Lecture_9_10.v2

Lecture_9_10.v2 - EEMB 129 LECTURE 9 2010 1 Two questions...

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Unformatted text preview: EEMB 129, LECTURE 9, 2010 1 Two questions that studies of individual meioses in fungi were able to determine DO INDIVIDUAL MEIOSES FOLLOW MENDELIAN SEGREGATION? If you make the cross Aa X aa The outcome is 50% Aa: 50% aa and we infer that in a single meiotic event, a diploid Aa cell produces 2 ‘A’ gametes and 2 ‘a’ gametes but, it could be possible that some other process could produce the results, say: 1/2 of the meioses produce only ‘A’ gametes and 1/2 produce only ‘a’ gametes. DO CROSS-OVERS OCCUR AT THE TWO OR FOUR-STRAND STAGE? If at the 2-strand stage: S A B-> A b-> A b a X b a B A b a B a B NPD Therefore ALL products of a single meiosis would be recombinant If at the 4-strand stage: S A B-> A B-> A B a b A B A b a X b a B a b a b T Thus, the occurrence of Tetratype asci when genes are linked shows that crossing over occurs at the 4-strand stage. EEMB 129, LECTURE 9, 2010 2 What happens with double cross-over events? Fig. 5.17 No crossing over produces PD Single cross-over events produce T Exp Freq (out of all double cross- over events, which are rare) Double cross-over events produce PD 1/4 T 1/2 NPD 1/4 - must be very rare Thus, PD>>NDP if the genes are linked, e.g., Fig. 5.17 RF = NPD + 1/2 T Total tetrads From Fig. 5.18: RF = 3 + (70/2) = 0.19 or 19% or 19 m.u. 200 PD = 127 NPD = 3 T = 70 Work with Neurospora & yeast showed that: Crossing over occurs at the 4-strand stage Recombination is reciprocal EEMB 129, LECTURE 9, 2010 3 I. Gene Mutation Chapter 7 A. Somatic versus Germinal B. Types of mutants C. Detection of mutants 1. Selective systems 2. X-chromosome 3. Mutagenesis As we have seen, the identification of variants is essential for a genetic analysis of a trait. For instance, if we are interested in the genetic basis of flower color we look for variants. Normal Red color – we can ask, genetically how complicated is the process producing red color. Look for variants (or mutants) – say we find two lines of white flowers. These can be crossed and if they produce red offspring, we say that the lines complemented each other and that there are at least two genes that are important for the production of red color. If, however the offspring are white then the white mutants are said to be allelic – or alleles at the same locus. However, we may not be sure if there is a third locus that is important for making red flowers – for this we need to find more mutants and determine whether they belong to specific loci or they represent mutants in a previously unrecognized step in color production. This can be a very intensive process if variants don’t appear often....
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This note was uploaded on 10/16/2010 for the course EEMB 129 taught by Professor Ajnarivera during the Spring '09 term at UCSB.

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Lecture_9_10.v2 - EEMB 129 LECTURE 9 2010 1 Two questions...

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