Animal Genetics and Population Genetics PDF

20 Principlesof Animal Genetics and Population Genetics 1. F does not resemble both the parents and a new phenotype (walnut)appears in the F,, which is due to interaction between two independently inherited dominant genes. These two new pheno...

20 Principlesof Animal Genetics and Population Genetics 1. F does not resemble both the parents and a new phenotype (walnut)appears in the F,, which is due to interaction between 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)appears which was not present in the parents and is due to interactíon of recessive alleles namely rpp. These peculiarities are not due to a new method of inheritance but it is simply due tothe circumstances that two pairs of alleles (Rr and Pp)control the same character ie., comb type or comb pattern in chicken. One of these new characters (walnutcomb)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 twvo genes diferent loci working together produce something at different from the efectof either one alone, they are satato be conplementary genes. EPISTASIS (Greek - standing upon) Epistasis is the term describing relationship between genes in differernt loci. The genotype at onelocus may influence the effect af the genotype (non allelic)at another locus. It may in fact prevent theexpression of other locus. Whenever a gene at one locus on a chromosome masks or prevents the expression ofa gene at anotherlocus the first gene is said to be 'epistatic' to the second and, the gene that is hidden is said to be 'hypostatic'. dominance, oneallele suppresses the expression of another allele In at the same locus (intra allelicinteraction). Dominanceis the interaction between different alleles of the same gene (ie., within a locus). Epistasis is interaction between genes that are not alleles ie., genes located in different loci (interallelic interaction). Epistasis is one importanttype of functional interaction between products of different genes. Therefore, the classical Mendelian ratio of 9:3:3;1 does not occur in all cases. It is found that most of the apparent exceptions could be by twoor explained by assuming that many characters are influenced more genes. Depending on thetypeof gene interaction thephenotypic ratios are modified in various ways.  Principles of Genetics 21 oVÁecessive epistasis Example: In common house mouse (Musmusculus) several varieties of coat colour exist. The standard or wild type is known as "agouti (the individual hairs are most part black with anarrow yellow band near the tip). The commonest and most familiar variation ís "albino'.Albinos always breed true. Another variation in coat colour that arose as a result of the disappearance of yellow pigment in the agoutiis "black' colour. Black is recessive to agouti. When pure "black' mice are crossed with 'albino' the progerny 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 Purnett square or checker board. P Black X Albino CCaa Cc AA Gametes cA CCAi all Agouti F Gametes CA CA When mated interse, the following genotypes appear. Gametes CA Ca CA ca CA CCAA CCAa CCAA CCAa Agouti Agouti Agouti Agouti Ca CCAa CCaa CCAa Ccaa Agouti Black Black Agouti 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  14 Principles of Animal Genetics and Genetics Population Red R'R' white Gametes RR R F R'R Roan F,xF Roan Roan Fagametes R R'R R'R RR RR RR Red Roan White Red: Roan: White Phenotypicratio 1: 2: 1 RR: RR: RR 1 :2: 1 Genotypic ratio In this case, the heterozygote has its own phenotype that is 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. The two genesseem to be of aboutequal strength in their determnining powers and a heterozygous individual shows characteristics of both the types. This is a situation in which the phenotypic expression of both alleles is observed in the heterozygote. Example 2 MN blood group in human being Parents M blood group N blood group MM Camets M) N MN (MN blood group) MN MN MM MN MN NN M type: MN rype: Ntype MM: MN: NN Phenotypic ratio 2 Genotypic ratio 1 :2 :1  Principles of Genetics 17 An explanation for the altered ratiocanbefound if we consider all the eggswhich were