Heredity ❮ 99
of the passing of traits from generation to generation. Its basic premise is that offspring are
more like their parents than less closely related individuals.
In this chapter, we begin by discussing some terms that will prove important to your
study of heredity. This is followed by an examination of Mendel’s law of segregationand
thelaw of independent assortment,including how they were discovered and how they can be
applied. We will examine the law of dominance,which arose from Mendel’s work, and we
will also discuss some exceptions to Mendel’s fundamental laws such as intermediate inher-
itance (incomplete dominance and codominance), multiple alleles, polygenic traits, epista-
sis, and pleiotropy.
In the next section, we will examine Thomas Morgan’s work on fruit flies, which paved
the way for the discovery of linked genes, genetic recombination, and sex-linked inheri-
tance. This discussion concludes with a look at gene linkage and linkage maps.
Finally, since chromosomes carry the vital genes necessary for proper development and
passage of hereditary material from one generation to the next, it is important to discuss the
types of chromosomal errors that can occur during reproduction. This includes the various
forms of nondisjunction, or the improper separation of chromosomes during meiosis
(which leads to an abnormal number of chromosomes in offspring). The chapter concludes
with an examination of the other major types of chromosomal errors: deletions, duplica-
tions, translocations, and inversions.
Terms Important in Studying Heredity
The following is a list of terms that will help in your understanding of heredity:
Allele:a variant of a gene for a particular character. For example, two alleles for fur color
could be B (dominant) and b (recessive).
F 1 : the first generation of offspring, or the first “filial” generation in a genetic cross.
F 2 : the second generation of offspring, or the second “filial” generation in a genetic cross.
Genotype:an organism’s genetic makeup for a given trait. A simple example of this could
involve fur color where B represents the allele for brown and b represents the allele for
black. The possible genotypes include homozygous brown (BB), heterozygous brown (Bb),
and homozygous black (bb).
Heterozygous(hybrid): an individual is heterozygous (or a hybrid) for a gene if the two
alleles are different (Bb).
Homozygous(pure): an individual is homozygous for a gene if both of the given alleles are
the same (BB or bb).
Karyotype:a chart that organizes chromosomes in relation to number, size, and type.
Nondisjunction:the improper separation of chromosomes during meiosis, which leads
to an abnormal number of chromosomes in offspring. A few classic examples of non-
disjunction-related syndromes are Down, Turner, and Klinefeltersyndromes.
P 1 : the parent generation in a genetic cross.
Phenotype:the physical expression of the trait associated with a particular genotype. Some
examples of the phenotypes for Mendel’s peas were round or wrinkled, green or yellow,
purple flower or white flower.
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