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Abnormal meiosis
Abnormal meiosis
• Sometimes mistakes occur during meiosis
– Segments of chromosome (block of genes) can be lost
or added – chromosome mutation / aberration
– Cell may be missing a chromosome or have an extra
chromosome (aneuploidy – incorrect number of
chromosomes)
– Plant cells have more than 2n sets of chromosomes in
nucleus (3n, 4n – called polyploidy)
• Examples of aneuploidy
– Down’s syndrome (3 x chromosome 21- trisomy)
– Turner’s syndrome (female has 1 X chromosome –
monosomy)
– Klinefelter’s syndrome (3 x sex chromosome – XXY)
T. Botha
Telematics Life Sciences 2011
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How aneuploidy happens
Aneuploidy – Trisomy – chromosome 21
• Chromosomes of homologous pair 21 do not
separate during meiosis
• One gamete (♀) may have two chromosomes
for chromosome 21 and the other (♀) does not
have a chromosome 21.
• Fertilisation: 2 x 21 (♀) + 1 x 21 (♂) = 3 x
chromosomes 21
• Extra chromosome 21 (trisomy) - baby
• This process is called non-disjunction (failure to
separate)
T. Botha
Telematics Life Sciences 2011
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The diagram below shows an abnormality during a certain
phase of meiosis with the number 21 chromosome pair
(labelled A).
1.
2.
3.
4.
What is the specific purpose of this phase of the cell division?
What specific genetic condition does this abnormality (shown by A) cause?
Give TWO symptoms of this genetic disorder.
People with this condition are often sterile. Explain why you think this is so.
T. Botha
Telematics Life Sciences 2011
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Karyotype
1
2
6
7
13
14
19
20
3
4
8
9
15
10
11
16
21
22
17
5
12
18
23
Karyotype of a person with a genetic disorder
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Telematics Life Sciences 2011
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How aneuploidy happens
Aneuploidy – Sex cells - Trisomy and Monosomy
• Chromosomes of homologous pair 23 do not separate
during meiosis
• One gamete (♀) may have two chromosomes for
chromosome 23 and the other (♀) does not have a
chromosome 23.
• Fertilisation: 2 chromosomes 23 (♀) + 1 chromosome (♂) =
3 chromosome 23
• Extra chromosome 23 (trisomy - XXY)
AND
• Fertilisation: 0 (♀) + 1 (♂) = 1 X chromosome 23
• Only ONE X chromosome (monosomy - X0)
T. Botha
Telematics Life Sciences 2011
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How aneuploidy happens
Polyploidy – Plant cells with more than the
diploid set of Chromosomes
• Chromosomes of all the homologous pairs of one
organism do not separate during meiosis
• One gamete (♀) may have two sets of chromosomes.
• Fertilisation: 2n (♀) + n (♂) = 3n (triploid)
OR
• Self pollination and Fertilisation: 2n + 2n = 4n (tetraploid)
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Telematics Life Sciences 2011
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Polyploidy
• Common in plants and rare in animals
• Can occurs spontaneously – natural
polyploids forming – sympatric speciation –
plant evolution
• Artificially - plant breeding for agriculture –
± 47 % of flowering plants are polyploids e.g.
wheat, corn, bananas and apples – specially
developed / breed
• Cells of polyploidy plants are bigger – results
in bigger fruits, flowers and plants.
Tetraploid apples can be twice the size of
diploid apples
T. Botha
Telematics Life Sciences 2011
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Beyond Simple Inheritance
Patterns
• Many genetic disorders and other traits
are inherited according to laws first
established by Gregor Mendel.
• Inheritance is often more complex,
providing exceptions to Mendel’s laws but
helping to explain an even wider variety in
patterns of gene inheritance.
T. Botha
Telematics Life Sciences 2011
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23-8
Beyond Simple Inheritance
Patterns
1.
Polygenic Inheritance
• Polygenic traits are governed by more
than one gene pair. On different loci.
• The inheritance of skin color, determined
by an unknown number of gene pairs, is
another example of polygenic inheritance.
• Height is another example of polygenic
inheritance
T. Botha
Telematics
Life Sciences 2011
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23-10
10
T. Botha
Telematics Life Sciences 2011
Multiple Allelic Traits
• More than two alternative alleles exist for a
particular gene locus.
• blood type is an example
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Telematics
Life Sciences 2011
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ABO Blood Types
•
•
•
•
•
•
•
•
Alleles – IA,IB,I
Genotype
IA IA
IA i
IB IB
IB i
IA IB
ii
T. Botha
Phenotype
A
A
B
B
AB
O
Telematics Life Sciences 2011
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12
• What are the possible blood types of children
from a mother with type A blood and a father
with type B blood?
T. Botha
Telematics Life Sciences 2011
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13
23-14
14
T. Botha
Telematics Life Sciences 2011
• Codominance -both alleles are equally
expressed in a heterozygote. (blood type
AB)
• Incomplete dominance - heterozygote
shows an intermediate phenotype (pink
snapdragon flowers; sickle cell anemia)
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Telematics Life Sciences 2011
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15
23-16
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Telematics Life Sciences 2011
Homohybrid Crosses
• In Homohybrid crosses, one trait is
considered.
• When performing crosses, the original
parents are called the parental generation,
or the P generation.
• All of their children are the filial generation,
or F generation.
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• If you know the genotype of the parents, it is
possible to determine the gametes and use a
Punnett square to determine the phenotypic
ratio among the offspring.
• This ratio is used to state the chances of a
particular phenotype.
T. Botha
Telematics Life Sciences 2011
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Monohybrid cross
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Telematics Life Sciences 2011
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Sex-Linked Traits
• Sex Determination• XX – female
• XY – male
• Traits controlled by genes on the X or Y
chromosomes are sex-linked
• An allele is termed X-linked.
T. Botha
Telematics Life Sciences 2011
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Hemophilia
• Hemophilia refers to the lack of one of
several clotting factors that leads to
excessive bleeding in affected individuals.
• Hemophiliacs bleed externally after injury,
but also bleed internally around joints.
• Hemorrhages can be stopped with blood
transfusions or a biotechnology clotting
factor.
T. Botha
Telematics Life Sciences 2011
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Color Blindness
• Three types of cones are in the retina
detecting red, green, or blue.
• Genes for blue cones are autosomal;
those for red and green cones are on the
X chromosome.
• Males are much more likely to have redgreen color blindness than females.
• About 8% of Caucasian men have redgreen color blindness.
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Telematics Life Sciences 2011
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X-Linked Disorders
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X-Linked Alleles
• The key for an X-linked problem shows the
allele attached to the X as in:
• XB = normal vision
• Xb = colour blindness.
• Females with the genotype XBXb are
carriers because they appear to be normal
but each son has a 50% chance of being
colour blind depending on which allele the
son receives.
• XbXb and XbY are both colorblind.
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Telematics Life Sciences 2011
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