Principles of Genetics PDF

Principles of Genetics 21 O Recessiveepistasis Example: In common house mouse (Mus musculus) several varieties of coat colour exist. The standard or wild type is known as 'agouti' (theindividual hairs are most partbl...

Principles of Genetics 21 O Recessiveepistasis Example: In common house mouse (Mus musculus) several varieties of coat colour exist. The standard or wild type is known as 'agouti' (theindividual hairs are most partblack with a narrow yellow band near the tip). The commonest and most familiar variation ís 'albino'. Albinos always breed true. Another varíation in coat colour that arose as a result of the disappearance of yellow pigment in the agouti is "black colour. Black is recessive to agouti. When pure "black' mice are crossed with 'albino' the progeny are all 'agouti. When F,are interse mated agouti, black and albino appear in the ratio of 9:3:4 in the progeny. This result can be explained by the fact that coat colour in this particular cross is controlled by two pairs of alleles occurring in two loci. The result can be explained with the help of Punnett square or checker board. P Black X Albino Caa cc AA Gametes CCA CcAa all Agouti F Gametes A When mated interse, the following genotypesappear. F,„F, Gametes CA Ca CA CA CCAA CCAa CCAA CeAa Agouti Agouti Agouti Agouti Ca CCAa CCaa CCAa Ccaa Agouti Black Agouti Black cA CCAA CCAa cCAA ccAa Agouti Agouti Albino Albino ca CcAa Ccaa cCAa Ccaa Agouti Black Albino Albino C_A_: Agouti :9 C_ aa :Black :3 cc :Albino 4 20 Principles of Animal Genetics and Population Genetics 1. F, does not resemble both the parents and a new phenotype (walnut) appears in the which is due to interaction between F, two independently inherited dominant genes. These two new phenotypes were the result of gene product interaction. 2. In the F, another new phenotype (single comb) was not present in the parents and is due toappears which interaction of recessive alleles namely rrpp. These peculiarities are not due to a new method of inheritance but it is simply due to the circumstances that two pairs of alleles (Rr and Pp) control the samecharacter ie., comb typeor comb pattern in chicken. One of these new characters (walnut comb) evidently results from an interaction between two independently inherited dominant genes and the other (single comb)results from the interaction of their two recessive alleles. When two genes at different loci working together produce something diferernt from the efect either one alone,they aresatd to be complementary of genes. EPISTASIS (Greek -standing upon) Epistasis is the term describing relationship between genes in differernt loci. The genotype at one locus may influence the effect af-he genotype (non allelic) atanotherlocus. It may in fact prevent the expression of other locus. Whenever a geneat one locus ona chromosome masks or prevents the expression of a gene at another locus the first gene is said to be epistatic to the second and, the gene that is hidden is said to be 'hypostatic. In dominance,one allelesuppresses the expression of another allele at the same locus (intra allelic interaction). Dominance is the interaction between different alleles of the samegene (ie., within a locus). Epistasis is interaction between genes that are not alleles ie., genes located in different loci (interallelicinteraction). Epistasis is one important type of functional interaction between products of different genes. Therefore, the classical Mendelian ratio of 9:3:3:1 does not occur in allcases. It is found that most of the apparent exceptions could be explained by assuming that many characters are influenced by two or more genes. Depending on the typeof gene interaction thephenotypic ratios are modified in various ways.  Principles of Genetics 19 The ratiosindicate that two pairs of genes or factors are governíng the comb pattern of fowls ie.a singlecharacter is governedby 2 genes. The walnut comb dependson the presenceoffwo dominant genes symbolized by R and K(interaction of dominant alleles K and p P. produces walnut); One o thesc genes alone R produces rose conbthe other P produces the pea comb, The combination of therecessive algjes in homozygous condition produces single comb. (interaction of - recessive alleles produces single comb). These are illustrated in the following checker board. Rose comb X Pea comb RR pp rPP GaRtes rP Fi (Walnut) Rr Pp Figanetes RP) Rp rP FixFi Gametes RP Rp rP RP RRPP RRPp RrPP RrPp Walnut Walnut WalnutWalnut P.RPp RRpP RrPp Rrpp Walnut Rose Wajnut rP RrPP Rrlp rPp Walnut Pea Pea Walnut rp Rrlp Rrpp rrp rpp Walnut Rosc Pea Single Walnut : Rose: Pea: Single 9: 3: 3: 1 RP Walnut comb R PP Rose comb rr P Pea conb rr pP Single comb The mode of inheritanceof genes for rose and pea comb does not Lusually differ from Mendeliau pattern. The differences that distinguish this andsimilarcases from ordinary dihybrid inheritance are asfollows.  