14-1 Human Heredity

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Summary

14-1 Human Heredity

In order to learn more about humans, biologists often use a
karyotype to analyze human chromosomes. A karyotype is a
picture of a cell’s chromosomes grouped in homologous pairs.
Humans have 46 chromosomes. Two of these, X and Y, are sex
chromosomes. Females have two X chromosomes (XX). Males
have one X and one Y chromosome (XY). The other 44 chromo-
somes are known as autosomes. All human egg cells carry a
single X chromosome. Sperm cells carry either X or Y chromo-
somes. Half of all sperm cells carry an X chromosome and half
carry a Y chromosome. This ensures that just about half of the
zygotes will be female and half will be male.

To study human inheritance, biologists use pedigree charts.
A pedigree chart shows relationships within a family. The
inheritance of a trait can be traced through the family. From this,
biologists may determine the genotypes of family members.

It is difficult to link an observed human trait with a specific
gene. Many human traits are polygenic, or controlled by many
genes. The environment also affects some traits.

The genes controlling blood type were among the first human
genes to be identified. A number of genes are responsible for blood
groups, but the two best known are the ABO blood groups and the
Rh blood group.

•Red blood cells can carry two antigens—A and B. Antigens
are molecules the immune system can recognize. A person
who has only antigen A has type A blood. A person who has
only antigen B has type B blood. A person who has both anti-
gens has type AB blood. A person who does not have either
antigen has type O blood. A single gene with three alleles
determines the ABO blood types.

•Red blood cells can also have the Rh antigen. People
with the Rh antigen are Rh positive. Those without it are
Rh negative. A single gene with two alleles determines
the Rh blood group.

There are many human genetic disorders. Some, including
PKU and Tay-Sachs disease are caused by recessive alleles.
Individuals must inherit a recessive allele from each parent. Other
disorders, such as Huntington disease, are caused by a dominant
allele. Huntington disease is expressed in any person who has the
allele. Still other disorders, such as sickle cell disease, are caused
by a codominant allele.

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Scientists are starting to learn which changes in the DNA
sequence cause certain genetic disorders. Sometimes, a small
change in the DNA of a single gene affects the structure of a
protein and causes a serious genetic disorder. This is the case
with cystic fibrosis and sickle cell disease.

14-2 Human Chromosomes

Genes on the X and Y chromosomes are said to be sex-linked.
They are inherited in a different pattern than are genes on
autosomes. Males have just one X chromosome. Thus all
X-linked alleles are expressed in males, even if they are
recessive. Some examples of sex-linked disorders are color-
blindness and hemophilia.

Most of the time, the mechanisms that separate chromosomes
in meiosis work well, but sometimes errors happen. The most
common error during meiosis is nondisjunction. Nondisjunction
is the failure of chromosomes to separate properly during meiosis.

If nondisjunction occurs, abnormal numbers of chromosomes
may find their way into gametes, and a disorder of chromosome
numbers may result.

•Down syndrome is an example of autosomal nondisjunction.
In this disorder, there is an extra copy of chromosome 21.

•Nondisjunction can also occur in sex chromosomes. Turner’s
syndrome and Klinefelter’s syndrome are two examples.

14-3 Human Molecular Genetics

Biologists use molecular biology techniques to read, analyze, and
change the DNA code of human genes. DNA analysis techniques
can be used in different ways.

•DNA analysis can be used to test parents for recessive alleles
that code for genetic disorders.

•DNA fingerprinting is a DNA analysis technique that can be
used to identify individuals.

The Human Genome Project is an ongoing effort to analyze
the human DNA sequence. Scientists are using the results of the
human genome project to help locate genes on human chromo-
somes. In addition, information about the human genome may be
used to help diagnose and treat disease.

Data from the human genome may be used to cure genetic
disorders by gene therapy. In gene therapy, an absent or faulty
gene is replaced by a normal, working gene. In one method
of gene therapy, a normal, working copy of a gene is attached
to viral DNA. Virus particles deliver this copy of the gene to
human cells. The human cells can then make proteins that
correct genetic defects.

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