Evolution And History

38 CHAPTER 2 | Genetics and Evolution

The process of meiotic division has important impli-
cations for genetics. Because paired chromosomes are
separated, two different types of new cells will be formed;
two of the four new cells will have one half of a pair of
chromosomes, and the other two will have the second half
of the original chromosome pair. At the same time, cor-
responding portions of one chromosome may “cross over”

homozygous Refers to a chromosome pair that bears identi-

cal alleles for a single gene.

heterozygous Refers to a chromosome pair that bears differ-

ent alleles for a single gene.

genotype The alleles possessed for a particular gene.

to the other one, somewhat scrambling the genetic mate-

rial compared to the original chromosomes.

Sometimes the original pair is homozygous, possess-

ing identical alleles for a specific gene. For example, if in

both chromosomes of the original pair the gene for A-B-

O blood type is represented by the allele for type A blood,

then all new cells will have the A allele. But if the original

pair is heterozygous, with the A allele on one chromosome

and the allele for type B blood on the other, then half of

the new cells will contain only the B allele; the offspring

have a 50-50 chance of getting either one. It is impossible

to predict any single individual’s genotype, or genetic com-

position, but (as Mendel originally discovered) statistical

probabilities can be established (see Figure 2.5).

Alzheimer’s disease

Karyotype with a Few Genetic Loci

Orofacial cleft

Huntington’s disease

Schizophrenia Beta chain of hemoglobin

Diabetes-associated

peptide (amylin)

Cystic fibrosis

Leptin (appetive regulation)

A-B-O blood

Tay-Sachs

Serotonin receptor

BRCA-1 (associated

with breast cancer)

Maple syrup urine

disease

Gigantism Sex-determining region

Androgen receptor

Cone dystrophy (deltonia, color blindness)

123 45

6

13

19 20 21 22

XY

14 15 16 17 18

789110 112

Figure 2.5 The twenty-three pairs of chromosomes humans possess include twenty-two pairs of
somatic or body chromosomes plus one pair of sex chromosomes for a total of forty-six chromosomes.
Of each pair, one is inherited from the individual’s mother and the other from the father. Each pair
of chromosomes has a characteristic size and shape. The genes coding for specific traits are located
on specified places on each chromosome as indicated here. In the lower right corner is the pair of sex
chromosomes typically found in males: a larger X chromosome (left) and smaller Y. Females typically
possess two X chromosomes. Offspring inherit an X chromosome from their mother but either an
X or a Y from their father, resulting in approximately equal numbers of male and female offspring
in subsequent generations. Though the Y chromosome is critical for differentiation into a male
phenotype, compared to other chromosomes the Y is tiny and carries little genetic information.