Question

1. The 1952 experiment performed by the Lederbergs in which they replica-plated ten million colonies of TI-phage sensitive E. coli to plates containing high concentrations of TI phase and found a small number of phage-resistant colonies showed that: A) a selecting agent can affect mutation rate in E. coli. B) mutations can arise spontaneously before exposure to the selecting agent. C) mutations in E. coli occur at a relatively high frequency, D) the mutation rate in E. coli fluctuates greatly from one generation to the next. E) the T1 phage can act as a mutagen as well as a selecting agent in E.coli. 2. A likely explanation for an abnormal human phenotype associated with a trisomy is: A) the presence of multiple recessive mutant alleles. B) the extra chromosome has typically undergone significant rearrangements. C) the absence of genes necessary for certain cellular processes. D) altered gene dosage (also known as genetic balance). E) the random inactivation of a complete autosome. 3. The virus that causes AIDS, HIV (Human Immunodeficiency Virus), derives from SIV (Simian Immunodeficiency Virus). SIV can only infect monkeys. People who hunted wild monkeys were not susceptible to SIV infection because the host range of the virus was species specific (i.e., restricted to monkeys). Then all of a sudden, SIV gained the ability to infect humans and the HIV/AIDS pandemic began. As a geneticist, how would you explain this? You may answer in very general terms. A full credit answer can be written in one word. 4. A man who is XYY is found to be colorblind (a recessive X-linked trait). Both his mother and father have normal vision. Write the relevant genotypes of the man's parents and explain how he might have ended up XYY and colorblind. 5. (5 points) If a person with Down's syndrome (trisomy 21) marries a person of normal karyotype, what is the chance that their first child will also be affected by Down's syndrome? 6. (3 points) The species Geneticus isalmostoveris is a diploid and has an x number of 12. How many chromosomes would be expected in the following? (a) a monosomic (b) a trisomic (c) a triploid

Answer 1

1. The 1952 experiment performed by the Lederbergs in which they replica plated 10 million colonies of T1- phage sensitive E. coli to plates containing high concentration of T1 - phage and found a small number of phage resistant colonies showed that:

B) Mutations can arise spontaneously before exposure to the selecting agent.

The replica plated colonies were not exposed to the selecting agent (here penicillin) before the experiment and some of the colonies still survived the antibiotic penicillin showing they developed resistance. So through this experiment the scientists Ester and Joshua Lederberg proved that mutations are random.

2. A likely explanation for an abnormal human phenotype associated with a trisomy is:

D) Altered gene dosage

Trisomy is extra chromosome in any of the 13, 18 or 21 chromosome which results in Trisomy 18 or 13 or 21.

We have to consider that recent research has shown that alterations in the gene dosage, around 50% increase, especially in Downs syndrome (trisomy 21 (chromosome)), results in abnormal human phenotype expression. The genetic balance is disturbed due to the increased gene expression in certain genes on particular chromosomes.

In certain cases the extra chromosome has typically undergone significant rearrangements - the chromosome can become attached to another chromosome egg or sperm and can become carrier but the individual is normal.

3. Mutations in the SIV have caused them to become evolved to infect humans. In the early age human beings were immune to the SIV infections as they had developed a mutation against them. It had taken several hundred years for the SIV to evolve through various mammals and cross species transmission to be able to adapt itself according to the human immune system and infect them.

4. Man who is XYY is color blind (recessive X linked trait). Both his mother and father have normal vision. Write the relevant genotypes of the parents and how he might have ended up being colorblind.

Ans.: Let us assume that the mother is a carrier for the colorblindness. Her genotype - XX. Since only one copy of the X chromosome is affected, she does not have color blindness.

Father has normal vision and we can assume that he is not a carrier of the disease. His genotype - XY.

Since male have only one X chromosome, if they inherit the affected X chromosome they will inherit the disease.

XYY is also called as 47, XYY syndrome or just YY syndrome. The person has an extra Y chromosome and is called a super male. Some may not exhibit any abnormalities while some can display learning disability. They are tall in stature. This is not an inherited disease, it is an abnormality arising due to error during cell division or when the egg is fertilized by a sperm carrying two Y chromosome.

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