Genetics Exam 2 Outline for Pierce Benjamin A 4 5 6 16 17 Genetics Essentials Concepts and Connections 2nd ed New York W H Freeman 2013 N pag Print Chapter 4 mother Maternal effect the phenotype of the offspring is determined by the genotype of the mother o In cytoplasmic inheritance the genes are usually inherited from the mother o In genetic maternal effect the genes are inherited from both parents but the phenotype of the child is determined by the genotype of its mother not by its own genotype o Frequently arises when substances within the egg are pivotal for early development Ex the shell coiling off the snail the direction of coiling depends on the genotype of the Direction of coiling is affected by the way the cytoplasm divides soon after fertilization which is determined by a substance produced by the mother and passed to the child in the egg s cytoplasm Extranuclear inheritance A form of non Mendelian inheritance in which a trait was transmitted from the parent to offspring through nonchromosomal cytoplasmic means o Ex during the union of gametes the mitochondrial DNA from the maternal parent is imparted to its offspring Hence when the mitochondrial DNA is expressed the resulting traits would be extranuclear in nature and determined largely as maternal effect Inherited diseases relating to mitochondria are called mitochondrial diseases Cytoplasmic inheritance inheritance of characteristics encoded by genes located in the cytoplasm o Because the cytoplasm is usually contributed all by one parent cytoplasmically inherited characteristics are inherited from a single parent o Mitochondrial diseases exhibit cytoplasmic inheritance Endosymbiotic theory explains the origin of chloroplasts and mitochondria and their double membranes o Postulates that chloroplasts and mitochondria are the result of years of evolution initiated by the endocytosis of bacteria and blue green algae According to this theory blue green algae and bacteria were not digested they became symbiotic instead o 2 prokaryotic cells one engulfs the other a new larger cell a double membrane can be found inside Chapter 5 Axel Hillmer 2005 o Pattern baldness o x chromosome hypothesis o conducted a linkage analysis study looked for association between the inheritance of pattern baldness and the inheritance of genetic variants known to be on the x chromosome the variants were SNPs single nucleotide polymorphisms positions in the genome where individuals vary in a single nucleotide studied inheritance of pattern baldness and SNPs in 95 families observed that they tended to be inherited together physical linkage on the same chromosome and segregate together in meiosis 5 1 5 2 linkage between genes is broken down by a process called recombination crossing over the amount of recombination between genes is directly related to the distance between them 1911 Thomas Hunt Morgan Alfred Sturtevant o Demonstrated in fruit flies that genes can be mapped by determining the rates of recombination between the genes o Hillmer and colleagues used this method with his pattern baldness experiment o Observed the gene for pattern baldness is closely linked to SNPs located at position p12 22 on x chromosome a region which encodes the androgen receptor gene that encodes a protein that binds male sex hormones o Further studies in 2008 found genes on chromosomes 3 and 20 to also contribute to pattern baldness This chapter covers the inheritance of genes located on the same chromosome They don t strictly obey Mendel s independent assortment They tend to be inherited together Ch 3 covered Mendel s principles of segregation independent assortment o Principle of segregation states that each individual diploid organism possesses two alleles for a characteristic each at the same position locus on each of the 2 chromosomes that separate in meiosis with one allele going into each gamete o Principle of independent assortment states that in the process of separation segregation the two alleles at a locus act independently of alleles at other loci o Independent separation of alleles results in recombination the sorting of alleles into new combinations o Mendel derived these principles by observing progeny of genetic crosses o Didn t know about biological processes 1903 Walter Sutton proposed a biological basis for Mendel called the chromosome theory of heredity which holds that genes are located on chromosomes There are many more genes than chromosomes so they should not all assort independently Linked genes genes located close together on the same chromosome o Belong to the same linked group o Linked genes travel together in meiosis into a gamete Mendel s pea plants showed independent assortment o Results in a 9 3 3 1 Non independent assortment example ratio 284 21 21 55 o The two dominant alleles lie close together on the same chromosome p 115 Normally genes close together on same chromosome segregate as a unit and are therefore inherited together However crossing over may occur genes switch between homologous chromosomes o Results in recombination breaking up the closely linked genes Special notation for crosses with linkage p 116 Complete linkage the genes are located very close together and don t exhibit crossing over o Rare o Often assumed for experiments Testcross illustrates effects of linkage o Traits appearing in progeny reveal which alleles were transmitted by the heterozygous parent P 118 FIG 5 4 complete linkage vs independent assortment Nonrecombinant parental gametes contain only original combinations of alleles present in the parents o Nonrecombinant parental progeny o No new combinations of the traits appear in offspring o The genes affecting the two traits are completely linked o New combinations could arise only if the physical connection between the genes was broken Recombinant gametes have new combinations of alleles With independent assortment nonrecombinant and recombinant gametes are made in equal proportions Recombinant progeny are formed by recombinant gametes Genes are incompletely linked if they exhibit crossing over Crossing over occurs during Prophase 1 of meiosis o Exchange of genetic material between nonsister chromatids o The chromatids that did not experience crossing over are unchanged and the gametes receiving these For each meiosis in which 1 crossing over occurs 2 nonrecombinant and 2 recombinant gametes are produced are the nonrecombinant gametes o Same result seen in independent assortment So when crossing over
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