Introduction aux lois mendéliennes de l'hérédité; application de l'analyse mendélienne à des problèmes de génétique incluant la cartographie des gènes et l'analyse de liaison, la bioinformatique et la génétique des populations. Introduction to Mendel's laws of inheritance; application of Mendelian analysis to problems in genetics including: gene mapping and linkage, molecular genetics, bioinformatics and population genetics.

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This study is a comparative investigation on the morphology of chromosome polytene extracted from Drosophila Virulis larvae and treated under heated and ambient conditions. The study reinforces that there are in fact notable differences in the chromosome structures between the cells that were incubated at high temperature (37° C) and the cells that remained at standard room temperature. The effect of heat tends to provoke pusffs which enlarged chromosomal regions where RNA transcription suddenly escalates due to heat. This phenomenon was characterized by augmented gene expressions when viewed under a microscope. Although the differences in the structures of chromosomes were not utmost due to some ambiguity in identifying chromocenter, the results were still significant – particularly at the level of the alternating pale and dark bands sequences. From the results, the conclusion made was that temperature is a remarkable factor that speeds up the synthesis of RNA on polytene active sites. 

This lab assesses Mendelian genetics by conducting chi-square analyses on phenotypic ratios obtained from

virtually simulating fruit fly crosses using Flylab as a software. Mendelian genetics can be summarized

with two laws: the law of segregation (Mendel’s first law) and the law of independent assortment (Mendel’s

second law). The former states that alleles of a gene separate from one another during gamete formation

while the latter states that genes separate independently (randomly) from one another during gamete

formation (Klug, Cummings & Spencer, 2009). The lab comes in two parts: part A and part B. In part A, a

monohybrid, dihybrid and trihybrid crosses were simulated. A monohybrid cross is a crossbreeding between

two individuals with varying alleles at one genetic locus. A dihybrid and a trihybrid cross differ from a

monohybrid cross in that the varying alleles are located on two genetic loci for a dihybrid cross and three

genetic loci for a trihybrid cross. In part B, a testcross is simulated in order to determine the genotype of a

first filial generation. A testcross is a crossbreeding between two individuals; one of the two being

homozygous recessive (mutant) and the other one being of a dominant phenotype but unknown genotype

(homozygous or heterozygous wild type). In this lab, a dominant phenotype refers to the observable wild

type trait expressed when at least one of the two alleles for a given gene is a dominant (wild type) allele

(Griffiths et al., 2004). A recessive phenotype, on the other hand, is the observable mutant trait expressed

when both alleles of the given are recessive (mutants). For both part A and part B of the lab, the mutation

studied are autosomal. Autosomal mutations are mutations for which the probability of inheritance is

independent of sex as they are only found on autosome chromosomes (non-sex chromosomes).

Based on Mendelian genetic, for any given type parental cross, there would be a specific phenotypic ratio

(Mendelian ratio) for the progeny. The hypothesis made for each experiment was that the experimental

phenotypic ratios would not significantly differ from the expected Mendelian ratios. This hypothesis was

indeed supported using chi-square statistics.

The mutation of interest for part 1 of the lab (lethal mutations) are ST and SB. ST stands for star eyes mutant phenotype (character: eye shape) while SB stands for stubble bristles mutant phenotype (character: bristle). 

Question: What is the pattern of inheritance in a dihybrid cross for which the female is heterozygous mutant for an autosomal lethal allele (ST+) and the male is homozygous wild type (++). 

Hypothesis: A cross between two parent that have different alleles (one that expresses the dominant trait and one that expresses the recessive trait) at one gene will produce offspring following Mendel’s first law. However, the homozygous mutant progeny will not survive and so phenotypic ratio will be rescaled among the surviving progeny. 

Question: 

Provided the occurrence of crossing-over, what will be the recombinant frequency and calculated relative map distance in a dihybrid cross for which the female is heterozygous wild type (RI+, +SS) and the male is homozygous mutant (RIRI, SSSS)? 

Hypothesis: 

A dihybrid cross between two parents that have different alleles (a wild type allele + expressing the dominant trait and two mutant alleles, RI and SS, expressing the recessive traits) at two gene loci will produce offspring following Mendel’s first and second law and particularly the law of genetic recombination since RI and SS genes are linked. 

Finding the PTC taster and non-taster allele frequencies, “p” (dominant allele) and “q” (recessive allele) respectively, for two different populations, lab section population (N=33) and lab day population (N=94). The allele frequency “p” and “q” were calculated by tallying the number of PTC tasters and non-tasters for both populations and then using the hardy Weinberg equilibrium equation since the assumption was that both populations were in Hardy Weinberg equilibrium1. 

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