Presentation on theme: "As you all know genes reside on chromosomes"— Presentation transcript:
1As you all know genes reside on chromosomes As you all know genes reside on chromosomes. This basic fact is called the chromosome theory of inheritance. However earlier in this century, the issue of where the units of heredity resided was fiercely debated.The notion the genes were located on the chromosomes came from the recognition that the behavior of Mendel's particles during meiosis parallels the behavior of chromosomes at meiosis.Mendel’s Laws of independent assortment imply that genes on the same chromosome are inherited together and genes on different chromosomes are inherited independently.
2Morgan discovered an exceptional white-eyed male. Sex-linkedAlthough these were convincing correlations, actual proof of the chromosome theory required the discovery of sex linkage.Remember, Mendel had found that reciprocal crosses produce equal results with respect to the progeny. In general geneticists confirmed his results.However exceptions did arise. The most famous exception was that discovered by Tomas Hunt Morgan in the fruit fly Drosophila melanogaster. Drosophila eyes are normally bright red.Morgan discovered an exceptional white-eyed male.He performed the following crosses:
4Somehow eye color was linked to sex X and Y chromosomesSomehow eye color was linked to sex The key to understanding this pattern of inheritance arose from work demonstrating that males and females of a given species often differ in the chromosome constitution.For example, they found that male and female Drosophila both have four chromosome pairs. However in males one of the pairs the members differed in size:Female Drosophila:Male Drosophila:
5If the gene for eye color resides on a X chromosome Sex chromosomesMorgan realized that difference in chromosome constitution was the basis of sex determination in Drosophila:Females produce only X-bearing gametes, while males produce X and Y-bearing gametes.2: 2If the gene for eye color resides on a X chromosomeThere is no counterpart for this gene on the Y chromosome
6F1 Females have 2 copies of the eye color gene males have one copy Formal explanationFemales have 2 copies of the eye color genemales have one copyW (red) is dominant over w (white)RedXWXWwhiteXwYF1
7Females have 2 copies of the eye color gene and males have one copy Formal explanationFemales have 2 copies of the eye color gene and males have one copyW (red) is dominant over w (white)RedXWXwXWYF2
8In the F1 all the females are red and all the males are white Formal explanationThe reciprocal crossWhiteXwXwRedXWYF1In the F1 all the females are red and all the males are white
10This explanation also accounts for the fact that equal numbers of male and female progeny are produced.With this explanation of sex determination, Morgan realized that hecould explain the inheritance patterns of eye color by assuming:The gene determining eye color resides on the X chromosome(red and white eyes represent normal and mutant alleles of this gene)2. There is no counterpart for this gene on the Y chromosomeThus females carry two copies of the gene, while males carryonly a single copy.
11Red-green color blindness Red-green color blindness means that a person cannot distinguish shades of red and green. Males are affected 16 times more often than females, because the gene is located on the X chromosome.In color-blind men, the green or red cones worked improperly.The genes for the red and green receptors were altered in these menX-linked red-color blindness is a recessive trait. Females heterozygous for this trait have normal vision. The color perception defect manifests itself in females only when it is inherited from both parents.By contrast, males inherit their single X-chromosome from their mothers and become red green color blind if this X-chromosome has the color perception defect.The dominant (normal) X chromosome is represented as XCB.The recessive (mutant) chromosome is represented as Xcb.Since males have only one X-chromosome, if this chromosome has the red-green color blind allele, the males will have the color perception defect.Females have 2 X-chromosomes. Both X-chromosomes must carry the mutant allele for the females to be color blind. Red-green color blind females are homozygous for the recessive allele.Females with one mutant allele and one normal allele are heterozygous "carriers". They are not color blind, but they can pass the color blindness to their children.
12Bridges a student of Morgan set up the cross outlined above in Sex determinationBridges a student of Morgan set up the cross outlined above inlarge numbersP cross:white females x red malesXwXw x XWYAs expected, he obtained red-eyed females (XwXW) and white-eyed males (XwY)
13These were called primary exceptional progeny About 1 in every 2000 progeny he obtained a white-eyed fertile female or a red-eyed sterile male.These were called primary exceptional progenyHow can these exceptional progeny be explained?Bridges suggested that occasionally during meiosis the X chromosomes fail to separate during meiotic division.ｷ Normal separation of the X chromosomesproduces Xw gametesｷ Failure of X chromosome separation(non disjunction) XwXw and nullo gametesautosomeautosomeXXdisjunctionNon-disjunction
15Bridges assumed that XXX and Y0 progeny die and the only two viable progeny types were XXY and X0 In this model sex is determined by the number of X chromosomes rather than the presence or absence of the Y chromosomeThis model makes a strong prediction that is genes reside on chromosome the exceptional red-eyed males should be X0 and the exceptional white eyed females should be XXY.THAT IS WHAT BRIDGES FOUND(something to think about: What classes of progeny would be expected from the cross XwXwY x XWY ?)
