SUPPELMENTARY PROBLEMS IN ADVANCED CYTOGENETICS

VARIATION IN CHROMOSOME NUMBER

8.13. Abyssinian oat (Avena abyssinica) appears to be a tetraploid with 28 chromosomes.  The common cultivated oat (Avena sativa) appears to be a hexaploid in this same series.  How many chromosomes does the common oat possess?

8.14. The European raspberry (Rubus idaeus) has 14 chromosomes.  The dewberry (Rubus caesius) is a tetraploid with 28 chromosomes.  Hybrids between these two species are sterile F1 individuals.  Some unreduced gametes of the F₁ are functional in backcrosses.  Determine the chromosome number and level of ploidy for each of the following: a) F₁ (b) F₁ backcrossed to R. idaeus, (c) F₁ backcrossed to R. caesius, (d) chromosome doubling of F₁ (R. maximus).

8.15. There are 13 pairs of chromosomes in Asiatic cotton (Gossypium arboreum) and also 13 pairs in an American species G. thurberi.  Interspecific crosses between arboreum and thurberi are sterile because of highly irregular chromosome pairing during meiosis.  The American cultivated cotton (Gossypium hirsutum) has 26 pairs of chromosomes.  Crosses of arboreum X hirsutum or thurberi X hirsutum produce triploids with 13 bivalents (pairs of chromosomes) and 13 univalents (single unpaired chromosomes).  How can this cytological information be used to interpret the evolution of hirsutum?

8.16. If two alleles, A and a, exist at a locus, 5 genotypic combinations can be formed in an autotetraploid : quadruplex (AAAA), triplex (AAAa), duplex (AAaa), simplex (Aaaa), nulliplex (aaaa).  Assume A exhibits xenia over a.  For each of these five genotypes determine the expected phenotypic ratio (A:a) when (a) the locus is tightly linked to its centromere(choromosomal assortment) and the genotype is selfed, (b) the locus is assorting chromosomally and the genotype is test crossed, (c) the locus is far from its centromere so that chromatids assort independently and the genotype is selfed, (d) the locus assorts by chromatids and the genotype is testcrossed.

8.17. The loci of genes A and B are on different chromosomes.  A dihybrid autotetraploid plant of genotype AAaaBBbb is self-pollinated. Assume that only diploid gametes are formed and that the loci of A and B are very close to their respective centromeres (chromosomal segregation).  Find the phenotypic expectations of the progeny.

8.18.    The flinty endosperm character in maize is produced whenever 2 or all 3 of the alleles in this triploid tissue are F.  In the presence of its alternative allele F’ in double or triple dose, a floury endosperm is produced.  White endosperm color is produced by a triple dose of recessive allele y, its dominant allele Y exhibiting xenia and producing yellow endosperm.  The loci   F and Y assort independently. (a) In crosses between parents of genotypes FF’Yy, what phenotypic ratio is expected in the progeny seed? (b) Pollen from a plant of genotype FF’Yy is crossed onto a plant of genotype FFyy.  Compare the phenotypic ratios produced by this cross with its reciprocal cross.

8.19.    The diploid number of the garden pea is 2n = 14, (a) How many different trisomics could be formed? (b) How many different double trisomics could be formed?

8.20.    The diploid number of an organism is 12.  How many chromosomes would be expected in (a) a monosomic, (b) a trisomic, (c) a tetrasomic, (d) a double trisomic, (e) a nullisomic,  (f) a monoploid, (b) a triploid, (h) an autotetraploid?

8.21.    Sugary endosperm of corn is regulated by a recessive gene s on chromosome IV and starchy endosperm by its dominant allele S.  Assuming n + 1 pollen grains are nonfunctional, predict the genotypic and phenotypic ratios of endosperm expected in the progeny from the cross of (a) diploid ss pollinated by trisomic-IV of genotype SSs,  (b) diploid Ss pollinated by trisomic - IV of genotype SSs.

8.22.    A dominant gene w+ produces yellow flowers in a certain plant species and its recessive allele w produces white flowers.  Plants trisomic for the chromosome bearing the color locus will produce n and n+ 1 functional female gametes, but viable  pollen has only the n number.  Find the phenotypic ratio expected from each of the following crosses:

 Seed Parent                Pollen Parent

 

                                 a)  + + w             X         + + w

                                 b) + w w             X         + + w

                                 c) + + w              X         + w

                                 d) + w w             X         + w

8.23 Shrunken endosperm is the product of a recessive gene sh on chromosome III of corn: its dominant allele sh produces full, plump kernels. Another recessive gene pr on chromosome V gives red color to the aleurone, and its dominant allele Pr gives purple. A diploid plant of genotypes Sh/sh, Pr/pr was pollinated by a plant trisomic for chromosome III of genotype Sh/Sh/sh,Pr/pr. If n+ 1 pollen grains are nonfunctional, determine the phenotypic ratio expected in the progeny endosperms.

8.24 Normal women possess 2 sex chromosomes (XX) and normal men have a single X chromosome plus a Y chromosome that carries male determiners.  Rarely a woman is found with marked abnormalities of primary and secondary sexual characteristics, having only 1 X chromosome (XO).  The phenotypic expressions of this monosomic – X state is called Turner syndrome.  Likewise, men are occasionally discovered with an XXY constitution exhibiting corresponding abnormalities called Klinefelter’s syndrome.  Color blindness is a sex-linked recessive trait.  (a) A husband and wife both had normal vision, but one of their children was a color-blind Turner girl. Diagram this cross, including the gametes that produced this child, (b) In another family the mother is color blind and the father has normal vision.  Their child is Klinefelter with normal vision. What gametes produced this child? (c) Suppose the same parents in part (b) produced a color-blind Klinefelter,  What gametes produced this child? (d) The normal diploid number of humans is 46.  A trisomic condition for chromosome 21 results in Down syndrome.  At least one case of Down-Klinefelter has been recorded.  How many chromosomes would this individual be expected to possess?

