genetics Yeast, neurospora, Tetrad-Analysis.pptx

genetics Yeast, neurospora, Tetrad-Analysis.pptx

1. Genetic Mapping: Tetrad Analysis

2. Tetrad Analysis Paired homologous chromosome is called a tetrad since they are having four chromatids. Tetrad analysis is a special mapping technique can be used to map the genes of haploid eukaryotic organisms which are the product of single meiosis event. Life cycle of Saccharomyces cerevisiae

3. Ordered Tetrad and Mapping of Gene-Centromere Ordered tetrad is used to calculate the distance between gene and centromere. Relative proportion of tetrad is used to calculate map distance rather than progeny. Genetic studies on Neurospora crassa , a fungus commonly known as orange bread mold, revealed that crossing over occurs after the duplication of chromosome rather than prior to the duplication. Neurospora produces ordered tetrad. Why Neurospora crassa was taken for the study? Meiosis occurs in diploid nucleus formed after the fusion of nuclei from two distinct strains. Haploid products of meiosis are called ascospores are contained in linear order in a tube like structure called ASCUS (presenting four products of single meiosis event). Ascospores germinate to produce haploid mycelium so the allele irrespective to dominant or recessive can be determined easily in this haploid phase. Neurospora crassa can be grown easily on synthetic medium. Haploid phase can be maintained asexually. The order of the four spores within an ascus reflects exactly the orientation of the four chromatids of each tetrad at the metaphase plate at Meiosis I. The spores can be isolated in the same order as they are in ascus.

4. Life Cycle of Neurospora crassa

5. Mapping of Gene by Tetrad Analysis : It is used to map of locus in relation to the centromere. In Neurospora crassa meiosis is followed by mitotic division resulting in to eight ascospores arranged in four pairs in ascus, each pair representing one of the products of meioisis. For ex. If a mating type A is crossed with a in N. crassa resulting zygote will be A/a. In First Case : When there is no crossing over between mating type locus and its centromere. The resulting ascospores will have genotype as follows: Since the two types of centromeres or alleles of a gene A and a segregate to different nuclear areas after first meiotic division, the segregation is called First Division Segregation (FDS).

6. In Second Case : If cross over occurs between gene and its centromere then A and a allele do not get separated until the second meiotic division. This separation is called as second division segregation (SDS).

7. Calculating Map Distance: Percentage of asci with SDS can be used to map gene with respect to its centromere. Number of SDS asci is halved in the formula because only half of the spores are actually product of cross over in asci. So to convert the tetrad data to cross over or recombination data percentage of second division segregation asci (SDS asci) is divided by 2. Map Distance = ½ X Number of SDS Asci X 100 Total number of Asci

8. Tetrad Analysis to Map Two linked Genes In some cases spores, the product of meiosis, don not retained in linear manner inside the single structure and get mixed. These types of tetrads are known as unordered tetrads. They are unable to suggest the consequence of first and second meiotic division. Unordered tetrad can be used to map genes in dihybrid cross. A diploid that is heterozygous for both the genes can be constructed by making appropriate cross. Then, after meiosis the resulting tetrads are analyzed.

9. Three tetrad types ( T, PD, NPD ) are produced when two gene are present on the same chromosome: Condition 1 : when no crossing over occurs or two strands double cross over occurs between two loci. Then PD (parental ditype) are produced. Condition 2 : A four strand double crossover between two genes results in two kinds of spores that do not resembles parents, are called NPD (non parental ditype).

10. Condition 3 : A two strand single cross over or three strand double cross over between two genes results in the formation of both parental and non parental or recombinant TT (Tetratype). Formation of three tetrad types ( T, PD, NPD ) can be seen when two gene are present on the different chromosomes like wise:

11. In tetrad analysis we analyze types of tetrads rather than progeny . The basic formula of genetic mapping is converted in terms of tetrad. Distance between two gene can be computed by the following formula: ½ T + NPD X100 Total tetrads References Peter J.Russell. iGenetics. Gardner, Simmons and Snustad. Principles of Genetics. https://www.slideserve.com/brice/rest-of- chapter- 5