Population Genetics Principles of Animal Genetics and 18 and elimination of lethal genes Detection be carried genes from the population could Elimination of lethal them (heterozygotes) and preventing out by identifying the carriers from further breeding. all are much casier to detect because Intermediate lethalgenes expression of the gene. theíndividuals willexhibit somephenotypic either in homozygous or Dominant lethals kill the individual is eliminated from the heterozygous conditions and therefore in which itaríses. population in the same generation stage. They ¥Recessive Jethals kill only when in homozygous the populatíon. Hcterozygous carrier very difficult to climinate from identify could be used as testers to parents that produce a lethal effect others in the population. INTERACTION OF GENES /COMPLEMENTARY Interaction of Genes pattern in chicken (domesticfowl) Example: Comb Rose comb Wyandotte (American) Pea comb Cornish (English) Single comb Leghorn (Mediterrenean)- - Walnut comb Malay (Asiatic) 'Rose' comb is dominant over 'single comb and similarly pca' combis dominant over 'single' comb. When the following crosscs are performed, Pea x Single Parents Rose Single Rose Pea Kose: Single Pea: Single 3:1 3:1 Rose Pea comb Walnut comb Walnut Walnut Walnut :Rose: Pea : Single 9:3 J:!  17 Principles of Genetics An explanation for the altered ratio can be found if we consider all the eggs which were incubated. Onefourth of these failed to hatch Hence it is which had dead embryos (at about 4h day of incubation). expression evident that creeper condition in chicken isa heterozygous of a gene which is lethal when homozygous. Example 3:Achondroplasia in cattle. Englanc and Dexter Dexter and Kerry ar< beef brceds)of fattlcof the Kerry. But it is (has short limbs) is a better beef producer than Dexters because these impossible to establish a pure breeding herd of in association with desirable characteristics of the breed result Dextersare crossed heterozygousexpression of lethalgene. When two the offspring are produced in the ratio of 1 Kerry Dexter bull :2 :1 dog calf. The bull dog calf usually dics. Parents Dexter Dexter Kk Kk Gameles KK Kk Kk bull dog Dexters kerry calf Phenotypic ratio 2 1 Recessivelethals Example :In Swedis(Holstein Friesian cattly, there is a recessive gene producing condition described as amputated' which when homozygous results in calves without legs and parTot jaws. Effects of lethal genes Somelethal genes induce gross abnormalities, some interfere with physiological processes and others kill or are not yet been identified. They produce lethaleffect in homozygousstate. Lethal action may be due to single gene or at times more than single gene. Expressedlethal elfect is due to environment, incomplete penetrance and variable expressivity and epistasis. In majority of cases,it is difficult to determine the typeof gene action.,  16 Principles of Animal Genetics and Population Genetics Thus the yellow mice provided the first demonstration where genes could be lethal in the honozygous state, A A':dominant gene foryellow fur, a:its recessive allele Parents Yellow x Yellow Gometes AAY aa Dies Yellow Non yellow Phenotypicratio 2 1 When a yellow is crossed to a non yellow (heterozygotewith a recessive homozygote) Parents Yellow Non yellow A'a aa Gametes A) A'a Yellow Non yellow Phenotypicratio I Example 2: Creeper condition in domestic fowl In domestic fowl, an intermediate lethal genewhen heterozygous gives rise to a peculiar condition called creeper. The creeper chicken have short andcrooked legs: they are unable to move normally and greep about without lifting their bodies off the ground. The samelethal genewhen homozygous causes death during the embryonicstages. When two creeper chicken are crossed they yield a peculiar ratio of 2:1. Parents Creeper Creeper Cpcp Cpcp Gametes CpCp Cpep Cpcp cpcp Dies Creepers Normal ratio 2 1 Phenoypic  Principles of Genetics 15 2.Incomplete dominance /Partialdorninance/ Blending inheritance Exahple :plumage colours in (Andalusian breed of chickers- black splashed white, blue If we cross a black chicken with splashed white the all blue. When blue chicken are crossed among themselves offspring are they give rise to black, blue and white in the ratio of 1:2: in offspring. Black Plumage X Splashed white plumage Gametes PF Blue Gametes Black Blue FF Splashed white Phenotypic ratio Black: Blue: White 1: 2: 1 Genotypic ratio FB FB 1: 2: 1 Lethal Genes Lethalgenesare those which cause death of the phenotype either during pregnancy or early postnatal life. Semi dominan) lethals intermediate lethal) :(Incompletely dominant lethal, ixample1 Yellow fur in mice In 1905,Cuenot,a French Zoologist found that when yellow mice were mated interse they never bred true to type and yielded yellow and non-yellow progeny in the ratio of 2:l. Backcross of yellow into non-yellow yielded a ratio of 11.Aftermore experimentation, it was eventually realized that all yéllow mice were heterozygote and those zygotes homozygous for yellow died at an early stage of gestation.  14 Principles of Animal Genetics and Population Genetics P Red white R'R' RR Gametes R F R'R Roan FxF, Roan Roan R'R R'R Figmetes R'R R'R RR RR Red Roan White Red: Roan: White RR: RR:RR Phenotypic ratio 1: 2: 1 Genotypic ratio In this case, the heterozygote has its own 1 1:2: phenotype that 15 different from both the homoygotes. In codominance, it is difficult to know which allele is dominant and which allele is recessive. It must be noted that in codominance for a single character the genotypic and phenotypicratios are similar. Thetwo genes seem to be of about equal strength in their determining powers and a heterozygous individual shows characteristics of both the types. This is a situation in which the phenotypicexpression of both alleles is observed in the heterozygote. Example 2 MN blood group in human being Parents Mblood group N blood group MM Cametes M) MN (MNblood group) MN MN MM MN MN N M type: MN ype: N type MM: MN:NN Phenotypic ratio i: 2 1: Genotypicratio I:2 : 1