Principles Of Animal Genetics And Population Genetics PDF

'36 Principles of Animal Genetics and Population Genetics Sex-linked inheritance and autosexing in chicken Thekind of sex-linked inheritance described in drosophila and ín man is characteristic of animal in which male sex is heterogametic one.In birds, moth...

'36 Principles of Animal Genetics and Population Genetics Sex-linked inheritance and autosexing in chicken Thekind of sex-linked inheritance described in drosophila and ín man is characteristic of animal in which male sex is heterogametic one.In birds, moths, butterflies, silkworm and in some fishes, different kind of sex linkagesoccur where malesare homogameticand females are heterogametic. Particularly in poultry sex chromosomes are deseribed by different letters males being 'ZZ'and females 'ZW. The general principle for autosexing in fowls is that homogametic sex (male) should have the recessive character in homozygous condition and heterogametic sex (female) should have the dominant character. Slow feathering -K Fast feathering -k Rapid feathering Slow feathering Male Female kk KW W Kk kW Slow feathering Rapid feathering Male female 4.Ylinked inheritance Y chromosomes are nearly (inert)and onlya few genes of X chromosonmes are present. In mammals, the corresponding Y chromosome has a locus known as histocompatibility complex. The individuals carrying Y chromosome are known to produce HY antigen. The inheritance of genes carried on Y chromosome called is holandric inheritance and the genes are called holandric genes. LSex-influencedtraits Certain characters/traits in farrm animals are inluenced. In this case, certain known to be sex peculiarities are noticed which distinguishes it from sex-limited traits, 1. The genes governing sex and not on sex influenced traits are carried chromosomes. on autosomes Principles of Genetics 37 2. Nodifference between reciprocal crosses in F, and F, 3. Dominance in the heterozygous condition depends onthe sex of the individual or the dominance of the allele may differ in the heterozygotesof the two sexes. Example : Horned condition in sheep Dorset horn - both males and females horned Suffolk - both males and females hornless (polled) Reciprocal cross Dorset Suffolk Suffolk Dorset 99 99 HH hh hh HH Gametes H H F1 Hh Hh Males Hh-horned Males Hh-Horned Females Hh- Hornless Females Hh– Hornless Fx Fi Hh Hh Hh Hh Gametes H) H HH Hh Hh hh HH Hh Hh hh F2 Males 3Horned :1Homless Males 3Homed :1 Hormless HH Hh Hh hh HH Hh Hh hh Fensales 1Hormed :3 Hornless Females IHomed :3 Homless. So F, andF,ratios must be read for males and females separateiy. The genefor horns is dominant in males and recessive in femaleswhile its recessive allele for hornless condition is dominant in females and recessive in males. 3.Sex-limited traits Genes fora characterare carried by both the sexes but thecharacter will be expressed only in one sex. The gene for sex-limited trait may be 38 Principles of Animal Genetics and Population Genetics carried both on sex chromosomes and autosomes. When the penetrance of a gene is zero in one sex then, the trait will be sex limited. Example: Milk production in cattle and all mammals, Egg production in poultry Prolificacy in rabbits, swine and goat etc., Plumage in Peacock. LINKAGE AND CROSSING OVER According to the law of random assortment, genes controlling different characters assort independently of each other and combine at random during fertilization. The segregation of one paír of alleles does not in any way affect the segregation of another pair of alleles. This will hold good aslong as the genes under question are located in different chromosomes. If two genes are located on the samne chromosome, then random assortment may not occur. The phenomenon of linkage was first discovered by Bateson and Punnet (1906)in sweet peas. Vbcfinition: Linkage is a phenomenon in which genes controlling different characters if located on the same chromosomne move together as a unit (ormove en bloc) at the time of gamete formation and thereis no random assortrment. Linkage is an exception to random assortment or indepen dentassortment. For example in Drosophila melanogaster, 4 pairs of chromosomes are there and so far several hundreds of genes have been identified. Hence, each pair of chromosomes should comprise few hundreds of genes and thesemove asa unit (group)at thetime of gameteformation. Hence, there are 4 pairs of chromosomes and 4 linkage groups in Drosophila and likewise in other species too. Usually inkage groups will not exceed the number of pairs of chromosomes. Eg: Inrosophila a'pair'of alleles "B and b are for 'gray'and "black body coloutrespectively. There is also ajpair of alleles V,, 'v affecting the length óf wings [Longand vestigial (shortened) wings]. T.H.Morgan made a dihybrid cross involving'bodycolour and 'wing length'.  