CHAPTER 8 Highlights 183
Key Concepts
Study CHAPTER HIGHLIGHTS
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The Origins of Genetics
Gregor Mendel bred varieties of the garden pea in an
attempt to understand heredity. Mendel observed that con-
trasting traits appear in offspring according to simple ratios.
In Mendel’s experiments, only one of the two contrasting
forms of a trait was expressed in the F 1 generation. The other
form reappeared in the F 2 generation in a 3:1 ratio.Mendel’s Theory
Different versions of a gene are called alleles. An individual
usually has two alleles for a gene, each inherited from a
different parent.
Individuals with the same two alleles for a gene are
homozygous; those with two different alleles for a gene
are heterozygous.
The law of segregation states that the two alleles for a trait
separate when gametes are formed. The law of independent
assortment states that two or more pairs of alleles separate
independently of one another during gamete formation.Studying Heredity
The results of genetic crosses can be predicted with the use
of Punnett squares and probabilities.
A test cross can be used to determine whether an individual
expressing a dominant trait is heterozygous or homozygous.
A trait’s pattern of inheritance within a family can be deter-
mined by analyzing a pedigree.Complex Patterns of Heredity
Traits usually display complex patterns of heredity, such as
incomplete dominance, codominance, and multiple alleles.
Mutations can cause genetic disorders, such as sickle cell
anemia, hemophilia, and Huntington’s disease.
Genetic counseling can help patients concerned about a
genetic disorder.4
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Key Terms
Section 1
heredity (162)
genetics (162)
monohybrid cross (164)
true-breeding (164)
P generation (164)
F 1 generation (164)
F 2 generation (164)Section 4
polygenic trait (177)
incomplete dominance (177)
multiple alleles (178)
codominance (178)Section 2
allele (167)
dominant (167)
recessive (167)
homozygous (167)
heterozygous (167)
genotype (168)
phenotype (168)
law of segregation (169)
law of independent assortment
(169)Section 3
Punnett square (170)
test cross (172)
probability (173)
pedigree (175)
sex-linked trait (175)BIOLOGYBIOLOGY
Unit 5—Heredity
Use this unit to review the key concepts
and terms in this chapter.