Presentation on theme: "Genetics Explain the basic rules and processes associated with the transmission of genetic characteristics."— Presentation transcript:
1GeneticsExplain the basic rules and processes associated with the transmission of genetic characteristics
2Describe the evidence for dominance, segregation and the independent assortment of genes on different chromosomes, as investigated by Mendel
3Introduction Heredity Genetics Gregor Mendel is the passing of traits from parents to offspringGeneticsis the study of the patterns of inheritance as hereditary characteristics or traitsGregor Mendelthe "father of modern genetics“an Austrian monkpublished his completely new and thoroughly documented ideas of inheritance in 1866
4IntroductionHis model was so simple that scientists who read it at that time considered it "trivial“it received little attention and no recognition until it was rediscovered in 1900 after his death (simultaneously by 3 different people)In the meantime,chromosomes had been namedtheir movements during mitosis and meiosis observed and described
5Introduction1902scientists realized that chromosomes moved precisely as reported by MendelOnce the connection between chromosomes and heredity was established, the science of genetics was rebornMendel was the first person to realize that genetic traits are inherited as separate particles
6Introduction He did not actually see these particles but he predicted their existence based on patterns of inheritanceHe proposed that organisms have a pair of "factors" for each traitone from each parentWe NOW know that the particles of inheritance are segments of DNAwhich we call genes
7Mendel's Experiments Mendel worked with garden peas available in many different varietiesE.g. pure breeding tall, pure breeding dwarfPea flowers contain both male and female parts and normally self-pollinatebut can easily be artificially cross-pollinated with other pea plantsMendel crossed plants of two varieties with contrasting traits (such as tall and dwarf) to see what would happen.
8Mendel's ExperimentsMendel worked with seven traits, each which occurred in two distinct formsMendel began by studying crosses involving only one trait at a timeExample: Flower colorP1: (parental generation)pure-breeding red flowered plants X pure-breeding white flowered plantsF1: (first filial generation)red-flowered hybrids (genetically mixed offspring)F2: (second filial generation)3/4 red-flowered and 1/4 white-flowered
9Mendel's LawsInherited characteristics are controlled by pairs of factorsgenesone from each parent.One gene may “mask” the effect of another.The gene which is expressed is dominant, while the one which is masked is recessive.Pairs of genes segregate during gamete formationso each sex cell contains only one member of a pair of genes.
10Terms Alleles Homozygous Heterozygous Two or more alternate forms of a gene, which produce contrasting effects for a certain traite.g. red (R) and white (r) for flower colour of peasHomozygoushaving two of the same alleleeg. red/red (RR)purebredHeterozygoushaving two different alleles eg. red/white (Rr)
11Terms Genotype Phenotype Purebred Hybrid the genetic makeup of an individualPhenotypethe expression of the genes, or appearance of an individualPurebredan organism having all homozygous gene pairsHybridan organism having at least one heterozygous gene pair
12Terms Monohybrid Dihybrid an organism having only one heterozygous gene pairDihybridan organism having two heterozygous gene pairs
13Monohybrid Cross Purebred Red Purebred White Phenotypes x Rr Rr Rr Rr GenotypesRrRrRrRrF1 Genotypes
14Monohybrid Punnet Square Genotype of Parent #2MaleFemaleRRGametes !!rRrRrGenotypeofParent #1rRrRr
15Monohybrid Cross Purebred Red Purebred White Phenotypes x Rr Rr Rr Rr GenotypesRrRrRrRrF1 GenotypesRrxRrF1 Self PollenateRRRrrRrrF2 Genotypes
16Monohybrid Cross Purebred Red Purebred White Phenotypes x Rr Rr Rr Rr GenotypesRrRrRrRrF1 GenotypesRrxRrF1 Self PollenateRRRrRrrrF2 Genotypes
17The phenotypic ratio for a monohybrid cross is always 3:1 dominant trait : recessive trait
18Predicting the Outcome of a Genetic Cross When we know the genotypes of parents used in a genetic cross, we can predict the genotypes of the offspring and their expected ratiosMust use a Punnett square is usednamed after some guy named PunnettA geneticistBasically it’s a just a fancy chart
19Monohybrid Punnet Square Genotype of Parent #2MaleFemaleRRGametes !!rRrRrGenotypeofParent #1rRrRr
21If an organism has the dominant phenotype, how can you determine whether it is homozygous or heterozygous?
