Genetics Final Oral Exam questions with verified correct answers.

Why was Mendel's approach to the study of heredity so successful? Mendel's approach to the study of heredity was so successful for many reasons. First, his choice in using the garden pea for his experiment was very advantageous, because these plants varied in height and in the appearance of their flowers, seeds, and pods. The pea plants were also very easily crossed, which worked in Mendel's favor, as he was able to manipulate the crosses and study the results easily. Mendel also began his experiments with true-breeding strains of pea plants that varied in only one of seven different characters. This allowed him to focus on that particular character individually. In which phases of mitosis and meiosis are the principles of segregation and independent assortment at work? Mendel's Law of Segregation can be explained by the segregation of homologs during Meiosis. In meiosis I, homologous chromosomes segregate from each other. Sister chromatids segregate in meiosis II, and this leads to the segregation of the alleles into separate gametes. Each gamete contains one copy of each type of chromosome. Mendel's Law of Independent Assortment can be explained by the random alignment of bivalents(a structure in which two pairs of homologous sister chromatids have synapsed (i.e., aligned) with each other) during Metaphase of Meiosis I. The chromosomes carrying Y or y alleles assort independently from chromosomes carrying R or r alleles. This leads to independent assortment of genes on non-homologous chromosomes. How is the chi-square goodness of fit test used to analyze genetic crosses? What does the probability associated with a chi square value indicate about the results of a cross? The chi-squared goodness of fit test is used to analyze genetic crosses by evaluating whether the predictions of a genetic hypothesis agree with data from an experiment or not. A low chi square values indicates a high probability that the observed deviations could be due to random chance alone. High chi square values indicate a low probability that the observed deviations are due to random chance alone. What characteristics of an organism would make it suitable for studies of the principles of inheritance? Name a few organisms that have these characteristics. Short generation time, large numbers of progeny from a single cross, easy to culture in lab, males and females are readily distinguished, having a relatively small genome, being large enough to be easily observed with a dissecting microscope. Mice, flies, worms, frogs, yeast, zebrafish. Explain Mendel's Laws to an educated non-scientist. Mendel's law of independent assortment: two different genes(or units of heredity) will randomly assort or group their alleles(or alternative form of a specific gene) during gamete(or reproductive cell) formation (if they are not linked). Mendel's law of segregation: the two copies of a gene segregate(or separate) from each other during transmission or transportation from parent to offspring. Distinguish the following terms: chromosome, chromatin, sister chromatid, and homologous chromosome. chromosome: the structures within living cells that contain the genetic material. Genes are physically located within the structure of chromosomes. chromatin: the complex of DNA and proteins that is found within eukaryotic chromosomes sister chromatid: pairs of replicated chromosomes that are attached to each other at the centromere. Sister chromatids are genetically identical. homologous chromosome: the two homologs of a chromosome pair are said to have similar size, banding pattern, and genetic composition Given three recessive traits, (1) explain how you would test for linkage; and (2) if you determine all three traits to be linked, explain the general approach to calculating the map distance between the loci. Three recessive traits are green seed color, wrinkled seed shape, and yellow pod color. To test for linkage, I would perform a three-factor cross, which is a cross in which the experimenter follows the outcome of three different characters. First I would cross two true-breeding strains that differ with regard to the three alleles for see color, seed shape, and pod color. Then, I would perform a testcross by mating F1 female heterozygotes to male individuals that are homozygous recessive for all three alleles. Then, I would collect data for the F2 generation. If the genes assorted independently, all eight combinations would occur in equal proportions. To determine linkage, I would then perform a chi squared test to test the hypothesis that the characters are segregating and assorting independently. If it is determined that all three traits are linked, I would then calculate the map distance between the loci. I would regroup the data according to pairs of genes to calculate map distance between two genes. I would then make a table in order to organize the total number of parental offspring for the paired genes, and the total number of nonparental or recombinant offspring with the paired genes. Then, using the formula for map distance which is ((number of recombinant offspring)/(total number of offspring))*100, I would determine the map distance in map units between the paired genes. I would repeat this process for all the paired genes. Based on these calculations, I could then construct the map of the genes. Would Mendel get the same results if he'd started his first cross between pure-breeding round green pea plants and pure-breeding wrinkled yellow pea plants, rather than the cross between pure breeding round yellow and wrinkled green plants? Yes, Mendel would have gotten the same results if he'd started his first cross between pure-breeding round green pea plants and pure-breeding wrinkled yellow pea plants, rather than the cross between pure breeding round yellow and wrinkled green plants. This is because the genotypes of the the F1 generations of both crosses would be the same. RRYY crossed with rryy produces all offspring with the genotype of RrYy, meaning the phenotype shown would be round and yellow. RRyy crossed with rrYY produces all offspring with the genotype of RrYy, meaning the phenotype shown would be round and yellow as well. What is the relationship between a locus and a gene? Between a gene and an allele? A gene is a unit of heredity that may influence the outcome of a trait in an organism. A locus is the physical location of a gene or other DNA segment within a chromosome. An allele is an alternative form of a specific gene. Explain the distinction between a sex-linked, sex-limited, and sex influenced trait. Sex-influenced inheritance is an inheritance pattern in which an allele is dominant in one sex but recessive in the opposite sex. An example of this is scurs in cattle. Sex-limited inheritance is an inheritance pattern in which a trait is found in only one of the two sexes. An example of this is feather plumage in male chickens. Sex-linked inheritance is an inheritance pattern involving sex-linked genes, which are found on the X or Y chromosome, but not both. An example of this is hemophilia. How does the sex determination in Drosophila differ from sex determination in humans? In humans, the X and Y chromosomes determine sex. Females have two X chromosomes, and males have one X and one Y chromosome. The Y chromosome (SRY gene)determines maleness. The MSY region on the Y chromosome is male specific, and includes the SRY gene that determines maleness, three male fertility genes that are required for spermatogenesis, and eight essential genes that are shared with the X chromosome that determine male viability. Femaleness is the default state in humans. In Drosophila, males are XY and females are XX, but the Y chromosome does not determine maleness. The ratio between the X chromosomes and the number of sets of autosomes determines maleness. Maleness is the default state brought about by the presence of only one X chromosome, because 2 X chromosomes are required to activate transcription of the Sxl gene in early Drosophila embryogenesis. Define and distinguish between expressivity and penetrance. Expressivity is the degree to which a trait is expressed. For example, flowers with deep red color have a high expressivity of the red allele. Penetrance is the proportion of organisms having a particular genotype that actually express the corresponding phenotype. If the phenotype is always expressed, penetrance is complete. If otherwise, it is incomplete. An example of this is polydactyly, which is an autosomal dominant trait, and some individuals carry the allele but don't express the trait. Explain the genetic phenomenon of epistasis. How would one test for it? Epistasis is an inheritance pattern where one gene can mask the phenotypic effects of a different gene. Epistatic interactions often arise because two (or more) different proteins participate in a common cellular function. An example is an enzymatic pathway. If there is a colorless precursor, with Enzyme C creating a colorless intermediate, and then Enzyme P creates a purple pigment. If an individual is homozygous for either recessive allele, it will not make any functional enzyme C or enzyme P, and therefore the flower will remain white. To test for epistasis, we would