16Sex chromosomes and sex In Drosophila, it is the number of X's that determine sex.In mammals it is the presence or absence of a Y chromosome that determines sex.SpeciesXXXYXXYXODrosophilafemalemaleHumans
17Bridges and Triploids White XwXwY red XWY Y XwXw Xw XwY XW XWY YY Red maleYYlethalXW XwXwXW XwRed femaleXW XwYY XwXwwhite femaleY XwWhite maleY XwY
18TriploidySpecies that are triploid, reproduce asexually (plant species)What are the consequences of triploidy during mitosis and meiosis?DiploidHaploidTriploidMitosis
19TriploidySpecies that are triploid, reproduce asexually (plant species)What are the consequences of triploidy during mitosis and meiosis?DiploidHaploidTriploidMitosis
20Meiosis and triploidsMeiosisIThis is for one chromosome. If there are n chromosomes in an organism, then balanced gametes (equal copies of all chromosomes) are very rare.
21Meiosis and triploidsMeiosisI4N3N2NTriploids produce unbalanced gametesThis is for one chromosome. If there are n chromosomes in an organism, then balanced gametes (equal copies of all chromosomes) is very rare.
22Seedless watermelonsTriploidy is useful in agriculture.Biological control:Cross a tetraploid watermelon with a diploid watermelon
23And triploid toadsTriploid toadsNature Genetics 30, (2002)Tetraploid green toads reproduce through diploid eggs and sperm cells.A new taxon was discovered at an isolated site in the Karakoram mountain range.Every wild toad caught from eight localities was triploidDid not find a single diploid or tetraploid Batura toad. Both males and females were found3N female 3N maleNelimination Nelimination2N 2N2N N N3N
24Sex in organismsSex chromosomes and sex linkage:In Drosophila, it is the number of X's that determine sex while in mammals it is the presence or absence of a Y chromosome that determines sex.Homogametic sex- Producing gametes that contain one type of chromosome (females in mammals and insects, males in birds and reptiles)Heterogametic sex- Producing gametes that contain two types of chromosomes (males in mammals and insects, females in birds and reptiles)SpeciesXXXYXXYXODrosophilafemalemaleMale (sterile)HumansBridges could tell genotype by where the sex chromosome went and therefore established that chromosomes carried genes
25Non-sex chromosomes are called autosomes Humans have 22 autosomes, Drosophila has 3Homogametic sex- XX- females in humansmales in birdsHeterogametic sex- XY- males in humansHemizygous gene present in one copy in a diploid organismHuman males are hemizygous for genes on the X-chromosome
26Sex linkedDescribed below is an actual case history of the use of pedigree analysis:After the death of a wealthy individual (II:3), a man claiming tobe his son (III:3) filed a paternity suit and claimed the inheritance.The deceased had only two living nephews (III:1 and III:2 whowere sons of his brothers (II:1 and II:2). In determining whether the man was actually the son and had the rights to the inheritance which of the following markers would be most usefulAutosomal X-linked Y-linked$$$
27Surname projectYYYYAll males in this pedigree will have the SAME Y-chromosome!!!
28Mendelian genetics in Humans: Autosomal and Sex- linked patterns of inheritance Obviously examining inheritance patterns in humans is much more difficult than in Drosophila because defined crosses cannot be constructed. In addition humans produce at most a few offspring rather than the hundreds produced in experimental genetic organisms such as DrosophilaIt is important to study mendellian inheritance in humans because of the practical relevance and availability of sophisticated phenotypic analyses.Therefore the basic methods of human genetics are observational rather than experimental and require the analysis of matings that have already taken place rather than the design and execution of crosses to directly test a hypothesisTo understand inheritance patterns in human genetics you often follow a trait for several generations to infer its mode of inheritance. For this purpose the geneticist constructs family trees or pedigrees. Pedigrees trace the inheritance pattern of a particular trait through many generations. Pedigrees enable geneticists to determine whether a familial trait is genetically determined and its mode of inheritance (dominant/recessive, autosomal/sex-linked)
29Termination of pregnancy Pedigree symbols:MaleFemaleSex Unknown5Affected individualDeceasedNumber of individualsSpontaneous abortionTermination of pregnancy
30Pedigree symbols:relationship lineSibship lineline of descentindividual’s linesconsanguinityMonozygoticDizygotic
31Characteristics of an autosomal recesssive trait: Patterns in pedigrees of Autosomal recessive traits:There are several features in a pedigree that suggest a recessive pattern of inheritance:Rare traits, the pedigree usually involves mating between two unaffected heterozygotes with the production of one or more homozygous offspring.2. The probability of an affected child from a mating of two heterozygotes is 25%3. Two affected individuals usually produce offspring all of whom are affected4. Males and females are at equal risk, since the trait is autosomal5. In pedigrees involving rare traits, consanguinity is often involved.In the pedigree shown below, an autosomal recessive inheritance pattern is observed:
32Characteristics of an autosomal dominant trait: 1. Every affected individual should have at least one affected parent.2. An affected individual has a 50% chance of transmitting the trait3. Males and females should be affected with equal frequency4. Two affected individuals may have unaffected children
33The following pedigree outlines the typical inheritance pattern found in red-green color-blindness. Does this fit an autosomal recessive or autosomal dominantpattern of inheritance?
34Characteristics of a sex- linked trait: Hemizygous males and homozygous females are affectedPhenotypic expression is much more common in males than in females, and in the case of rare alleles, males are almost exclusively affectedAffected males transmit the gene to all daughters but not to any sonsDaughters of affected males will usually be heterozygous and therefore unaffected.Sons of heterozygous females have a 50% chance of receiving the recessive gene.