22Test Cross You conduct a test cross. For example: Mate it with an organism of a known genotype and see what you get.The only known genotype that is observable is HOMOZYGOUS RECESSIVE (rr)For example:determine whether a red-flowered pea plant is homozygous or heterozygous
23Test Cross P: Red flowered X White-flowered R? rr F1: Suppose the cross produces all red flowers Rr Rr Rr RrIf no offspring showing the recessive phenotype are produced, the unknown parent must be …homozygous
24Purebred Red Purebred White Phenotypes x Rr Rr Rr Rr RR rr Genotypes F1 Genotypes
25Test Cross P: Red flowered X White-flowered R? rr F1: Suppose the cross produces 50% red flowers and 50% white flowersThe only way a white flower could appear is if it received a recessive allele from the unknown parent.The unknown parent must be …HETEROZYGOUS
27Dihybrid CrossIn addition to his monohybrid crosses, Mendel performed dihybrid crosses of plants with two different pairs of contrasting allelesIn one experiment, Mendel crossed plants homozygous for seeds that were both round and yellow with plants homozygous for wrinkled, green seeds
28Dihybrid Cross All the F1 offspring had round, yellow seeds Self-fertilization of the F1 plants produced and F2 generation of seeds with the following phenotypes:315 round yellow108 round green101 wrinkled yellow32 wrinkled green
29Dihybrid Cross To find the ratio among the F2 phenotypes, we take the number of offspring in the smallest category and divide it into the number of offspring in the other categories:then the quotient is rounded to the nearest whole number
30Dihybrid CrossThus Mendel determined the phenotypic ratio in the F2 generation to be 9:3:3:1This is the ratio typical of a dihybrid cross in which both pairs of alleles show a dominant-recessive relationship
31Dihybrid CrossMendel explained these data by assuming that the genes governing seed color and seed shape move independently during gamete formationIn the process of independent assortment, each pair of alleles behaves as it would in a monohybrid cross - independently of the other pair A dihybrid can produce four possible gene combinations (with equal probability)If the alleles are:R = round r = wrinkledY = yellow y = greenThe possible gamete combinations areRY Ry rY ry
32Dihybrid Cross RRYY x rryy RY ry RrYy RY, Ry, rY, ry Parent: P Gametes:F1:F1 Gametes:F2:RRYY x rryy RY ryRrYy RY, Ry, rY, ry9/16 round, yellow (R_Y_)3/16 round, green (R_yy)3/16 wrinkled, yellow (rrY_)1/16 wrinkled, green (rryy)
37Compare ratios and probabilities of genotypes and phenotypes for dominant and recessive, multiple, incomplete dominant, and codominant allelesExplain the relationship between variability and the number of genes controlling a trait
38Multiple AllelesThe genes which we have studied so far have only two different alleles:Many genes actually exist in more than two allelic forms,Although only two genes control coat colour in rabbits it is controlled by a series of four alleles for the same gene:C Full colourCch Chinchillach Himalayanc Albino
39Multiple AllelesThe dominance hierarchy of these alleles is C > Cch > ch > cDetermine the genotypes for the following phenotypes:Phenotype Possible GenotypeFull colorChinchillaHimalayanAlbinoC + any of the 4Cch+ anything but Cch ch, ch cc c
40Incomplete DominanceIn some cases, a heterozygous organism shows a blending of genes because neither gene is dominant:this is termed incomplete dominancefor example, in snapdragons, neither the red nor the white allele is dominant…
41Red Flowers x White Flowers Incomplete DominanceParent:F1:F2:Red Flowers x White FlowersPink Flowers¼ Red flowers½ Pink flowers¼ White flowersRR x rrRr, Rr, Rr, RrRRRrrr
42CodominanceIf two different alleles each contribute to a phenotype, they are termed codominantOne of the best-known example of codominant genes occurs in humans, and determine ABO blood typesThere are three possible alleles:IAIBi
43Codominance Blood Type PhenotypePossible GenotypesAIA IA, IA iBIB IB, IB iABIA IBOi i
49Chromosome MappingExplain the influence of gene linkage and crossing over on variability
50Chromosomal Theory of Inheritance Chromosome MappingChromosomal Theory of InheritanceGenes are located on chromosomesChromosomes undergo segregation during meiosisChromosomes assort independently during meiosisEach chromosome contains many different genes
52Chromosome MappingEach chromosome contains hundreds or thousands of genesGenes located on the same chromosome are inherited together,they are part of a single chromosome that is passed along as a unitsuch genes are said to be linked
53Chromosome Mappingduring meiosis, chromosomes may exchange segments of DNA by crossing-overA A a aB B b bC C c cD D d dE E e eF F f f
54Chromosome Mappingduring meiosis, chromosomes may exchange segments of DNA by crossing-overA A a a A a A aB B b b B B b bC C c c C C c cD D d d D D d dE E e e E e E eF F f f F f F f
56Chromosome MappingThe closer two genes are on a chromosome, the fewer the possible points of crossover are between them, and the less frequently such a cross-over will occurIn other words, if two genes are close together on a chromosome it is likely they will stay together and not be exchanged between chromatids during meiosisTo determine the location of genes along a chromosome is called MAPPING a chromosome
57Chromosome Mapping A chromosome map indicates The order in which specific genes occur on a chromosomeThe distances between the genes
58Chromosome Mapping Example: In Drosophila, the following data was obtained from genetic crosses: 13% recombination between bar eye and garnet eyeHigh percentage recombination indicates that these two genes are far apart from each otherHigh likelihood that crossing over will occur between these two genes.7% recombination between garnet eye and scalloped wingsThese two genes are closer together than bar eye and garnet eye6% recombination between scalloped wings and bar eye
59Chromosome Mapping This data can be used to map the chromosome: Bar eye Scalloped wings Garnet eye6 units units13 units
61Sex DeterminationThere are two chromosomes involved in the determination of sex of most animals,the sex chromosomes (X & Y)Any other chromosome not involved in sex determination is called an autosomefor example, humans have 22 pairs of autosomes and 1 pair of sex chromosomesin most mammals, females are homozygous and have 2 X chromosomes, while males are heterozygous and carry an X and a Y chromosome
62Sex Determination Female = XX Male = XY Females can produce eggs carrying only an X chromosome,Males produce sperm carrying either an X or a Y chromosome (50% of each)Thus, it is the male who determines the sex of his offspring !
