BIOL 112 Lecture Notes - Lecture 12: Y Chromosome, Phenotypic Trait, Chromosome

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21 Jul 2016
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!Conclusion: heredity inherited through discrete units (alleles)
!Mendel's discoveries were ignored until the discovery of chromosomes (discrete
structures that could be responsible for this phenomenon)
o!If genes are on different chromosomes, then they can segregate
independently during meiosis (arrangement across metaphase plate)
o!However, they don't HAVE to be on different chromosomes - chiasma
(genetic recombination) during meiosis (?)
o!But the law apparently only applies to genes on different chromosomes
!Usually can't see alleles, but in the case of sex chromosomes, you sort of can (XX
vs. XY)
o!Y is kind of dominant - XXY is male, X is female
!Red-green quarter blindness: sex-linked (X), recessive
o!Colour-blind females have affected fathers
o!Colour-blind males usually have unaffected parents (colour-blind dad = red
herring; colour-blind mother = unlikely)
o!There is no colour-blind gene on the Y chromosome - too small I guess
!Wild-type (predominant, >99%) vs. mutant alleles (sometimes purposely-induced
mutations)
o!Polymorphic alleles - > 1% of population
!Autosomes (non-sex chromosomes) and allosomes (sex chromosomes)
!For genes on the same chromosome:
o!Experiment with fruit flies, either two genes are on the same chromosome or
not
o!Results: the genes are on the same
o!Draw the Punnett squares for both possibilities, results matched the same-
chromosome situation
o!Also tells you how the genes are arranged on the chromosomes (e.g. both
dominants on the same chromosome)
o!HOWEVER, you also get some evidence that the genes are NOT linked to the
same chromosome
o!Of course this is due to genetic recombination (crossover etc)
!!Recombination occurs at random points on the chromosome
!!So the probability or rate of recombination occurring between two
genes depends on their distance from each other
!!The further apart two genes are, the more likely that they will be
separated
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