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Berkeley MCELLBI 140 - Lecture Notes

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Amacher Lecture 2, 9/05/08LECTURE 2: Chromosomes and InheritanceReading: Ch. 4, p. 81-88; 105-110Many of the classic papers mentioned in this and previous lectures are available athttp://www.esp.org/foundations/genetics/classical/Problems: Ch. 4, solved probs I, II; also 12, 24, 27, 33, 34, 38, 39Announcements:**The first quiz will be given next week in section that will cover material through today’s lecture. The GSIs haveagreed that students with legitimate, excused absences can take the quiz in a different section during the week ofthe quiz as long as you arrange it with your GSI ahead of time.**No calculators are allowed on quizzes or exams.**We will cover pedigree analysis on Monday. If you are having difficulties with the assigned pedigree problems,please re-attempt them after Monday’s lecture.**See 9/3 Lecture notes for correction to parentl genotype/phenotype in the sweet pea example.**DSP students should contact the DSP office as soon as possible.**My Office Hours will be Thursdays, 1:30 – 3:30 on 9/4, 9/11, 9/18, and 10/2. There will be no office hours onSept 25th due to a scheduling conflict.Today, we’ll finish discussing epistasis (see notes from 9/3/08), as well as discuss the evidencethat confirms that genes reside on chromosomes.THE CHROMOSOMAL THEORY OF INHERITANCEEvidence that genes reside in the nucleusAbout the time Mendel began his experiments, cytologists were using microscopy tofollow the union of paternal and maternal gametes during fertilization in sea urchins and in frogs.Because the nuclei are the only elements that are contributed equally from the male and femalegametes, it was hypothesized that the nucleus contained the genetic material.Evidence that genes reside in chromosomesIn the 1880s, technological advances allowed microscopists to follow chromosomes inthe nucleus during mitosis and meiosis. During mitosis, each daughter cell inherited the samenumber and type of chromosomes as the parent cell. During meiosis, each of the gametesreceives only one chromosome of each chromosome pair and the chromosomes assortindependently as predicted by Mendel. Furthermore, the union of maternal and paternal gametesduring fertilization yields a zygote with two complete chromosomal sets, and the maternally- andpaternally-derived chromosomes are alike in size and shape (with the exception of the sexchromosomes in most species.) This suggested that chromosomes carry the hereditaryinformation.Sexual identity is correlated in many organisms with the inheritance of particularchromosomes. Several cytologists (McClung, Sutton, Boveri, Wilson) at the beginning of the1900s suggested that in insects, there was a relationship between specific chromosomes and thedetermination of sex. For example, Sutton showed that in the great lubber grasshopper, therewere 11 pairs of autosomes in both females and males, but females carried an additionalmatched set (XX) of chromosomes and males carried only one X. Sutton missed the Ychromosome in males, but others to follow showed that in many sexually reproducing species,not just insects, that two distinct chromosomes (sex chromosomes) are the basis for sexdetermination.Amacher Lecture 2, 9/05/08Sexual identity in other organisms:In humans, females carry 23 pairs of chromosomes, 22 autosomal pairs and two Xchromosomes, whereas males carry 22 autosomal pairs and one X and one Y chromosome. Thefruit fly Drosophila also uses an XX=female, XY=male strategy, although in flies it is the ratioof X to autosomes that ultimately determines sex, not the presence or absence of the Y. In birds,it is the female that has the unmatched set, designated WZ, and males are ZZ. In some insectspecies, the female is XX, but the male is XO, having only one sex chromosome. Wilson pointedout this difference in the early 1900’s, and used this to suggest that the “accessorychromosomes” (sex chromosomes) probably themselves weren’t the actual sex determinants, butthat they acted in a quantitative manner to determine sex.Based upon his observations, Sutton proposed that the chromosomes carried Mendel’sunits of heredity. But confirmation depended upon showing that specific traits were inheritedwith specific chromosomes.Validation of the Chromosome Theory of InheritanceThomas Hunt Morgan describes an X-linked gene for eye color in DrosophilaIn 1910, a white-eyed male fly arose in a stock of red-eyed flies in Morgan’s laboratory.Morgan called the gene responsible for eye color the white gene, because a mutation destroyingits function gives white eyes. The wild-type allele is designated w+ (red eyes) and the mutantallele is designated w. (This is different symbolism than what we used for Mendel's peas).Morgan’s crosses:Generation Genotypes:P: red-eyed female x white-eyed male Xw+/X w+ x Xw/YF1: all red-eyed Xw+/Xw and Xw+/YF1: red-eyed female x red-eyed male Xw+/Xw x Xw+/YF2: 3:1 red eyes:white eyes Xw+/X w+ ; Xw+/Xw ; Xw+/Y ; Xw/Y(all white-eyed flies are male!)[If the two traits (eye color and sex) assorted independently, then one would expect that half thewhite-eyed flies would be female and half would be male.]Reciprocal cross:Cross: white-eyed female x red-eyed male Xw/Xw x Xw+/YProgeny: red-eyed daughters and white-eyed sons Xw+/Xw and Xw/YNote also that this cross gives a different result that the parental cross above; Mendel neverobserved differences in his reciprocal crosses.The white gene is X-linked and the Y chromosome does not carry an allele of this gene for eyecolor. Thus, male flies are hemizygous for the white gene.Nondisjunction (Rare meiotic mistakes)Calvin Bridges, one of Morgan’s students, repeated Morgan’s crosses on a larger scale.When he crossed white-eyed females (Xw/Xw) to red-eyed males (Xw+/Y), he observed that theAmacher Lecture 2, 9/05/08progeny was almost always red-eyed daughters and white-eyed sons, except about 1 in 2000males had red eyes and about the same proportion of females had white eyes. He hypothesizedthat these exceptions were caused by a failure of the X chromosomes to separate during meiosis(nondisjunction) in the mothers. This leads to gametes carrying 2 X chromosomes or no Xchromosomes.The cross is: Xw/Xw (white-eyed female) x Xw+/Y (red-eyed male)The Punnett square for the exceptional cases looks like this:Xw+YXw XwXw Xw / Xw+(lethal)Xw Xw / Y(white female)OXw+ / O(red male, sterile)Y / O(lethal)Cytologically, Bridges showed that the white-eyed exceptional females


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Berkeley MCELLBI 140 - Lecture Notes

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