116 Chapter 3

Understanding Concepts

1. Figure 5 shows sperm cell production following meiosis.(a) Which cells do not contain homologous pairs?(b) If the chromosome number for cell A is 12, indicate the

chromosome number for cell C.

2. Use Figure 6 to answer the questions below.(a) Which process(es) identify mitosis? Explain your answer.(b) Which process(es) identify meiosis? Explain your answer.

Making Connections

3. King Henry VIII of England had some of his wives executed fornot producing sons. Indicate why a little knowledge of meiosismight have been important for Henry’s wives.

4. A microscopic water animal called daphnia can be reproducedfrom an unfertilized egg. This form of reproduction is asexualbecause male gametes are not required. Indicate the sex of theoffspring produced. Explain your answer.

Sections 3.8 Questions

3. All gametes produced by meiosis have haploid chromosome numbers.

4. Homologous chromosomes are similar in shape, size, gene arrangement,and gene information.

5. Crossing over is the exchange of genetic material between homologouschromosomes that occurs during meiosis.

6. Sex chromosomes are pairs of chromosomes that determine the sex of anorganism. All other chromosomes are autosomes.

parent 1 parent 2

process W process X

1set

process Y

process Z

embryo

2sets

2sets

2sets

2sets

1set1

set

2sets

2sets

Figure 6

The processes and number of sets of chromo-somes involved in the production of an embryoin humans

AA

BB BB

CC CC CC CC

Figure 5

Sperm cell production in humans

3.9 Abnormal Meiosis:Nondisjunction

Meiosis, like most processes of the body, is not immune to mistakes.Nondisjunction occurs when two homologous chromosomes move to the samepole during meiosis. The result is that one of the daughter cells will be missingone chromosome while the other will retain an extra chromosome. Cells thatlack genetic information, or have too much information, will not function prop-erly. Nondisjunction can also occur in any cell during mitosis where separationof chromatids is involved, but the effects are most devastating during the for-mation of sex cells in meiosis.

In humans, nondisjunction produces gametes with 22 and 24 chromosomes(Figure 4, page 118). The gamete with 24 chromosomes has both chromosomesfrom one of the homologous pairs. If that gamete joins with a normal gamete of23 chromosomes from the opposite sex, a zygote containing 47, rather than 46,chromosomes will be produced. The zygote will then have three chromosomes inplace of the normal pair. This condition is referred to as trisomy. However, if thesex cell containing 22 chromosomes joins with a normal gamete, the resultingzygote will have 45 chromosomes. The zygote will have only one of the chromo-somes rather than the homologous pair. This condition is called monosomy.Once the cells of the trisomic or monosomic zygotes begin to divide, each cell ofthe body will contain greater or less than 46 chromosomes.

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3.9

Nondisjunction Disorders

Nondisjunction is associated with many different human genetic disorders.Compare the chromosomes of a male shown in Figure 1(a), with the chromo-somes of a female who has Down syndrome, shown in Figure 1(b). Notice howthe chromosomes are arranged in pairs. Such a picture of the chromosomes isknown as a karyotype chart. In about 95% of cases, a child with Down syn-drome has an extra chromosome in chromosome number 21. This trisomic dis-order is produced by nondisjunction; the person has too much geneticinformation.

The trisomic condition is referred to as a syndrome because it involves agroup of disorders that occur together. People with Down syndrome (Figure 2)can be identified by several common traits, regardless of race: a round, full face;enlarged and creased tongue; short height; and a large forehead. It has been esti-mated that 1 in 600 babies is born with Down syndrome. Down syndrome is

trisomy: the condition where there arethree homologous chromosomes in placeof a homologous pair

monosomy: the condition where there is asingle chromosome in place of a homologouspair

Down syndrome: a trisomic disorder inwhich a zygote receives three homologouschromosomes for chromosome pair number 21

karyotype chart: a picture of chromo-somes arranged in homologous pairs

(a) Karyotype chart of a male with 46 chromosomes. Notice that thechromosome pair number 23 is not homologous. Males contain an Xand a Y chromosome. They act as a 57

(b) Down syndrome female. Note the trisomy of number 21. Downsyndrome affects both males and females.

Figure 2

People with Down syndrome have a widerange of abilities.

Figure 1

118 Chapter 3

generally associated with mental retardation, although people with this conditionretain a wide range of mental abilities. Statistics indicate that the risk of having ababy with Down syndrome increases with the age of the mother. A woman in herforties has a 1 in 40 chance of having a child with Down syndrome. This rate is 25times greater than for a woman in her twenties (Figure 3).

