BSC1005 Florida State University UNIT 3 STUDY GUIDE GENETICS HUMAN INHERITANCE Contents Section I Mendelian Genetics 3 Section II Chromosomes and the Cellular Bases of Inheritance 6 Section III Expansion of Mendel s Principles 8 Section IV The Chemical Basis of Inheritance From the Structure of DNA to Human Gene Therapy 13 2 Section I Mendelian Genetics Theories of Inheritance Blending Theory old belief system The characteristics of each parent were somehow blended in the offspring as liquids will blend when mixed together yielding an offspring intermediate between the two parents Doctrine of the Fixity of Species Grounded in the religious belief that each species of animal and plant was created individually by God and was fixed and unchanging in its characteristics Mendel s Particulate Theory of Inheritance new belief system Mendel hypothesized that inheritance patterns were due not to the blending of traits but to the passing of discrete particulate factors from parents to offspring Each child receives factors from one parent and some from the other Factors genes Factors genes responsible for a single trait and were passed on from one generation to the next during sexual reproduction Advantages of studying genetics using pea plants Like animals plants reproduce sexually by the union of gametes gametes sperm eggs zygote egg fertilized by sperm Unlike animals plants have both male female sex organs Therefore can self fertilize by union of own OR cross pollinate by union of gametes from separate gametes plants Mendel s Results Study used alternate pure bred varieties of pea plants which differed in only one visible trait Inbred self fertilized pea plants were used 3 If plants of two different varieties are crossed the result is a hybrid which displays a new mixture of traits Main traits observed Trait of plant height Tall plant vs dwarf plant Trait of seed pea color Green vs yellow Trait of flower color Red vs white Results of pure bred red flowered variety crossed with white flowered variety P1 parents F1 first filial generation aka offspring of P1 hybrids Results F1 hybrids 100 red flowered Results of F1 hybrids red flowered when self fertilized F2 second filial generation Results F2 hybrid 75 red 25 white The factor for white flowers had not been lost it was merely masked for one generation Mendel found that all traits observed were inherited in the same fashion as flower color Interpretation of Mendel s Results Results interpreted based on three basic assumptions known together as Mendel s Law of Random Segregation 1 Each trait controlled by pair of factors aka genes Genes that code for alternative forms of the Inheritance is determined by chance it relies on the mathematical laws of same trait ex red vs white flowers are known as alleles 2 In each alternative pair of alleles for a given trait one is dominant and the other is recessive 4 3 In the formation of sex cells gametes each member allele of a gene pair segregates or separates from the other allele in a random fashion random segregation Each gamete receives only one of the two alleles An organism is homozygous for a trait when it carries two identical alleles for that trait ex RR or rr Heterozygous two different alleles for a given trait ex Rr Gen Ph otype genetic makeup enotype physical appearance Phenotype of red flowers the dominant trait R in pea plants can indicate 2 possible genotypes Rr OR RR Phenotype of white flowers the recessive trait r in pea plants indicates only ONE possible genotype rr Understanding Mendel s Results Pure bred red flower RR pure bred white flower rr Both homozygous When RR plant makes gametes each receives only one of the two alleles for each gene pair Thus all gametes formed by pure bred self fertilized RR plant are genetically identical to the parent The same principle is true for the pure bred rr plant When pure bred RR and pure bred rr are crossed their gametes combine to form the next generation F1 Since each parents only forms one type of gamete there is only one possible outcome for this cross Rr F1 hybrids are heterozygous for the trait flower color F1 hybrid genotype Rr F1 hybrid phenotype red flowers The F2 Generation F2 hybrids are self fertilized 4 different allele combinations possible RR Rr rR and rr Fema R le r side Mal R e side r R r R r F1 hyb rid 5 Ga met es RrRrRrRr Because rR Rr only 3 genotypes actually result The Punnett square Predicts the frequencies with which each genotype will occur Punnett squares show the 4 possible outcome combinations of alleles for a given trait based on the parents genotypes Though there are 3 genotypes present there are only 2 phenotypes for the F2 hybrid offspring R R R rR rR r R r r r Phenotype red 75 25 RR 50 Rr Phenotype white 25 Dominant and Recessive Genes in Humans Albinism abnormality in which brown pigment in skin melanin is not produced Albinos carry 2 copies of the recessive allele homozygous aa Individuals heterozygous for albinism Aa have normal skin pigmentation but they are carriers of the trait they have a 50 50 chance of passing on the recessive allele on to each of their children Offspring of two albino parents are always also albino Other common human diseases similarly due to recessive genes include hemophilia sickle cell disease muscular dystrophy and cystic fibrosis CF Sickle cell anemia red blood cells become sickle shaped when they are deprived of oxygen People with sickle cell have an altered for of hemoglobin Probability in Genetics Each organism has two alleles for each trait therefore there is a 50 50 chance that any particular allele will be passed on through sexual reproduction 3 Laws of Probability 1 Principle of Independence In a random process the outcome of each event is independent of all other events 2 Product Rule The probability that two or more independent events will both or all occur is the product of their independent probabilities 6 3 The Sum Rule The probability that two or more independent events will occur is the sum of their individual probabilities Section II Chromosomes and the Cellular Bases of Inheritance Cells Chromosomes and Cell Division All living organisms are made up of cells nucleus round central body inside a cell which contains genetic information DNA The genes in the nucleus control the cell s activities via metabolism Development of cellular specialization is also controlled by genes in the nucleus cytoplasm gelatinous substance surrounding the nucleus containing the
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