39 Principles of Genetics Black body & X Gray body & Long wing vestigial wing bb V,V: BB V, Vg Gametes (bV, B v) Fi BbVgVs Gray body long wing TestCrossing Instead of producing F,through interse mating in F he made a test cross with homozygous recessive (bbv v).Test cross is almost invariably used to determine linkage and crossing over because less number of progeny is sufficient to find out random assortment. Fr Gray long 9 Black vestigial (homozygous recessive) BbVg vg X bb v Ve Black long Gray vestigial Ifrandom assortmentis there Gray long,Gray vestigial, black long and black vestigial should appear in the ratio of 1:1:1:1.But instead only 'gray vestigial' and "black long' ie. parental types, occurred in equal ratio of 1: 1. That means, there is no rarndom assortment and the genes appear to be linked (located on the same chromosome). This can be represented diagrammatically as follows: ie. writing the linked genes together, Black long Gray vestigial (V) V) (b X (B v) (Bv) Gametes Bv Fi (Bv)(bV) Gray long 40 Principles of Animal Genetics and Population Genetics Test cross Gray long ? Black vestigeal (B y, )(6Vg) X (bvj(b v) Gametes (By) (6 Vg) (bv,) Progeny ) (Bv, (bv) (bVg)(bv) Gray vestigial Black long The test crossprogeny phenotypically resembled parental generation (ie. Black body long wing and Gray body vestigial wing). From theresults, it is clear that geneslocated in thesame chromosome move together atthe timeof gamete formation. The linkage is complete. Advantages of the test cross The test cross is almost invariably used to determine linkage and Cross over values. This type of cross brings out all the recessive genes, which are present in the chromnosome of hybrids. If we cross the F, among themselves, the recessive characteristics would show in approximately 1/4h of the offspring rather than ½ as in the case with the test cross. Thus a greater numberof offspring would be necessary in order to feel reasonably certain of a representative sample. Also, it would be more difficult to calculate cross over values between genes in F,progeny,because allowance would have to be made for the larger number of dominant characteristics. Incomplete linkage and crossing over Morgan made a reciprocal cross by exchanging sexes of F, and the test cross parent. Gray long black vestigial F, (B vg)(bVg) X (b vg) (bvg) In thís case, the following types and numbers were obtained by T.H.Morgan. Phenotype Number obtained Gray vestigial 1294 Black long 1418 Parental type (Non-cross over type) -2712 Gray Long 283 Black vestigial 241 Crossover type(Recombinant type)- 524 3236  Principles of Genetics 41 Parentaltype : 2712/3236 X 100= 83.8 % Recombinant type : / 524 3236 X 100 = 16.2 or 16 % Gametes Gametes o Genotype Phenotype Number obtained (Bv) (bv) (B v)(bv) Gray vestigial 1294 (6V) (bVJ(bv) Black long 1418 (BV) (BV)(bv) Gray long 283 - (ov) (bv)(bv) Black vestigial 241 In the reciprocal cross, the linkage is not complete. In the case of female Drosophila, recombination has taken place as a result of crossing over.[Crossing over is the exchange of genctic material between non-sister chromatids of homologouschromosomes]. If male F, Drosophila isused in the cross the linkage is complete,as there is no chiasmata formation (crossing over is absent in male drosophila) during meiosis (spermatogenesis). That is why only parental-type progeny are produced whereas if,female F,Drosophila is used there is crossing over as a result of chiasmata formation during meiosis (Oogenesis). Hence, both parental type gametes and cross over (recombinants) gametes are produced and all four types of progeny appearbut in unequal proportions. In otherwords, 'linkage is complete ifF, male Drosophila is used; but linkage is incomplete if F female Drosophilaisused in the cross.There isalso recombination as a result of cross over. Complete linkage is a situation peculiar to Drosophila and a few other species and in all other species mostly incomplete linkage alone is observed. One of the indicators of linkage is that percentage parental typeswill be more than cross over or recombinant types. Features ofcrossing over 1. The loci of the genes on a chromosomeare arranged in a linear scquence. 2. The two alleles in a locus in a heterozygote occupy corresponding positions in homologous chromosomes. 3. Crossing over involves breakage of each of two homologous chromosomes and cxchange of parts. 4. Crossing over occurs at pachytene stage of meiosis after synapsis. 5. The probability thatcrossing overbetwecn two loci increasewith increase in distance bctween two loci on the chromosome. 42 Principles of Animal Genetics and Population Genetics Morgan inferred that crossing over is an indicator of relative distance between genes under study. If they are located close to each other the per cent ofcrossing over willbe less. If they are located farther then per cent crossing over will be more. However, the percent cross over between any two genes will not be 50per cent or more. At 50 per cent and beyond one cannot differentiate between random assortmernt and crossing over. The relative distance between two genes isexpressed in map units or CentiMorgan. One Centi Morgan (cM)is % equal to 1 rossing over. Thus rOSsingoveris an exception tolinkage. Couplingphase and repulsion phase Coupling phase: If two dominant genes or two recessive genes are located in the same chromosomethen itis known as coupling phase orcis position. Eg. B V, or b v Repulsion phasc: If one dominant and one recessive gene (or one recessive and one dominant gene) are located on the same chromosome, then it is known as repulsion phase or transposition. Eg. Bv, or b V Practical Utility: (of