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Genetics notes week 2

by: Joseph Notetaker

Genetics notes week 2 BIO 235

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Joseph Notetaker
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About this Document

These notes go over the basics of Meiosis, the work of Gregor Mendel, and a brief understanding of human sex chromosomes.
Lazlo Kovacs
Class Notes
Biology, Genetics




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This 6 page Class Notes was uploaded by Joseph Notetaker on Friday September 2, 2016. The Class Notes belongs to BIO 235 at Missouri State University taught by Lazlo Kovacs in Summer 2015. Since its upload, it has received 51 views. For similar materials see Genetics in Biology at Missouri State University.


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Date Created: 09/02/16
Genetics Notes Week 2 Mendalian Genetics Section 2.3: Sexual Reproduction produces genetic variation -Meiosis in animals and plants- pages 35-39 Section 3.2: Monohybrid crosses: segregation and dominance What monohybrid crosses reveal- pages 50-53 The molecular nature of alleles- pages 53 Predicting the outcome of genetic crosses- pages 53-56 Meiosis o First Mechanism of meiosis responsible for new allele combinations: Crossing over o Reciprocal exchange of chromosomal segments between homologs o Second mechanism of meiosis responsible for new allele combinations o Random orientation of chromosomes in metaphase I  The genetic information of the genome is reshuffled at prophase I and metaphase I o Meiosis results in a combinatorial change of the genetic information which creates genetic diversity o Genetic diversity in a species increases the possibility that some individuals in a population will be successful at meeting challenges of a changing environment  Cheetahs have very low genetic diversity due to a bottleneck in their population o Spermatogenesis follows the typical process of meiosis o Oogenesis on the other hand results in only one cell being produced o The secondary Oocyte is the only one that is haploid o Cells undergoing Oogenesis are stored in a prophase I state. This state is highly unstable and is the cause of miscarriages and children with down syndrome. o These risks increase as the woman ages o In plants things work a bit differently o In algae and mosses the haploid gametophytic generation is larger than the diploid sporophytic generation o In Plants meiosis produces an entire haploid generation (gametophyte). This generation then gives rise to gametes. o In ferns, the haploid gametophytic generation is morphologically different and it lives independently from the diploid sporophytic generation. The sporophytic generation is larger than the gametophytic generation o Humans have been using genetic techniques for millennia before they understood the natural laws that govern hereditary o Erroneous explanations of heredity at Mendel’s time: o Blending theory of inheritance  Factors that control hereditary forever change in the progeny – blend together in each new generation o Theory of preformationism  Only the male parent determines the traits of the progeny Gregor Mendel: o Hybridizes garden pea while carefully controlling self-and cross- pollination. (Garden peas are great because they produce many offspring, have relatively short life cycles, and have clearly identifiable traits). o Uses true-breeding lines (heritage varieties). o Studies clear-cut alternative traits that come in mutually exclusive forms. o Counts the number of progeny that display the alternative forms, thinks in statistical terms and tests hypotheses. o Performs reciprocal crosses. Terms: o Hybridization: o The mating or crossing between two individuals that have different characteristics  Purple flowered plant X White flowered plant o Monohybrid Cross: o Mating between two individuals that have different traits in a single character o Dihybrid Cross: o Mating between two individuals that have different traits in two characters o Hybrids: o The offspring that result from such a mating Mendel Studied Seven Characters That Bred True o The morphological characteristics of an organism are termed characters-a general term of category o The term trait describes the specific properties of a character  Seed color is a character, yellow or green is a trait o A variety that produces the same trait over several generations is termed a true-breeder o The seven characters used in the experiment are:  Seed color (Yellow/Green)  Seed shape (Round/Wrinkled)  Seed coat color (Gray/White)  Pod color (Yellow/Green)  Pod shape  Flower position  Stem length Mendel’s Breakthrough o Mendel singles out a certain characteristic in the offspring of the parent generation making it the norm. This created the F1 generation. o Mendel observes that the parents’ characteristics re-appear in the F2 generation. o Mendel counts the number of progeny showing the different traits. o This process disproved the blending hypothesis. Mendel’s data showed the 3:1 ratio that we see today. How did Mendel explain his results in biological terms? o