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Molecular Biology and Genetics
MBG 2040
Christine Schisler

2040 Midterm Notes Lecture 1: • Only mammals have x and y chromosomes, many other species do not • ~300 genes on human y chromosome and >1000 genes on human x chromosome • Autosomal recessive traits: examples of this include: classic albinism, sickle cell anaemia and cystic fibrosis • Recessive mutations often involve a loss of gene function resulting in one of the following: a)Null/Amorphic Alleles: a non-functional protein is produced OR no protein is produced b)Hypomorphic Alleles: a poorly functioning protein is produced OR reduced amounts of a normally functioning protein is produced • Albinism is caused when a person is homozygous for a null allele of the tyrosinase gene(which encodes one of the enzymes needed to synthesize melanin) • The wild type allele produces a functional polypeptide, where as the recessive amorphic loss-of-function allele does not produce a functional polypeptide and in turn produces a severe mutant phenotype of the allele(due to the fact there is a mutation in the protein encoding region OR if it is found in the regulatory region no protein will be present) • In contrast a recessive hypomorphic allele produces a partially functional polypeptide(the mutation is found in the regulatory region) or can produce reduced amounts of a functional polypeptide. Both result in a mild mutant phenotype(not as severe as above) • Autosomal dominant trait: examples of this include: Huntington Disease, Polydactyly and Neurofibromatosis • Dominant mutations- often involve gain/change of gene function: a)Dominant Hypermorphic allele- negative phenotypic consequences due to the over production of a normal protein OR due to the production of a protein with increased activity levels b)Neomorphic allele- negative phenotypic consequences due to the presence of an altered protein that has a new function OR when the altered protein interferes with the wild type protein(dominant- negative allele) • Phenocopy Effect: The environment agent changes(the phenotype) but NOT THE GENOTYPE. Examples: of this would include dying hair, nose jobs, boob jobs etc • Structural Gene -Pedigree rules: -know it is autosomal recessive if normal parents produce an affected offspring -heterozygosity value is reduced in autosomal recessive traits by ½ with eachAAmating -in autosomal recessive traits, individuals who marry into the pedigree areAAunless there is evidence of the contrary -to find probability its general: (prob. mom has certain genotype)x(prob. dad has certain genotype)x(punnet square) = ?? -in autosomal dominant traits, individuals who marry in are assumed aa -in autosomal dominant traits affected individuals are assumed heterozygous Lecture 2: • Cells are specialized for a specific area, could potentially make every protein instead make specialized ones • Eukaryotic chromosomes made of chromatin- which is a complex of DNAand protein. These pieces of chromatin can alternate between tight and loose compaction (highly compacted during mitosis and meiosis-only time chromosomes are visible…not visible if not highly compacted) • -chromosomes can be found in certain regions of the nucleus called chromosome territories • -Karyotype- the number and appearance of chromosomes from a eukaryotic somatic cell(don’t use sex cells as they are too hard to access). They are arranged and numbered by size from largest to smallest, this helps scientists quickly identify chromosomal alterations. Can’t see individual DNAstrands, # of genes in a given chromosome or the presence or location of small mutations. Can’t be taken during interphase as the chromosomes are uncoiled, long and thin. In Giemsa staining the dark bands are often constitutive heterochromatic regions. Euchromatin-white part Constitutive Heterochromatin- black part • Much of the y chromosome is heterochromatic consisting of highly repetitive non-coding DNA o -the y chromosome has the SRY gene which encodes TDF-testis determining factor • In birds females are ZW and males are ZZ-W is the female producing gene-female birds produce Z & W eggs • Unfertilized turkey eggs can develop into offspring when the chromosomes in the eggs are doubledtoo large to mate (need them large for consumer demand)heat shock them to fertilize themresults in all male offspring(ZZ) • Sex determining gene not defined to one chromosome in reptiles and amphibianscan be XX/XY or ZZ/ZW or no sex chromosomes • If turtle eggs incubated above certain temps will result in all female offspring • Bees o -Queen: diploid female can lay fertilized eggs or unfertilized eggs o -Drone: haploid male o -Worker: diploid female • In the drosophila male determining genes are on the autosomes, female determining genes are on the x chromosome-eye colour also maps to x chromosome • Humans born with only one X gene-female with turner syndrome • Humans born with XXY gene-male with klinefelter syndrome • Recessive x-linked traits v.s. x-linked dominant traits More likely to see an affected male vs female in