BICD 100 Lecture Notes - Silene Dioica, Dihybrid Cross, Mendelian Inheritance

Which of these statements is incorrect?

What is the relative genetic distance between two linked genesif the recombination frequency is 0.49?

What statement best explains the distortion in Mendelian ratiosobserved by Bateson & Punnett in 1905? (Reminder: they found anoverrepresentation of F2 offspring showing both dominant orrecessive phenotypes, and an underrepresentation of offspringdisplaying one dominant and one recessive phenotype)

When determining the relative genetic distance between twogenes, why is dihybrid back-cross preferable over traditionaldihybrid cross?

Why do we map genes?

“The wrinkled seed character that Mendel studied was R versus r on l inkage group V. The character ‘green versus yellow pod’ is unambiguously Gp versus gp , also on linkage group V. Linkage between these two loci can be detected. In Mendel’s study of two- and three- factor crosses he used approximately 600 F 2 individuals. He did not present data on the combination of RR Gp Gp crossed with rr gpgp … [When] lines JI15 and JI399 [ are crossed ], the recombination fraction between the R locus … and Gp is 36 %, resulting in an expected segregation ratio of 9 . 6 : 2.4 : 2.4 : 1 . 6 rather than 9 : 3 : 3 : 1. Mendel would have needed about 200 plants in the ‘smaller number’ to have a 5 % statistically significant deviation from independent assortment.

Furthermore, linkage group V in pea, most likely corresponding to chromosome 3 , behaves unusually in this cross because the number of chiasmata is never greater than one; usually two or three occur. The recombination fraction calculated above is therefore the smallest that Mendel could have encountered, so it is unlikely that genetic l inkage would have been discernable in any of the crosses that Mendel examined. ”

• Explain the 9.6:2.4:2.4:1.6 ratio expected with a 36% recombination between the R and Gp loci in the cross between lines JI15 X JI399.

Hints: (i) What extent (%) of recombination would give you a perfect 9:3:3:1 ratio? (ii) You may consult the Hall et al. (1997) paper if necessary (file [6] in Minicourse 1 folder).

• Does the number of chiasmata (two or three in many other Pisum sativum cultivars but 1 or 0 in the JI lines) explain Mendel’s results? Hint: What are chiasmata? What is their genetic function?

• Can we still trust Mendel’s conclusions about independent segregation of characters?

What is a genome? What is a genome made of? Describe the bacterial genome. How do bacteria reproduce? What is binary fission? Distinguish between a chromosome and chromatin. Where are chromosomes located in eukaryotic cells? What is the chromosome made of? What is a centromere? What is the centromere's role? What are sister chromatids and how do they arise? Distinguish between somatic cells and germ (gametes) cells. Describe the cell cycle. What are the different phases? What occurs during S phase? What is interphase? How is the cell cycle controlled?

Describe mitosis. What are the phases of mitosis? What occurs in each of the phases of mitosis? What is cytokinesis? How does this differ between plant and animal cells? What is a kinetochore? Where does it form? What is its role? What is the role of microtubules in mitosis? What is a centriole and what is it composed of? What is a centrosome and what is its role in mitosis? What is the metaphase plate and why is this important? What is the mitotic spindle, its role and its makeup? What defines a diploid cell? How many copies of each chromosome is in a diploid cell? What about a haploid cell? Why do the daughter cells of mitosis always have the same complement of chromosomes?

Describe meiosis. What are the phases of meiosis? What occurs in each of the phases of meiosis? Distinguish between meiosis I and meiosis II and describe how each of these processes compare to mitosis. Why do animal cells undergo meiosis? What is a gamete? What is a zygote? What are recombination, crossing-over, and chiasma? When and why does this occur? What is meant by 'homologous pair of chromosomes'? Why don't the daughter cells of meiosis have the same genetic makeup? What controls the segregation of chromosomes into the gametes?

What are genetic characters? What are genetic traits? What is an allele? What is a locus? How many alleles at each locus would a human cell have and why? Distinguish between genotype and phenotype. What is meant by a dominant gene? What is meant by a recessive gene? How do these differ? What is the genotype of a homozygous dominant organism? What is its phenotype? What is the genotype of a homozygous recessive organism? What is its phenotype? What is the genotype of a heterozygous organism? What is its phenotype? What is a genetic cross? Distinguish between the P, F1, and F2 generations. What is a true-breeding line? What is the genotype and phenotype of a cross between homozygous dominant and homozygous recessive parents? If the F1 progeny are crossed, what would be the genotype and phenotype of the F2 generation? What would be the phenotypic ratio? What would be the genotypic ratio? Be ready to generate Punnet square tests or probability analyses of crosses involving more than one genetic locus. Describe a dihybrid cross where two alleles are homozygous dominant in one parent and homozygous recessive in the other. What would be the phenotypic ratio of the F2 generation from this dihybrid cross? What is meant by independent assortment? Be prepared to calculate probabilities resulting from different crosses.

Distinguish between complete and incomplete dominance. How would incomplete dominance alter the results of a hybrid cross? What would the resulting F2 generation phenotype and genotype ratios be? What is codominance? How does this affect the results of crosses? Be prepared to determine likely blood types of parents based on the blood types of their offspring. What is a polygenic trait? What is a carrier state for a genetic disease? What is meant by linked genes? How does this affect expected outcomes of genetic crosses? How is gene linkage used for chromosomal mapping? Distinguish between autosomes and sex chromosomes. What sex chromosomes are found in women? What sex chromosomes are found in men? What are sex-linked traits? Be prepared to give the results of crosses that involve sex-linked recessive traits. What is the process of nondisjunction? What are the likely results? Why is this important? What is a Barr body? Recognize the genetic abnormality of Downs Syndrome, Turner's Syndrome, and Klinefelter's Syndrome.