Each trait carries two copies of a unit of inheritance, one inherited from the mother and the other from the father. o Alternative forms of traits are called alleles o The maternal and paternal alleles for each trait separate (segregate) during gamete formation, and then reunite at random, one from each parent, at fertilization. o Came up with the idea that some traits were dominant and others were recessive. In Summary: 1. A pea plant contains two discrete hereditary factors (units of inheritance or genes) for a given character, one from each parent 2. The two factors may be identical or different 3. During gamete formation, the paired factors for a given character segregate randomly so that half of the gametes receive the other i. This is called Mendel’s Law of Segregation Dominant/recessive traits and dominant/recessive alleles: o Disappearance of traits in F1 generation and reappearance in the F2 generation disproves the hypothesis that traits blend o A character must have two forms that can each breed true o One form must be hidden (masked) when plants with each trait are interbred o The form of the character that appears in F1 is dominant. Dominant trait is caused by a dominant allele (variant of the gene). An allele is dominant when its effect can be seen, no matter what other allele it is paired with. o The form of the character that is hidden in F1 is recessive. Recessive trait is caused by a recessive allele (variant of a gene). An allele is recessive when its effect can only be seen when it is paired with another recessive allele. Terms: o Phenotype:  Observable characteristic of an organism o Genotype:  Pair of alleles present in an individual o Homozygous:  Two alleles of trait are the same (YY or yy) o Heterozygous:  Two alleles of trait are different (Yy) Basic Rules of Probability: o Independent events- probability of two events occurring together  What is the probability that both A and B will occur?  Solution: Determine probability of each and multiply one with the other. This is the PRODUCT RULE o Mutually exclusive events- probability of one or another event occurring  What is the probability of A or B occurring?  Solution: Determine the probability of each and add them together. This is the SUM RULE Punnett Square: o A Punnett Square can explain the genotypes of a monohybrid cross, making it a good tool for probability o Homozygous Dominant: 25% o Heterozygous: 50% o Homozygous Recessive: 25% Section 3.3: Dihybrid Crosses Section 4.1: Sex id determined by a number of different mechanisms Section 4.2: Sex-linked characteristics are determined by genes on the sex chromosome Section 4.3: Dosage compensation Two or more phenotypes are inherited at once o Mendel performs dihybrid crosses: Simultaneous inheritance of two unrelated traits (Crossing individual plants that differ in two characters  Character 1: Seed texture  Character 2: Seed color Important points: o There are new combinations not found in the original parents o Each individual character is still inherited in a 3:1 ratio  Now the phenotypes come in a different ratio  9:3:3:1 (out of a total of sixteen)  This ratio comes from the multiplication rule (3/4*1/4, 1/4*1/4, etc.) o Mendel Explained that these results came about because the alleles segregate independently of each other  The Law of Independent Segregation  During Gamete formation different alleles segregate independently of each other Law of Independent Assortment: o During the second phase of Meiosis (Metaphase I) the chromosomes randomly orient between the cells. This is the Law of independent assortment o In order for the traits to be independent, they have to be on different chromosomes or far apart on the same chromosome In 1900, Carl Correns, Hugo deVries, and Erich von Tschermak rediscover and confirm Mendel’s laws o At the time biologists didn’t study statistics, so most biologists didn’t understand his theories o This coincided with Darwin’s theory of evolution o Nobody understood what DNA was at the time In humans, sex is determined by the XX-XY system o Females are Homogametic (XX) o Males are Heterogametic (XY)  The Y chromosome is largely heterochromatic  Heterochromatin is condensed and inactive  Important regions include:  Pseudo autosomal region: region of shared X-Y homology allows the Y chromosome to find the X chromosome  SRY-sex determining region Y: encodes testis determining factor (TDF) for male development o X-linked inheritance  Certain traits are governed by genes on the sex chromosomes  The white-eye allele in Drosophila is one example of an X- linked trait o This provided instrumental evidence for the chromosomal theory of inheritance  A pedigree for an X-linked disease shows that it is mostly males that are affected with their mothers as carriers  Examples of X-linked genetic disorders are Duchenne muscular dystrophy and green-red color blindness o Abnormal variation of sex chromosome number  XXY, XXYY, XXXY: Klienfelter syndrome; males traits  X0: Turner syndrome; female traits  XXX, XXXX, XXXXX: Ploy-X chromosome; female traits


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