63Sex LinkageCompare the pattern of inheritance produced by genes on the sex chromosomes to that produced by genes on autosomes, as investigated by Morgan and others.
64every white-eyed fly was male! A True StoryOne day a geneticist named Thomas Hunt Morgan discovered a mutant white-eyed male fly among his hundreds of red-eyed flies. He bred the white-eyed male with several red-eyed females and observed the offspring: as expected, all the F1 flies had red eyes, showing red to be the dominant allele. He allowed the F1 generation to interbreed freely and observed the F2 generation. Again, as expected, the ratio of red-eyed flies to white-eyed flies was 3:1, except that …every white-eyed fly was male!
65Explanation … X-chromosome Morgan concluded that the gene controlling eye-colour was carried on the…X-chromosome
66Sex LinkageLike other chromosomes, the sex chromosomes carry many genesSome of the regions of the X-chromosome have a homologous region on the Y- chromosomeThere are also large non-homologous portions:That is, the X chromosome carries some genes that have no counterparts on the Y chromosome
67Sex LinkageGenes for sex-linked traits are carried on the X chromosome but not on the Y chromosomeTherefore in a male, the gene on the X chromosome is expressed whether it is dominant or recessiveIn a female, she must have two recessive alleles to have the recessive phenotype
68Fly Solution R = Red r = white XR XR = Red eyed Female Xr Xr = White eyed FemaleXR Y = Red eyed MaleXr Y = White eyed Male
69Fly Solution P White Eyed Male F1 F2 XR XR x Xr Y XR Xr x XR Y XR XR
70Example of hemophilia trait – (h) recessive disease
71Sex Linkage Some sex-linked traits in humans are Colour vision HemophiliaDuchenne's Muscular Dystrophy
73Sex Influenced GenesThe main role of sex hormones is to influence the reproductive system and its related organs,These hormones, however, also affect many other parts of the bodyGenes that are expressed to a greater or lesser degree as a result of the level of sex hormones are called sex-influenced genes.
74Sex Influenced Genes These genes are usually located on the autosomes Males and females with the same genotype may differ greatly in phenotype because the levels of sex hormonesFor example:A bull may have a gene for high milk production, but he will not produce milk because he has low levels of female hormones.
75Sex Influenced GenesIn humans, the gene for male pattern baldness is autosomal and sex-influencedA man will become bald even if he has only one allele for baldness, becausethe male sex hormones somehow stimulate the expression of the alleleIn a woman, however, the allele acts as a recessive allele so that she must have two balding genes before she loses her hair
77Lethal AllelesIf An organism has a mutation that destroys the genetic code for a protein essential to life, the organism will often die prematurely.This gene that fails to code for a functional protein is called a lethal alleleIt is possible for lethal alleles to be dominant, but most are rapidly eliminated from a population because they cause death before the individual carrying the allele reproduces.
78Lethal AllelesAn exception of a lethal dominant allele that remains in a population is the one responsible for Huntington's Disease in humans, because this allele is not expressed until later in life ( years of age)An example of a recessive lethal allele in humans is the one for Brachydactyly:Heterozygotes have a short middle finger bone that makes the fingers appear to have only two bones instead of threeHomozygous babies lack fingers and have abnormal development of the skeleton that result in death in infancy
84PedigreesBecause geneticists are unable to manipulate the mating patterns of people, they must analyze the results of matings that have already occurred.As much information as possible is collected about a family's history for a particular trait, and this information is assembled into a family tree describing the interrelationships of parents and children across the generationsThis is called a pedigree
86PedigreesFrom a pedigree you should be able to determine if a particular trait is dominant or recessive, sex-linked or autosomalA pedigree not only helps us understand the past but also helps us predict the futureGeneticists, physicians, and genetic counselors use pedigreesfor analysis of genetic disordersto advise prospective parents of genetic risks involved Complete the pedigree studies on page 632 and page 611 as a class.