Transmission Genetics PDF

G. GENETICS 16. TRANSMISSION GENETICS 16.1 Mendelian genetics  Definition of the terms gamete, gene , allele, dominant and recessive alleles, homozygote, heterozygote, phenotype, genotype, filial generation (P 1, P2, F1, F2) , types of crosses (test...

G. GENETICS 16. TRANSMISSION GENETICS 16.1 Mendelian genetics  Definition of the terms gamete, gene , allele, dominant and recessive alleles, homozygote, heterozygote, phenotype, genotype, filial generation (P 1, P2, F1, F2) , types of crosses (test cross, back cross, reciprocal cross, selfing) and pure breeding. 16.1.1 Monohybrid - Monohybrid cross and its result. - Mendel’s first law (Law of Segregation) and its relation to meiosis - Calculations of genotypic and phenotypic ratios (Punnet square method) 16.1.2 Dihybrid - Dihybrid cross and its results - Mendel’s second law (Law of Independent Assortment) and its relation with meiosis - Calculations of genotypic and phenotypic ratios until F 2 generation (Punnet square/fork methods) - 16.2 Modification of Mendelian genetics  Crosses that result in ratios differing from the classic Mendelian 3:1 and 9:3:3:1 ratios 16.2.1 Codominance  Definition  Example of inheritance: MN blood group in humans  Calculations of genotypic and phenotypic ratios 16.2.2 Incomplete dominance  Definition  Example of inheritance: Antirrhinum (snapdragon) flower colour  Calculations of genotypic and phenotypic ratios 16.2.3 Multiple alleles  Definition  Example of inheritance: human ABO blood group  Calculations of genotypic and phenotypic ratios 16.2.4 Lethal genes  Definition  Example of inheritance: coat colour in mice  Calculations of genotypic and phenotypic ratios 16.2.5 Polygenes  Definition  Example of inheritance: height in humans 16.2.6 Linked genes  Definition of linked genes and sex-linked genes  Effect of crossing over on ratio of dihybrid crosses  Parental and recombinant phenotypes  Examples : Drosophila eye color and haemophilia in humans  Calculations of genotypic and phenotypic ratios  Pedigree analysis  Sex determination in humans 16.2.7 Epistasis  Definition and examples only 16.3 Genetic mapping  Calculations of distance between two loci based on percentage of crossing over  Examples of calculations for Drosophila  Determining the relative position of a gene on a chromosome based on percentage of crossing over Genetics  Is the study of heredity, i.e the transmission of characteristics or traits from one generation to another.  Modern genetics is concerned with the study of genes  Genes are the units of heredity that control the characteristics of organisms  A gene is made up of DNA with a particular nucleotide sequence 16. Transmission genetics 16.1 Mendelian Genetics Definition of genetics terms Genetics terms description examples i. Gamete - a specialized male or female Sperm reproductive cell with a..................... set or................. of chromosomes formed by.......................... ii. Gene - the basic unit of inheritance for a flower position given characteristics iii. Allele - alternative forms of the same gene A or a responsible for determining contrasting characteristics iv. Locus - position of an allele within a DNA molecule v. Homozygous - the diploid condition in which the AA or alleles at a given locus are identical vi. Heterozygous - the diploid condition in which the alleles at a given locus are............... vii. Phenotype - the characteristics of an individual axial , terminal usually resulting from the interaction between the genotype and the environment in which development occurs viii. Genotype - the genetic constituent of an organism AA,......,....... with respect to the alleles ix. Dominant - the allele which influences the appearance of the phenotype even in the presence of an alternative allele x. Recessive - the allele which influences the appearance of the phenotype only in the presence of another identical allele xi. F1 generation - the generation produced by crossing homozygous parental stocks xii. F2 generation - the generation produced by crossing two F1 organisms  - Genes are located on chromosomes - In diploid organism, chromosomes exist in pairs (homologous pairs), one of each pair having come from the mother, the other from the father A homologous pair of chromosome Mendel’s Work  Mendel’s scientific investigations on inheritance began in the summer of 1856.  Mendel’s success was due, in part, to his careful choice of experimental organism, the garden pea, Pisum sativum which has the following advantages over other species. i. there were several varieties available had quite distinct characteristics ii. the plants were easy to cultivate iii. the reproductive structures were completely enclosed by the petals so that the plant was normally self-pollinating , which led to the varieties producing the same characteristics generation after generation, a phenomenom known as pure-breeding iv. artificial cross-breeding between varieties was possible and resulting hybrids were completely fertile.   