8.25. Colorless aleurone of corn kernels is a trait governed by a recessive gene c and is in the same linkage group (IX) with another recessive gene wx governing waxy endosperm.  In  1931 Creighton and McClintock found a plant with one normal IX chromosome, but its homologue had a knob on one end and a translocated piece from another chromosome on the other end.  A dihybrid colored, starchy plant with the heteromorphic IX chromosome shown below was testcrossed   to a colorless, waxy plant with normal chromosomes.  The results of this experiment provided cytological proof that genetic crossing over involves an actual physical exchange between homologous chromosome segments.  Diagram the results of this cross, showing all genotypes and phenotypes.

8.26. Nipple-shaped tips on tomato fruit is the phenotypic expression of a recessive gene nt. on chromosome v. A heterozygous plant (Nt/nt) that is also heterozygous for a reciprocal translocation involving chromosomes V and VIII is testcrossed to a plant with normal chromosomes.  The progeny were 48 normal fruit, fertile: 19 nipple fruit, fertile: 11 normal fruit, semisterile : 37 nipple fruit, semisterile.  What is the genetic position of the locus of gene Nt with respect to the point of translocation?

8.27. Given a pericentric inversion heterozygote with 1 chromosome in normal order (1 2 3 4 . 5 6 7 8) and the other in the inverted order (1 5 . 4 3 2 6 7 8 ), diagram the first anaphase figure after a 4-strand double crossover occurs: one crossover involved the regions between 4 and the centromere (.); the other cross over occurs between the centromere and 5.

8.28. A 4 - strand double crossover occurs in an inversion heterozygote.  The normal chromosome order is (.1 2 3 4 5 7 8); the inverted chromosome order is (.1 2 7 6 5 4 3 8).  One crossover is between 1 and 2 and the other is between 5 and 6.  diagram and label the first anaphase figures.

8.29. Diagram and label the first anaphase figure produced by an inversion heterozygote whose normal chromosome   is (.a b c d e f g h ) and with the inverted order (.a b f e d c g h).  Assume that a 2-strand double crossover occurs in the regions c-d and e-f.

8.30. A chromosome with segments in the normal order is (.a b c d e f g h).  An inversion heterozygote has the abnormal order (.a b f e d c g h).  A 3-strand double cross over occurs involving the regions between a and b and between d and e.  Diagram and label the first and second anaphase figures.

8.31.    Given the pairing figure for an inversion heterozygote with 3 crossovers as indicted on page 201, diagram the first anaphase. 

8.32.    Four races of a species are characterized by variation in the segmental order (a-h) of a certain chromosome.  A fifth race, with still a different chromosomal order, is postulated to have existed in the past but is now extinct.  Explain the evolutionary origin of these races in terms of single inversion differences.  Hint: See problem 8.10.

8.33.    A species of the fruit fly is differentiated into five reaces on the basis of differences in the banding patterns of one of its giant chromosomes.  Eight regions of the chromosome are designated a-h.  If each of these races is separated by a single overlapping inversion, devise a scheme to account for the evolution of the five races: (1) a d g h f c b e,  (2) f h g d a c b e, (3) f h c a d g b e (4) f h g b c a d e, (5) f a d g h c b e. 

VARIATION IN THE NUMBER OF CHROMOSOME SEGEMENTS

8.34.    In higher animals, even very small deficiencies, when homozygous, are usually lethal.  A recessive gene w in mice results in an abnormal gait called “waltzing”.  A waltzing male was crossed to several homozygous normal females.  Among several hundred offsprings one was found to be a waltzer female.  Presumably, a deficiency in the chromosome carrying the W⁺ allele caused the waltzing trait to appear as pseudodominant. The pseudodominant waltzer female was then crossed to a homozygous normal male and produced only normal offspring.  (a) List 2 possible genotypes for the normal progeny from the above cross.  (b) Suppose that 2 males, one of each genotype produced in part (a) were backcrossed to their pseudo dominant mother and each produced 12 zygotes.  Assuming that homozygosity for the deletion is lethal, calculate the expected combined number of waltzer and normal progeny.

 

1.      Given a pericentric inversion heterozygote with 1 chromosome in normal order

 (1 2 3 4 . 5 6 7 8) and the other in the inverted order (1 5 . 4 3 2 6 7 8 ), diagram the first anaphase figure after a 4-strand double crossover occurs: one crossover involved the regions between 4 and the centromere (.); the other cross over occurs between the centromere and 5.

2. Nipple-shaped tips on tomato fruit is the phenotypic expression of a recessive gene nt on chromosome v. A heterozygous plant (Nt/nt) that is also heterozygous for a reciprocal translocation involving chromosomes V and VIII is testcrossed to a plant with normal chromosomes.  The progeny were 48 normal fruit, fertile: 19 nipple fruit, fertile: 11 normal fruit, semisterile : 37 nipple fruit, semisterile.  What is the genetic position of the locus of gene Nt with respect to the point of translocation?