Module 5 Lesson 4 - 3

Nondisjunction

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Read page 567

After reviewing meiosis and gamete production, you should understand better how errors might occur in the process. Sometimes, homologous chromosomes or sister chromatids fail to separate when they form haploid gametes. This is known as nondisjunction, which leads to gametes with either too many or too few chromosomes. Because a very small segment of chromosome can contain many genes, chromosomal disorders can have dramatic effects or they can even be fatal.

The figure below explains how nondisjunction can occur in meiosis I and II with very different results. Which one leads to more gametes with an abnormal number of chromosomes?

In scenario A, the homologous chromosomes fail to separate during meiosis I. This results in two gametes that lack copies of the chromosome (n-1) and two gametes that have an extra copy of a chromosome (n+1). All resulting gametes are abnormal.

In scenario B, the homologous chromosomes separate properly during meiosis I. However, the sister chromatids fail to separate during meiosis II. This results in one gamete with an extra copy of a chromosome (n+1), one gamete that lacks a copy of a chromosome ( n-1), and two gametes that contain a single copy of the chromosomes (n). Two gametes are normal and two gametes are abnormal.

The incidence of nondisjunction in female gametes increases as a woman ages. For example, the chances of having a Down syndrome child at age 25 is 1 in 1500 pregnancies, by age 30 the risk is 1 in 910, and by age 45 it is 1 in 30 pregnancies. A specific cause for this is unknown. Given that the oocytes have been suspended in prophase I for so many years, likely the apparatus to complete meiosis properly has deteriorated. New research also indicates that a woman's womb can become less selective for genetically compromised embryos, making it more likely to carry an embryo with a genetic disorder to term.

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Watch this animation on nondisjunction:

Notice that nondisjunction in meiosis I leads to all abnormal offspring, but nondisjunction in meiosis II leads to half the offspring with a normal chromosome count and the other half with too many or too few chromosomes.

Genetic Testing

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You may recall from Unit B the various technologies used to test a fetus for genetic problems. Three common DNA testing procedures are amniocentesis, cordocentesis, and chorionic villus sampling. Each differs in when it can be done, which material it tests, and how quickly the answers are available. Each test is the same in terms of purpose, which is to identify an abnormal number of chromosomes.

Name	Description of Procedure	Advantages	Risks
Amniocentesis	A sample of amniotic fluid is tested.	Can identify genetic disorders and birth defects before birth	A small risk of miscarriage or injuring the fetus
Chorionic Villus Sampling	A sample of tissue from the placenta is tested.	Can be done earlier than amniocentesis	A small risk of miscarriage and infection in the site where the sample was taken
Cordocentesis	A sample of fetal blood is taken from the umbilical cord.	Can be used when ultrasound, amniocentesis, and CVS are inconclusive	A risk of bleeding on the site where the blood sample was taken

Ethical Concerns

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Some groups within society object to prenatal tests because of the possible risks to the fetus or because of the future actions parents may take in their pregnancy. Science is very good at seeking knowledge. However, science or scientists do not have the role of deciding what should be done with that knowledge. Instead, this role is the responsibility of society.

What are some ethical concerns that can arise from prenatal genetic testing? What is the tension or debate that focuses on prenatal testing?

Benefits

• Genetic testing can identify chromosomal disorders and can help individuals make informed decisions about the future health of their family members.
• Certain birth defects can be detected early, and early detection can provide more time for parents to decide on the treatment immediately after birth.

Drawbacks

• The potential risks of the prenatal testing may not be worth the benefits.
• Prenatal testing can be inaccurate and provide false positive test results.

Think about other potential benefits and drawbacks involved in prenatal testing.

The Stem Cell Research Debate

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As you saw in Unit B, embryonic stem cells from the blastocyst stage are pluripotent. In other words, these stem cells are capable of developing into more specialized cells. Because the inner cell mass of a blastocyst has the regenerative potential to form any organs and tissues, it provides new possibilities to treat diseases such as cancer, heart disease, and diabetes through further research.

Stem cell research holds much promise for finding new treatments to many serious diseases. It may generate greater ethical concern than prenatal testing does because of the certain death it causes to the blastocyst from which these cells are taken. Although blastocysts cannot be created for research purposes, discarded embryos from in-vitro fertilization procedures can be donated for human stem cell research in Canada.

By starting with young, undifferentiated cells, scientists may be able to generate any number of missing or malfunctioning cells in adults. However, the collection of these cells destroys the blastocysts from which they are taken.

From your reading of the text on page 527, and from your own research on Canada's guidelines for stem cell research, take a position on whether Canada's guidelines give enough freedom to scientists while still respecting ethical concerns.

Watch and Listen

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Watch the following news feature 

from CBC on stem cell research and Canada's guidelines on when research can be done with these cells.