Chapter 23: Patterns of Gene
Inheritance
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Mendel’s Laws
Gregor Mendel was an Austrian monk
who in 1860 developed certain laws of
heredity after doing crosses between
garden pea plants.
Gregor Mendel investigated genetics at
the organismal level.
Examples of traits that can be observed
at the organismal level include facial
features that cause generations to
resemble each other.
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Mendel working in his garden
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Gregor Mendel
Gregor Mendel combined his farmer’s
skills with his training in mathematics.
Mendel’s law of segregation states that
each individual has two factors (called
genes today) for each trait.
Alternative forms of a gene having the
same position on a pair of homologous
chromosomes and affecting the same
trait are now referred to as alleles.
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Today we know that alleles occur at the
same loci (position) on a chromosome.
The factors segregate during the
formation of the gametes and each
gamete has only one factor from each
pair.
Fertilization gives each new individual
two factors again.
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Gene locus
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The Inheritance of a Single Trait
A capital letter indicates a dominant
allele, which is expressed when
present.
An example is W for widow’s peak.
A lowercase letter indicates a recessive
allele, which is only expressed in the
absence of a dominant allele.
An example is w for continuous hairline.
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Widow’s peak
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Genotype and Phenotype
Genotype refers to the genes of an
individual which can be represented by
two letters or by a short descriptive
phrase.
Homozygous means that both alleles are
the same; for example, WW stands for
homozygous dominant and ww stands
for homozygous recessive.
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Heterozygous means that the members
of the allelic pair are different—for
example, Ww.
Phenotype refers to the physical or
observable characteristics of the
individual.
Both WW and Ww result in widow’s peak,
two genotypes with the same
phenotype.
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Gamete Formation
Because homologous pairs separate
during meiosis, a gamete has only one
allele from each pair of alleles.
If the allelic pair is Ww, a gamete would
contain either a W or a w, but not both.
Ww represents the genotype of an
individual.
Gametes are represented by W or w.
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One-Trait Crosses
In one-trait crosses, only one trait such
as type of hairline is being considered.
When performing crosses, the original
parents are called the parental
generation, or the P generation.
All of their children are the filial
generation, or F generation.
Children are monohydrids when they are
heterozygous for one pair of alleles.
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If you know the genotype of the parents, it
is possible to determine the gametes and
use a Punnett square to determine the
phenotypic ratio among the offspring.
When a monohybrid reproduces with a
monohybrid, the results are 3 : 1.
This ratio is used to state the chances of a
particular phenotype.
A 3 : 1 ratio means that there is a 75%
chance of the dominant phenotype and a
25% chance of the recessive phenotype.
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Monohybrid cross
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One-Trait Crosses and
Probability
Laws of probability alone can be used to
determine results of a cross.
The laws are:
(1) the probability that two or more
independent events will occur together
is the product of their chances
occurring separately, and
(2) the chance that an event that can
occur in two or more independent ways
is the sum of the individual chances.
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In the cross of Ww x Ww, what is the
chance of obtaining either a W or a w
from a parent?
Chance of W = ½, or chance of w = ½
The probability of these genotypes is:
The chance of WW = ½ x ½ = ¼
The chance of Ww = ½ x ½ = ¼
The chance of wW = ½ x ½ = ¼
The chance of ww = ½ x ½ = ¼
The chance of widow’s peak (WW, Ww,
wW) is ¼ + ¼ + ¼ = ¾ or 75%.
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The One-Trait Testcross
It is not always possible to discern a
homozygous dominant from a
heterozygous individual by inspection
of phenotype.
A testcross crosses the dominant
phenotype with the recessive
phenotype.
If a homozygous recessive phenotype is
among the offspring, the parent must
be heterozygous.
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One-trait testcross
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The Inheritance of Many Traits
Independent Assortment
The law of independent assortment
states that each pair of alleles
segregates independently of the other
pairs and all possible combinations of
alleles can occur in the gametes.
This law is dependent on the random
arrangement of homologous pairs at
metaphase.
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Segregation and independent
assortment
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Two-Trait Crosses
In two-trait crosses, genotypes of the
parents require four letters because
there is an allelic pair for each trait.
Gametes will contain one letter of each
kind in every possible combination.
When a dihybrid reproduces with a
dihybrid the results are 9 : 3 : 3 : 1.
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Dihybrid cross
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Two-Trait Crosses and
Probability
It is possible to use the two laws of
probability to arrive at a phenotypic
ratio for a two-trait cross without using
a Punnett square.
The results for two separate monohybrid
crosses are as follows:
Probability of widow’s peak = ¾
Probability of short fingers = ¾
Probability of straight hairline = ¼
Probability of long fingers = ¼
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The probabilities for the dihybrid cross:
Probability of widow’s peak and short
fingers = ¾ x ¾ = 9/16
Probability of widow’s peak and long
fingers = ¾ x ¼ = 3/16
Probability of straight hairline and short
fingers = ¼ x ¾ = 3/16
Probability of straight hairline and long
fingers = ¼ x ¼ = 1/16
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The Two-Trait Testcross
A testcross is done when it is not known
whether a dihybrid individual is
homozygous dominant or
heterozygous for both or one of the
traits under consideration.
