Heredity: passing
traits from one generation to the next.
Molecular
Basis of Heredity
Although some human traits
are controlled by a single gene, most human characteristics are controlled
by several genes. A polygenic traitis
controled by two or more genes. Skin and eye color are both influenced
by the additive effects of from three to six genes. Each of these
genes results in the formation of a certain amount of a pigment called
melanin. The more of the genes that are expressed, the darker the
color produced.
39. This human karyotype
is unusual because chromosome set -
A. 5 has chromosomes of different shapes
B. 10 is missing genetic material
C. 14 has enlarged centromeres
D. 21 has extra genetic material
Multiple-allele traits
are controlled by three or more alleles of the same gene. Human blood
type is an example.
There are three forms of the blood type gene, A,
B, and O. An individual's blood type is determined by two of
the three genes.
Blood Type
Rh Type
How Many Have It
Percentage
O
+
1 person in 3
37.4%
44%
O
-
1 person in 15
6.6%
A
+
1 person in 3
35.7%
42%
A
-
1 person in 16
6.3%
B
+
1 person in 12
8.5%
10%
B
-
1 person in 67
1.5%
AB
+
1 person in 29
3.4%
4%
AB
-
1 person in 167
.6%
Sex-linked traits have
genes located on a sex chromosome. Genes on the X chromosome are X-linked
and genes on the Y chromosome are Y-linked. When genes are X-linked,
males are much more likely to show recessive traits since there is
no complementary gene on the Y chromosome. Color blindness and hemophilia
are examples.
Sex-influenced traits are
located on autosomes, but express themselves differently in the sexes
because of sex hormones. Pattern baldness is an example.
22. Human blood type is
controlled by multiple alleles. How many forms of the human blood
type gene are there?
A. 3
B. 6
C. 8
D. 12
23. What is the rarest blood type?
A. O -
B. A -
C. B -
D. AB-
24. What is the most common blood type?
A. O +
B. A +
C. B +
D. AB +
25. Given a sample of 100 people, how many will have a B blood
type?
A. 10
B. 9
C. 2
D. 1
Use this
table to answer questions 22 through 25.
Blood Type
Rh Type
How Many Have It
Percentage
O
+
1 person in 3
37.4%
44%
O
-
1 person in 15
6.6%
A
+
1 person in 3
35.7%
42%
A
-
1 person in 16
6.3%
B
+
1 person in 12
8.5%
10%
B
-
1 person in 67
1.5%
AB
+
1 person in 29
3.4%
4%
AB
-
1 person in 167
.6%
The
Human Genome Project
In
1990, the U.S. Department of Energy and the National Institutes
of Health formally began a multidisciplinary project to map
and sequence the human genome (all the genes available on
chromosomes of the species). While the project was originally
planned for 15 years, rapid technological advances the was
accelerated and completed in spring of 2003. Project
goals
were to discover all the approximately 100,000 human genes
and make them accessible for further biological study.
A news conference was held in June of 2000 to announce the
completion of over 99% of the human genome. Scientists stated
that they had mapped the order in which most of the nitrogen
bases occuring in the DNA molecules making up each of the
twenty three pairs of human chromosomes. The press release
from this news conference predicted that cures for most genetic
diseases and disorders will be found within a few years. While
this project is a great step forward in our understanding
of human genes, there is still a very long way to go before
mastering the subject. To put the current phase of the human
genome project in prospective, think of it in this way:
The information in human chromosomes
is like a complete, unabridged dictionary for human proteins.
Each of the terms in the dictionary represent a protein
gene. The letters in the words making up the definition
of each term represent the DNA bases that make up the gene.
At the present time, the human genome project has given
us a dictionary with two major weaknesses:
1. The dictionary is not in alphabetical
order.
2. All the letters in the dictionary are in the correct
order, but there are no breaks between the words.
While this dictionary is much better than
nothing, there is still a lot of work to be done before
we can easily use it.
Genetic engineering: the application of molecular
genetics for practical purposes.
What is a clone?
Basically, clones are organisms which have identical genetic
material. Today, the term "clone" is most often
associated with a genetic engineering process.
However, using the definition of clone above, identical
twins are clones. The new plant that grows from a plant
cutting is a clone. When you look at it this way, clones
have been around for a very long time. Scientists have been
using "lower" animals for years in artificial
cloning experiments, but it wasn't until 1996 that Dolly
came into the world.
Genetic engineering can be used to identify genes
for specific traits or to transfer
genes from one oranism to another organism. DNA
can be cut at specific sites on
the molecule using bacterial enzymes called restriction
enzymes.
This allows
individual genes to be isolated. Once the gene is
isolated, a cloning vector is
used to clone the gene and transfer it from one
organism to another. These
vectors are found in many bacteria and consist of
a ring of DNA that is in
addition to its main chromosome. This ring of DNA
is known as a plasmid.
Recombinant DNA is a combination of DNA from two
or more sources. One of the best applications of recombinant DNA
today is in the production of insulin.
The gene for human insulin is placed into the plasmid of bacteria
cells and those cells produce insulin. This
process has helped lower the cost of insulin needed to treat diabetes.
Gene therapy treats a genetic disorder by introducing
a gene into a cell or by correcting a defect in a cell's genome.
Gene therapy has been in use since 1990. There are still obstacles
today regarding how and where to insert genes safely into an organism's
chromosomes. But gene therapy is being used to treat cystic fibrosis,
hemophilia, AIDS, and several forms of cancer. As more is learned
about the human genome, gene therapy may one day provide permanent
cures for genetic disorders.
Genetic disorders:
The 23 pairs of human chromosomes contain over 40,000 genes. This
complexity provides for many possible variations. The chromosome
location of many disease-causing genes are known. While our ability
to intrepret the complex human genome is increasing, we are still
far from mastering it. Today, gene testing is used to identify some
problems. However, population sampling and pedigree studies are
still used to track many genetic problems.
Population sampling:
Sampling is done by carefully selecting a small group of individuals
to represent the whole population. Data from studying these individuals
is applied to the whole population statistically.
Pedigree studies:
A pedigree study produces a chart that traces a "history"
of the traits of several generations. These studies frequently
identify the carrier of a gene that is not visible.
Test your knowledgeof
genes and chromosomes.
In the 1930's, Eugenics
was
a thriving science.
1. What was this science?
2 . Why isn't this science around today?
The diagram at the top of this page
is a karyotype made by matching pairs of human chromosomes, then
arranging the pairs from largest to smallest.
When everyone at your table is finished with the assignment, do
the following as a group:
a. Ask your science facilitator for a copy of one of these pictures
of human chromosomes.
b. Cut out the individual chromosomes.
c. Pair them by size and paste the pairs onto a table like the one
pictured at the top of this page.
d. Does your karyotype agree with one of these?
e. Show your completed karyotype to your facilitator.
Molecular
Basis of Heredity
is
controled by two or more genes. Skin and eye color are both influenced
by the additive effects of from three to six genes. Each of these
genes results in the formation of a certain amount of a pigment called
melanin. The more of the genes that are expressed, the darker the
color produced. 
There are three forms of the blood type gene, A,
B, and O. An individual's blood type is determined by two of
the three genes.
