8.01 Nondisjunction may happen in either division I or division II of meiosis.

(a) Assuming nondisjunction occurs in meiosis I for the sex chromosomes in a male, what sperm genotypes would be possible?

Answer: Possible sperm are XY or OO (no sex chromosomes present at all)

(b) If each of these possible sperm were to fertilize a normal egg, what are the possible results?

Answer: Possible zygotes are XXY or XO

(c) Answer questions a and b again but assume that the nondisjunction happens in meiosis II.

Answer: Possible sperm are XX or YY (the normal X and Y gamete would also be possible assuming that the nondisjunction affects only one of the secondary spermatocytes). Possible zygotes when each of these sperm fertilizes a normal egg are XX, XY, XXX, or XYY.

8.02 In humans, there are several well known conditions that result from nondisjunction and thus have incorrect numbers of chromosomes. For each of the following, how many autosomes, X chromosomes, and Y chromosomes would be present in each cell?

(a) An male who is trisomic for chromosome number 21 (Down Syndrome)

Answer: 45 autosomes + X + Y

(b) An female who is trisomic for chromosome number 21 (Down Syndrome)

Answer: 45 autosomes + XX

(c) A female who is monosomic for the sex chromosomes (Turner Syndrome)

Answer: 44 autosomes + X

(d) A male who is trisomic for the sex chromosomes (Kleinfelter Syndrome)

Answer: 44 autosomes + XXY

8.03 Chromosome abnormalities such as duplications, deletions, and inversions can happen by breakage and rejoining of chromosomes.

(a) Using diagrams similar to those in the text, show how a reverse duplication might be generated.

Answer:

(b) Show how a displaced duplication might be generated.

Answer:

8.04 Polyploids can result in greater numbers of phenotypes and genotypes for a single gene when compared with the same gene in a diploid. Consider the following questions:

(a) How many copies of a gene does an autotetraploid have?

Answer: Autotetraploids have four copies of each gene

(b) Assuming that there are two kinds of alleles possible at a particular locus, A and a, there would be five possible genotypes. What are they?

Answer: Genotypes: AAAA, AAAa, AAaa, Aaaa, and aaaa

(c) If A is dominant to a, how many possible phenotypes would you expect?

Answer: Two phenotypes. The first four of the genotypes listed above would give the dominant phenotype; the last one would give the recessive phenotype.

8.05 Polyploids have various degrees of fertility.

(a) Why are autopolyploids almost always sterile while allopolyploids are frequently fertile?

Answer: Autopolyploids have more than two copies of each chromosome so synapsis cannot happen normally. (How would four chromosomes pair and then segregate evenly?) Allopolyploids have two sets of nonhomologous chromosomes. If a spontaneous chromosome doubling occurs, the resulting cell can behave like a diploid during meiosis and produce viable gametes.

(b) If an allopolyploid is formed by fertilization of a haploid egg of one species by a haploid sperm of a different species, would you expect the resulting hybrid to be fertile? If not, what event might cause it to become fertile at some later time?

Answer: No, at first the hybryd would be sterile, but it may become fertile later if chromosome doubling occurs.