( Modification of Mendelian ratios ) General Genetic Bio 221 Lab 8 ( Modification of Mendelian ratios )

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 interactions 1- 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 )

Type of non-mendelian 1- 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 .

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

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. 

* 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 .

Incomplete dominance example 4- Sickle cell anemia is recessive disease . In heterozygous which have abnormal red blood cells under some conditions .

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 .

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 genes A gene that alters the phenotypic expression of another gene.

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 , with 2 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 anemia 2- Dominant lethal genes . The ratio 2:1 . EX: Huntington disease 3- Semi or Sub Lethal genes . EX : Hemophilia 4- Synthetic Lethal genes. 5- Conditional lethal genes. EX: favism allele

Kind of Lethal Genes 1- 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 2:1 EX: 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

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 .

Epistasis The 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 event Epi-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 squash 2- Dominant duplicate Epistasis : (15:1 ) from ( 9: 3:3 :1 ) EX: Kernel Color in wheat 3- Recessive Epistasis: (9:4:3 ) from ( 9: 3: 3 :1 ) EX: mice coat color 4- Recessive duplicate Epistasis : (9:7) from ( 9: 3:3 :1 ) EX: Flower Color in sweet pea

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 . Genotype Phenotype Enzymatic Activites 9 W_G_ White Dominant white allele negates effect of G allele 12 3 W_gg 3 wwG_ yellow Recessive color allel allows yellow allele expression 3 1 wwgg Green Recessive color allel allows green allele expression 1

1- Dominant Epistasis : (12:3:1 ) from ( 9: 3 :3 :1 ) EX: Fruit color in squash when one gene is dominant gene 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) F1 WwYy (white) (selfed) F2 White:Yellow:Gree n 12 : 3 : 1 ♂/♀ WY Wy wY wy WWYY WWYy WwYY WwYy WWyy Wwyy wwYY wwYy wwyy

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 . Genotype Phenotype Enzymatic Activites 9 A_B_ Colored Kernels Functional enzymes from both genes 15 3 A_bb Functional enzymes from A genes 3 aaB_ Functional enzymes from B genes 1 aabb White Kernels Non Functional enzymes from both genes 1

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 wheat when either gene is dominant Gene A = Enzyme AA Gene B = Enzyme BB AABB aabb AaBb 9 الجينين يعملان AAbb 3 يعمل A aaBB يعمل B 1 لايعمل 15

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 C When 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) mice F1 BbCc( black)

Recessive Epistasis: (9:4:3 ) from ( 9: 3: 3 :1 ) EX: mice coat color when one gene is homozygous recessive

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 . Genotype Phenotype Enzymatic Activites 9 C_P_ Colored : anthocyanin produced Functional enzymes from both genes 9 3 C_pp Flowes white : no anthocyanin produced p enzyme non-functional 7 3 ccP_ c enzyme non-functional 1 ccpp C and p enzymes non-functional

; 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 recessive Gene1 =C Gene2=P CCPP Colored ccpp white CcPp عمل كلا الجينين 9 CCpp لايعمل انزيم P 3 ccPP لايعمل انزيم C لايعمل 1 7 White