crossing over) Relative distance between genescan be estimated and chromosome maps can be constructed. In Drosophila, there are four pairs of chromosomes and there are 4 linkage groups. It means that inkage groups will never exceed the numberof pairs of chromosomes. Double crossoverCoincidence and Interference matter to add to Double cross over In principle, it is rather simple until all the genes belonging the map position of moreand more genes, For this purpose, to the same linkage group have been entered. known loci at least two previously experiments are planned so that Then, using the in crosses. and one or mnore new ones are involved fact that the distarce between two genes a andc is either the sum (ab b-c distances, it is determined + bc) or the difference of the a-b and and c before a or after c. whether the new gene lies between a b C b Interference and coincidence the occurrence of cross over in In most of the higher organisms, reduces the probability of another one part of a chromosome actually Principles of Genetics 43 cross over in an immediate adjacent region of the chromosome,Thís is called interference and the net result is the observationof fewer double crossover typesthan would beexpected according to the map distances. The strength of interference varies in different segments of the chromosome and is usually expressed as coefficient of coincidences, It is the ratio between the observed and expected double cross over. Per cent observed double cross overs Coefficient of Coincidence Per cent expected double cross overs Coincidence is complement of interference. Coincidence + Interference =1. When interference iscomplete (1.0), no double crossovers will be observed and coincidence becomes zero. When we observe allthe double crossovers expected, coincidence is unity and interference becomes zero. SEXDETERMINATION Theories of sex determination : 1. Sex determination by environment 2.. X-O method of sex deternination 3. Geneic balance theory or Ratio theory of sex determination 4. Sex determination by male haploid 5. ZW method 6. Sex determination in mammals KSex determinationby environment Example: Marine worm [Boniellia viridis] Boniellia exhibitsa most extreme example of sexual dimorphism. Females are several inches long having a long bifurcated proboscis. The males are ciliated and are minute creatures, which live within the reproductive tract of fenmales. The fertilized eggs become tiny Swimming larvae, The larvae that settles in the mud becomesa free On theother hand, if newly hatched worms female. (larvae) are released into water containing matured temales some of the young worms gets a chanceto attach to the proboscis of females. They finally develop males and migrate down to the into reproductivetract of female. It is thoughtthat there is something in the secretions absorbed by each 44 Principles of Animal Genetics and Population Genetics male from the proboscis of femalewhich cause genes for maleness to be expressed. Hence, the environment decides the larva to become male when contacts the mature female and the water environment makes the larva to become female. 2.X-0method of sex detemination Example: Squash bug [Anasa tristis] E.B. foundthat the female bugs had 22 chromosomes in Wilson somatic cells and they form a haploid set of 11 at meiosis. Alleggs therefore bear the haploid set 11. Males, on the other hand have of only 21 chromosomesin their somatic cells.At meiosis, there areonly 0 pa and an odd accessory chromosome. Wilson suggested this odd chromosomebecalled X-chromosome. About one half ofthesperms carry this X-chromosome (10+1) and other half does not carry this X-chromosome (10+0). Fertilisation oftheeggbya sperm carrying 11chromosomes (10+1) will result in fermale. Fertilisation by a sperm with 10 chromosomes (10+0) will result in male. This is known as X-O method of sex determination. Gamete Gamete Female :20A +>XX- 10A +X Male :20 A+X- 10A + X -10A +0 Fertilization: (10A +X) +(10A +X) -Female (10A + + (10A +0) >X) -male Determination by sexchromosomes and autosomes(Ratio theoryor Geneic balance theoryl Example: Drosophila melanogaster This fly has 3 pairs ofautosomesand one pair of sex chromosomes in somatic cells. Females have 2 X chromosomes.Male has 1 X and 1 Y.It was first thought that the fertilisation of an ovum with X sperm will results in female and with Y sperm results in males. C.B.Bridges obtained flies with XO chromosome complement ie,Y chromosome was missing due to non-dysjunction. These were phenotypic males but these flies were sterile.In another instance,XXY Aies were obtained which were normally fertile. Therefore, it appeared that Y chromosomme have no role in determining sex in drosoplhilia except carrying genes for fertility.Therefore, it was postulated that X chromosomes have  45 Principles of Genetics of autosomes preponderance for female tendency and the haploid set have preponderance of male tendency. male and fernale Every individual in its genoty pe have both is decided by the balance ie., potentiality. Which sex actually develops preponderance of female tendency or male tendency. genes that In drosophila, the X carries more female determining toward femaleness. The incline the development of the individual it towards autosomescarry male