parents
must have the disorder
• If the disorder is recessive, neither parent has to
have the disorder because they can be
heterozygous (hidden recessive gene). They are
called carriers.
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Interpreting a Pedigree Chart
To determine if the pedigree chart show an
autosomal or X-linked disease
• If most of the males in the pedigree are affected,
then the disorder is X-linked
• If it is a 50/50 ratio between men and women, then
the disorder is autosomal
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Rules of Inheritance
If a trait is autosomal dominant
• Appears in both sexes with equal frequency
• Both sexes transmit the trait to their offspring
• Does not skip generations
• Affected offspring must have an affected parents
• Unaffected parents do not transmit the trait
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Rules of Inheritance
If a trait is autosomal recessive
• Both parents are carriers
• Appears in both sexes equally
• Affected offspring usually have unaffected parent
• Appears more frequently among children of
consanguine (blood relatives) marriages
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Rules of Inheritance
If a trait is sex-linked dominant
• Both males and females are affected
• Does not skip generations
• Affected daughter must have an affected parent
• Affected son must have an affected mother
• Affected fathers will always pass the trait to their
daughter
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Rules of Inheritance
If a trait is X-linked recessive
• More males than females are affected
• Affected sons are usually born to unaffected
moms, therefore the trait skips generations
• It is never passed from father to son
• All daughters of affected fathers are carriers
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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
common
Regions where the sickle
cell allele is common
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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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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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