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Chapter Nine Patterns of Inheritance Key Concepts
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Genetics is the study of inherited characteristics (genetic traits) and the genes
that affect those traits
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A phenotype is the specific version of a genetic trait that is displayed by an
individual
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Diploid cells have two copies of every gene: one copy inherited from the male
parent the other from the female parent homozygotes have the same two
alleles for a gene heterozygote have two different alleles
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Mendel’s laws of inheritance help us predict the phenotypes of offspring from
the known genotypes of the parents
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Many aspects of an organism’s phenotypes are determined by multiple genes
that interact with one another and with the environment so offspring with
identical genotypes can have very different phenotypes
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Chapter Nine Patterns of Inheritance Vocab
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The law of independent assortment states that when gametes form, the two
copies of any given allele are sorted independently of any two alleles of other
genes
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Incomplete dominance of alleles produces an intermediate phenotype in the
heterozygote
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Codominance occurs when the effect of both alleles is equally visible in the
phenotype of the heterozygote
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The situation in which a single gene influences two or more distinctly different
traits is called pleiotropy
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The term epistasis applies when the phenotypic effect of the alleles of one gene
depends on the presence of certain alleles for another, independently inherited
gene
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Traits governed by the action of more than one gene are polygenic traits
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Geneticists estimate there are more than a dozen genes that control melanin
production in our skin, which, when coupled with environmental influences,
results in continuous variation in the trait
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Complex traits are those that cannot be predicted using Mendel’s laws of
inheritance; complex traits display often display continuous variation in a
population
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Chapter Nine Patterns of Inheritance Summary
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1 principles of genetics: an overview
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Diploid individuals generally have two copies of each gene one inherited
from the male parent the other from the female parent
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The genotype is an individual’s allelic makeup the phenotype is the
specific version of an observable trait that is the individual displays
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The different alleles of a gene arise by mutation and by specifying
different versions of the same protein they generate heredity
differences among organisms
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Some mutations are harmful many have little effect and a few are
beneficial
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Mendel’s experiments with pea plants led him to reject the old notion
of blending inheritance instead they suggested that the inheritance
characteristics of organisms are controlled by specific units of
inheritance which we know as genes
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Alleles of genes account for the variation in genetic traits
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Alleles can be dominant or recessive
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fertilization combines gametes in random manner to give rise to
diploid offspring
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3 Mendel’s laws of inheritance
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The law of independent assortments says that when gametes from
during meiosis the alleles of one gene are distributed independently of
the alleles of a different gene located on a different chromosome
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4 extensions of Mendel’s laws
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Many alleles show incomplete dominance or codominance
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Alleles for one gene can suppress the alleles of another gene
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Most traits are polygenic (governed by 2 or more genes)

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