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Ch.14: Mendel and the Gene Idea & Ch.15: Chromosomal Basis of…
Ch.14: Mendel and the Gene Idea & Ch.15: Chromosomal Basis of Inheritance
Alterations of chromosome
number or structure cause some genetic
disorders
Abnormal Chromosome Number
Nondisjunction
In
nondisjunction,
pairs of homologous
chromosomes do not separate normally during
meiosis
As a result, one gamete receives two of the same
type of chromosome, and another gamete receives
no copy
Aneuploidy
Offspring with this condition have an abnormal
number of a particular chromosome
Aneuploidy
results from the fertilization of gametes
in which nondisjunction occurred
A
monosomic
zygote has only one copy of a
particular chromosome
A
trisomic
zygote has three copies of a particular
chromosome
Polyploidy
Polyploidy
is a condition in which an organism has
more than two complete sets of chromosomes
– Triploidy (3n) is three sets of chromosomes
– Tetraploidy (4n) is four sets of chromosomes
Polyploidy is common in plants, but not animals
Polyploids are more normal in appearance than
aneuploids
Alterations of Chromosome Structure
Breakage of a chromosome can lead to four types
of changes in chromosome structure:
–
Deletion
removes a chromosomal fragment
–
Duplication
repeats a segment
–
Inversion
reverses orientation of a segment within a
chromosome
–
Translocation
moves a segment from one
chromosome to another
Human Disorders Due to Chromosomal
Alterations
Alterations of chromosome number and structure
are associated with some serious disorders
Some types of aneuploidy appear to upset the
genetic balance less than others, resulting in
individuals surviving to birth and beyond
These surviving individuals have a set of
symptoms, or syndrome, characteristic of the type
of aneuploidy
Down Syndrome
(Trisomy 21)
Down syndrome
is an aneuploid condition that
results from three copies of chromosome 21
Aneuploidy of Sex Chromosomes
Nondisjunction of sex chromosomes produces a
variety of aneuploid conditions
Klinefelter syndrome
is the result of an extra
chromosome in a male, producing
XXY
individuals
About one in 1,000 males is XYY; these males do
not exhibit any syndrome
XXX
females occur with a frequency of about one
in 1,000
Monosomy X, called
Turner syndrome
, produces
X0
females, who are sterile; it is the only known
viable monosomy in humans
Large-scale chromosomal alterations in humans
and other mammals often lead to spontaneous
abortions (miscarriages) or cause a variety of
developmental disorders
Plants tolerate such genetic changes better than
animals do
Disorders Caused by Structurally Altered
Chromosomes
The syndrome cri du chat (“cry of the cat”), results
from a specific deletion in chromosome 5
A child born with this syndrome is severely
intellectually disabled and has a catlike cry;
individuals usually die in infancy or early childhood
Certain cancers, including chronic myelogenous
leukemia (CML), are caused by translocations
of chromosomes
Mendel used the scientific
approach to identify two laws of inheritance
Mendel’s Experimental, Quantitative Approach
A heritable feature that varies among individuals
(such as flower color) is called a
character
Each variant for a character, such as purple or
white color for flowers, is called a
trait
He started with varieties that were
true-
breeding
(plants that produce offspring of the
same variety when they self-pollinate)
Hybridization
In a typical experiment, Mendel mated two
contrasting, true-breeding varieties, a process
called
hybridization
The true-breeding parents are called the
P
generation
The hybrid offspring of the P generation are called
the
F1 generation
When F1 individuals self-pollinate or cross-pollinate
with other F1 hybrids, the
F2 generation
is
produced
The Law of Segregation
These alternative versions of a gene are called
alleles
If the two alleles at a locus differ, then one,
the
dominant allele
, determines the organism’s
appearance
The other, the
recessive allele
, has no noticeable
effect on appearance
The
law of segregation
: the two alleles for
a heritable character separate (segregate) during
gamete formation and end up in different gametes
An organism with two identical alleles for a gene is
called a
homozygote
It is said to be
homozygous
for the gene
controlling that character
An organism with two different alleles for a gene is
a
heterozygote
and is said to be
heterozygous
for
the gene controlling that character
We distinguish between an organism’s
phenotype
, or physical appearance, and its
genotype
, or genetic makeup
The Law of Independent Assortment
Using a dihybrid cross, Mendel developed
the law
of independent assortment
It states that each pair of alleles segregates
independently of any other pair of alleles during
gamete formation
Many Human Traits follow
Mendelian Patterns of Inheritance
Pedigree Analysis
A
pedigree
is a family tree that describes the
inheritance of a trait across generations
Pedigrees can be used to make predictions about
future offspring
Recessively Inherited Disorders
These range from relatively mild to life-threatening
The Behavior of Recessive Alleles
Recessively inherited disorders show up only in
individuals homozygous for the allele
Carriers
are heterozygous individuals who carry
the recessive allele but are phenotypically normal
Most individuals with recessive disorders are born
to carrier parents
