Genetics- Exam 2 Material

What is a Barr body? How is its structure different from that of other chromosomes in the cell? How does the structure of a Barr body affect the level of X-linked gene expression?

A Barr body is a mammalian X chromosome that is highly condensed. It is found in somatic cells with two or more X chromosomes. Most genes on the Barr body are inactive.

How many Barr bodies would you expect to find in humans with the following abnormal compositions of sex chromosomes XYY XXY XXX X0 (a person with just a single X chromosome)

A. One B. Zero C. Two D. Zero

Genes that cause Prader-Willi syndrome and Angelman syndrome are closely linked along chromosome 15. Although people with these syndromes do not usually reproduce, let's suppose that a couple produces 2 children with Angelman syndrome. The oldest child (Pat) grows up and has 2 children with Prader-Willi syndrome. The second child (Robin) grows up and has 1 child with Angelman syndrome 1.) Are Pat and Robin's parents both normal or does one of them have Angelman or Prader-Willi syndrome? If one of them has a disorder, explain why it is the mother or the father. 2.) What are the sexes of Pat and Robin? Explain.

A. Pat and Robin's mother is abnormal. We know this because Pat and Robin both have Angelman syndrome. The AS gene is inactivated in the sperm, so both children must have inherited the deletion from their mother. Therefore, they did not get the gene from their mother, and the gene from their father is normally inactivated. This causes them to have Angelman syndrome. We do not actually know if Pat and Robin's mother has AS or PWS. We only know she has the deletion. Their mother could have either AS or PWS, depending on whether their mother inherited the deletion from Pat and Robin's grandmother or grandfather. B. Pat is a male because he has children with PWS. He transmitted the chromosome carrying the deletion to his two children, and the mother of Pat's children normally inactivates the PW gene in the egg. Therefore, both children have PWS. As in the answer to part A, we know Robin is a female because she has a child with AS.

With regard to the number of sex chromosomes, explain why dosage compensation is necessary

Dosage compensation refers to the phenomenon that the level of expression of genes on the sex chromosomes is in males and females, even though they have different numbers of sex chromosomes. In many species it seems necessary so the balance of gene expression between the autosomes and sex chromosomes is similar between the two sexes.

When does the erasure and reestablishment phase of genomic imprinting occur? Explain why it is necessary to erase an imprint and then reestablish it in order to always maintain imprinting from the same sex of parent.

Erasure and reestablishment of the imprint occurs during gametogenesis. It is necessary to erase the imprint because each sex will transmit either inactive or active alleles of a gene. In somatic cells, the two alleles for a gene are imprinted according to the sex of the parent from which the allele was inherited.

What is the difference in the meaning between the terms genetic recombination and crossing over?

Genetic recombination is a term that refers to a new combination of alleles in an offspring. Crossing over is a physical event that involves the exchange of pieces of genetic material between two chromosomes.

Among different species, describe 3 distinct strategies for accomplishing

In mammals, one of the X chromosomes is inactivated in females; in Drosophila, the level of transcription on the X chromosome in males is doubled; in C. elegans, level of transcription of the X chromosome in hermaphrodites is decreased by 50% of that of males.

What is mitotic recombination? A heterozygous individual (Bb) with brown eyes has one eye with a small patch of blue. Provide two or more explanations for how the blue patch may have occurred.

Mitotic recombination is crossing over between homologous chromosomes during mitosis in somatic cells. Mitotic recombination is one explanation for a blue patch. Following mitotic recombination, the two chromosomes carrying the b allele could segregate into the same cell and produce the blue color. Another reason could be chromosome loss; the chromosome carrying the B allele could be lost during mitosis.

When true-breeding mice with brown fur and short tails (BBtt) were crossed to true-breeding mice with white fur and long tails (bbTT), all F, offspring had brown fur and long tails. The F offspring were crossed to mice with white fur and short tails. What are the possible phenotypes of the F2 offspring? Which F2 offspring are recombinant, and which are non-recombinant? What are the ratios the F2 offspring if independent assortment is taking place? How are he ratios affected by linkage?

The four phenotypic categories for the F2 offspring are: brown fur, short tails; brown fur, long tails; white fur, short tails; and white fur, long tails. The recombinants are brown fur, long tails, and white fur, short tails. The F2 offspring will occur in a 1:1:1:1 ratio if the two genes are not linked. In other words, there will be 25% of each of the four phenotypic categories. If the genes are linked, there will be a lower percentage of the recombinant offspring.

What is the spreading phase of X-chromosomes inactivation? Why do you think it's called a spreading phase? Discuss the role of the Xist gene in the spreading phase.

The spreading stage is when the X chromosome is inactivated (i.e., condensed) as a wave that spreads outward from the X-inactivation center (Xic). The condensation spreads from Xic to the rest of the X chromosome. The Xist gene is transcribed from the inactivated X chromosome. It encodes an RNA that coats the X chromosome, which subsequently attracts proteins that are responsible for the compaction.

Describe the molecular process of X-chromosome inactivation. This description should include the 3 phases of inactivation and the role of Xic. Explain what happen to X chromosomes during embryogenesis, in adult somatic cells, and during oogenesis.

X-chromosome inactivation begins with the counting of Xics. If there are two X chromosomes, in the process of initiation, one is targeted for inactivation. During embryogenesis, this inactivation begins at the Xic locus and spreads to both ends of the X chromosome until it becomes a highly condensed Barr body. The Tsix gene plays a role in the choice of the X chromosome that remains active. The Xist gene, which is located in the Xic region, remains transcriptionally active on the inactivated X chromosome. It is thought to play an important role in X-chromosome inactivation by coating the inactive X chromosome. After X-chromosome inactivation is established, it is maintained in the same X chromosome in somatic cells during subsequent cell divisions. In germ cells, however, the X chromosomes are not inactivated, so an egg can transmit either copy of an active (noncondensed) X chromosome.

Describe when X-chromosomes inactivation overs and how this leads to phenotypic results at the organism level. In your answer you should explain why XCI causes results such as variegated coat patterns in mammals. Why do 2 different calico cats have their patches of orange and black fur in different places? Explain whether or not a variegated coat pattern due to XCI could occur in marsupials.

X-chromosome inactivation in heterozygous females produces a mosaic pattern of gene expression. During early embryonic development, some cells have the maternal X chromosome inactivated and other cells have the paternal X chromosome inactivated; these embryonic cells will divide and produce billions of cells. In the case of a female that is heterozygous for a gene that affects pigmentation of the fur, this produces a variegated pattern of coat color. Because it is a random process in any given animal, two female cats will vary as to where the orange and black patches occur. A variegated coat pattern could not occur in female marsupials due to X-chromosome inactivation because the paternal X chromosome is inactivated in the somatic cells of females.

When applying a chi square approach in a linkage problem, explain why an independent assortment hypothesis is used

n independent assortment hypothesis is used because it enables us to calculate the expected values based on Mendel's ratios. Using the observed and expected values, we can calculate whether or not the deviations between the observed and expected values are too large to occur as a matter of chance. If the deviations are very large, we reject the hypothesis of independent assortment.