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Genetics notes week 4

by: Joseph Notetaker

Genetics notes week 4 BIO 235

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This last section of notes before our first exam goes over some of the ways in which Mendelian inheritance doesn't work. These include Epistasis, Complementary Gene Action, and Gene Linkage
Lazlo Kovacs
Class Notes
Biology, Genetics
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This 5 page Class Notes was uploaded by Joseph Notetaker on Friday September 16, 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 23 views. For similar materials see Genetics in Biology at Missouri State University.


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Date Created: 09/16/16
Genetics Notes Week 4 Recessive Epistasis:  Bombay Phenotype:  Normally sugars in human blood attach to proteins (II,H)  In type O blood the proteins are there and can accept sugars from other blood types (ii,H)  The Bombay Phenotype has none of these proteins (ii,hh) Dominant Epistasis: o The effect of a dominant allele masks the phenotypic effect of the dominant allele of another gene. o Indicated by the 12:3:1 ratio o Three phenotypic classes Complementary Gene Action: o One of the two phenotypes emerge only if both genes are present in at least one dominant allele  Genes working in tandem to produce a particular trait o Indicated by the 9:7 ratio o Two phenotypic classes o Duplicate recessive epistasis  When one gene is present in a recessive allele, it prevents the expression of another gene o The loss of one enzyme within the phenylpropanoid pathway can result in the loss of anthocyanin synthesis o Complementation test  A method of discovering whether two mutations are in the same or separate genes  The missing genes compensate for each other in the offspring resulting in an offspring with the trait Huntington disease: o A progressive nerve degeneration, usually beginning about middle age, that results in severe physical and mental disability and ultimately in death o Affects both sexes o Every affected person has an affected parent o ½ the offspring of an affected individual are affected Autosomal recessive: o Albinism: the absence of pigment in the skin, hair, and iris of the eyes. The trait affects both sexes o Most affected persons have parents who are not themselves affected; the parents are heterozygous for the recessive allele and are called carriers o Approximately ¼ of the children of carriers are affected o The parents of affected individuals are often relatives Patterns of inheritance: o Recessive:  Two normal heterozygous individuals will have, on average, 25% of their offspring affected  Two affected individuals will produce 100% affected offspring  Horizontal pattern of inheritance (found scattered throughout a pedigree) o Dominant:  An affected individual will have inherited the gene from at least one affected parent  If one parent has the symptom, there is a 50% chance that his/her child will develop the disease  Vertical pattern of inheritance (clearly passed from parent to child on the pedigree) o Two genes assort independently when they are located on different (non- homologous) chromosomes o This is because the orientation of bivalents in prophase I is random o We EXPECT that genes that are located on the same chromosome travel together as contents of “one package” o We EXPECT the same combination of alleles showing up in the progeny as the parental chromosomes o DESPITE our expectation, genes on the same chromosome do not always travel as contents of “one package” o The homologous chromosomes recombine with one-another o Crossing Over Chiasmata: o Evidence of exchange between chromosomes o Shows up as a dark, concentrated area of chromatin under a microscope Note: The Y chromosome doesn’t go through crossing over so 1 Y chromosome can pass through several generations The biological basis of recombinant genotypes: o Crossing-over between non-sister chromatids takes place during prophase I o In the diplotene phase of meiosis, chiasmata indicate the site where crossing-over occurred and segments of the non-sister chromatids were exchanged between the chromosomes  Very frequent (but never occurs in more than 50% of the DNA)  Two genes can also assort independently if they are located far apart on the same chromosome o To determine the linkage (or lack thereof), we must know what the genotype of the gametes are. In-practice, we observe the phenotype of a progeny from which the genotype is inferred o A typical mating scheme to infer the genotype of gametes:  Select true breeding lines that are:  Homozygous dominant to one trait, but…  Homozygous recessive to the other trait o Dihybrid Cross: o Two genes: ANL, YGR o First cross a ygr ANL/ ygr ANL plant and a YGR anl/YGR anl plant together to create a heterozygous generation o Then cross the F1 with a homozygous recessive plant o Creates 4 possible gametes: o ygr ANL (parent) 49% o YGR anl (parent) 49% o ygr anl (recombinant) 1% o YGR ANL (recombinant) 1% o Dominant alleles have an effect on the genotype. As usual, recessive alleles only show up in the absence of dominant alleles The production of an equal number of parental and recombinant indicates unlinked genes When genes are linked the parental genotypes vastly outnumber the recombinant genotypes o Genetic Linkage: o Operational Definition: Parental types are more frequent than recombinant types o Molecular definition: The association of genes located near one-another on the same chromosomes o Discovered by Bateson, Saunders and Punnett in 1905: certain genes in sweet pea did not follow segregation ratios predicted by Mendel o Recombination Frequencies of paris of genes reflect the distances between them o The Farther apart on the chromosome, the higher the probability that a recombination takes place o Recombination frequency: The percentage of the total progeny that are recombinant type  Map Units  Map units=percentage of recombinant offspring  In the example above it is 2% because 2% of the offspring are recombinant  Remember that there are ALWAYS 4 phenotypes 2 parent 2 recombinant o Two point crosses: Comparisons help establish relative gene positions o Genes are arranged in a line along a chromosome Problem: o The gentic distance is massively underestimated when genes are far apart o Genetic distances are not additive recombination will never be above 50% o This is due to to something called double crossover an event in which non-sister chromatids exchange DNA and then exchange some of it back in a different cross When the distance between two genes is large: o The likelihood of multiple crossovers increases o The observed frequency of recombinant offspring tends to underestimate the actual distance between the genes The expected frequency of double cross-over events can be calculated using the product rule: o The expected frequency of double cross-over events in a chromosomal region is the product of the frequencies of single crossover events o These can be calculated using three-point crosses involving three genes Interference: o The number of double crossovers may be less than expected. o Occurrence of one crossover reduces likelihood that another crossover will occur in adjacent parts of the chromosome o Chromosomal interference-crossovers do not occur independently o Interference is not uniform among chromosomes or even within a chromosome


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