Introduction to Genetic Analysis 488

Introduction to Genetic Analysis 488 - 44200_15_p481-520...

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487 15.1 Changes in chromosome number somes segregate normally, producing diploid gametes. The fusion of gametes at fertilization regenerates the tetraploid state. If trivalents form, segregation leads to nonfunctional aneuploid gametes, and hence sterility. What genetic ratios are produced by an autote- traploid? Assume for simplicity that the tetraploid forms only bivalents. If we start with an A/A/a/a tetraploid plant and self it, what proportion of progeny would be a/a/a/a ? Obviously we Frst need to deduce the fre- quency of a/a gametes because this is the only type that can produce a recessive homozygote. The a/a gametes can arise only if the pairings are both A with a , and then the a alleles must both segregate to the same pole. Let’s calculate the frequencies of the possible outcomes by means of the following thought experiment. Consider the options from the point of view of one of the a chro- mosomes faced with the options of pairing with the other a chromosome or with one of the two A chromo- somes; if pairing is random there is a chance that it will pair with an A chromosome. If it does, then the pairing of the remaining two chromosomes will necessarily also be A with a because those are the only chromosomes re- maining. With these two A -with- a pairings there are two
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This note was uploaded on 01/10/2011 for the course BIOL BIOL taught by Professor Johnson during the Spring '08 term at Aberystwyth University.

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