Chromosomal Basis of
Inheritance
Lecture 13
Fall 2008
1
The Chromosomal Basis of Inheritance
Chromosomal Theory of Inheritance
– Genes are found in specific locations on
chromosomes
– The behavior of chromosomes during meiosis and
fertilization accounts for inheritance patterns
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•
•
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Mendel published in 1866
Mitosis worked out in 1875
Meiosis worked out in 1890s
Early 1900’s, Chromosomal Theory of
Inheritance
• 1953 – Structure of DNA (and how it accounts
for the ability of cells to pass on genes)
discovered
2
The Chromosomal Basis of Inheritance
Fig. 15.2
Sex-linked Genes
3
• Thomas Hunt Morgan
– 1907 established fruit fly lab
– Drosophila melanogaster
• Ideal model organism
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–
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Fast breeding
Fig.
Many offspring
Chromosomes easy to see with light microscope
Four chromosomes
• 3 autosomes
• Sex chromosomes
– Female = XX
– Male = XY
15.3
4
Sex-linked Genes
• Created first mutant after 2+ year of breeding
– White eyed male
• Naming conventions in flies
– Wild type
• Trait that is found most often in nature
– Mutant phenotypes
• Alternatives to the wild types
– Gene takes its symbol from the
first mutant discovered
• w = white eyes (mutant)
• w+ = red eyes (wild type)
Fig. 15.3
5
Sex-linked Genes
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•
•
•
Red eyed female and white eyed male crossed
F1 = all red eyes
F2 = 3:1 ratio
White eyed flies were all males
Fig. 15.4
6
Sex-linked Genes
• Allele for white eye
must be on X
chromosome
• No corresponding
allele on Y
chromosome
• First experiment
linking a gene to a
specific chromosome
Fig. 15.4
7
Sex Linked Genes
Sex linked genes
– Any gene located on a sex chromosome
• Female XX (mammals, flies)
• Male XY
• Y chromosome has few genes
– Most genes related to “maleness” (testes
formation, sperm production)
• X has many more genes than Y
– Only small regions at end of Y homologous
with X
• Most sex linked genes on X
• Many recessive sex linked traits show up
more often in males
– Example: white eyed fruit flies
Fig. 15.5
Sex Linked Genes
• Hemizygous: individual has only one member of a
chromosome pair
8
X chromosome inactivation
• One X chromosome
inactivated in every cell
– Chromosome condensed, so
unavailable for transcription
of DNA
• Barr body
• Random and independent
• Female heterozygous for
sex-linked trait, half of cells
express one trait and half
the other trait
Fig. 15.8
9
10
Linked Genes & Crossing Over
Linked Genes
• Genes located on the same chromosome that
tend to be inherited together
• Often found close together on a chromosome
• Violates Mendel’s Law of Independent
Assortment
Linked Genes & Crossing Over
• High frequency of
parental genotypes
suggested that
genes are linked
• Presence of other
combinations
suggested that
genes only partially
linked
11
Linked Genes & Crossing Over
Crossing over
• Exchanging of genetic
material between nonsister chromatids
• Crossing over results in
genetic recombination
• Parental types
– Phenotype matches parent
• Recombinant Type
– Phenotype different from
parents
• Can prevent linked genes
from traveling together
• Review Fig. 15.10 in text
12
Linkage Maps
• Genetic map
– Ordered list of the genetic loci along a particular
chromosome
– The farther apart two genes are, the higher the
probability that a crossover will occur between them
and therefore the higher the recombination frequency
• Assumes crossing over random event
– Linkage map
• Genetic map based on recombination frequencies
Fig. 15.11
14
15
Linkage Maps
Limitations to linkage maps
• Genes may be so far
apart they are considered
unlinked even though they
travel on the same
chromosome
• Portrays order along a
chromosome, but not
precise location
– Crossover frequency not
uniform over length of
chromosome
Fig. 15.12
Genetic Disorders
• Nondisjunction
– Members of a pair of homologous chromosomes do
not separate properly during meiosis 1
– Sister chromatids fail to separate during meiosis 2
Fig. 15.13
16
Genetic Disorders
• Aneuploidy
– Zygote with abnormal number of chromosomes
– Due to aberrant gamete uniting with normal gamete
– Monosomic (2n-1)
• Zygote missing a chromosome
– Trisomic (2n+1)
• Zygote has an extra chromosome
• Downs syndrome – extra chromosome 21
• Polyploidy (3n, 4n)
– More than two complete chromosome sets in all
somatic cells
– Common in plants
Fig. 15.13
17
Alterations of Chromosome Structure
• Deletion
– Fragment of chromosome lost
• Duplication
– If fragment attached to sister chromatid,
causes repeated section
• Can occur during meiosis
• Results from unequal crossing over
Fig. 15.15
18
Alterations of Chromosome Structure
• Inversion
– Fragment detached, reattached to same chromosome
but in reverse
• Translocation
– Segment from one chromosome moved to a
nonhomologous chromosome
• Reciprocal translocation
• Nonhomologous chromosomes exchange
segments
Fig. 15.15
19
Alterations of Chromosome Structure
• Can also occur in Mitosis
• Chronic myelogenous lukemia
• Reciprocal translocation
– Daughter cells now have “Philadelphia
chromosome”
Fig. 15.17
20
Genomic Imprinting
• Genomic imprinting
– Variations in phenotype depending on whether an
allele is inherited from the male or female parent
– Occurs during gamete formation
– One allele silenced
– Species specific for particular gene
• E.g., insulin-like growth factor
• Only paternal allele is expressed
Fig. 15.18
21
Genomic Imprinting
Fig. 15.18
22
23
Inheritance of Organelle Genes
• Extranuclear genes
– Mitochondria and chloroplasts
• Organelles reproduce independently
– Binary fission
• Organelles passed down through maternal line
• E.g., variegated leaves
– Mutations in plastid genes controlling
pigmentation
• E.g., defects in electron transport
chain
– Mitochondrial genes code for
proteins in ETC
Fig. 15.19