Chromosomes, Mapping, and the
Meiosis-Inheritance Connection
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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Drosophila have 4 pairs of chromosomes - 3
pairs of autosomes and 1 pair of sexchromosomes (X and or Y)
Sex determination in Drosophila is based on
the number of X chromosomes
2 X chromosomes = female
1 X and 1 Y chromosome = male
T.H. Morgan isolated a mutant white-eyed
Drosophila - a white-eyed male
–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 whiteeyed flies in a 3:1 ratio
– Closer examination showed all whiteeyed flies were male, there were no
white-eyed females!
– Morgan concluded that the eye color
gene is linked to the X chromosome
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Sex Determination
Sex Determination
Sex determination in birds is based on the
number of Z chromosomes
2 Z chromosomes = male
1 Z and 1 W chromosome = female
Sex determination in Drosophila is based on the
number of X chromosomes
2 X chromosomes = female
1 X and 1 Y chromosome = male
1 X and 0 Y chromosome = male
Sex determination in many insects is based
on haploid/diploid make-up
2N= female
1N = male
Sex determination in mammals is based on
the presence of a Y chromosome
2 X chromosomes = female
having a Y chromosome (XY) = male
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Sex Determination
Sex Determination
Earthworms and many plants: both male &
female in the same organism
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.
Some reptiles: temperature of embryo at a
certain stage determines the sex
Some organisms shift sex depending on
circumstances: too many males in the
population, some turn into females and
vice versa
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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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Genetic Mapping
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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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
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)
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
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
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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