An Introduction to DNA and Chromosomes
Part 8

A closer look at what makes up the human genome...

What is genetic variation, and why is it important?

Recall that each of the four daughter cells resulting from meiosis is unique. You can see in Figure B-20 in Part 7 that each daughter cell has different combinations of chromosomes. Unlike the daughter cells resulting from mitosis, the products of meiosis are not identical to each other or to the parent cell. By creating distinctive germ cells each with only one chromosome of each kind (remember this is called haploid) the genetic information of the parent cell is reshuffled. This reshuffling or "recombination" is accomplished through two aspects of meiosis. The first is called independent assortment, which refers simply to the fact that each homologous pair of chromosomes is separated into different daughter cells. Thus, the two alleles of any given gene in the parent cell—one on each of the homologous chromosomes—do not maintain any association with each other as meiosis works its way along. Each daughter cell receives a random mixture of maternal and paternal chromosomes, which leads to a huge number of possible combinations. Theoretically, a daughter cell could have 8,400,000 different combinations of chromosomes! But independent assortment is not the only means of creating genetic variation. In the section on prophase I, remember how we discussed the process of crossing over? When homologous chromosomes overlap and trade some of their genetic material, many, many more combinations of alleles are possible in the resulting daughter cells. Crossing over thus contributes to genetic recombination.

So why is variation important? Geneticists are not sure exactly why, but they generally agree that species, such as humans, that reproduce sexually (and make use of independent assortment and recombination) have a competitive advantage over species that reproduce asexually and basically clone themselves. Sexual reproduction leads to immense genetic variation, and therefore immense variation in the individuals that are produced. Evolutionary theory suggests that when environments are highly variable there is an advantage to producing variable offspring: then it is likely that at least some of the offspring will be able to survive the environmental challenges that arise. (For more information on the genetics of populations, click here).

Unfortunately, there is a drawback to the complexity of meiotic division. Sometimes, it does not proceed correctly and the resulting gametes are abnormal. One kind of mistake that can be made is nondisjunction. Nondisjunction occurs when either homologous chromosomes or sister chromatids fail to separate properly. This situation can occur during either anaphase I or anaphase II, and results in one gamete that lacks a particular chromosome and another one that has two copies. Usually these gametes do not survive, but nondisjunction of certain chromosomes can still produce viable offspring. The offspring will, however, have serious deficits. Down’s Syndrome or "trisomy 21" is a relatively common nondisjunction disorder that results from an extra copy of chromosome 21. Other problems that can arise from meiosis, though they are more unusual, are expansions and contractions of genes. These occur during crossing over, in which there is not an equal exchange of genetic material. An expansion of the CAG series in the Huntington gene, for example, can (but only very rarely) lead to a spontaneous appearance of the HD allele in a child, when neither parent had HD. (For more information on expansions in HD, click here).

Genetic variation can occur by means other than independent assortment and recombination as well. Mutations, or changes in the genetic code, appear frequently in humans and are an important source of variation. Expansions and contractions of genes are considered to be mutations, but there are also other kinds. (For more information on mutations, click here). Yet more variation comes from transposable elements. Transposable elements are relatively small pieces of DNA that can exist separately from chromosomes and can be inserted in new locations in the genome. Other species have evolved many creative ways of increasing genetic variation. Many plants, for example, have an elaborate life cycle that includes a longer, more significant single-chromosome (haploid) stage (For more information on plants, click here).

In conclusion, genetic variation is crucial to the evolution and survival of all species. Its advantages to living organisms have encouraged the evolution of complex and elegant processes of chromosome shuffling within dividing cells during the process of meiosis. These same processes also serve to make each of us genetically unique, except in the special case of identical twins.

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-S. Fu, 8-04-01 (sections 0-5)
-C. Tobin 4-25-06 (6-8)

To learn more about DNA, a number of resources exist on the web:

1. Visit The Tech for a good tutorial loaded with pictures.
2. Australia's "Cooperative Research Centre for Discovery of Genes for Common Human Diseases" (Gene CRC) web site has some fabulous tutorials at various levels of understanding.

1. Alberts, B., Bray, D., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. "Essential Cell Biology: An Introduction to the Molecular Biology of the Cell". Garland Publishing, Inc., 1998.
   This textbook covers many topics in molecular biology. It provides a great deal of detail, but it is quite dense.
2. McGraw Hill Online Learning Center. Mitosis Movie. http://highered.mcgraw-hill.com/sites/0073031216/student_view0/exercise13/
   mitosis_movie.html
   This tutorial is short and straightforward, and shows mitosis with animation.
3. Price, H., Snustad, D., & Simmons, M. "Principles of Genetics: Study Guide and Problems Workbook" (2nd ed.). John Wiley & Sons, Inc., 2000.
   Contains some helpful outlines of the concepts of mitosis and meiosis. It also has exercises that help solidify understanding.

Last Modified: 08/04/2008

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