and a recessive allele by the lowercase form of the same letter (
Dominant alleles are expressed, or make themselves “known,”
when they are present in either single or double dose. For reces-
sive alleles to be expressed, they must be present in double dose,
that is, the homozygous condition. Returning to our thumb ex-
ample, a person whose genetic makeup includes either the gene
(the homozygous dominant condition) or the gene pair
(the heterozygous condition) will have double-jointed thumbs.
(the homozygous recessive condition) is
needed to produce tight thumb ligaments.
Genotype and Phenotype
A person’s genetic makeup is referred to as his or her
o-tīp). Te way that genotype is expressed in the body is
no-tīp). For example, the double-
jointed condition is the phenotype produced by a genotype of
Check Your Understanding
When a geneticist orders a karyotype, why are cells in
metaphase studied instead of those in interphase?
What term refers to chromosomes other than our sex
Is an allele represented by a capital letter presumed to be
dominant or recessive?
Harold is homozygous for the dominant alleles
, and heterozygous for
. He is blond and blue-
eyed and has a very hairy chest. Which of these descriptions
refer to his phenotype?
For answers, see Appendix H.
of Genetic Variation
Describe events that lead to genetic variability of gametes.
Before we examine how genes interact, let us consider why (with
the possible exception of identical siblings) each of us is one of
a kind, with a unique genotype and phenotype. Tis variability
reﬂects three events that occur before we are even a twinkle in our
parents’ eyes: independent assortment of chromosomes, cross-
over of homologues, and random fertilization of eggs by sperm.
and Independent Assortment
As we described in Chapter 27, each pair of homologous chro-
mosomes synapses during meiosis I, forming a tetrad. Tis
happens during both spermatogenesis and oogenesis. Because
chance determines how the tetrads align (line up) on the meio-
sis I metaphase spindle, maternal and paternal chromosomes
are randomly distributed to daughter nuclei.
As illustrated in
, this simple event leads to an
amazing amount of variation in gametes. Te cell in our exam-
ple has a diploid number of 6, and so three tetrads form. As you
can see, the possible combinations of alignments of the three
tetrads result in eight gamete possibilities. Because the way each
tetrad aligns is random, and because many cells are undergoing
meiosis simultaneously, each alignment and each type of gam-
ete occurs with the same frequency as all others.
±wo important points about metaphase of meiosis I: (1)
Te two alleles determining each trait are
resulting from independent assortment.
During metaphase of meiosis I, the tetrads
of homologous chromosomes align
independently of other tetrads. The large
circles depict the possible alignments in a
mother cell having a diploid number of 6. The
small circles show the gametes arising from
each alignment. Some gametes contain all
maternal or all paternal chromosomes; others
have various combinations of maternal and