x-linked dominant traits • Blue sensitive colour controlled by autosomal trait vs red/green which is sex linked • Barr Body- dark staining structure inside the nucleus. Males have no Barr bodies and females have one BUT X0 females have no barr body and males with XXY have 1 barr body a barr body is an inactivated X chromosome-the genes of this x chromosome are not being transcribed as it is highly compactedthe inactivation of one of the x chromosomes in females equalizes x-linked gene expression between males and females this inactivation occurs in early embryonic development-the choice of which x is inactivated is randomtherefore there are 2 cell possibilities in female mammals: Maternal x is active and paternal x is Barr body or vice versamammal females are mosaics, a mixture of the 3 types of options above are found in the bodythe proportions of these two cells varies from female to female • Coat colour in cats follows a similar pattern with x-inactivation, 2 alleles of one x-linked gene control the presence of black and orange pigmentmale offspring can be black or orange females are the same HOWEVER they can also be tortoiseshell or calico(the heterozygous form of this) the pattern for this is due to the expression of the gene throughout the coatdue to barr bodies • Turner syndrome when the female is X0-she is therefore sterile as females require two transcriptionally active X chromosomes in the cells of the reproductive organs or she won’t develop ovaries • The Hardy-Weinberg equation: p + 2pq + q =1 2 Lecture 3: • Number of recombinants/total number of offspring x 100 = map units • For a dihybrid test-cross there are three possible scenarios: o Genes Aand B are independently assorting-4 classes of offspring with equal frequencies o Genes Aand B are tightly linked-2 classes of offspring with equal frequencies o Genes Aand B are linked and crossing over occurs betweenAand B—4 classes of offspring with 2 large and 2 small o Linked genes can be tracked from generation to generation Lecture 4: • Incomplete dominance/semi-dominance/partial dominance –phenotype of the heterozygote can be distinguished from the phenotype of both homozygotes-ex white and red snap dragons make pink snap dragon • Hybrids do not breed true-ex palomino x palomino= sorrel palomino and cremello • Alleles can be associated with certain breeds or varieties of a species o S > s >s > s solid > Irish spotting > piebald > extreme white • Irish spotting irregular areas of white spotting • Piebald dark pigmentation on the head, rump and saddle-brittany spaniels, landseer newfounlands • C > c > c > c >c full colour > Burmese > Siamese > blue eyed albino> albino b h • Tonkinese c c • Blood types: o A, B, O andAB • Due to difference in sugars present the immune system can recognize wrong one in blood transfusion- i encodes for a non-functional transferase which therefore means it does NOT have a sugar so it is universal and can be given to any blood type • AB blood type is co-dominant, both alleles are fully expressed in the heterozygote • Alleles can also be lethal o Homozygous dominant tail dominant cats o Dexter cattle homo dominant lethal o Yellow mice homo dominant lethal o Creeper chickens homo dominant lethal • Variable Expressivity-range of phenotypes can be expressed by a gene o Seen in many diseases such as polydactyly, Neurofibromatosis, Cleft hand-foot syndrome, cystic fibrosis • Polydactyly is a rare autosomal dominant disease that shows variable expressivity • Piebaldism is a rare autosomal dominant disease that shows variable expressivity • Neurofibromatosis is a rare autosomal dominant disorder that shows variable expressivity • Huntington’s disease is a rare autosomal dominant disorder-variable expressivity in terms of onset time of the disease • Allelic heterogeneity-many disease causing alleles some are associated with severe phenotype others with a milder one • Incomplete penetrance- sometimes an allele is not expressed even though it is present in an individual • Phenocopy/Phenocopy effect-environmentally caused phenotype which mimics one caused by genes ex plastic surgery, colour contacts, dying ones hair etc • Pleiotropy-a single gene can be responsible for a number of distinct and seemingly unrelated phenotypic effects • Gene Interaction- different combinations of alleles from two or more genes can result in different phenotypes, because of interactions between their products at the cellular or biochemical level • Epistasis is a type of gene interaction which one gene seems to prevent the phenotypic expression of another gene-if one gene has an overriding effect on the phenotype it is said to be epistatic to the other genes involved Lecture 5: • Expression of traits varies from males to females ex. Male pattern baldness o Male pattern baldness is a sex-influenced autosomal trait o Females homozygous for pattern baldness allele show thinning in the crown area • Sex limited traits-only one sex expresses the trait ex milk production, development of penis, uterus,
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