Transcript Chromosomal theory of inheritance

Chromosomes, Mapping, and the
Meiosis-Inheritance Connection
Chapter 13
Chromosome Theory
Chromosomal theory of inheritance
- developed in 1902 by Walter Sutton
- proposed that genes are present on
chromosomes
- based on observations that homologous
chromosomes pair with each other during
meiosis
- supporting evidence was provided by work
with fruit flies
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Chromosome Theory
T.H. Morgan isolated a mutant white-eyed
Drosophila
red-eyed female X white-eyed male gave a
F1 generation of all red eyes
Morgan concluded that red eyes are
dominant
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Chromosome Theory
Morgan crossed F1 females X F1 males
F2 generation contained red and white- eyed
flies but all white-eyed flies were male
testcross of a F1 female with a white-eyed
male showed the viability of white-eyed
females
Morgan concluded that the eye color gene is
linked to the X chromosome
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Sex Chromosomes
Sex determination in Drosophila is based on the
number of X chromosomes
2 X chromosomes = female
1 X and 1 Y chromosome = male
Sex determination in humans is based on the
presence of a Y chromosome
2 X chromosomes = female
having a Y chromosome (XY) = male
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Sex Chromosomes
In many organisms, the Y chromosome is
greatly reduced or inactive.
genes on the X chromosome are present in
only 1 copy in males
sex-linked traits: controlled by genes
present on the X chromosome
Sex-linked traits show inheritance patterns
different than those of genes on
autosomes.
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Sex Chromosomes
Dosage compensation ensures an equal
expression of genes from the sex
chromosomes even though females have
2 X chromosomes and males have only 1.
In each female cell, 1 X chromosome is
inactivated and is highly condensed into a
Barr body.
Females heterozygous for genes on the X
chromosome are genetic mosaics.
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Chromosome Theory Exceptions
Mitochondria and chloroplasts contain
genes.
traits controlled by these genes do not follow
the chromosomal theory of inheritance
genes from mitochondria and chloroplasts
are often passed to the offspring by only
one parent
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Chromosome Theory Exceptions
Maternal inheritance: uniparental (oneparent) inheritance from the mother
the mitochondria in a zygote are from the
egg cell; no mitochondria come from the
sperm during fertilization
in plants, the chloroplasts are often inherited
from the mother, although this is species
dependent
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Genetic Mapping
Early geneticists realized that they could
obtain information about the distance
between genes on a chromosome.
- this is genetic mapping
This type of mapping is based on genetic
recombination (crossing over) between
genes.
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Genetic Mapping
To determine the distance between genes:
- dihybrid organisms are testcrossed
- offspring resembling the dihybrid parent
result from homologues that were not
involved in the crossover
- offspring resulting from a crossover are
called recombinant progeny
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Genetic Mapping
The distance between genes is proportional to
the frequency of recombination events.
recombination
frequency
recombinant progeny
=
total progeny
1% recombination = 1 map unit (m.u.)
1 map unit = 1 centimorgan (cM)
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Genetic Mapping
Multiple crossovers between 2 genes can
reduce the perceived genetic distance
progeny resulting from an even number of
crossovers look like parental offspring
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Genetic Mapping
Determining the order of genes can be done
with a three-point testcross
the frequency of double crossovers is the
product of the probabilities of each
individual crossover
therefore, the classes of offspring with the
lowest numbers represent the double
crossovers and allow the gene order to be
determined
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Genetic Mapping
Mapping genes in humans involves
determining the recombination frequency
between a gene and an anonymous
marker
Anonymous markers such as single
nucleotide polymorphisms (SNPs) can
be detected by molecular techniques.
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Human Genetic Disorders
Some human genetic disorders are caused
by altered proteins.
the altered protein is encoded by a mutated
DNA sequence
the altered protein does not function
correctly, causing a change to the
phenotype
the protein can be altered at only a single
amino acid (e.g. sickle cell anemia)
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Human Genetic Disorders
Some genetic disorders are caused by a
change in the number of chromosomes.
nondisjunction during meiosis can create
gametes having one too many or one too
few chromosomes
fertilization of these gametes creates
trisomic or monosomic individuals
Down syndrome is trisomy of chromosome
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Human Genetic Disorders
Nondisjunction of sex chromosomes can
result in:
XXX triple-X females
XXY males (Klinefelter syndrome)
XO females (Turner syndrome)
OY nonviable zygotes
XYY males (Jacob syndrome)
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Human Genetic Disorders
genomic imprinting occurs when the
phenotype exhibited by a particular allele
depends on which parent contributed the
allele to the offspring
a specific partial deletion of chromosome 15
results in:
Prader-Willi syndrome if the chromosome is
from the father
Angelman syndrome if it’s from the mother
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Human Genetic Disorders
Genetic counseling can use pedigree
analysis to determine the probability of
genetic disorders in the offspring.
Some genetic disorders can be diagnosed
during pregnancy.
amniocentesis collects fetal cells from the
amniotic fluid for examination
chorionic villi sampling collects cells from
the placenta for examination
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