Biology Now, 2e

138 ■ CHAPTER 08 Chromosomes and Human Genetics

GENETICS

is male or female. All other chromosomes are

called autosomes.

Autosomes are homologous chromosomes

exactly alike in terms of length, shape, and the

genes they carry (Figure 8.4). Human auto-

somes are labeled with the numbers 1 through

22 (for example, chromosome 4). Sex chromo-

somes are assigned letter names; in humans,

males have one X chromosome and one Y

individual genes or by abnormalities in chromo-

some number or structure.

Every species has a characteristic number

of chromosomes: humans have 23 pairs of

homologous chromosomes, for a total of 46,

while mosquitoes, for example, have only

3 pairs, or 6 in total. One of the 23 pairs of

human chromosomes consists of the sex chro-

mosomes; they determine whether a person

Figure 8.4

A human karyotype

To r e s e a r c h c h r o m o s o m a l a b n o r m a l i t i e s , a s ci e n t i s t w i l l t a ke a p h o t o g r a p h

of a cell’s chromosomes during mitosis and then pair up each set of

homologous chromosomes to create a karyotype. In humans, the autosomes

are numbered 1 through 22, and the sex chromosomes are designated X or Y.

Q1: Is this the karyotype of a male or a female?

Q2: How would the karyotype of a person with Down syndrome

differ from this karyotype?

Q3: The size of a chromosome correlates roughly with the number

of genes residing on it. Why are an extra copy of chromosome 21 and

a missing Y chromosome two of the least damaging chromosomal

abnormalities?

Figure 8.3

Pedigree of a family with a history of Wiskott-Aldrich syndrome

As in Figure 8.2, circles represent females and squares represent males in this pedigree. Individuals affected by WAS are

shaded red.

Q1: How many male and how many female descendants (individuals that did not join the family by marriage) does

generation IV of Aldrich’s pedigree contain?

Q2: What proportions of the male and female descendants in generation IV were affected by the disorder?

Q3: Why did Aldrich hypothesize that the disease was X-linked? (You will need to read ahead to answer this question.)

I

II

III

IV

V

VI

Generation