Turner syndrome occurs when sex chromosomes undergo nondisjunction(Figure 4). This monosomic disorder produces a female with a single X chromo-some. In the egg cell, both homologous X chromosomes move to the same poleduring meiosis I. When the egg with no X chromosome is fertilized by a normalsperm cell with an X chromosome, a zygote with 45 chromosomes is produced.Individuals with Turner syndrome appear female, but do not usually develop sex-ually, and tend to be short and have thick, widened necks. About 1 in every 3000female babies is a Turner syndrome baby. Most Turner syndrome fetuses are mis-carried before the 20th week of pregnancy.

Klinefelter syndrome is caused by nondisjunction in either the sperm or egg.The child inherits two X chromosomes—characteristic of females—and a single

normal female normal male normal female normal male

normal meiosis nondisjunction nondisjunction normal meiosis

Klinefeltersyndrome

Turnersyndrome

trisomicfemale

Turnersyndrome

Figure 4

Nondisjunction disorders in humans

Klinefelter syndrome: a trisomic dis-order in which a male carries an XXY condition

Turner syndrome: a monosomic disorderin which a female has a single X chromosome

21–25

Age of Mother in Years

26 –30 31–35 36–40 41–45 46–500

20

40

60

80

Num

ber o

f Cas

es /

1000

Birt

hs

Figure 3

Incidence of Down syndrome

xxxx xyxy xxxx xyxy

xx xx xxxx

xxyxxy xoxoxxxxxx xoxo

xy x y

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Y chromosome—characteristic of males. The child appears to be a male at birth;however, as he enters sexual maturity, he begins producing high levels of femalesex hormones. Males with Klinefelter syndrome are sterile. It has been estimatedthat Klinefelter syndrome occurs, on average, in 1 of every 500 male babies.

Karyotype Charts

Technicians obtain a karyotype chart by mixing a small sample of tissue with asolution that stimulates mitotic division. A different solution is added which stopsdivision at metaphase. Since chromosomes are in their most condensed formduring metaphase—their size, length, and centromere location are most dis-cernible—it is the best phase in which to obtain a karyotype. The metaphase cellsare placed onto a slide and then stained, so that distinctive bands appear. A pho-tograph of the chromosomes is taken. The image is enlarged, and each chromo-some is cut out and paired up with its homologue. Homologous chromosomesare similar in size, length, centromere location, and banding pattern. Finally, allthe pairs are aligned at their centromeres in decreasing size order. The sex chro-mosomes are always placed last.

Sample Problem

Figure 5 shows the incomplete karyotype of a human.

1 2 3 4 5

6 7 8 9 10 11 12

13 14 15 16 17 18

19 20 21 22 X Y

Figure 5

The karyotype chart of a human. Noticeseveral chromosomes are missing.

120 Chapter 3

1. Identify where chromosomes a to f (Figure 6) should be in the karyotypechart.

2. This person has either Down syndrome or Klinefelter syndrome. Identifythe placement of chromosome g (Figure 7) to identify which disorder thepatient has.

Solution

• Start by scanning the karyotype chart to see which pairs are missing achromosome. Pairs 3, 5, 8, 15, and 16 need a partner.

• Match the most obvious chromosomes first: the longest, shortest, or mostdistinctively banded chromosomes.

• For chromosome matches that are not as obvious, look carefully at thebanding pattern and location of the centromere.

• Always pay attention to the X and Y chromosomes. In Figure 5, the missingchromosome might be X or Y. If it is Y, it will have to be found throughelimination since it will not match X.

1. a, 5 b, 8 c, 16 d, Y e, 15 f, 3

2. Klinefelter syndrome is the XXY condition. Chromosome g is too short to bean X chromosome, so the patient must have Down syndrome. Chromosomeg belongs at number 21.

Practice

Understanding Concepts

1. What is nondisjunction?

2. Differentiate between monosomy and trisomy.

3. What is Down syndrome?

4. What is a karyotype?

5. What is Turner syndrome?

Research in Canada: Dr. Irene Uchida

Down syndrome was first described by the English doctor John Down in 1866.By the early 1930s, geneticist L.S. Penrose had shown a link between the inci-dence of Down syndrome and the mother’s age. The Canadian researcher Dr.Irene Uchida (Figure 8) was intrigued by the higher incidence of nondisjunctiondisorders, specifically Down syndrome, in the babies of older women.

Dr. Uchida found a 1930s study on fruit flies, which showed that exposuresto high dosages of radiation increased the frequency of nondisjunction disor-ders. Although meiosis in fruit flies is not identical to that in humans, Dr. Uchidadecided to pursue the radiation link for clues in her research on Down syn-drome. During her first set of experiments, Dr. Uchida verified the original fruitfly experiments. She found that the greater the exposure to radiation, the higherwas the incidence of chromosome abnormalities. Assuming that the results forfruit flies would hold true for other chromosomes, Dr. Uchida hypothesized thatthe oocytes of older women have been exposed to radiation for a longer periodof time and, therefore, are more likely to suffer chromosome damage.

