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CHAPTER OUTLINE
23.1 Mendel’s Laws
The science of genetics explains the process of inheritance and also why there are variations
between offspring from one generation to the next.
Gregor Mendel
Gregor Mendel developed the basic principles of inheritance after performing a series of
experiments. He kept careful and complete records of plant crosses and concluded that
the plants transmitted distinct factors to offspring. The factors that control traits are called
genes and genes are found on chromosomes. Chromosomes are found in pairs, called
homologous chromosomes, one of which is from the mother, the other from the father;
they are similar in the types of genes they contain. Alternate forms of a gene for the same
trait are called alleles. On the basis of his studies Mendel formulated the law of
segregation. It states that each individual has two factors for each trait, which separate
during the formation of the gametes and each gamete contains only one factor from each
pair of factors. Fertilization gives each new individual two factors for each trait.
The Inheritance of a Single Trait
The phenotype of an individual refers to the individual’s actual appearance. The
genotype refers to the alleles the chromosomes carry that are responsible for that trait. A
capital letter indicates a dominant allele and a lowercase letter indicates a recessive
allele. Dominant means that this allele will mask the expression of the recessive allele
when they are together in the same organism. If the two members of the allelic pair are
the same, the organism is said to be homozygous. If they are different, the organism is
said to be heterozygous.
Gamete Formation
The gametes have only one allele for each trait in accordance with Mendel’s law
of segregation.
One-Trait Crosses
A Punnett square can be used to keep track of all the alleles in a one-trait, or
monohybrid, cross; every possible combination of gametes occurs within the
squares.
One-Trait Crosses and Probability
Another method of calculating the expected ratios uses the rules of probability or
chance. The product rule of probability states that the chance of two or more
independent events occurring together is the product of their chance of occurring
separately, while the sum rule of probability states that it is the sum of the
individual chances.
The One-Trait Testcross
In a testcross an organism showing the dominant phenotype, but unknown
genotype, is crossed with a homozygous recessive individual to determine its
genotype.
The Inheritance of Two Traits
When the homologues separate during meiosis, each gamete receives one member from
each pair of homologues. The homologues separate independently.
Independent Assortment
Mendel’s second law is the law of independent assortment. It states that each
pair of factors separates independently and that all possible combinations of
factors can occur in the gametes.
Two-Trait Crosses
A dihybrid genotype is when the individual is heterozygous in two regards. A
9:3:3:1 phenotypic ratio is always expected for a dihybrid cross when simple
dominance is present.
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Two-Trait Crosses and Probability
It is also possible to use the product rule and the sum rule of probability to
predict the results of a dihybrid cross.
The Two-Trait Testcross
A two-trait testcross occurs when an individual expressing the dominant allele
for two traits is crossed with a homozygous recessive for both traits.
23.2 Pedigree Analysis and Genetic Disorders
Many traits and disorders are genetic in origin and follow Mendel’s laws.
Patterns of Inheritance
A pedigree is a chart of a family’s history with regard to a particular genetic trait. Males
are designated by squares and females by circles. Individuals with the trait, the affected
individuals, are usually shaded.
Pedigrees for Autosomal Disorders
It is possible to decide if an inherited condition is due to an autosomal dominant
or recessive allele by studying a pedigree. A person is a carrier if they are
unaffected but capable of passing on a genetic trait or disorder.
Autosomal Recessive Disorders
Inheritance of two recessive alleles is required before an autosomal recessive
disorder will appear. Examples include Tay-Sachs disease, cystic fibrosis,
phenylketonuria, and sickle cell disease.
Autosomal Dominant Disorders
Inheritance of only one dominant allele is necessary for an autosomal dominant
genetic disorder to appear. Examples include Marfan syndrome and
Huntington disease.
23.3 Beyond Simple Inheritance Patterns
There are types of inheritance patterns other than simple dominant or recessive inheritance.
Incomplete Dominance and Codominance
Incomplete dominance occurs when the heterozygote is intermediate between the two
homozygotes. Codominance occurs when alleles are equally expressed in a
heterozygote.
Incompletely Dominant Disorders
Familial hypercholesterolemia is an incompletely dominant disorder.
Multiple Allele Inheritance
Even though a gene may exist in several allelic forms, each person has only two of the
possible alleles.
ABO Blood Types
Three alleles for the same gene control the inheritance of ABO blood types, they
determine the presence or absence of antigens on red blood cells.
Polygenic Inheritance
Polygenic inheritance occurs when a trait is governed by two or more genes (sets of
alleles). The result is a continuous variation of phenotypes.
Skin Color
Skin color is the result of pigmentation produced by cells called melanocytes in
the skin and is an example of a polygenic trait that is likely controlled by many
pairs of alleles. A bell-shaped curve is a common identifying characteristic of a
polygenic trait.
23.4 Environmental Influences
Environmental factors, such as nutrition or temperature, can also influence the expression of
genetic traits. Investigators try to determine what percentage of various human traits is due to
nature (inheritance) and what percentage is due to nurture (the environment).
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