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1. Independent segregation at metaphase I Each pair of chromosomes independently aligns at the
cell equator; equal probability of the maternal or paternal chromosome going to a pole
The number of combinations for chromosomes packaged into gametes is 2n where n = haploid number of chromosomes
2. Random fertilization The combination of each unique sperm with each
unique egg increases genetic variability
3. Genetic recombination (crossing-over)
3 Ways to Achieve Genetic Variation Through Sexual Reproduction
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Two equally probablearrangements ofchromosomes at
metaphase I
Possibility 1 Possibility 2
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Two equally probablearrangements ofchromosomes at
metaphase I
Possibility 1 Possibility 2
Metaphase II
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Two equally probablearrangements ofchromosomes at
metaphase I
Possibility 1 Possibility 2
Metaphase II
Combination 1
Gametes
Combination 2 Combination 3 Combination 4
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Homologous chromosomes can carry different versions of genes
Separation of homologous chromosomes during meiosis can lead to genetic differences between gametes
– Homologous chromosomes may have different versions of a gene at the same locus
– One version was inherited from the maternal parent, and the other came from the paternal parent
– Since homologues move to opposite poles during anaphase I, gametes will receive either the maternal or paternal version of the gene
Copyright © 2009 Pearson Education, Inc.
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Brown coat (C); black eyes (E) White coat (c); pink eyes (e)
Offspring (next page)
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Tetrad in parent cell(homologous pair of
duplicated chromosomes)
Coat-colorgenes
Chromosomes ofthe four gametes
Meiosis
PinkWhite
BlackBrown
Eye-colorgenes
C
e
E
c
C
e
E
c
C
e
E
c
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Genetic recombination is the production of new combinations of genes due to crossing over
Crossing over involves exchange of genetic material between homologous chromosomes
– Nonsister chromatids join at a chiasma (plural, chiasmata), the site of attachment and crossing over
– Corresponding amounts of genetic material are exchanged between maternal and paternal (nonsister) chromatids
Crossing over further increases genetic variability
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Centromere
ChiasmaTetrad
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Breakage of homologous chromatids
Coat-colorgenes
Eye-colorgenes
C
(homologous pair ofchromosomes in synapsis)
E
c e
Tetrad
C E
c e
Joining of homologous chromatids2
C E
c e
Chiasma
1
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Separation of homologous chromosomes at anaphase I
C E
c e
Chiasma
Separation of chromatids at anaphase II andcompletion of meiosis
C E
c e
c E
C e
c e
c E
C E
C e
Parental type of chromosome
Gametes of four genetic types
Recombinant chromosome
Parental type of chromosome
Recombinant chromosome
4
3
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Changing Chromosome Number or Structure:
Generally not a good thing
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A karyotype shows stained and magnified versions of chromosomes
– Karyotypes are produced from dividing white blood cells, stopped at metaphase
– Karyotypes allow observation of
– Homologous chromosome pairs
– Chromosome number
– Chromosome structure
8.19 A karyotype is a photographic inventory of an individual’s chromosomes
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Packed redand white bloodcells
CentrifugeBloodculture
Fluid1
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Packed redand white bloodcells
CentrifugeBloodculture
Fluid1
Hypotonicsolution
2
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Packed redand white bloodcells
CentrifugeBloodculture
Fluid1
Hypotonicsolution
2
3
Fixative
Whitebloodcells
Stain
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4
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Centromere
Sisterchromatids
Pair of homologouschromosomes
5
http://learn.genetics.utah.edu/content/begin/traits/karyotype/
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Trisomy 21 involves the inheritance of three copies of chromosome 21
– Trisomy 21 is the most common human chromosome abnormality
– An imbalance in chromosome number causes Down syndrome, which is characterized by
– Characteristic facial features
– Cardiac defects
– Mental deficits
– Variation in characteristics
– Association with Alzheimer’s Disease
– The incidence increases with the age of the mother
8.20 CONNECTION: An extra copy of chromosome 21 causes Down syndrome
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Infa
nts
wit
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ow
n s
ynd
rom
e(p
er 1
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bir
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Age of mother
90
70
60
50
40
30
20
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80
20 40353025 5045
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Nondisjunction is the failure of chromosomes or chromatids to separate during meiosis
– During Meiosis I
– Both members of a homologous pair go to one pole
– During Meiosis II
– Both sister chromatids go to one pole
Fertilization after nondisjunction yields zygotes with altered numbers of chromosomes
Accidents during meiosis can alter chromosome number
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Nondisjunctionin meiosis I
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Nondisjunctionin meiosis I
Normalmeiosis II
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Nondisjunctionin meiosis I
Normalmeiosis II
n + 1
Gametes
Number of chromosomes
n + 1 n – 1 n – 1
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Normalmeiosis I
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Nondisjunctionin meiosis II
Normalmeiosis I
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Nondisjunctionin meiosis II
Normalmeiosis I
Gametes
Number of chromosomes
n + 1 n – 1 n n
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Sex chromosome abnormalities tend to be less severe as a result of
– Small size of the Y chromosome
– X-chromosome inactivation
– In each cell of a human female, one of the two X chromosomes becomes tightly coiled and inactive
– This is a random process that inactivates either the maternal or paternal chromosome
– “Barr-body” formation
Abnormal numbers of sex chromosomes do not usually affect survival
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What does a Barr body look like?
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Polyploid species have more than two chromosome sets
– Observed in many plant species
– Seen less frequently in animals
Example
– Diploid gametes are produced by failures in meiosis
– Diploid gamete + Diploid gamete Tetraploid offspring
– The tetraploid offspring have four chromosome sets
New species can arise from errors in cell division
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Polyploidy.html
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Structure changes result from breakage and rejoining of chromosome segments
– Deletion is the loss of a chromosome segment– Duplication is the repeat of a chromosome
segment – Inversion is the reversal of a chromosome
segment– Translocation is the attachment of a segment
to a nonhomologous chromosome; can be reciprocal
Altered chromosomes carried by gametes cause birth defects
Chromosomal alterations in somatic cells can cause cancer
Alterations of chromosome structure can cause birth defects and cancer
Copyright © 2009 Pearson Education, Inc.
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Deletion
Inversion
Duplication
Homologouschromosomes
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Reciprocaltranslocation
Nonhomologouschromosomes
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Chromosome 9
“Philadelphia chromosome”
Activated cancer-causing gene
Reciprocaltranslocation
Chromosome 22