BIOL 112 Lecture Notes - Lecture 12: Mendelian Inheritance, Probability Distribution, Y Chromosome
16 May 2018
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BIOL 112- Lecture 14- Beyond Mendelian Genetics
According to the law of independent assortment and our understanding of
meiosis, to assort independently genes must be on different chromosomes
However, we have more than 23 genes ! some are linked (sex linked)
Example: red green color blindness appears in 1⁄4 of children ofunaffected
parents
"!Overwhelmingly appears in males (6% vs. 1%)
"!Only males of unaffected parents are colorblind
"!Colorblind females have affected fatherso If sex and colorblindness assort
independently, the following proportions would be seen; however, actual ratio is 50%
normal females, 25% normal males, 25% affected males, implying link
The Y chromosome always has the recessive colorblindness allele
o!Y chromosome is small and doesn’t contain many genes
o!Doesn’t contain the gene for colorblindness, which is itself a kind of allele: hemizygous
Law of independent assortment only applies to genes on different chromosomes
-!They are either unlinked or linked with dominant alleles on the same chromosome, or
they’re linked where the dominant alleles are on different chromosomes
Results show that genes are linked in cis, but when they don’t act in cis, the proportions follow
the trans linkage !crossing over during meiosis separated them
Recombination rate: measure of distance (centiMorgans cM)
-!Occurs randomly, if the genes are closer, there is a lesser chance that recombination
will occur between them
-!Calculated as the rate of recombinant offspring divided by total offspring
-!The more recombination, the farther apart the genesLinkage group: genes linked
together on the same chromosome
Gene mapping: describes methods used to identify the locus of a gene and distance
between genes
Locus: synonym for gene, referring to the fact that genes are partially determined by their
location on the chromosome
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