. Neurofibromas are tumors of the skin that can arise
when a skin cell that is originally NF1+/ NF1− loses
the NF1+ allele. This wild-type allele encodes a functional protein (called a tumor suppressor), while the
NF1− allele encodes a nonfunctional protein.
A patient of genotype NF1+ / NF1− has 20 independent tumors in different areas of the skin. Samples
are taken of normal, noncancerous cells from this
patient, as well as of cells from each of the 20 tumors.
Extracts of these samples are analyzed by a technique
called gel electrophoresis that can detect variant
forms of four different proteins (A, B, C, and D) all
encoded by genes that lie on the same autosome as
NF1. Each protein has a slow (S) and a fast (F) form
that are encoded by different alleles (for example, AS
). In the extract of normal tissue, slow and fast
variants of all four proteins are found. In the extracts
of the tumors, 12 had only the fast variants of proteins
A and D but both the fast and slow variants of proteins B and C; 6 had only the fast variant of protein A
but both the fast and slow variants of proteins B, C,
and D; and the remaining 2 tumor extracts had only
the fast variant of protein A, only the slow variant of
protein B, the fast and slow variants of protein C, and
only the fast variant of protein D.
a. What kind of genetic event described in this chapter could cause all 20 tumors, assuming that all the
tumors are produced by the same mechanism?
b. Draw a genetic map describing these data, assuming that this small sample represents all the types
of tumors that could be formed by the same mechanism in this patient. Show which alleles of which
genes lie on the two homologous chromosomes.
Indicate all relative distances that can be estimated.
Note that NF1 is one of the genes you can map in
c. Another mechanism that can lead to neurofibromas
in this patient is a mitotic error producing cells with
45 rather than the normal 46 chromosomes. How
can this mechanism cause tumors? How do you
know, just from the results described, that none of
these 20 tumors is formed by such mitotic errors?
d. Can you think of any other type of error that could
produce the results described?
It is given that neurofibromas are skin tumors that can occur when a skin cell that is originally NF1+/NF1 loses the NF1+ allele. This wild-type allele codes for a functional tumor-suppressor protein. The NF1- allele codes for a non-functional protein.
Form the given information, a patient of genotype NF1+/NF1- has 20 independent tumors in different areas of the skin. Samples are collected from the normal, noncancerous cells as well as from each of the 20 tumors of this patient. These samples are analysed by gel electrophoresis technique. There are three types of tumors found in this patient after the analysis. Mitotic crossing-over can develop individuals in which different cells have different genotypes. It has major direct as well as indirect consequences on human health. Therefore, mitotic recombination can cause these types of tumors in this patient.
It is given that all three types of tumors are homozygous for NF1- allele. There are variant forms of four different proteins found. These proteins are encoded by genes that present on the same chromosomes as NF1. Each of these proteins has a slow and fast form that is encoded by different alleles.
Pictorial representation: Fig.1 represents the genetic map
The extracts of normal tissues have slow and fast variants for all four proteins. In the extracts of cancerous cells, 12 tumors (tumor type 1) had only the fast variants of proteins A and D. Proteins B and C had both the variants; 6 tumors (tumor type 2) had only fast variant of protein A but proteins B, C, and D had both the variants. The remaining 2 tumors (tumor type 3) extracts had only fast variants of protein A, only slow variant of protein B, protein C had both variants, and protein D had only the fast variant. The mitotic recombination leads to genes to become homozygous that are located farther from the centromere. The genes that are present between the recombination event and the centromere or on the other side of the centromere stay heterozygous. It can be observed that all thr...
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