When Meiosis Goes Wrong – Paste questions and write answers in your labbook
1. Define: Nondisjunction
2. Define: Autosome
3. What happens in most failures of meiosis?
4. Define Trisomy 21 and describe the features of a person with this defect.
5. How does the age of parents affect the incidence of Trisomy 21?
6. What is a Barr body?
7. The second paragraph on the back page uses the term “phenotypically”. Look up
phenotype and genotype and write the definitions in your journal.
8. What is the most common sex chromosome abnormality in the U.S.?
9. Look back through your notes and see if you can discover how genetic abnormalities
are diagnosed.
10. Soon it will be possible to correct many genetic abnormalities. When do you think it
is morally correct to do so? Is it ever wrong to make genetic corrections?
When Meiosis Goes Wrong – Paste questions and write answers in your labbook
1. Define: Nondisjunction
2. Define: Autosome
3. What happens in most failures of meiosis?
4. Define Trisomy 21 and describe the features of a person with this defect.
5. How does the age of parents affect the incidence of Trisomy 21?
6. What is a Barr body?
7. The second paragraph on the back page uses the term “phenotypically”. Look up
phenotype and genotype and write the definitions in your journal.
8. What is the most common sex chromosome abnormality in the U.S.?
9. Look back through your notes and see if you can discover how genetic abnormalities
are diagnosed.
10. Soon it will be possible to correct many genetic abnormalities. When do you think it
is morally correct to do so? Is it ever wrong to make genetic corrections?
When Meiosis Goes Wrong – Paste questions and write answers in your labbook
1. Define: Nondisjunction
2. Define: Autosome
3. What happens in most failures of meiosis?
4. Define Trisomy 21 and describe the features of a person with this defect.
5. How does the age of parents affect the incidence of Trisomy 21?
6. What is a Barr body?
7. The second paragraph on the back page uses the term “phenotypically”. Look up
phenotype and genotype and write the definitions in your journal.
8. What is the most common sex chromosome abnormality in the U.S.?
9. Look back through your notes and see if you can discover how genetic abnormalities
are diagnosed.
10. Soon it will be possible to correct many genetic abnormalities. When do you think it
is morally correct to do so? Is it ever wrong to make genetic corrections?
When Meiosis Goes Wrong
Meiosis is a much more complicated process than mitosis. When you consider all the phases of chromosome pairing, crossing over, and double
divisions, you shouldn't be surprised to learn that frequently something goes wrong. In humans, for instance, about a third of all pregnancies
spontaneously abort in the first two or three months. When the expelled embryos are examined, it turns out that most of them had the wrong
number of chromosomes. Failure of the chromosomes to separate correctly at meiosis is called nondisjunction.
But not all failures of meiosis result in early miscarriage. There are late miscarriages and still-births of severely malformed fetuses. Even worse,
about one live-born human baby in 200 has the wrong number of chromosomes, accompanied by severe physical and/or mental abnormalities.
Apparently, nondisjunction can occur with any chromosomes. Having only one autosome (an autosome is any chromosomes other than the X and
Y sex chromosomes) of a pair instead of the normal two is invariably fatal. Having three instead of two is almost always fatal, resulting in
spontaneous abortion or death in infancy. There are a few important exceptions. For example, you can survive with three of the tiny chromosome
21. About one baby in 600 has three copies of chromosomes 21. Such persons may grow into adulthood, but they have all kinds of abnormalities.
The syndrome s known both as Trisomy-21 and Down Syndrome, after the 19th century physician who first described. it. Other characteristics of
the syndrome are general pudginess, rounded features, and a rounded mouth in particular, an enlarged tongue which often protrudes and various
internal disorders. Trisomy-21 individuals also have a characteristic barklike voice and unusually happy, friendly disposition. The "happiness" is a
true effect of the extra chromosome and not a result of their (usually) extremely low IQs, because other kinds of serious mental defectives are
usually, by most indications, miserable.
Trisomney-21 occurs most frequently among babies born to women over 35 years old, affecting up to 2% of such births. The age of the father
apparently has a much smaller effect. We can guess that the much prolonged prophase I of the human oocyte (egg) might have something to do
with this. Remember, the egg would have been arrested in this stage for 35 years.
Wrong numbers of X and/or Y chromosomes in humans result in live babies and abnormal adults. There are many varieties of sex chromosome
conditions. The shorthand designations of the most common are as follows:
XX
XY
XO
XXY
XYY
XXX
Normal female (2 X chromosomes)
Normal male (one X, One Y)
Turner's syndrome female (one X, no Y)
Klinefelter's syndrome (2 X's, 1 Y)
Extra Y or XYY syndrome male
Trisomy-X or XXX female
In addition to the above, there are many more extreme situations, such as XXYY, XXXY, XXXYY, XXXX, XXXXX, XXXXYY, and so on, each
having its own distinguishing characteristics. However, we can make four generalizations: First, one must have a least one X chromosome to live.
Second, the presence of a Y causes the individual to develop as a male, and the absence of a Y causes the individual to develop as a female. Third,
(and this is probably why these syndromes are not fatal), all but one of the X chromosomes will condense into heterochromatin, and be visible as a
Barr body when stained, so that XO females lack a Barr body, XXY males have one, XXX females have two, and XXXXXYY males have four,
and so on. Fourth the more sex chromosomes a person has, the taller he or she will be, so the XO females are tiny, while XXX, XXY and XYY
individuals are usually much taller than chromosomally normal men and women and XXYY men are huge.
XO individuals are phenotypically female, but do not develop ovaries. They remain sexually immature as adults, unless given hormones. XXY
males are tall and have small, imperfect testes and low levels of male hormones. There may be female-like breast development and somewhat
feminine body contours. XXX females are tall and frequently sterile, but appear otherwise normal. XYY males appear normal except for their
extreme height and for a tendency toward severe acne. They are also generally sterile. On the average, they have somewhat reduced IQs and in
common with other low-IQ groups, they average significantly increased criminal arrest records. At one time, there was speculation that XYY males
had so-called "genetic criminal tendencies", but further analysis showed that an XYY male is no more likely to be arrested than an XY or XXY
male of the same IQ.
Actually, all three types of individuals with three sex chromosomes (XXX, XXY, and XYY) in addition to being taller than average, have
somewhat lower IQ scores, though the distributions overlap the population mean of 100. XXX females and XYY males cannot usually be
diagnosed except by karyotyping (creating a picture of one's chromosomes) so these relatively innocuous conditions have not been well-studied;
the trisomy-X condition is most often found when a patient complains that she seldom or never menstruates, but the condition XYY men is usually
not discovered unless some researcher happens to include them in a population survey. XXY males (Klinefelter's syndrome males), however, make
up a substantial percentage of the institutionalized mentally retarded and are relatively easily diagnosed. Persons with four or more sex
chromosomes, such as XXXY are usually severely retarded.
The Frequency of Some Common Sex Chromosome Abnormalities in the United States
Syndrome
Number of
Frequency per live
Estimated Number
chromosomes
births
living in the U.S. in 1974
Triple X (XXX)
47
1/1000
200,000
Turner's (XO)
45
1/2500
85,000
Klinefelter's (XXY)
47
1/850
530,000
Double Y (XYY)
47
1/1000
850,000
XYY
47
1/1000
Adapted from G.J. Stine. Biosocial Genetics (New York: Macmillan. 1977). Updated 2013.