Ex: Albinism is a recessive condition characterized by a
lack of pigmentation in skin and hair
Cystic Fibrosis
Cystic fibrosis
is the most common lethal genetic
disease in the United States, striking one out of
every 2,500 people of European descent
The cystic fibrosis allele results in defective or
absent chloride transport channels in plasma
membranes, leading to a buildup of chloride ions
outside the cell
Sickle-Cell Disease: A Genetic Disorder with
Evolutionary Implications
Sickle-cell disease affects one out of 400 African-
Americans
It is caused by the substitution of a single amino
acid in the hemoglobin protein in red blood cells
In homozygous individuals, all hemoglobin is
abnormal (sickle-cell)
Heterozygotes (said to have sickle-cell trait) are
usually healthy but may suffer some symptoms
Dominantly Inherited Disorders
Dominant alleles that cause a lethal disease are
rare and arise by mutation
Ex: Achondroplasia is a form of dwarfism caused by a
rare dominant allele
Ex: Huntington’s disease is a degenerative disease of
the nervous system
Multifactorial Disorders
Many diseases, such as heart disease, cancer,
alcoholism, and mental illnesses, have both genetic
and environmental components
No matter what our genotype, our lifestyle has a
tremendous effect on phenotype
Sex-linked genes exhibit unique
patterns of inheritance
The Chromosomal Basis of Sex
A gene that is located on either sex chromosome is
called a
sex-linked gene
Genes on the Y chromosome are called Y-linked
genes
Genes on the X chromosome are called
X-linked
genes
; the human X chromosome contains about
1,100 genes
Inheritance of X-Linked Genes
X chromosomes have genes for many characters
unrelated to sex
Many Y-linked genes are related to sex
determination
For a recessive X-linked trait to be expressed,
– a female needs two copies of the allele (homozygous)
– a male needs only one copy of the allele (hemizygous)
X-linked recessive disorders are much more
common in males than in females
Some disorders caused by recessive alleles on the
X chromosome in humans:
– Color blindness (mostly X-linked)
– Duchenne muscular dystrophy
– Hemophilia
X Inactivation in Female Mammals
In mammalian females, one of the two X
chromosomes in each cell is randomly inactivated
during embryonic development
The inactive X condenses into a
Barr body
If a female is heterozygous for a particular gene
located on the X chromosome, she will be a mosaic
for that character
Inactivation of an X chromosome involves
modification of the DNA and proteins bound to it
called histones
One of the genes there becomes active only on the
chromosome that will be inactivated
The gene is called XIST (X-inactive specific
transcript)
Inheritance patterns are often
more complex than predicted by simple
Mendelian genetics
Extending Mendelian Genetics for a Single
Gene
Degrees of Dominance
Complete dominance
occurs when phenotypes of
the heterozygote and dominant homozygote are
identical
In
incomplete dominance
, the phenotype of F1
hybrids is somewhere between the phenotypes of
the two parental varieties
In
codominance
, two dominant alleles affect the
phenotype in separate, distinguishable ways
The Relationship Between Dominance and
Phenotype
Tay-Sachs disease
is a fatal inherited disorder; a
dysfunctional enzyme causes an accumulation of
lipids in the brain
– At the organismal level, the allele is recessive
– At the biochemical level, the phenotype (that is, the enzyme activity level) is incompletely dominant
– At the molecular level, the alleles are codominant
Frequency of Dominant Alleles
One baby out of 400 in the United States is born
with extra fingers or toes
This condition, polydactyly, is caused by a
dominant allele, found much less frequently in the
population than the recessive allele
Multiple Alleles
Ex: The four phenotypes of the ABO blood
group in humans are determined by three alleles
for the enzyme that attaches A or B carbohydrates
to red blood cells: IA, IB, and i
Pleiotropy
Most genes have multiple phenotypic effects, a
property called
pleiotropy
Ex: pleiotropic alleles are responsible for
the multiple symptoms of certain hereditary
diseases, such as cystic fibrosis and sickle-cell
disease
Extending Mendelian Genetics for Two or More
Genes
Some traits may be determined by two or more
genes
Epistasis
In
epistasis
, expression of a gene at one locus
alters the phenotypic expression of a gene at a
second locus
Ex: in Labrador retrievers and many other
mammals, coat color depends on two genes
One gene determines the pigment color (with
alleles B for black and b for brown)
The other gene (with alleles E for color and e for no
color) determines whether the pigment will be
deposited in the hair
Epistatic interactions produce a variety of ratios, all
of which are modified versions of 9:3:3:1
Polygenic Inheritance
Quantitative variation usually indicates
polygenic
inheritance
, an additive effect of two or more
genes on a single phenotype
Height is a good example of polygenic inheritance;
over 180 genes affect height
Ex: Skin pigmentation in humans is also controlled by
many separately inherited genes
Nature and Nurture: The Environmental Impact
on Phenotype
Another departure from simple Mendelian genetics
arises when the phenotype for a character depends
on environment as well as genotype
Traits that depend on multiple genes combined with
environmental influences are called
multifactorial