Transcript AP Bio Ch. 15 Chromosomal basis of

AP Biology
Chromosomal Basis of Inheritance
Mitosis and Meiosis solved


Cytologists worked out the process of
mitosis in 1875, and the process of
meiosis in 1890s.
As cytologists and geneticists shared
information, as well as with the
development of better microscopes, the
process became better understood.
Chromosome theory of inheritance


According to this theory, Mendelian
genes have specific loci on
chromosomes.
It is the chromosome that undergoes
segregation and independent
assortment.

Thomas Hunt Morgan


THM was an embryologist at Columbia
University. He was the first to associate a
specific gene with its location on a
chromosome.
He studied Drosophila, or the fruit fly,
because they were small, reproduced rapidly,
and had unique characteristics that could be
studied (they have only four pairs of
chromosomes).

RED EYE
Wild-type vs. Mutant phenotypes


The normal phenotype for a character,
or the phenotype most common in the
population, is called the wild type. Ex)
red eyes in Drosophila
Traits alternative to the wild type, such
as white eyes, are called mutant
phenotypes.
WHITE EYE

Sex-linked genes vs linked genes


Genes located on the sex chromosome
are called sex-linked genes.
Genes located on the same
chromosome tend to be inherited
together. Such genes are said to be
linked genes.
Genetic Recombination



Genetic recombination is the production of
offspring with new combinations of traits
inherited from two parents.
Meiosis and random fertilization generates
genetic variation among offspring of sexually
reproducing organisms.
Mendel’s Law of Independent Assortment
illustrated this.

Recombination due to crossing-over
Parental types vs. recombinants


Offspring from a cross whose
phenotype matches one of the parents
is called a parental type.
Offspring who have different
combinations of traits from the parents
are called recombinants.

Linked genes & independent assortment


Linked genes do not assort independently
because they are located on the same
chromosomes and tend to move together
through meiosis and fertilization.
Crossing-over accounts for recombination in
linked genes. Occasionally the linkage
between the genes is broken as homologous
chromosomes exchange segments.
Gene Maps



One of Thomas Morgan’s students, Alfred
Sturtevant, noticed a relationship between
linked genes and recombination.
He developed a genetic map, which is an
ordered list of the location of certain genes
located along a particular chromosome.
He suggested that the farther apart two
genes are on the chromosome, the higher the
rate of recombination of those genes will be.

Linkage maps



Linkage maps are genetic maps based on
recombination frequencies.
Linkage maps do not exactly match the actual
chromosome because the frequency of
crossing over is not uniform over the length
of the chromosome.
Map units do not have absolute size, but can
be used to show relative sequence of genes
on the chromosome.

Constructing a genetic map


The method used for mapping genes on
chromosomes assumes that the
probability of crossing over between
two genes is proportional to the
distance between them.
Recombinations are converted to map
units, and used to determine the
sequence of genes.
Cytological maps

Cytological maps locate genes with
respect to chromosomal features, such
as stained bands.
Sex Chromosomes



In humans, there are two varieties of sex
chromosomes, X and Y.
Females have the genotype XX, and males
are XY.
The sex of offspring is determined by the
male parent. Mother gives the offspring an X
chromosome, and the father gives either an X
or a Y chromosome.
SRY Gene


Researchers have discovered a region
of the Y chromosome called the “sexdetermining region of Y” or SRY gene.
It is responsible for the normal
development of testes, and may be a
biochemical trigger for the development
of secondary sex characteristics in
males.

Sex determination in animals




X-Y system: depends on whether sperm carries an
X or Y chromosome Ex) humans and horses
X-O system: females are XX, males are XO Ex)
crickets
Z-W system: sex chromosomes present in the
ovum, males are ZZ and females are ZW Ex) hawks
and other birds
Diploid-haploid: no sex chromosomes, females
develop from fertilized ova (diploid), and males
develop from unfertilized eggs (haploid) Ex) ants
X-linked alleles




More males than females have disorders
inherited through sex-linked recessive alleles.
Males have only one sex chromosome, so any
male receiving the recessive allele from his
mother will express the trait.
Females will only express the trait if they
receive the recessive allele from both parents.
For example, a girl will be color-blind only if
she has a color-blind father and a mother
who is a carrier.

