When breeders want to develop new varieties of bananas or improve existing varieties by, for example, using crosses to introduce a gene resistant for Black Sigatoka or some other disease, they need seeds. So, they induce the plant to become diploid, make the crosses and selections, and induce the new variety to, once again, take up its triploid ways.

Consider mitosis and meiosis. Speculate on how breeders might manipulate banana ploidy. What points in the cell processes could they manipulate to produce these results?

One more thing: Yes, the little black dots in a banana are shriveled, aborted seeds. Also, next time you are eating a banana, play with it - you can get it to split in three lengthwise sections. I wonder if you could get a diploid banana to do that.

Asexual and Sexual Life Cycles
Hydra Budding

Concept 13.3 Meiosis reduces the number of chromosome sets from diploid to haploid

Meiosis Animation

Concept 13.4 Genetic variation produced in sexual life cycles contributes to evolution

Origins of Genetic Variation
How Can the Frequency of Crossing Over Be Estimated?

This article discusses the significance of a research article by Katajima et al. in the same issue. The authors of the latter have found a protein called shugoshin in a species of yeast that seems to cause cohesion of homologous chromosomes during meiosis I.

• de Lange, T. "Ending Up with the Right Partner." Nature, April 23, 1998.

Addresses the question of how homologous chromosomes find each other during meiosis.

• Gould, S. J. "Ghosts of Bell Curves Past." Natural History, February 1995.

A famous evolutionary biologist argues against a best-selling book about the genetic contribution to intelligence.

• Haber, J. E. "Searching for a Partner." Science, February 6, 1998.

Contains a summary of recent discoveries about crossing over and synapses in different organisms.

• Martinez-Perrez, E., and G. Moore. "Promiscuous Maize Chromosomes." Science, January 2, 2004.

This Perspectives article puts into context a research article by Pawlowski et al. showing that homologous pairing is apparently not necessary in order for recombination to occur.

• Ridley, M. "Is Sex Good for Anything?" New Scientist, December 4, 1993.

Did disease play an important role in the evolution of sex?

• Sluder, G., and D. McCollum. "The Mad Ways of Meiosis." Science, July 14, 2000.

A signaling protein called Mad2 seems to be essential for the proper segregation of chromosomes during meiosis

-apsis 5 juncture (synapsis: the pairing of replicated homologous chromosomes during prophase I of meiosis)

auto- 5 self (autosome: the chromosomes that do not determine gender)

chiasm- 5 marked crosswise (chiasma: the X-shaped, microscopically visible region representing homologous chromosomes that have exchanged genetic material through crossing over during meiosis)

di- 5 two (diploid: cells that contain two homologous sets of chromosomes)

fertil- 5 fruitful (fertilization: process of fusion of a haploid sperm and a haploid egg cell)

haplo- 5 single (haploid: cells that contain only one chromosome of each homologous pair)

homo- 5 like (homologous: like chromosomes that form a pair)

karyo- 5 nucleus (karyotype: a display of the chromosomes of a cell)

meio- 5 less (meiosis: a variation of cell division which yields daughter cells with half as many chromosomes as the parent cell)

soma- 5 body (somatic: body cells with 46 chromosomes in humans)

sporo- 5 a seed; -phyt 5 a plant (sporophyte: the multicellular diploid form in organisms undergoing alternation of generations that results from a union of gametes and that meiotically produces haploid spores that grow into the gametophyte generation)

syn- 5 together; gam- 5 marriage (syngamy: the process of cellular union during fertilization)

tetra- 5 four (tetrad: the four closely associated chromatids of a homologous pair of chromosomes)

1. Explain in general terms how traits are transmitted from parents to offspring.
2. Distinguish between asexual and sexual reproduction.

The Role of Meiosis in Sexual Life Cycles

1. Distinguish between the following pairs of terms:
   • somatic cell and gamete
   • autosome and sex chromosome
2. Explain how haploid and diploid cells differ from each other. State which cells in the human body are diploid and which are haploid.
3. Explain why fertilization and meiosis must alternate in all sexual life cycles.
4. Distinguish among the three life-cycle patterns characteristic of eukaryotes, and name one organism that displays each pattern.
5. List the phases of meiosis I and meiosis II and describe the events characteristic of each phase.
6. Recognize the phases of meiosis from diagrams or micrographs.
7. Describe the process of synapsis during prophase I and explain how genetic recombination occurs.
8. Describe three events that occur during meiosis I but not during mitosis.

Origins of Genetic Variation

1. Explain how independent assortment, crossing over, and random fertilization contribute to genetic variation in sexually reproducing organisms.
2. Explain why heritable variation is crucial to Darwin ’s theory of evolution by natural selection