incubated. One fourth of these failed to hatch which had dead embryos (at about 4h day of incubation). Hence it is evident thatcreeper condition in chicken is a heterozygous expression of a gene which is lethal when homozygous. Example 3 :Achondroplasia in catte. Dexter and Kerry ar becf brceds)of(fattlc ofEnglan) and DexteL. (has short limbs) is a better beef producer than the Kerry. But impossible to establish a pure breeding herd of Dexters because these desirable characteristics of the breed result in association with heterozygous expression of lethal gene. When two DexterS arecrossed the offspring are produced in the ratio of 1 Kerry Dexter 1 bull :2 : dog calf. The bull dog calf usually dies. Parents Dexter X Dexter Kk Kk Gameles KK Kk Kk bull dog) Dexters kerry calf Phenotypic ratio 2 (Recessivelethals Example :In Swedist(Holstein Friesian cattly there is arecessive gene producing condition described as amputated' which when homozygous results in calves without legs and parrot jaws Effects of lethal genes Some lethal genesinduce gross abnormalities, some interferewith physiological processesand others kill or are not yet been identified. They produce lethl effect in homozygous state. Lethal action may be due to singlegene or at times more than single gene. Expressed lethal effect is due to environmnent, incomplete penetrance and variable expressivity and epistasis. In majority of cases,itis difficult to determine the type of gene action.,  Principles of Genetics 15 2.Incomplete dominance/Partial dominance/ Blending inheritance Exahple : plumage colours in (Andalusian breed of {chicker- black, splashed white, blue Ifwe cross a black chicken with splashed white the offspring are all blue. When blue chicken are crossed among themselves they give rise to black, blue and white in the ratio of 1:2:1 in offspring, Black Plumage F F" Splashed white plumage Gametes Blue Fx F F F Ganete F Black Blue F* F Splashed white Phenotypic ratio Black: Blue: White 1: 2: 1 Genotypic ratio FB FB 1: F F* 2: 1 SLethal Genes genes are thosewhich cause death of the phenotype either Lethal during pregnancy orearly postnatal life. intermediate lethal) : Semi dominant lethals (Incompletely dominant lethal, ixample 1 Yellow fur in mice In 1905,Cuenot, a French Zoologist found that when yellow mice were mated interse they never bred trueto type and yieldedyellow and non-yellow progeny in the ratio of 2:1. Backeross of velow into non-yellow yielded a ratio of 11. After more experimentation, it was eventually realized that állyéllow mice were heterozygote and those zygotes homozygous for yellow died at an early stage of gestation. *  16 Principles of Animal Genetics and Population Genetics Thus the yellow miceprovided the first demonstration where genes could be lethal in the homozygous state. A':dominant gene for yellow fur, a:its recessive allele Parents Yellow x Yellow Aa Aa Gometes aa Dies Yeilow Non yellow Phenotypicratio 2 1 When a yellow is crossed to a non yellow (heterozygote with 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 gene when heterozygous gives rise to a peculiar condition called creeper. The creeper chicken have short andcrooked legs: they are unable to move normally and çreep about without lifting their bodies off the ground. The same lethal gene when homozygous causes death during the embryonicstages. When two creeper chicken are crossed they yield a peculiarratio of 2:1. Parents Creeper Creeper Cpcp Cpcp Gametes CpCp Cpep Cpep cpcp Dies Creepers Normal 2 1 Phenotypic ratio  14 Principles of Animal Genetics and Population Genetics Red R'R white Gametes RR R F R'R Roan FxF Roan Roan R'R Fagametes R'R R R'R' RR RR RR Red Roan White Red: Roan: White Phenotypicratio 1: 2: RR: RR: RR 1 1 Genotypicratio 1 :2: :2 In this case, the heterozygote has its own phenotype that is different from both the homoygotes. In codominance, it is know which allele is dominant and which difficult to allele is recessive. It must be noted that in codominance for a single character the genotypic and phenotypicratios are similar. The two genesseem to be of aboutequal strength in their determnining powers and