BIOL 2000 Lecture Notes - Lethal Allele, Auxotrophy, Wild Type
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Causes death of the individual when homozygous
When heterozygous they can still live.
The death is usually not noted as it creates an assumption.
Changes the Mendel 1:3:1 ratio to a 1:2 ratio
o Ex. Yellow coat in mice is a recessive lethal allele
Yellow x yellow = 1black: 2 yellow: 1 death
o Ex. Tailless, a recessive lethal allele in cats
ML / M = tailless
ML/ML = lethal
Interactions of genes in pathways
Objective 3: Understand how Beadle and Tatum’s experiments led to the idea
that multiple genes may control multiple steps in a biosynthetic pathway
o Connected a genetic defect to a defect in chemical reactions in the
George Beadle and Edward Tatum
o One-gene-one-enzyme (polypeptide) model
o Isolated arginine auxotrophs in Neuspora
o Auxotoph- strain bale to grow only on supplemented media
o Prototoph (wild type)- able to grow on minimal media
o First xrayed to make mutants. Then wild type cross with wild type of
opposite mating type grew into fruiting bodies and into microscopic
ascospores that were dissected and transferred one by one to culture
o Plated onto:
Mutagenized spores transferred to complete media, then
Asexual spores transferred to minimal media
No growth on minimal media identifies nutritional
mutant (if it grew on complete media)
Auxotrophs that cannot grow, tested for growth on various
To try and figure out the specific mutation that lead to
the inability to grow.
If it can grow when given amino acids, that is the issue.
Then put it on the various amino acids to see what
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