Duchenne Muscular Dystrophy



Effects 1/3500 males in the U.S.
Caused by the absence of a key muscle
protein called dystrophin.
Characterized by a progressive
weakening of the muscles and loss of
coordination.
Hemophilia



Caused by the lack of a certain protein
required for blood clotting.
Hemophiliacs bleed excessively when
injured.
Historical note: There was a high
incidence of hemophilia in the royal
families of Europe throughout history,
due to the intermarriage in families.
Why are calico cats female?


The allele for gene color in cats is
located on the X chromosome, one for
black, and one for orange.
Females can inherit two X
chromosomes and both alleles, but
males can only inherit one of the
alleles.

XIST gene


XIST is the X-inactive specific transcript.
May initiate X-inactivation.
Nondisjunction


Nondisjunction is an accident in mitosis or
meiosis in which both members of a pair of
homologous chromosomes or both sister
chromatids fail to move apart properly.
In meiosis, this would result in one gamete
receiving two of the same type of
chromosome and another gamete receiving
no copy.

Polyploidy



Polyploidy describes the condition in which
an organism has more than two complete
sets of chromosomes. Most common in
plants, and can result in increased crop
production.
Triploidy- diploid gamete joins with a normal
haploid gamete, resulting in a 3n cell.
Tetraploidy- failure of a 2n zygote to divide
after replicating its chromosomes.

Tetraploid mouse species
Chromosomal alterations




Deletion- chromosomal segment is
removed.
Duplication- segment is repeated.
Inversion- reverses a segment within a
chromosome.
Translocation- moves a segment from
one chromosome to another,
nonhomologous chromosome

Down Syndrome (Trisomy 21)


Results from nondisjunction during
gamete formation, so there is an extra
chromosome 21 (trisomy)
Characterized by altered facial features,
short stature, heart defects,
susceptibility to respiratory infections
and mental retardation.


Klinefelter’s Syndrome



Males have an extra X chromosome
Individuals have male sex organs, but
the testes are abnormally small
Male is sterile

Turner Syndrome



Females are XO. Must have an X
chromosome for embryo to survive.
Sex organs do not mature, and
secondary sex characteristics fail to
develop.
Females are sterile and usually short in
stature.
Cri du Chat syndrome


Deletion in chromosome five.
Characterized by mental retardation,
small head with unusual facial features,
and a cry that sounds like the mewing
of a distressed cat.
Chromosomal translocation


Chromosomal translocation is the
attachment of a fragment from one
chromosome to another,
nonhomologous chromosome.
Linked to the cause of chronic
myelogenous leukemia, some cancers,
and some cases of Down’s syndrome.
Genomic Imprinting



The same alleles have different affects
on offspring depending on whether they
arrive in the zygote from the egg or the
sperm.
Parental affect on gene expression.
Fragile X is most inherited from the
mother rather than the father.

Prader-Willi vs. Angelman syndrome




Both diseases are caused by the deletion of
a segment of chromosome 15.
If the allele comes from the father, the
disorder is Prader-Willi, and if from the
mother the disorder is Angelman’s syndrome.
Prader-Willi- mental retardation, obesity,
short stature, small hands and feet
Angelman’s- jerky movements, motor and
mental deficits, sponataneous laughter at
inappropriate times
Extranuclear genes



Found on small circles of DNA in mitochondria
and chloroplasts (and other plastids).
Passed on to daughter organelles, codes for
some of the organelle’s proteins
Maternal pattern of inheritance; mitochondria
passed on by the zygote come from the
cytoplasm of the ovum
Mitochondrial myopathy




Mutation in the mitochondrial DNA coding for
proteins involved in ATP synthesis.
Symptoms are related to muscle functioning,
where large numbers of mitochondria are
found.
Symptoms include muscle weakness,
intolerance of exercise, and muscle
deterioration.
Mitochondrial myopathy may be associated
with diabetes, heart disease, and Alzheimer’s
disease as well.