Module 5 Lesson 3 - 3

Sources of Genetic Variation 1 — Crossing Over

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Read page 566

Meiosis goes further than simply shuffling the genetic deck. It actually makes new cards! Recall that, during prophase I of meiosis, the homologous chromosomes pair up. When the homologous chromosomes pair up,  the genes on the chromatids are aligned perfectly with each other and form a very tight pairing called synapsis. When the two homologous chromosomes form close groups, they are referred to as tetrads.

Those groups are so close and so tight that sometimes small pieces from two non-sister homologous chromatids break off and fuse with the other chromatid. This is known as crossing over, and it results in a chromosome with a gene combination never seen before. This is the first source of variation in meiosis. 

Read the description of crossing over on page 566 and consider the illustration in figure 16.14. Without crossover, no chromosomes would be possible with upper case A, B  and a lower case c.

Sources of Genetic Variation 2 — Independent Assortment

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Read page 565 - 566

Recall that our somatic cells, in the process of dividing, are made up of chromosome pairs with two sets of chromosomes. Originally, one of those sets came from the male gamete or sperm and the other from the female gamete or egg. Then, when our cells went through meiosis, they did not separate the original sets; instead, new sets were formed.

The image with the chromosome arrangements shows four possible genetic arrangements. It shows a cell with two pairs of replicated homologous chromosomes. When the cell goes through meiosis, each alignment shown is equally likely. The end result is 4 gametes with unique combinations.

This is the second source of variation in meiosis, and it occurs in metaphase I of meiosis. During this alignment of homologous pairs along the equator of the cell, each pair is free to arrange itself independent of other pairs. This is known as independent assortment.

The shuffling is far greater in humans when you consider that our cells have 23 pairs, not just four! By looking at 23 chromosome pairs, we can see that the result could be 2²³ or more than 8 million genetically unique gametes.

Read about "Independent Assortment" starting at the bottom of page 565 and examine the graphic on page 566 of your textbook for another example.

What are alleles?

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The diagram above uses colour and size to clarify differences in chromosomes. Homologous chromosomes are the same size and have the same kinds of genes. They are different in colour to illustrate that they contain different forms of genes. These different forms of the gene are known as alleles.

Because many alleles are possible on each chromosome, commonly alleles are represented with letters instead of colour. Thus, an upper case letter (for example, "B") and a lower case letter (for example, "b") shown on a chromosome are different alleles of the same gene (they are the same letter). For example, the gene that codes for the hair colour has various forms: blond, black, brown, and red hair colours. In other words, the hair colour gene has various alleles for blond, black, brown, and red.

Homologous chromosomes have the same letters (genes), but the letter case (alleles) can vary. The significance of this practice of using upper and lower case letters is reviewed in Module 6 when studying the results of potential crosses.

Watch and Listen

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Meiosis has three key components as highlighted above: reductional division, crossing over during synapsis, and the independent assortment of homologous chromosomes. The following animation, Unique Features of Meiosis, is a beneficial summary of those features.  While you are watching, give close attention to how the resulting gametes are different from their parent cells. You may wish to make summary notes, a flow chart, or a labeled diagram to summarize this information.

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Work through the following meiosis tutorial and quiz   

Each problem has hints and explains why each given answer is correct. The questions go slightly beyond our lesson, so do not worry if you miss a few.

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Although the end results of mitosis and meiosis are unique, they follow similar steps when sorting chromosomes. You must have a clear understanding of the differences of these types of cellular division. The following animation shows a step-by-step comparison of mitosis to meiosis 

when starting with the same cell.   Choose the non-flash version

Comparing Fraternal and Identical Twins

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Read pages 570 - 571

 

You have just read about how meiosis produces unique gamete cells. This important part of the human life cycle ensures variation within our species. Given the incredible amount of variation that can result, how is it that identical twins exist? Is it some kind of faulty meiosis?

Identical twins start from a single egg and a single sperm. Those gametes unite to form a single diploid zygote. As the zygote begins to divide through mitosis, a disruption may occur and cause the creation of two cell masses instead of one cell mass. At this stage, cells have not begun to specialize. Any cell is capable of becoming anything in the human body. They are known as totipotent. Because they have not specialized, and conditions are perfect for growth and development, each new mass grows to become a full person. Because twins started from the same sperm and egg, they are genetically identical.

Fraternal twins are a different story. They are the result of two eggs being released at roughly the same time. During fertilization, each egg receives its own sperm nuclei, and the result is two separate zygotes. Each of these zygotes is genetically unique, and they are no more related than any other brother and sister are.