Mendelian Genetics - Heredity Part II

Mendelian genetics - Heredity Part IIIntroduction to heredityHereditary traits and GeneticsMonohybrid Inheritancegenea section of dna at a specific location on the chromosomecontains information that determines a traitmendelian traitsingle trait determined by 1 genesomatic cellany cell in the body apartfrom gametesGameteSex cellhereditary trait characteristic that can bepassed on from one generation to anotherexamples:wet or dry earwaxattached / detached earlobeskin colourhair coloureye colourblood typeface shapechin shapeability to roll tongueinheritance of 1 characteristic with2 contrasting forms controlled by a single geneeach gene is made up of a pair of allelesalleles can be dominant or recessivehomologous chromosomesexist in pairs whereone chromosome comes from the male parentone chromosome comes from the female parentboth have the exactsame gene locigeneunit of inheritance found on a particular locus of a chromosomesmall portion of DNA in a chromosome that controlsa particular characteristic or protein in an organismallelesalternative version of a gene that occupy thesame locus on a pair of homologous chromosomesPhenotypetraits of an organism that can be seeninfluenced by genotype + environmentGenotypegenetic make-up of an organism that isinherited from its parentsHomozygous organism (for a certain trait)happens if the 2 alleles controllingthe trait are the samepossible combinations:homozygous dominant (TT)homozygous recessive (tt)Heterozygous organism (for a certain trait)happens if the 2 alleles controllingthe trait are not the samepossible combination: Ttcan exist in 2 forms:dominantrecessivedominant allele will express itself inboth homozygous dominant and heterozygous conditionsrecessive allele will only express itselfin a homozygous recessive genotypeMendel's Monohybrid ExperimentMendel cross-bred pure-bred Tall pea plantswith pure-bred dwarf plantsF1 Generation hybrids allturned out to be tall plantsAfter the F1 offspring self -fertilized and produced seeds,F2 Generation plants came upRatio of tall plants to dwarf plants: 3 : 1(approx)why?genes are responsible for thiseach charactersitic is controlled bya pair of factors in the cellsIf the 2 factors are different, only thedominant factor will express itselfGenetic modelscan be used to explain how allelesare passed on to offspringpredict the traits that will be displayed by the offspringboth the tall and short plants are pure bredhowever, only the tall alleles were dominantand the short alleles were recessiveso when the 2 dominant tall gametes fused withthe 2 recessive short gametes, the dominant alleleswere expressed in heterozygous conditions while therecessive alleles were only expressed in homozygousconditions this resulted in 1 dwarf and 3 tall hybrid plantsSex Determination in humanssex chromosomeschromosomes that determine thesex of an organismAutosomeschromosomes in cells otherthan the sex chromosome2 types of sex chromosomes:X chromosomeY chromosomesex cellscells that produce gametes by meiosissomatic cellsother cells in the bodyhumans have:22 pairs of autosomes1 pair of sex chromosomes in each cellsXY - maleXX - femalemale gametescontain either the X orY chromosomeFemale gametescontain only the X chromosomehow is the sex of the zygote determined?if an X - carrying sperm fertilizes the ovumthe zygote will be femaleif a Y - carrying sperm fertilizes the ovumthe zygote will be maleequal chance that the offspringcould be male or femaleVariationdifferences in traits between individuals of the same speciestraits of an individual are dependent on interactionsbetween the genes and the environmentgenetic variationinheritablevariations due to environmentnot inheritabletypes of variationscontinuousdeals with a range of phenotypescontrolled by many genesgenes show additive effectaffected by environmental conditionsexamples: discontinuousdeals with a fewclear-cut genotypesgenes do not show additive effectnot affected by environmental conditionsexamples: eye colour, blood groupalleles of a single gene combine andthe combined affects show out as thephenotypeoccurs when 2 or more genes contribute to the final phenotypeheight skin colourcaused by different levels ofmelanin in our skinmore alleles for melanin, darker skin (vice versa)Mutationschange in structureof a gene (sickle - cell anemia)or in the chromosome number (Down's syndrome)occurs as a result of error during the replication of the gene or chromosomesomatic mutationsoccur in normal body cellscannot be inheritedmutations can be inherited by the next generationif they occur in cells that give rise to gametesdominant mutationseasily detectedrecessive mutationsmay not be detectable for generationspeople will be carriers of it withoutanyone's knowledgefactors that increase the rate of mutationsradiationchemicalschromosome mutationschange in structure ornumber of chromosomesexampleDown's syndromeaka Trisomy 21people with this syndrome havean extra copy of chromosome 21they have 47 chromosomes on totalnormal humans have 46how does this happen?in rare cases when one of the eggs has 2 copiesof chromosome 21zygote formed has 3 copies of chromosome21, so the child will have Down's syndromechromosome mutation in the gametes of a femaleparent can produce a child with Down's syndromegene mutationschange in the structure of DNA of a geneproduces variation between individualsas it results in new alleles of genesexamplesalbinismsickle - cell anemiacaused by a mutation ina recessive alleleabsence of the pigment melanin results inreddish - white skin, white hair and pink eyesalbinos get sunburnt easily as they are very sensitive to sunlightthey can't look at the sun directlycaused by mutation in the gene controlling haemoglobin productionmutated gene is recessive, so it's only expressedin homozygous recessive conditionshow is the person affected?sickle - shaped red blood cells have low oxygen carryingcapacity and tend to clump together (and block blood flow)fatal disease - sufferersusually die youngmalaria caused by plasmodiummodifies red blood cells to obtainnutrients, escape destruction by spleesheterozygotes with both sickle - shapedand normal cells have an advantagethey are protected from malariathey won't die due to lackof oxygensuch people are commonin areas where malaria is prevelant like West AfricaMutation and Selectionmutations can be:harmfulbeneficialindividuals with harmful mutationswill be eliminatedindividuals with beneficial mutation on the otherhand may leave more offspring than normal individualsnature "selects" organisms with more favourablecharacteristics to survive and reproducemutagenscan greatly increase the rate of mutationsrate of spontaneous mutatopn isextremely low in mutagens' absenceexamplesradiationchemicalsUV lightX - rayalpha and beta radiationgamma raysmustard gasformaldehydelysergic acid dielthylamide (LSD)may arise due to:crossing over and independent assortment of chromosomes and during meiosismutation in genetic materialprovides new alleles to the gene poolfor natural selection to act ongenetic variation is important to helporganisms adapt and survive in changingenvironments
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