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Asexual Reproduction
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Vegetative propagation
Binary Fission
Budding
Sporogenesis
Chromosomes and
Inheritance
Chapter 15
Meiosis
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Oogenisis
Spermatogenisis
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Chromosome Theory of Inheritance
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Mendelian genes have
specific loci along
chromosomes, and it’s
the chromosomes that
undergo segregation
and independent
assortment.
Thomas Hunt Morgan
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First to associate a specific gene with a
specific chromosome
Drosophila melanogaster = fruit fly
Fruit Flies
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Wild type = normal character phenotype
Mutant = alternative traits
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Morgan’s cross:
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White-eyed male with a Red-eyed female
All the F1 offspring had red eyes
What would you conclude?
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The red allele is dominant to the white allele
But then…
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Crosses between F1 offspring produced 3:1
phenotypic ratio in F2 offspring
The white-eyed trait appeared only in males
Morgan concluded that a fly’s eye color was
linked to its sex
sex-linked gene
The gene with the
white-eyed mutation is
on the X chromosome
linked genes
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Tend to be inherited together
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the chromosome is passed along as a unit
Genetic recombination
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Offspring with new combinations of traits
inherited from two parents
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Result from:
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independent assortment of genes located on
nonhomologous chromosomes
crossing over of genes located on homologous
chromosomes
SRY Gene
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Anatomical signs of sex first appear when the
embryo is about 2 months old
Presence of SRY gene - sex determining
region of the Y chromosome)
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generic embryonic gonads are modified into
testes
Sex Linked Genes
Heterozygous females will be carriers
Any male receiving the recessive allele from his mother will
express the trait
Examples
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Duchenne muscular dystrophy
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Absence of an X-linked gene for a key muscle
protein, called dystrophin
Characterized by a progressive weakening of the
muscles and loss of coordination
Hemophilia
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Absence of one or more clotting factors
Prolonged bleeding because clots form slowly
Barr body
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During female development, one X
chromosome per cell condenses into a
compact barr body
This inactivates most of its genes
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Reactivated in ovarian cells that produce ova
Females consist of a mosaic of cells
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some with an active paternal X
others with an active maternal X
Alfred Sturtevant
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Chromosome map
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Constructed using crossing over of linked genes
Ordered list of the genetic loci along a particular
chromosome
body color and wing shape are usually inherited together
because their genes are on the same chromosome
linkage map
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used recombination frequencies from fruit fly
crosses to map the relative position of genes
along chromosomes
frequency of recombinant offspring reflected
the distances between genes on a chromosome
Genetic recombination
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Parental types = phenotypes that match the original
parents
Recombinants = new combination of parental traits
dihybrid cross  combination of traits that did not
match either parent
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crosses between hybrid plants produces four phenotypes
map units
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One map unit is equivalent to a 1%
recombination frequency
map units
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Some genes on a chromosome are so far apart
that a crossover between them is virtually
certain.
In this case, the frequency of recombination
reaches is its maximum value of 50%
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the genes act as if found on separate
chromosomes and are inherited independently
Nondisjunction
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Problems with the meiotic spindle cause
errors in daughter cells
Aneuploidy
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Trisomic cells  three copies of a particular
chromosome type
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2n + 1 total chromosomes
Monosomic cells  only one copy of a
particular chromosome type
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2n - 1 chromosomes
Polyploidy
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Organisms with more than two complete sets
of chromosomes
Relatively common among plants and much
less common among animals
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fishes and amphibians have polyploid species
Aneuploidy vs. Polypoidy
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One extra or missing chromosome upsets the
genetic balance during development more
than does an entire extra set of chromosomes
changes in chromosome structure
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deletion  a chromosome fragment lacking a
centromere is lost during cell division
duplication  a fragment becomes attached
as an extra segment to a sister chromatid
changes in chromosome structure
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inversion  a chromosomal fragment
reattaches to the original chromosome in the
reverse orientation
translocation  a chromosomal fragment
joins a nonhomologous chromosome
Outcome
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Most of these alterations are so disastrous that
the embryos are spontaneously aborted long
before birth
Down syndrome
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3 copies of chromosome 21
affects 1 in 700 children born in the US
Sex Chromosome Disorders
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Klinefelter’s syndrome
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XXY male
Occurs 1 in every 2000 live births
have male sex organs, but are sterile
Turner’s syndrome
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XO female (monosomy)
occurs 1 in every 5000 births
produces phenotypic, but immature females
Genomic Imprinting
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A gene on one homologous chromosome is
silenced, while its allele on the homologous
chromosome is expressed
Depends on whether the gene resides in a
female or a male
Genomic Imprinting
The same alleles may have
different effects on
offspring, depending on
whether they arrive in the
zygote via the ovum or via
the sperm