1. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings To study human chromosomes microscopically, researchers stain and display them as a karyotype –A karyotype usually shows 22 pairs of autosomes and one pair of sex chromosomes ALTERATIONS OF CHROMOSOME NUMBER AND STRUCTURE 8.19 A karyotype is a photographic inventory of an individual’s chromosomes

2. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Preparation of a karyotype Figure 8.19 Blood culture 1 Centrifuge Packed red And white blood cells Fluid 2 Hypotonic solution 3 Fixative White Blood cells Stain 45 Centromere Sister chromatids Pair of homologous chromosomes

3. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings This karyotype shows three number 21 chromosomes An extra copy of chromosome 21 causes Down syndrome 8.20 Connection: An extra copy of chromosome 21 causes Down syndrome Figure 8.20A, B

4. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Abnormal chromosome count is a result of nondisjunction –Either homologous pairs fail to separate during meiosis I 8.21 Accidents during meiosis can alter chromosome number Figure 8.21A Nondisjunction in meiosis I Normal meiosis II Gametes n + 1 n – 1 Number of chromosomes

5. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings –Or sister chromatids fail to separate during meiosis II Figure 8.21B Normal meiosis I Nondisjunction in meiosis II Gametes n + 1n – 1nn Number of chromosomes

6. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Fertilization after nondisjunction in the mother results in a zygote with an extra chromosome Figure 8.21C Egg cell Sperm cell n + 1 n (normal) Zygote 2n + 1

7. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Nondisjunction can also produce gametes with extra or missing sex chromosomes –Unusual numbers of sex chromosomes upset the genetic balance less than an unusual number of autosomes 8.22 Connection: Abnormal numbers of sex chromosomes do not usually affect survival

8. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Table 8.22 With some female features underdeveloped

9. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Chromosome breakage can lead to rearrangements that can produce genetic disorders or cancer –Four types of rearrangement are deletion, duplication, inversion, and translocation 8.23 Connection: Alterations of chromosome structure can cause birth defects and cancer

10. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 8.23A, B Deletion Duplication Inversion Homologous chromosomes Reciprocal translocation Nonhomologous chromosomes