Objectives
🧬 Summarize how the process of meiosis produces genetic recombination.
🧬 Explain how gene linkage can be used to create chromosome maps.
🧬 Analyze why polyploidy is important to the field of agriculture.
Vocabulary
genetic recombination: new combination of genes produced by crossing over and independent assortment.
polyploidy: having one or more extra sets of all chromosomes, which, in polyploid plants, can often result in greater size and better growth and survival.
Genetic Recombination
During meiosis, genes are combined in new ways. Genetic recombination occurs when crossing over and independent assortment produce new combinations of genes.
Recall that independent assortment occurs in meiosis when chromosomes separate randomly. The number of possible gene combinations due to independent assortment can be calculated using the formula 2n, where n equals the number of chromosome pairs.
Pea plants have 7 pairs of chromosomes. The possible combinations of these chromosomes would be 27, or 128. Fertilization further increases the number of combinations. During fertilization, any possible male gamete can fertilize any possible female gamete. The number of combinations after fertilization would be 2n times 2n. For peas, this number is 16,384, or 128 128. In people, the possible combinations of chromosomes are over 70 trillion. Crossing over increases genetic recombination even more.
Gene Linkage
Chromosomes contain many genes. Genes that are located close together on the same chromosome are said to be linked. This means they usually travel together during gamete formation. Linked genes do not segregate independently. They are an exception to Mendel’s law of independent assortment.
Occasionally, linked genes separate due to crossing over. Crossing over occurs more frequently between genes that are far apart than between genes that are close together.
Genes that are linked together on the same chromosome usually travel together in the gamete.
What does a chromosome map show?
The relationship between crossing over and chromosome distance is very useful. The distance between two genes can be estimated by the frequency of crossing over that occurs between them. Scientists use cross-over data to create a drawing of genes along a chromosome. The drawing, called a chromosome map, shows the order of genes on a chromosome. The first chromosome maps were published in 1913 for fruit-fly crosses.
Polyploidy
Most organisms have diploid cells—cells with two chromosomes in each cell. Some species have polyploid cells. Polyploidy means the cells have one or more extra sets of all chromosomes. For instance, a triploid organism has three complete sets of chromosomes in each cell. It is designated 3n. Polyploidy occurs in only a few animals, such as earthworms and goldfish sh. It is always lethal in humans.
Polyploidy is common in flowering plants. Polyploid plants are often bigger and more vigorous. Many food plants, such as wheat (6n), oats (6n), and sugarcane (8n), are polyploidy.
Summary
🧬 Genetic recombination involves both crossing over and independent assortment.
🧬 Early chromosome maps were created based on the linkage of genes on the chromosome.
🧬 Polyploid plants are selected by plant growers for their desirable characteristics.
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