BIOL 1030H Lecture Notes - Lecture 9: Genetic Distance, Mendelian Inheritance, Punnett Square
26 Jun 2018
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consists of X and Y chromosomes
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X has about 1000 genes compared to 50 in Y
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Tips of the arms share small regions of homology as this is where they pair up during meiosis
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Human Sex Chromosomes
Meiosis in a male results in a 1:1 ratio of X bearing and Y bearing sperm
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Meiosis in a female result in X bearing eggs only
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Random fertilization results in a 1:1 ration
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Segregation of the Sex Chromosomes
Located on only x chromosomes
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F1 generation will result in dominant phenotype
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F2 generation will result in ONLY males having the mutation and the male ratio will be 1:1
(mutation:non-mutation)
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It occurs because in males there is only one x chromsome and when the mutation is present
on the x chromosome, they have no other x chromosome to cover it up as females do
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Fathers always pass onto daughters but will not show up in females because mothers x
chromosome covers it up, sons will always get y chromosome so therefore they are not
infected
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Heterozygous carrier female ends up with a 3:1 ratio (non:mutated)
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Create punnet square
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X linked Genes
Nondisjunction: Evidence that genes reside on chromosomes
Normal chromosome separation results in each egg containing a single X chromosome
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In a cross with white eyed females, normal chromosome separation results in female progeny
with red eyes and males with white eyes
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Most XY males receive their X chromosomes from their mother
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Normal
Nondisjunction results in eggs with either two X chromosomes or no X chromosome
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In a cross with white eyed females nondisjunction of the X chromosomes results in XXY female
progeny with white eyes and XO male progeny with red eyes
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Rare XO males receive their X chromosome from their father
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Nondisjunction
For a rare X linked recessive traits, most affected individuals are male
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The offspring of an affected male are usually not affected
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The sister of an affected male can have affected sons
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The heterozygous daughters of affected males can have affected sons
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X linked recessive mutations in humans
All sons of males with the traits also show the trait
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Only males exhibit the trait
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Females do not inherit or transmit the trait
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Only fathers can pass on because they only ever pass on the Y chromosome, no daughters ever
have it, no mothers will pass it on, all sons have it
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Y linked genes
Inheritance of Mitochondrial and Chloroplast DNA
- Inherited mitochondrial diseases are often associated with muscle weakness reflected deficient
production of ATP, the red patches in the microscopic image result from clumps of defective
mitochondria in muscle fibres observed in one form of the trait
Sex Chromosomes, Organelles and Linkage
March 21, 2018
6:18 PM
Biology 1030 Page 1
Document Summary
Human sex chromosomes consists of x and y chromosomes. X has about 1000 genes compared to 50 in y. Tips of the arms share small regions of homology as this is where they pair up during meiosis. Meiosis in a male results in a 1:1 ratio of x bearing and y bearing sperm. Meiosis in a female result in x bearing eggs only. F2 generation will result in only males having the mutation and the male ratio will be 1:1 (mutation:non-mutation) It occurs because in males there is only one x chromsome and when the mutation is present on the x chromosome, they have no other x chromosome to cover it up as females do. Fathers always pass onto daughters but will not show up in females because mothers x chromosome covers it up, sons will always get y chromosome so therefore they are not infected. Heterozygous carrier female ends up with a 3:1 ratio (non:mutated)
