Honors Biology Supplemental Notes
Control of Development and Gene Regulation
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I. Dominant and Recessive Genes
A. General
1. genes in which a disorder or abnormality is recessive often code
for an enzyme. In a heterozygous individual, enough enzyme is
made for the individual to be normal.
2. genes in which a disorder is dominant often code for a structural
protein. Even with one normal allele, not enough protein is made.
B. Mutations (click for pictures)
1. point mutations - the wrong base (fig. 10.16, p. 199)
2. deletions and insertions lead to a frameshift mutation
3. chromosomal rearrangements (p. 148 fig. 8.23):
a. duplication
b. inversion
c. translocation
d. transposons - "jumping genes" discovered by B.
McClintock
e. sometimes duplicated chromosomes can be
beneficial, by leading to genes with new functions
4. mutations can be traced through history as they are altered by
natural selection.
C. examples
1. cone pigments in eyes are sex-linked recessive. The gene is on
the X chromosome; if one normal gene is present, not enough
pigment is made, and the person might be partially color-blind
(click for protein structure)
2. cystic fibrosis is a recessive disorder. The normal gene codes
for a protein that allows Chloride ions to pass through the cell
membrane (in other words, it is a specific cell membrane "gate").
One normal gene makes enough membrane proteins for the person
to be normal. In a homozygous recessive person, the gates don't
work properly. The person has a buildup of mucus in the lungs as a
result.
3. retinoblastoma - this gene acts as a recessive. It makes a
protein that is a tumor suppresser gene in eyes. It prevents tumors
by controlling cell division. In a heterozygous individual, the one
good gene suppressers tumors. However, chances are good that an
occasional mutation will leave one or two cells without protection,
and these cells could develop tumors. Thus, the disease itself is
dominant.
4. marfan syndrome is a dominant disorder. The gene in question
makes a protein called fibrillin, which is an elastic material that
makes up connective tissue. It holds up the eyelids, composes
blood vessels and arteries, and covers bones. Heterozygous persons
do not make enough normal protein to have normal connective
tissue.
5. p53 is another cancer suppresser gene, and is dominant. The
product is a portion of a protein stops cell division in damaged
cells, to prevent the spread of damage. The protein actually has
four parts that must fit together. Each part is made by the same
gene. In a heterozygous individual, only 1/16 of the proteins will
be completely normal, so the disorder is dominant. [1/16 = (1/2)4]
6. Huntington's disease is an autosomal dominant disorder. It is
caused by a series of repeated segments of CAG – 35 to 100 extra
repeats, on chromosome #4. The gene produces as-yet unknown
toxic proteins that degrade the brain - thus even one copy of the
disease gene is dangerous and the trait is dominant.
*you can view the structures of all these proteins at the protein
databank
also check out: hemophilia and sickle-cell anemia
Many mutations affect communication between cells. Learn more
here.
II. Gene control switches

the problem: since every cell in every part of you, (or in a plant or bacteria
or salamander, for that matter) contains the same 23 pairs of
chromosomes, with the same hundreds of thousands of genes, why are
your cells not all identical?
A. operons (click for pictures) in prokaryotes (p. 210-211)
1. sometimes an enzyme or other molecule will bind to a promoter
region of a gene, causing it to "turn on" (see figure 11.1B). The
lactose operon is an example - wehn lactose sugar is present, the
enzymes to digest it are produced.
2. in other cases, the product of a gene will bind to the promoter
region and turn it off. When enough product has been made, the
gene automatically turns off. When more needs to be made, no
more products will be present to turn the gene off, so it "starts up"
again. This is called negative feedback.
B. Eukaryotes - RNA processing (click for pictures) (figure 11.6, p. 215 and 11.9
p. 217)
1.promoter regions -- help identify the beginning of the gene (a
high percentage of eukaryote DNA is non-transcribed or "junk
DNA".
2. introns - intervening sections of mRNA that are removed before
the mRNA leaves the nucleus
3. exons - sections of mRNA that are expressed. Cells can control
the protein product by cutting out certain parts of the transcribed
gene before it is translated.
4. this process allows one gene to have several different products,
depending on how the RNA is "spliced" (p. 215, fig. 11.6)
5. regulatory proteins
C. Homeobox genes and other master switches
1. "master switches" are genes that turn on many other genes. One
gene can be responsible for the expression of many traits.

interactive on gene control switches
2. SRY gene (sex determining region on the Y chromosome)

this gene appears to be the pivotal region that builds testes
in males, which produce testosterone, which leads to
development of other male characteristics. It is possible for
a person to be XY, but missing the SRY gene, and thus be
female.
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
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there are other genes that are involved in the development
of males. Some individuals are XY females because they
lack a receptor for testosterone
more on SRY and errors in sex determination - Nova
special
other situations can affect sexual development as
well. More on this here. | and check out this link for an
explanation of how genital development works.
3. homeobox genes in fruit flies (Drosophila) (p. 218, fig. 11.10)
a. discovered by Edward Lewis; further work done
by Christiane Nusslein-Vollhard (awarded the
Nobel Prize in 1995)
b. these genes control the placement of large
structures, such as legs or eyes. Mutants will form
extra legs in the place of eyes, for example. They
appear to work like the operon regions of
prokaryotes - the proteins that they produce can turn
on or off other genes.
c. humans appear to have similar control
mechanisms, which homeobox genes that have
remarkably a similar DNA sequence and placement
among the chromosomes. Homeobox or Homeiotic
genes have been found in mice and frogs as well.



view an animation/video that explains these
similarities
another interactive animation
read more: The Genes we Share with Yeast,
Flies, Worms, and Mice
4. what turns the homeobox genes on and off? The answer is still
out there, but researchers have discovered some clues. The
concentration of certain proteins in the developing embryo is
capable of turning these genes on and off. In this way, features
such as the orientation of appendages and organs (right-left, topbottom) can be controlled. Some of these concentration gradients
are already present in the egg before fertilization, so the sperm has
nothing to do with them.
5. Environmental or "epigenetic" factors can also affect
development, as twin studies show: Check out a slide show
6. DNA packing helps regulate gene expression (fig. 11.3 p. 213)
7. In female mammals, one X is inactive in each cell (fig. 11.4)
8. proteins can also be activated after they are transcribed
9. signals from outside the cell also activate genes. They can come
from organs or from adjoining cells/tissue
D. Genetics of cancer – p. 224-227 – involves genes that regulate cell growth
Note: some of the links on this page are to the Access Excellence site, operated by the National Health Museum. Go
here for information on using these images.