Presentation on theme: "( Modification of Mendelian ratios )"— Presentation transcript:
1 ( Modification of Mendelian ratios ) General GeneticBio 221 Lab 8( Modification of Mendelian ratios )
2 Modification of Mendelian ratios Most traits are non-mendelian , meaning they do not follow the rules of mendelian traits . Almost all traits in humans are non-mendelian. The trait is determined by :1- more than one gene .2- or the environmental effects of the trait .Allelic or gene interactions1- The gene in an organism can interact in many different ways .2- The effect of dominant allele is diluted or modified so that the phenotypic expression of the concerned trait in a hybrid (Heterozygous ) is distinguishable from both parental types ( Homozygous )
3 Type of non-mendelian1- in complete ( partial , semi , intermediate ) dominance :A type of inheritance in which one allele for a specific trait is not completely dominant over the other allele. This results in a combined phenotype (expressed physical trait). and character is intermediate between the parents* Phenotype and genotype ratios in F2 generation are 1:2:1 .* EX : Red shaded chrysanthemum breed with white shaded chrysanthemum , the resulting is pink . The mixture of colors shows incomplete dominance between colors .
4 2- Co - dominance ( Mosaic dominance ) : * Both allelic genes of a genetic trait are equally expressive , there is complete lack of dominance . When the heterozygote organism shows the characteristics of two dominant alleles .* Phenotype and genotype ratios in F2 generation are 1:2:1 .* EX:1- White cat and black cat generate offspring that the dominant alleles share dominance .2- Red and white coat colors in short-horn cattle expressions ( Phenotype) , and they are homozygous dominant and homozygous recessive genotypes .The hybrids exhibit a roan coat color is intermediate
5 Incomplete VS Codominance With incomplete dominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype that is a blending of the parental traits. With codominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype in which both of the parental traits appear together.
6 * EX: ( codominace)3- The AB blood group is due to co-dominance . AB group is controlled by the genes A and B . The A and B are equally dominant . A produces antigen A and B produces antigen B .
7 Incomplete dominance example 4- Sickle cell anemia is recessive disease . In heterozygous which have abnormal red blood cells under some conditions .
8 Multiple alleles-Which a population has more than two alleles in it .-Multiple alleles increases the number of different phenotypes.-Multiple alleles can be dominant, recessive or co-dominant to each other.EX: ABO blood groups in humans , in which there are 3 possible alleles A,B or IA and IB ( co dominant ) , and i. type O blood is recessive .
9 Polygenic inheritance More than one gene affecting a trait . Most traits in humans are polygenic , such as weight , height . IQ , personality ,( skin color is environmental determined and determined by 5 genes , each with 2 alleles giving 9 phenotypes . )Modifier genesA gene that alters the phenotypic expression of another gene.
10 Mendelian Ratios and Lethal genes In 1905 , Lucien Cuenot observed unusual patterns when studying inheritance of a coat color gene in mice .After mating 2 yellow mice , observed that the offspring never showed a normal 3:1 phenotype but observed 2:1 , with2 yellow mice and one non-yellow mice.This is lethal genes cause the death of the organisms that carry them .Sometimes , death is not immediate it may take yeas depending on the gene .Kind of Lethal Genes :1- Recessive Lethal genes . The ratio 3:0 . EX : Cystic fibrosis and Sickle cell anemia2- Dominant lethal genes . The ratio 2:1 . EX: Huntington disease3- Semi or Sub Lethal genes . EX : Hemophilia4- Synthetic Lethal genes.5- Conditional lethal genes. EX: favism allele
11 Kind of Lethal Genes1- Recessive Lethal genes .That occur in dominant or recessive traits but they do not cause death unless an organism carries 2 copies of lethal allel . The ratio 3:0 .EX : Sickle cell anemia .2- Dominant lethal genes .Are expressed in both homozygotes and heterozygotes . The ratio 2:1EX: Huntington disease , a neurological disorder in humans ( chromosome 4 ) .3- Semi or Sub Lethal genes .The allele responsible for hemophilia is carried on the X chromosome , affected mainly in males , and they inherit the allele from their mothers .Hemophilia = is affected in individual bleed as longer period of time until clotting occurs , this means that normally minor wounds can be fatal in a person .4- Synthetic Lethal genes .Some mutations are only lethal when paired with second mutation
12 5- Conditional lethal genes EX: favism allele that common among people of Mediterranean , African and Southeast Asian . The disease was named because when affected individuals eat fava beans , they develop hemolytic anemia , a condition in which red blood cells break apart and block blood vessels . But that disease is resistant to malaria because it is difficult for malaria to multiply in cells with deficient a mount of glucose 6 phosphate dehydrogenase .
