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586 Chapter 17 NEL

4. Always pay attention to the X and Y chromosomes. In Figure 5, on the previous page,
   the missing chromosome might be X or Y. If it is Y, it will have to be found through
   elimination since it will not match X.
   a, 5 b, 8 c, 16 d, Y e, 15 f, 3

WEBActivity

Web Quest—Modelling Mitosis and Meiosis

Cellular division is one of the most critical processes an organism regularly undergoes.

Unfortunately, errors during cellular division can result in a number of genetic syndromes such

as Down syndrome, Turner syndrome, Klinefelter syndrome, and XYY syndrome. In this Web

Quest, you will explore normal and abnormal cellular division. You will use the knowledge that

you gathered to create an animation or presentation that shows exactly how abnormal cellular

divisions occur.

• Nondisjunction occurs when two homologous chromosomes move to the
  same pole during meiosis. In humans, this produces gametes with 22 and
  24 chromosomes.
  
  
  • Trisomy: a zygote containing 47 chromosomes; causes human genetic
    disorders such as Down syndrome and Klinefelter syndrome
  
  
  • Monosomy: a zygote containing 45 chromosomes; causes Turner syndrome
  
  
  
  


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

SUMMARY Abnormal Meiosis

Section 17.4 Questions

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?

6.Use a diagram to illustrate how nondisjunction in meiosis I

(2n= 4) differs from nondisjunction in meiosis II.

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Practice

1. This person has either Down syndrome or Klinefelter syndrome. Identify the
   placement of chromosome g (Figure 7) to identify which of these two
   disorders the patient has.

Figure 7

g

Karyotyping

There are a number of human

genetic disorders that involve

nondisjunction. In this Virtual

Biology Lab, you will construct

karyotype charts and use them to

predict genetic disorders, in much

the same way as a genetic

counsellor might.

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+ EXTENSION