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) 1 T. Botha Telematics Life Sciences 2011
Transcript
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)
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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
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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. What is the specific purpose of this phase of the cell division? 2. What specific genetic condition does this abnormality (shown by A) cause? 3. Give TWO symptoms of this genetic disorder. 4. People with this condition are often sterile. Explain why you think this is so.
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Karyotype
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6 7 8 9 10 11 12
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19 20 21 22 23
Karyotype of a person with a genetic disorder
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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.
• 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
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• 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.
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Beyond Simple Inheritance Patterns
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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
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Multiple Allelic Traits
• More than two alternative alleles exist for a particular gene locus.
• blood type is an example
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ABO Blood Types
• Alleles – IA,IB,I• Genotype Phenotype• IA IA A• IA i A• IB IB B• IB i B• IA IB AB• i i O
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• What are the possible blood types of children from a mother with type A blood and a father with type B blood?
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• Codominance -both alleles are equally expressed in a heterozygote. (blood type AB)
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.
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Monohybrid cross
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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.
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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.
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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 red-green color blindness than females.
• About 8% of Caucasian men have red-green color blindness.
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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.