13 EpistasisThe effects of one gene are modified by one or several other genes .Hypo-static : is one whose phenotype is masked by the expression of an allele at a separate locus, in an epistasis eventEpi-static : The gene whose phenotype is expressed.Epistasis can be contrasted with dominance which is an interaction between alleles at the same gene locus . The interaction between two or more genes to control a single phenotype and identify and recognize the 9:3:3:1 that results of crossing 2 dihybrids produced a modified mendelian .Kind of Epistasis :1- Dominant Epistasis : (12:3:1 ) from ( 9: 3 :3 :1 ) EX: Fruit color in squash2- Dominant duplicate Epistasis : (15:1 ) from ( 9: 3:3 :1 ) EX: Kernel Color in wheat3- Recessive Epistasis: (9:4:3 ) from ( 9: 3: 3 :1 ) EX: mice coat color4- Recessive duplicate Epistasis : (9:7) from ( 9: 3:3 :1 ) EX: Flower Color in sweet pea
14 Kind of Epistasis : 1- Dominant Epistasis . * Definition :Complete dominance at both gene pairs ; when one gene is dominant , it hides the effects of other gene . And the ratio is 12:3: 1* EX: Fruit color in squash .GenotypePhenotypeEnzymatic Activites9 W_G_WhiteDominant white allele negates effect of G allele123 W_gg3 wwG_yellowRecessive color allel allows yellow allele expression31 wwggGreenRecessive color allel allows green allele expression1
15 1- Dominant Epistasis : (12:3:1 ) from ( 9: 3 :3 :1 ) EX: Fruit color in squash when one gene is dominantgene 1= ( white dominant (WW) , colored (ww) ) .gene 2= ( yellow is dominant (YY) , green ( yy ) ) .The effect of dominant gene ’Y’ is masked by the dominant gene ’W’ (epistatic gene)P WWYY X wwyy(white) ↓ (green)F WwYy(white) (selfed)F2 White:Yellow:Gree n12 : 3 : 1♂/♀WYWywYwyWWYYWWYyWwYYWwYyWWyyWwyywwYYwwYywwyy
16 2- Dominant duplicate Epistasis . * Definition :Complete dominance at both gene pairs ; when either gene is dominant , it hides the effects of other gene . And the ratio is 15 : 1* EX: Kernel Color in wheat .GenotypePhenotypeEnzymatic Activites9 A_B_Colored KernelsFunctional enzymes from both genes153 A_bbFunctional enzymes from A genes3 aaB_Functional enzymes from B genes1 aabbWhite KernelsNon Functional enzymes from both genes1
17 when either gene is dominant Gene A = Enzyme AA Gene B = Enzyme BB Dominant duplicate Epistasis : (15:1 ) from ( 9: 3:3 :1 ) EX: Kernel Color in wheatwhen either gene is dominantGene A = Enzyme AAGene B = Enzyme BBAABBaabbAaBb9الجينين يعملانAAbb3يعمل AaaBBيعمل B1لايعمل15
18 Black mice ( BBCC) X Albino (bbcc) mice F1 BbCc( black) 3- Recessive Epistasis .*Definition: Complete dominance at both gene pairs ; when one gene is homozygous recessive , it hides the effects of other gene . And the ratio is 9:4:3* EX: mice coat color .in some casses recessive allele c masks the effect of dominant allele (B) .When black mouse crossed to albino mouse , only black progeny were produced , but in F2 , 9 black , 3 cream and 4 albino (9:3:4) were produced .Coat color in mouse is controlled by 2 dominant genes B and CWhen B is dominant , mouse color will be black .When both recessive genes b and c are homozygous recessive , albino mice are produced .In this case when c is homozygous it masked the effect of the B gene.Black mice ( BBCC) X Albino (bbcc) miceF BbCc( black)
19 Recessive Epistasis: (9:4:3 ) from ( 9: 3: 3 :1 ) EX: mice coat colorwhen one gene is homozygous recessive
20 4- Recessive duplicate Epistasis . * Definition : Complete dominance at both gene pairs ; when either gene is homozygous recessive , it hides the effects of other gene . And the ratio is 9: 7* EX: Flower Color in sweet pea .. The following table explanation for the ratio 9:7 because either the genes can provide the wild type phenotype .GenotypePhenotypeEnzymatic Activites9 C_P_Colored : anthocyanin producedFunctional enzymes from both genes93 C_ppFlowes white : no anthocyanin producedp enzyme non-functional73 ccP_c enzyme non-functional1 ccppC and p enzymes non-functional
21 ; when either gene is homozygous recessive Gene1 =C Gene2=P Recessive duplicate Epistasis : (9:7) from ( 9: 3:3 :1 ) EX: Flower Color in sweet pea; when either gene is homozygous recessiveGene1 =CGene2=PCCPPColoredccppwhiteCcPpعمل كلا الجينين9CCppلايعمل انزيم P3ccPPلايعمل انزيم Cلايعمل17White