a heterozygous individual shows characteristics of both the types. This is a situation in which the phenotypic alleles is observed in the expression of both heterozygote. Example 2 MN blood group in human being Parents M blood group N blood group MM Gamets M) MN (MN blood group) MN MN MM MN MN NN M type: MN rype: Ntype MM:MN: NN Phenotypic ratio 2 Genotypic ratio12:I  Principles of Genetics 17 Anexplanation for thealtered ratio can be foundif we consider. all eggswhich were incubated, One fourth of these failed to hatch the which had dead embryos (at about 4h day of incubation). Hence it is evident that creeper condition in chicken is a heterozygous expression of a genc which is ethal when homozygous. Example 3:Achondroplasia in cattte. and Kerry Dexter ar beef brceds)of fattlc of England) and Dexter (has short limbs) is a better beef producer than the Kerry. But it is impossible to a pure breeding herd of Dexters because these establish desirable characteristics of the breed result in association with heterozygous expression of lethal gene. When two Dexters arecrossed the offspring are produced in the ratio of 1 Kerry 2 Dexter dog calf. The bull dog calf usually dics. 1 bull : : Parents Dexter Dexter Kk Kk Gametes KK Kk Kk bull dog Dexters kerry calf Phenotypic ratio 2 Recessive lethals Example :In SwedisHolstein Friesian cattlo,there is recessive a gene producing condition described as amputated which when homozygous results in calves without legs and parrot jaws. Effects of lethal genes Some lethal genes induce gross abnormalities, some interferewith physiological processes and others kill or are not yet been identified. They produce lethal effect in homozygous state. Lethal action may be dueto single gene or at times more than single gene. Expressed lethal effect is due to environment, incomplete penetrance and variable expressivity and epistasis. In majority of cases, it isdifficult to determine the type of gene action.,  Principles of Genetics 15 2.Incomplete dominance/Partial dominance/ Blending inheritance Exahple : plumage colours in (Andalusian breed of {chicker- black, splashed white, blue Ifwe cross a black chicken with splashed white the offspring are all blue. When blue chicken are crossed among themselves they give rise to black, blue and white in the ratio of 1:2:1 in offspring, Black Plumage F F" Splashed white plumage Gametes Blue Fx F F F Ganete F Black Blue F* F Splashed white Phenotypic ratio Black: Blue: White 1: 2: 1 Genotypic ratio FB FB 1: F F* 2: 1 SLethal Genes genes are thosewhich cause death of the phenotype either Lethal during pregnancy orearly postnatal life. intermediate lethal) : Semi dominant lethals (Incompletely dominant lethal, ixample 1 Yellow fur in mice In 1905,Cuenot, a French Zoologist found that when yellow mice were mated interse they never bred trueto type and yieldedyellow and non-yellow progeny in the ratio of 2:1. Backeross of velow into non-yellow yielded a ratio of 11. After more experimentation, it was eventually realized that állyéllow mice were heterozygote and those zygotes homozygous for yellow died at an early stage of gestation. *  16 Principles of Animal Genetics and Population Genetics Thus the yellow mice provided the first demonstration where genes could be lethalin the homozygous state, A': dominantgene for yellow fur, a: its recessive allele Parents Yellow x Yellow Aa Gometes AA Dies Yellow Non yellow Phenotypicratio 2 1 When a yellow is crossed to a non yellow (heterozygote with a recessive homozygote) Parents Yellow Non yellow A'a Gametes A'a Yellow Non yellow 1 Phenotypic ratio I Example2:Creeper condition in domesticfowl In domestic fowl, an intermediate lethal gene when heterozygous gíves rise to a peculiar condition called creeper. The creeper chicken and have short andcrooked legs: they are unable to move normally çreep about without liftingtheirbodies off the ground.The samelethal gene when homozygous causes death during the embryonic stages. When two creeper chicken are crossed they yield apeculiar ratio of 2:1. Parents Creeper Creeper Cpcp Cpcp Gametes CpCp Cpp Cpep cpcp Creepers Normal Dies 2 1 Phenotypic ratio