A 1960 survey, conducted by Dr. Uchida at the Children’s Hospital inWinnipeg, indicated that women who had been exposed to radiation prior toconception were more likely to have children with Down syndrome. Similarstudies carried out in other hospitals supported Dr. Uchida’s findings. In 1968,

Figure 8

Dr. Irene Uchida

a b c d e f

Figure 7

Figure 6

g

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Dr. Uchida carried out a larger study with women who had been exposed to radi-ation. Using a list of patients who had had abdominal X-ray examinations, orfluoroscopies, from the Winnipeg General Hospital between 1956 and 1959, Dr.Uchida compiled a second list of women who had given birth to a child withDown syndrome. The larger study supported her hypothesis that X rays may beone of the causes of Down syndrome.

How do you think the results of Dr. Uchida’s research influenced the way Xrays are now used and administered?

Frontiers of Technology: Prenatal Testing

Developments in medical technology now make it possible for doctors to detectgenetic disorders like Down syndrome even before the baby is born. Tests forchromosomal abnormalities are generally offered to women over the age of 35.

The most widely performed technique, called amniocentesis, involves drawingfluid from the sac surrounding the developing fetus with a syringe, using ultra-sound for guidance (Figure 9). The fluid, called amniotic fluid, also contains cellsfrom the developing fetus. When these cells are treated with special stains, thechromosomes can be made visible for microscopic examination. A cameramounted to the microscope is often used to take a picture of the chromosomes.Amniocentesis can be performed in the 11th week of pregnancy, although it issafer to wait until about 14 weeks.

A complementary technique, called karyotyping, discussed earlier in thechapter, compares the number, size, and shape of homologous chromosomes. Achromosome count of 47, for example, would indicate the existence of a nondis-junction disorder. By comparing homologous chromosomes, physicians canidentify the specific disorder or syndrome.

Another technique, chorionic villus sampling (CVS), draws cells from theouter membrane (chorion) surrounding the embryo. This procedure can be per-formed as early as eight weeks into pregnancy.

For women under 35, a more recent test called the multiple marker screen(MMS) identifies women who may be at higher risk of having a baby with agenetic disorder. The MMS is performed at 15 to 18 weeks of pregnancy, usingthe mother’s blood. One such marker is a substance called alphafetoprotein, orAFP. Elevated levels of AFP in the mother’s blood may indicate neural tubedefects in the baby that result in malformations of the baby’s spine or skull.Amniocentesis is recommended to verify the AFP levels. An ultrasound is per-formed to confirm the age of the baby and physical formation.

Nondisjunction

1. Nondisjunction occurs when two homologous chromosomes move to thesame pole during meiosis. In humans, this produces gametes with 22 and24 chromosomes.• trisomy—a zygote contains 47 chromosomes—Down syndrome or

Klinefelter syndrome• monosomy—a zygote contains 45 chromosomes—Turner syndrome

2. A karyotype chart is a picture of chromosomes arranged in homologouspairs in descending order by size, with the sex chromosomes placed last.

3. Amniocentesis involves withdrawing amniotic fluid, which can then bekaryotyped to look for nondisjunction disorders.

Figure 9

Ultrasound can be used to locate the positionof the fetus within the uterus. It uses theenergy from sound waves bouncing off thefetus to make its image.

122 Chapter 3

Understanding Concepts

1. Use a diagram to illustrate how nondisjunction in meiosis I differsfrom nondisjunction in meiosis II.

Making Connections

2. State advantages and disadvantages of ultrasound and amnio-centesis.

3. Amniocentesis, like many other advances in reproductive tech-nology, raises many moral and ethical questions. The knowledgeof a disorder can help the parents and medical staff prepare forthe birth of the child, and can help the parents prepare for theneeds of a child with a genetic disorder. Do you think the tech-nique should be used? Would you place any limits or restrictionson its use?

4. Ultrasound, amniocentesis, chorionic villus sampling, and themultiple marker screen test all give information about the fetus.Research to find out how improved scientific understandingabout fetuses and amniotic fluid contributed to the developmentof each technique.Follow the links for Nelson Biology 11, 3.9.

5. Do some research about Dr. Uchida’s work, or find out about otherCanadian contributions to research and technology in genetics.Follow the links for Nelson Biology 11, 3.9.

Section 3.9 Questions

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