Exam 12: The Chromosomal Basis of Inheritance and DNA: The Genetic Material

Suppose you are carrying out a series of crosses with an insect where the mechanism of sex determination is unknown. You discover a mutant female with short bristles and decide to cross it with a wild type male that has normal bristles. Half of the F₁ progeny have short bristles but all of these short-bristled F₁ progeny are males. Based on these results, a valid hypothesis would be

In humans, red-green color blindness is caused by a recessive allele on the X chromosome. Therefore, if a female who is a carrier for the allele that causes color blindness marries a male who is color blind, the probability that their first child will be a girl who is color blind is 25%. This assumes normal dosage compensation where the X that is inactivated in females varies randomly from cell to cell. But suppose the normal mechanism of dosage compensation is altered so that the X that is inactivated in any given female is random but the same X is inactivated in every cell of her body. In this case, what is the probability that their first child would be a girl with color blindness? ( Express the probability as a percent. Enter the number only without the percent sign. For example, enter 100% as 100 and enter 12.5% as 12.5 )

Huntington's disease is caused by a single dominant allele. It is a lethal disease, yet it persists in the human population. Which of the following statements best describes why?

In some human populations, the proportion of individuals who are heterozygous for the sickle cell allele is much higher than would be expected by chance alone. Why?

In humans, the male has an X and a Y sex chromosome. The human female has two X chromosomes. In birds, the female has a Z and a W sex chromosome while the male has two Z chromosomes. Which of the following statements is accurate about which parent determines the gender of the offspring?

In the fruit fly Drosophila, there is a dominant gene for normal wings and its recessive allele for vestigial wings. At another gene locus on the same chromosome, there is a dominant gene for red eyes and its recessive allele for purple eyes. A male that was heterozygous at both gene loci was mated with a female that was homozygous for both recessive alleles and the following results were observed among the offspring: Normal wings and red eyes - 420 Vestigial wings and red eyes - 80 Normal wings and purple eyes - 70 Vestigial wings and purple eyes - 430 According to these data, what is the distance, in centimorgans, between these 2 gene loci? (Enter the number only without the units. For example, 100 cM would be entered as 100)

Genetic maps are based on recombination frequencies. Because both odd and even numbers of crossovers can occur between any 2 gene loci, as the physical distance between two loci increases, the maximum recombination frequency levels off at 50%. However, suppose you discovered a species where only an even number of crossovers can occur between any two gene loci. In this case, as the physical distance between two loci increases, you would expect the maximum recombination frequency to

In Drosophila, dosage compensation is controlled by the male-specific lethal (MSL) complex consisting of MSL proteins and roX RNAs. Based on what you know about dosage compensation, the role of the MSL complex in males would be to

Below is a pedigree for a particular human trait. Affected individuals are illustrated by the shaded in circles or squares. This particular pedigree does not yet have a particular shading pattern for carriers. As a result, some of these individuals may be carriers for the trait. What is the mechanism of inheritance of this trait?

Suppose a new form of DNA polymerase III is discovered that does not require a primer to begin synthesis of a new strand. All other properties of the enzyme remain unchanged. If this new enzyme were to be expressed in the same organism that normally expresses DNA polymerase III, what would no longer be necessary to completely replicate all of the cellular DNA?

Why isn't mitochondrial DNA a unique identifier?

Each origin of replication in eukaryotic cells is recognized by an origin recognition complex, which allows the assembly of the pre-replicative complex including the replicative helicase. This complex ultimately allows the recruitment of DNA polymerase. After DNA polymerase is recruited to an origin and replicates the DNA in that region during S phase, the replicative helicase cannot be loaded onto the DNA in that region again until early G1 due to a variety of regulatory mechanisms. Why is it important that, once used, the replicative helicase cannot be loaded onto the DNA again until G1 of the cell cycle?

You decide to repeat the Meselson-Stahl experiment, except this time you plan to grow the cells on light ¹⁴N medium for many generations and then transfer them to heavy ¹⁵N medium and allow them to grow for 2 additional generations (2 rounds of DNA replication). If the semiconservative model of DNA replication is correct, what is the expected distribution of DNA in the density gradient after two rounds of replication?

Suppose a new species of bacteria was found that contained a form of DNA polymerase III that can add new nucleotides to either the 5' end or the 3' end of an existing strand. All other properties of the DNA polymerase III enzyme remain unchanged. In addition, these bacteria contain all of the other enzymes and proteins relevant to DNA synthesis that are found in E. coli. How would DNA replication in these new bacteria differ from replication in E. coli?

At an autosomal gene locus in humans, the allele for brown eyes is dominant over the allele for blue eyes. At another gene locus, located on the X chromosome, a recessive allele produces colorblindness while the dominant allele produces normal color vision. A heterozygous brown-eyed woman who is a carrier of colorblindness marries a blue-eyed man who is not colorblind. What is the probability that their first child will be a blue-eyed female who has normal color vision? (Enter the probability as a percent. Enter the number only without the percent sign. For example, enter 100% as 100 and enter 12.5% as 12.5)

A married couple has a son with sickle-cell disease. If this couple has more children, what is the likelihood that a daughter will inherit sickle-cell disease?

You decide to repeat the Meselson-Stahl experiment, except this time you plan to grow the cells on light ¹⁴N medium for many generations and then transfer them to heavy ¹⁵N medium and allow them to grow for 2 additional generations (2 rounds of DNA replication). If the conservative model of DNA replication is correct, what is the expected distribution of DNA in the density gradient after one round of replication?

How did the development of anonymous markers aid in the production of a human genetic map?

In 1910, Morgan did a series of experiments with the fruit fly Drosophila, an organism where females are XX and males are XY. When a mutant male fly with white eyes was crossed with a wild type female with red eyes, none of the F₁ progeny had white eyes but 18% of the F₂ progeny had white eyes. Unexpectedly, all of these white-eyed F₂ flies were males. Based on these results, Morgan concluded that white eyes is caused by a recessive X-linked allele. Suppose Morgan has found that half of the F₁ progeny had white eyes but all of these white-eyed F₁ flies were females. In this case, a valid hypothesis would be

You are working with a strain of E. coli that has a mutation in the DNA polymerase II gene, such that the encoded protein is exonuclease deficient, but is otherwise normal. How do you predict that this would most likely affect the activity of this enzyme?