Check Your Understanding
Which fetal testing procedure depends on analyzing amniotic
What noninvasive imaging procedure is used to determine
some aspects of fetal development? (You many want to
A Closer Look
in Chapter 1, pp. 16–17, for this.)
For answers, see Appendix H.
In this chapter, we have explored some of the basic principles
of genetics, the manner in which genes are expressed, and the
means by which gene expression can be modiﬁed. Consider-
ing the precision required to make perfect copies of genes and
chromosomes, and the incredible mechanical events of meiotic
division, it is amazing that genetic defects are as rare as they are.
Perhaps, aFer reading this chapter, you have a greater sense of
wonder that you turned out as well as you did.
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Genetics is the study of heredity and mechanisms of gene
The Vocabulary of Genetics
A complete diploid set of chromosomes is called the karyotype of an
organism; the complete genetic complement is the genome. A person’s
genome consists of two sets of instructions, one from each parent.
Gene Pairs (Alleles)
Genes coding for the same trait and found at the same locus on
homologous chromosomes are called alleles.
Alleles may be the same or diﬀerent in expression. When the
allele pair is identical, the person is homozygous for that trait;
when the alleles diﬀer, the person is heterozygous.
Genotype and Phenotype
Te actual genetic makeup of cells is the genotype; phenotype is
the manner in which those genes are expressed.
Sexual Sources of Genetic Variation
Chromosome Segregation and Independent Assortment
During meiosis I of gametogenesis, tetrads align randomly on
the metaphase plate, and chromatids are randomly distributed to
the daughter cells. Tis is called independent assortment of the
homologues. Each gamete receives only one allele of each gene pair.
Each diﬀerent metaphase I alignment produces a diﬀerent assortment
of parental chromosomes in the gametes, and all combinations of
maternal and paternal chromosomes are equally possible.
Crossover of Homologues and Gene Recombination
During meiosis I, all four chromatids of each tetrad cross over at
one or more points and exchange corresponding gene segments.
Te recombinant chromosomes contain new gene combinations,
adding to the variability arising from independent assortment.
Te third source of genetic variation is random fertilization of
eggs by sperm.
Types of Inheritance
Dominant genes are expressed when present in single or double
dose; recessive genes must be present in double dose to be
±or traits following the dominant-recessive pattern, the laws of
probability predict the outcome of a large number of matings.
Genetic disorders more oFen reﬂect the homozygous recessive
condition than the homozygous dominant or heterozygous
condition because dominant genes are expressed and, if they are
lethal genes, the pregnancy usually results in miscarriage. Genetic
disorders caused by dominant alleles include achondroplasia and
Huntington’s disease; recessive disorders include cystic ﬁbrosis
and ²ay-Sachs disease.
Carriers are heterozygotes who carry a deleterious recessive gene
(but do not express the trait) and have the potential of passing it
on to oﬀspring.
In incomplete dominance, the heterozygote exhibits a phenotype
intermediate between those of the homozygous dominant and
recessive individuals. Inheritance of sickle-cell trait is an example
of incomplete dominance.
Multiple-allele inheritance involves genes that exist in more
than two allelic forms in a population. Only two of the alleles are
inherited, but on a random basis. Inheritance of ABO blood types
is an example of multiple-allele inheritance in which the
alleles are codominant.
²raits determined by genes on the X and Y chromosomes are
said to be sex-linked. Te small Y chromosome lacks most genes
present on the X chromosome. Recessive genes located only
on the X chromosome are expressed in single dose in males.
Examples of such X-linked conditions, passed from mother to
son, include hemophilia and red-green color blindness.
Polygene inheritance occurs when several gene pairs interact to
produce phenotypes that vary quantitatively over a broad range.
Height, intelligence, and skin pigmentation are examples.