EXAM 3
Multiple Choice (30 pts total). Correct answers are in bold type.
1. Homologous chromosomes segregate towards opposite poles of a dividing cell during
A. mitosis
B. meiosis I
C. meiosis II
D. fertilization
E. binary fission
2. The zygote contains the ________ number of chromosomes.
A. haploid
B. diploid
C. polyploid
D. both A and B are true
E. none of the above are true
3. Whether an allele is dominant or recessive depends on
A. How common the allele is, relative to other alleles
B. whether it is inherited from the mother or father
C. which chromosome it is on
D. whether it or another allele determines the phenotype when they are both
present
E. whether or not it is linked to other genes
4. All of the offspring of a white hen and a black rooster are gray. The simplest explanation for this pattern of inheritance is
A. pleitropy
B. sex linkage
C. linkage
D. independent assortment
E. incomplete dominance
5. A biochemist measured the amount of DNA in cells growing in the laboratory and found the quantity of DNA in a cell doubled
A. between prophase and anaphase of mitosis
B. between the G1 and G2 phases of the cell cycle
C. during mitosis
D. between prophase I and prophase II of meiosis
E. during cytokinesis
6. Which of the following phases of mitosis is essentially the opposite of prophase in terms of nuclear changes?
A. telophase
B. metaphase
C. S phase
D. interphase
E. anaphase
7. A picture of a dividing cell of a mouse showed 19 chromosomes, each consisting of two sister chromatids. During which of the following stages of cell division could this picture have been taken?
A. prophase of mitosis
B. telophase II of meiosis
C. prophase I of meiosis
D. anaphase of mitosis
E. prophase II of meiosis
8. The mating of 2 individuals that carry different alleles for the same trait is called
A. a monotrait cross
B. a monohybrid cross
C. a test cross
D. a dihybrid cross
E. none of the above
9. Which of the following represent the possible genotypes resulting from a cross between an individual homozygous for black hair (BB) and an individual heterozygous for black hair (Bb)?
A. BB and Bb
B. BB, Bb and bb
C. BB only
D. Bb only
E. bb only
10. During prophase I, each chiasmata represents
A. the site at which homologous chromosomes are united
B. the site at which maternal and paternal homologs are united
C. a newly formed haploid gamete
D. a site of crossing over
E. none of the above
Problems/Brief essays
1. In hamsters, fur-color genes and eye-color genes are linked. Brown fur (B) is dominant to white fur (b) and pink eyes (P) are dominant to black eyes (p). Assuming crossing over does not occur, what are the phenotypic and genotypic ratios of the offspring of the following mating? (5 pts)
BBPP X bbpp
One parent can only produce BP gametes, and the other parent can only produce bp gametes. Thus, all of the offspring would have genotype BbPp and would have brown fur and pink eyes.
2. Your uncle dies and leaves you his racehorse, Dingle. You want to put the horse out to stud in order to make some money. However, in looking over the stud book, you discover that Dingle's grandfather exhibited a rare clinical disorder that leads to brittle bones. This disorder results from homozygosity of a recessive allele (dd). If Dingle is heterozygous for that allele, it will not be possible to use him for a stud, lest the genetic defect be passed on. How would you determine whether dingle carries the allele? Describe your tests and the potential outcomes. (10 pts)
You should test mate Dingle with a diseased horse as follows:
D D d Dd Dd d Dd Dd If the offspring are not diseased, Dingle is homozygous.
D d d Dd dd d Dd dd If the offspring are 1:1 normal to diseased, Dingle is heterozygous.
3. Adult height in humans is at least partially hereditary: tall parents tend to have tall children. But humans come in a range of sizes, not just tall and short. Name and give a 1-2 sentence explanation of an extension of Mendel's model that could produce the hereditary variation in human height. (5 pts)
The best answer to this question is polygenic inheritance- when multiple and independent pairs of genes have additive effects on the same characteristic.
Partial credit was given for incomplete dominance and and codominance.
4. In 2-4 senetences, explain 2 sources of genetic variability seen in sexually reproducing populations of organisms. (10 pts)
The best answer is :
1. Synapsis and crossing over in prophase I can result in genetic recombination.
2. The random distribution of maternal and paternal homologs into daughter cells during meiosis I results in new gene combinations.
Partial credit was given for polygenic inheritance, codominance, incomplete dominance, even though they are not strictly sources of genetic variability, but rather describe the relationship between genotype and a phenotype.
5. A cell contains 6 chromosomes in the diploid state. Draw that cell in metaphase I of meiosis. Label the following: sister chromatids, homologous chromosomes, centrioles, mitotic spindle, kinetochores. (10 pts)
See page 230 of your text. Your text shows 4 chromosomes, the question asked for 6 chromosomes.
Short Answer (30 pts total)
1. The impact of a single gene on more than one characteristic is called pleitropy.
2. A fruit fly somatic cell contains 8 chromosomes. This means that 16 different
combinations of chromosomes are possible in its gametes.
3. An organism with a diploid number of 40 will have 20 chromosomes in their
gametes and 40 chromosomes in their somatic cells.
4. Dosage compensation in mammals generally involves inactivation of one of
the two X chromosomes in the female. During interphase, a dark spot of chromatin, called
a Barr body, is visible at the edge of the nucleus of each female
mammalian cell.
5. The sex chromosomes of a human male are X and Y. The sex chromosomes
of a human female are X and X.
6. Sister chromatids are tightly associated in the region of their DNA called centromeres.
7. Cytokinesis is the division of the cytoplasm of the cell to form two cells.
8. Assuming the sex chromosomes do not undergo synapsis, how many synaptonemal
complexes would you find during meiosis in a human cell? 22
9. A cell's chromosome complement is called its karyotype.
10. Mendel's three principles are of Dominance, Independent Assortment, and segregation.