determining genes and inclínenumber of X maleness. The deciding factor is the ratio between the egg. chromosomes and of sets of autosomes in the fertilized The Geneic balance is governed by the ratio of the number of X chromosomes to the number sets of autosomes of A]in zygotes at that is to develop into a any individual fertilisation. This ratio is 1.0 in 1 female and 0.5 in a normal male. If the ratio is intermediate between and 0.5 the resulting individual is an intersex. Super females have a females, and super ratio of 1.5, which is higher than that in normal males a have ratio of 0.33, which is lower than in normal males. Chromosome X Haploid Sex index Phenotypic chromosome setof X/A sex complement autosome 2A +XX 3 2 1,5 Super female (sterie) 2A+XX 2 2 1.0 Normal diploid (fermale) 3A +XX 2 0.67 Intersex 2A +XY 1 2 0.5 Normal diploid male 3A +XY 3 0.33 Super male (sterile) 4,Sex determination by male haploid Exanple: Bees, wasps, Hymenopteron insects. In honeybee, the fertilized eggs produce diploid females (32) and unfertilizedeggundergosasexual process of embryonic development known as parthenogenesis and produces haploid (16) but fully fertile male. The bee produces sperm by mitosis rather than meiosis (yielding functional sperm per spermatocyte). The diploid females develop into workers and queen bee and haploid males into drones. 46 Principles of Animal Genetics and Population Genetics Males Females 16 32 16 16 16 32 16 Female Male 52Wmethod sexdetermination of A rooster has two alike sex chromosomes, which is designated ZZ while, ahen has two unlike sexchromosomes, which isdesignated as "ZW". Allsperms would carry 'Z'chromosomes but an egg would carry either "Zor W in approximately equal numbers. Sex determination therefore depends upon which of thetwo types of eggs is fertilized. 6.Sex determinationin mammals [XYmethod] Now it is known that a gene called SRY (sex determiring region on the Y chromosome)located on theshort-arm of the Y chromosome isresponsible for the determination of sex in mammals.The imnportance ofSRY gene isthat itappears to provide the initialsignal,which causes the undifferentiated gonad in an embryo into a testes rather than an ovary. The principle is that female is a default sex. In other words, if the undifferentiated gonad does not receive any message from SRY region, it developsasovary whosehormones produce secondary sexual characteristics. IfSRY genes sendsmessage theundifferentiated gonad develops into a testes whose hormones produce secondary sexual characteristics. SEX CHROMATIN In 1949, Barr and Bertram reported sexual dimorphism in the the female nerve cells of cat based on the presence of sex chromatin in and its absence in the male. They observed darkly stained bodies and has these bodies were called Barr bodies. Since then sex chromatin number of been described in cells from various tissues in female and in different species. It is actually an X chromosome that islate replicating highly condensed. during mitosis and subsequently has become very the number of X The number of barr bodies is usually one less than chromosomes. XX-one barr body; XY -NobarT body: XXX- Two barr body  47 Principles of Genetics males will Normal females will have oné barr body and normal have no barr body at all. Individuals of XXX phenotype will have two barr bodies. This is so because only one X chromosome is actíve and the otherX chromosome condenses into sex chromatin or Barr body. This condensed X chromosome is the sex chromatin(Barr body) and is inactive. This X chromosome inactivation compensates for differences in gene dosage between males and females in relation to X- linked genes. The limitation of sex chromatin studies is that they do not differentiate between individuals >XX and XY. X CHROMOSOME INACTIVATION In most mammals, the female sex chromosomesare two large X chromosomes whereas the male sex chromosomnes are a large X chromosomeand a smnall Y chromosome.Based on evidencefrom the autosomal characteristics both genes at a site on homologous chromosomes are active at the same time and the product of outputof the gene is fixed. It is to be expected that the female output from X linked geneswould be twice that of male (because of two X chromosomes in female and onein male). However it isnot so and the explanation put forward by Lyon in 1961 is that early in the embryonic life of females each cell independently and randomly inactivateseither the maternally derived or paternally derived X chromosomeand that all the descendents of that cell will have the same X chromosome inactivated. This is known as Lyon hypothesis. DOSAGE COMPENSATION : Example coat colour in cats. The result of X inactivation in female is that each femalecell has the same quantity of gene product from X-linked genes as do male cells. Thus inactivation is a mechanism that compensates for the difference in gene dosage between males and females in relation to X linked genes. This effect of X chromosomeinactivation is called dosage compensation. Note: Sex chromatin not observed in birds. The dosage is compensation for Z-linked genes does not occur in birds. An enzynne locus onZ chromosome of domestic chicken (andother species of birds) in males produce twice as much of the enzyme as a female does.