A cross of a person heterozygous for
both traits with a homozygous
recessive person produces a 1 : 1 : 1 : 1
ratio.
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Two-trait testcross
23-28
Genetic Disorders
Patterns of Inheritance
When studying human disorders,
biologists often construct pedigree
charts to show the pattern of
inheritance of a characteristic within a
family.
The particular pattern indicates the
manner in which a characteristic is
inherited.
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Pedigree charts represent males as
squares and females as circles.
Recessive and dominant alleles have
different patterns of inheritance.
Genetic counselors construct pedigree
charts to determine the mode of
inheritance of a condition.
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Autosomal recessive pedigree
chart
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Autosomal dominant pedigree
chart
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Autosomal Recessive Disorders
Tay-Sachs Disease
Tay-Sachs disease is common among
United States Jews of central and
eastern European descent.
An affected infant develops neurological
impairments and dies by the age of
three or four.
Tay-Sachs results from a lack of
hexosaminidase A and the storage of
its substrate in lysosomes.
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Cystic Fibrosis
Cystic fibrosis is the most common lethal
genetic disorder among Caucasians.
A chloride ion transport protein is defective
in affected individuals.
Normally when chloride ion passes
through a membrane, water follows.
In cystic fibrosis patients, a reduction in
water results in a thick mucus which
accumulates in bronchial passageways
and pancreatic ducts.
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Cystic fibrosis therapy
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Phenylketonuria (PKU)
Individuals with phenylketonuria lack an
enzyme needed for the normal
metabolism of phenylalanine, coded by
an allele on chromosome 12.
Newborns are regularly tested for elevated
phenylalanine in the urine.
If the infant is not put on a phenylalaninerestrictive diet in infancy until age seven
when the brain is fully developed, brain
damage and severe mental retardation
result.
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Autosomal Dominant Disorders
Neurofibromatosis
Small benign tumors, made up largely of
nerve cells, occur under skin or on
various organs.
The effects can range from mild to
severe, and some neurological
impairment is possible; this disorder is
variably expressive.
The gene for this trait is on chromosome
17.
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Huntington Disease
Individuals with Huntington disease
experience progressive degeneration of
the nervous system and no treatment is
presently known.
Most patients appear normal until middle
age.
The gene coding for the protein
huntingtin contains many more repeats
of glutamines than normal.
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Huntington disease
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Beyond Simple Inheritance
Patterns
Polygenic Inheritance
Polygenic traits are governed by more
than one gene pair.
Several pairs of genes may be involved
in determining the phenotype.
Such traits produce a continuous
variation representing a bell-shaped
curve.
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Polygenic inheritance
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Skin Color
The inheritance of skin color, determined
by an unknown number of gene pairs,
is a classic example of polygenic
inheritance.
A range of phenotypes exist and several
possible phenotypes fall between the
two extremes of very dark and very
light.
The distribution of these phenotypes
follows a bell-shaped curve.
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Polygenic Disorders
Many human traits, like allergies,
schizophrenia, hypertension, diabetes,
cancers, and cleft lip, appear to be due
to the combined action of many genes
plus environmental influences.
Many behaviors, such as phobias, are
also likely due to the combination of
genes and the effects of the
environment.
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Multiple Allelic Traits
Inheritance by multiple alleles occurs
when more than two alternative alleles
exist for a particular gene locus.
A person’s blood type is an example of a
trait determined by multiple alleles.
Each individual inherits only two alleles
for these genes.
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ABO Blood Types
A person can have an allele for an A
antigen (blood type A) or a B antigen
(blood type B), both A and B antigens
(blood type AB), or no antigen (blood
type O) on the red blood cells.
Human blood types can be type A (IAIA or
IA i), type B (IBIB or IBi), type AB (IAIB), or
type 0 (ii).
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Inheritance of blood type
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Incompletely Dominant Traits
Codominance means that both alleles are
equally expressed in a heterozygote.
Incomplete dominance is exhibited when
the heterozygote shows not the
dominant trait but an intermediate
phenotype, representing a blending of
traits.
Such a cross would produce a
phenotypic ratio of 1 : 2 : 1.
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Incomplete dominance
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Sickle-Cell Disease
Sickle-cell disease is an example of a
human disorder controlled by
incompletely dominant alleles.
Sickle cell disease involves irregular,
sickle shaped red blood cells caused by
abnormal hemoglobin.
HbA represents normal hemoglobin; and
HbS represents the sickled condition.
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HbAHbA individuals are normal; HbSHbS
individuals have sickle-cell disease and
HbAHbS individuals have the
intermediate condition called sickle-cell
trait.
Heterozygotes have an advantage in
malaria-infested Africa because the
pathogen for malaria cannot exist in
their blood cells.
This evolutionary selection accounts for
the prevalence of the allele among
African Americans.
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Chapter Summary
Alleles are alternative forms of a gene
located at one site on a chromosome;
alleles determine the traits of
individuals.
Chromosomes and their alleles separate
and assort independently when
gametes form; this increases variety
among offspring.
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Many genetic disorders and other traits
are inherited according to laws first
established by Gregor Mendel.
Inheritance is often more complex,
providing exceptions to Mendel’s laws
but helping to explain an even wider
variety in patterns of gene inheritance.
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