Cytogenetics Practice Problems Help (page 3)

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Solution 5

Cytogenetics Solved Problems

The existence of a morphologically normal X chromosome in recombinant male progeny with carnation eyes provides cytological proof that genetic crossing over is correlated with physical exchange between homologous chromosomes in the parents. Similarly, all other phenotypes correlate with the cytological picture. Chromosomal patterns other than the ones shown above may be produced by crossing over outside the inverted region.

Practice 6

The centromere of chromosome V in corn is about 7 map units from the end. The gene for light yellow (virescent) seedling (v) is 10 map units from this end, and a gene called brevis (bv) that shortens internode length is 12map units from this end. The break point of a translocation (T) is 20map units from this end. A translocation heterozygote involving chromosomes Vand VIII of genotype + bv t/v + T is pollinated by a normal (nontranslocated, t) plant of genotype v bv t/v bv t. If gametes are formed exclusively by alternate segregation from the ring of chromosomes formed by the translocation heterozygote, predict the ratio of progeny genotypes and phenotypes from this cross (considering multiple crossovers to be negligible).

Solution 6

First let us diagram the effect that crossing over will have between the centromere and the point of translocation. We will label the ends of chromosome V with 1-2, and of chromosome VIII with 3-4. A cross-shaped pairing figure is formed during meiosis.

Cytogenetics Solved Problems

Alternate segregation produces half functional and half nonfunctional (duplication-deficiency) gametes. Note that the nonfunctional gametes derive only from the crossover chromatids. Thus, recovery of chromatids that experience a crossover between the centromere and the point of translocation is prevented. The combination of genes in this region of the chromosome is prevented from being broken up by crossing over and are thus transmitted as a unit. This situation is analogous to the block of genes within an inversion heterozygote that are similarly held together as a genetic unit. Noncrossover chromatids will form two types of functional gametes with equal frequency: + bv t and v + T. Expected zygotes are: 1/2 + bv t/v bv t = brevis, homozygous for the normal chromosome order and 1/2 v + T/v bv t = virescent, heterozygous for the translocation.

Practice 7

Shrunken endosperm of corn is governed by a recessive gene sh and waxy endosperm by another recessive wx. Both of these loci are linked on chromosome 9. A plant that is heterozygous for a translocation involving chromosomes 8 and 9 and that developed from a plump, starchy kernel is pollinated by a plant from a shrunken, waxy kernel with normal chromosomes. The progeny are

Cytogenetics Solved Problems

(a) How far is each locus from the point of translocation? (b) Diagram and label the pairing figure in the plump, starchy parent.

Solution 7

  1. The point of translocation may be considered as a gene locus because it produces a phenotypic effect, namely, semisterility. The conventio nal symbol for translocation is T, and t is used for the normal chromosome without a translocation. Gene order in the parents must be in order for double crossovers to produce the least frequent phenotypes:
      + + T = plump, starchy, semisterile ear
      sh wx t = shrunken, waxy, normal ear
  2. The map distances are calculated in the usual way for a three-point testcross.

      Distance shwx = (82 + 49 + 6 + 3)/573 = 24:4 map units
      Distance wxT = (17 + 40 + 6 + 3)/573 = 11:5 map units
      Distance shT = 24:4 +11:5 = 35:9 map units
  3. Cytogenetics Solved Problems

Practice 8

An inversion heterozygote possesses one chromosome in the normal order:

and one in the inverted order: Cytogenetics Solved Problems

A four-strand double crossover occurs in the areas f-e and d-c. Diagram and label the first anaphase figures.

Solution 8

A somewhat easier way to diagram the synapsing chromosomes when crossing over is only within the inversion as shown below. This is obviously not representative of the actual pairing figure. Let the crossover in the c-d region involve strands 2 and 3, and the crossover in the e-f region involve strands 1 and 4.

Cytogenetics Solved Problems

Practice 9

Data from Drosophila studies indicate that noncrossover (NCO) rings are recovered in equal frequencies with NCO rods from ring-rod heterozygotes. What light does this information shed on the occurrence of sister-strand crossing over?

Solution 9

Let us diagram the results of a sister-strand crossover in a rod and in a ring chromosome.

  1. Rod chromosome:
  2. Cytogenetics Solved Problems

  3. Ring chromosome:
  4. Cytogenetics Solved Problems

The double bridge at anaphase will rupture and produce nonfunctional gametes with duplications or deficiences. These would fail to be recovered in viable offspring. The fact that both rings and rods are recovered with equal frequency argues against the occurrence of sister-strand crossing over.

Modern techniques (involving autoradiography with labeled thymidine or fluorescence microscopy of cultured cells that have incorporated 5-bromodeoxyuridine in place of thymine) reveal that some sister-strand exchanges occur by a repair mechanism when DNA is damaged. One of the initiating steps that transforms a normal cell to a cancer cell is DNA damage. Hence, screening chemicals for their ability to induce sister-strand exchanges is one method for detecting potential cancer-inducing agents (carcinogens).

Practice 10

Yellow body color in Drosophila is produced by a recessive gene y at the end of the X chromosome. A yellow male is mated to an attached-X female (X^X) heterozygous for the y allele. Progeny are of two types: yellowfemales and wild-type females. What insight does this experiment offer concerning the stage (two strand or four strand) at which crossing over occurs? (Reminder: the Y chromosome does not determine maleness in Drosophila; see Genic Balance, Chapter 5).

Solution 10

Let us assume that crossing over occurs in the two-strand stage, i.e., before the chromosome replicates into two chromatids.

Cytogenetics Solved Problems

The yellowmale produces gametes with either a y-bearingXchromosome or one with theYchromosome that is devoid of genetic markers. Trisomic X (X^XX) flies seldom survive (superfemales). Those with (X^XY) will be viable heterozygous wild-type attached-X females. Crossing over fails to produce yellow progeny when it occurs in the twostrand stage.

Let us assume that crossing over occurs after replication of the chromosome, i.e., in the four-strand stage:

Cytogenetics Solved Problems

The appearance of yellow females in the progeny is proof that crossing over occurs in the four-strand stage.

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