The seven basic characteristics, or traits, that Mendel was interested in were: flower colour, flower position, seed colour, seed shape, pod shape, pod colour, and stem length 16.3.1 Monohybrid Monohybrid inheritance  Inheritance of a single characteristics Observations Parents axial flowers x terminal flower F1 all axial flower F2 651 axial flowers 207 terminal flowers F2 ratio 3 : 1 On the basis of these results Mendel drew the following conclusions:  Since the original stock were pure breeding, the axial variety must have possessed two axial factors (alleles) and the terminal variety two terminal factors  The F1 generation possessed one factor from each parent which were carried by the gametes  These factors do not blend in the F1 generation but retain their individuality  The axial factor is dominant to the terminal factor which is recessive - The separation of the pair of parental factors, so that one factor is present in each gamete, became known as Mendel’s first law or the principle of segregation which states that : “ the characteristics of an organism are determined by alleles which occur in pairs. Only one of a pair of such alleles can be represented in a single gamete” Mendel’s principle of segregation of factors (alleles) A and a described in terms of the separation of homologous chromosomes which occurs during meiosis Monohybrid cross o A cross between parents that differ in one characteristics controlled by the alleles for one particular gene.  The ratio of dominant phenotypes to recessive phenotypes of 3:1 is called monohybrid ratio Q1 If a pure strain of mice with brown-coloured fur are allowed to breed with a pure strain of mice with grey- coloured fur they produce offspring having brown-coloured fur. If the F1 mice are allowed to interbreed they produce an F2 generation with fur colour in the proportion of three brown-coloured to one grey. (a) Explain fully these results. (b) What would be the result of mating a brown coloured heterozygote from the F2 generation with the original grey-coloured parent? Test cross o Is a cross between a homozygous recessive and an organism of dominant phenotype but unknown genotype. o Is used to determine the unknown genotype which shows dominace for a trait either homozygous dominant or heterozygous. o For example in the fruit fly, Drosophila, long wing is dominant to vestigial wings. o The genotype of a long wing Drosophila may be homozygous (LL) or heterozygous (Ll) o The long wing fly is testcrossed with a double recessive (ll) vestigial wing fly. o If the test cross offspring are all long wing the unknown genotype is homozygous dominant. o A ratio of 1 long wing: 1 vestigial wing indicates that the unknown is heterozygous. Q2. Why is it not possible to use a homozygous dominant organism (such as TT) in a test cross experiment to determine the genotype of an organism showing the dominant phenotype? Illustrate your answer fully using appropriate genetic symbols. Backcross o An individual of unknown genotype is cross-fertilised with a pure breeding parental trait either homozygous dominant or the homozygous recessive. o If the backcross uses the parental homozygous recessive then the backcross is also the ………. ……. Reciprocal cross o Involves a cross fertilisation done twice, using the opposite gametes. o Both crosses in a reciprocal cross produced equal numbers of dominant and recessive forms of the trait studied. o The inheritance of the trait is thus not due to the types of gametes used but on the alleles present in the gametes. 16.3.2 Dihybrid  Dihybrid inheritance : inheritance of two pairs of contrasted characteristic or inheritance involving a genetic cross between parents that differ in two characteristic controlled by alleles for two particular genes at different loci.  Mendel’s experiment : using pea shape and pea cotyledon colour.  Pure breeding (homozygous) plants having round and yellow peas were crossed with pure breeding plants having wrinkled and green peas.  The F1 generation seeds were round and yellow.  Self-pollination of the F1 plants gave the following results: 315 round and yellow 101 wrinkled and yellow 108 round and green 32 wrinkled and green  The proportions of each phenotype approximated to a ratio of 9:3:3:1, which is known as the dihybrid ratio.  Two deductions were made from these observations: i. two new combinations of characteristic had appeared in the F2 generation: …………………………………………………… and ……………………………………………………. ii. the ratios of each pair of allelomorphic characteristic appeared in the monohybrid ratio of ……………….., that is 423 round to 133 wrinkled and 416 yellow to 140 green.  On the basis of these results Mendel was able to state that the two pairs of characteristic (seed shape and colour) , whilst combining in the F1 generation separate and behave independently from one another in subsequent generations.  This forms the basis of Mendel’s second law or the principle of independent assortment, which states that: ‘any one of a pair of characteristic may combine with either one of another pair.’ Mendel’s principle of independent assortment of factors (alleles) A,a,B,b described in terms of the separation of homologous chromosomes which occur during meiosis Example of a dihybrid cross: Example of a dihybrid test cross 7/2004 (a) State Mendel’s First and Second laws. Explain the conditions under which these laws are applicable. [6 marks] (b) By using suitable genetic symbols, explain the use of a recessive homozygous organism in a breeding experiment to determine the genotype of an organism showing a dominant phenotype. [9 marks] 7/2005 In pea plants, yellow seed colour which is controlled by Y allele is dominant over green seed colour which is controlled by y allele, and smooth seed which is controlled by R allele is dominant over wrinkled seed which is controlled by r allele. In a cross between parents with genotypes YYrr and yyRR, determine: (a) the phenotype of each parent (b) the genotype and phenotype of the resulting F1 (c) the genotype and phenotype of F2 and their ratios Nov.6/1984 (a) - Ya - kerana F1 menghasilkan satu jenis progeny sahaja yang fenotipnya sama dengan induk (b) Simbol: H alel bulu hitam (dominant) h alel bulu merah (resesif) T alel tanpa tanduk (dominant) t alel bertanduk (resesif) Fenotip P1 hitam, tanpa tanduk X merah, bertanduk Genotip P1 HHTT X hhtt Gamet HT HT ht ht F1 genotip HhTt F1 fenotip hitam, tanpa tanduk F1 x F1 HhTt X HhTt Gamet HT Ht hT ht HT Ht hT ht F2 (menggunakan Punnet square) gamet Nisbah fenotip: 73 bulu hitam, tanpa tanduk : 23 bulu hitam, bertanduk: 26 bulu merah, tanpa tanduk: 8 bulu merah, bertanduk 9 bulu hitam, tanpa tanduk : 3 bulu hitam, bertanduk: 3 bulu merah, tanpa tanduk: 1bulu merah, bertanduk (i) pasangan: D 7/2004 Answer: (a) Mendel’s Law of Segregation states that: - the characteristics of an organism are determined by alleles which occured in pairs - only one of a pair of alleles can be present in a single gamete - only applicable in complete dominance - in a diploid organism Mendel’s Law of Independent Assortment states that:  - either of a pair of allele may combine randomly with either of another pair - applies only to gene pairs on different pairs of homologous chromosomes/ provided that the genes concerned are not linked by being on the same chromosome (b) - the technique is known as a test cross - if all test cross offspring show the dominant trait, the unknown genotype is homozygous dominant - a phenotypic ratio of 1 dominant trait: 1 recessive trait, the unkown is heterozygous Let: T represent dominant allele t represent recessive allele Test cross of a homozygous dominant parent Test cross phenotype: dominant (homozygous) x recessive Test cross genotype: TT x tt Gametes: T T t t Offspring genotype: Tt Tt Tt Tt Offspring phenotype: all dominant (heterozygous) A test cross of a heterozygous dominant parent Test cross phenotype: dominant (heterozygous) x recessive Test cross genotype: Tt x tt Meiosis Gametes: T t t t Offspring genotype: Tt Tt tt tt Offspring phenotype: 1 dominant (heterozygous) : 1 recessive 7/2005 Answer: (a) YYrr = yellow and wrinkled yyRR = green and smooth (b) F1 = YyRr = all yellow and smooth (c) F1 parent/ F1 x F1 F1 phenotype yellow and smooth x yellow and smooth F1 genotype YyRr x YyRr Gamete YR Yr yR yr YR Yr yR yr F2  Gamete YR Yr yR Yr YR YYRR YYRr YyRR YyRr Yr YYRr YYrr YyRr Yyrr yR YyRR YyRr yyRR yyRr yr YyRr Yyrr yyRr yyrr arrengement of gamete – horizontal + vertical correct cross (column) 1/16 YYRR yellow and smooth 2/16 YYRr yellow and smooth 9/16 yellow and smooth 2/16 YyRR yellow and smooth 4/16 YyRr yellow and smooth 1/16 YYrr yellow and wrinkled 3/16 yellow and wrinkled 2/16 Yyrr yellow and wrinkled 1/16 yyRR green and smooth 3/16 green and smooth 2/16 yyRr green and smooth 1/16 yyrr green and wrinkled 1/16 green and wrinkled 16.2 Modification of Mendelian genetics  Crosses that result in ratios differing from the classic Mendelian 3:1 and 9:3:3:1 ratios 16.2.1 Codominance o both alleles in a heterozygous individual are being expressed phenotypically o Example: MN blood group in humans o Two alleles LM and LN determine the three blood groups  Incomplete dominance produces an F2 generation exhibiting a …………….. ratio for both genotype and phenotype. Nov.86 K2/6 Sistem kumpulan darah pada manusia dikawal oleh gen yang terdapat pada kromosom nombor 9. Sistem ini merupakan satu contoh dimana interaksi alel diilustrasikan oleh alel berganda.Terdapat enam gabungan yang mungkin bagi alel-alel dalam system ini, iaitu AA,AO,BB,BO,AB dan OO. Daripada enam gabungan ini, empat kumpulan darah yang berbeza di kenali iaitu A, B,AB dan O. Gen A dan gen B menunjukkan dominan ringkas apabila masing-masing bergabung dengan gen O, tetapi gen A dan gen B menunjukkan kodominan apabila gen-gen ini bergabung dengan satu sama lain. (a) Apakah yang dimaksudkan dengan alel berganda? ……………………………………………………………………………………………… ……………………………………………………………………………………………… (b) Berapakah bilangan alel dalam sistem diatas?................................................................................................................................................ (c) Lukiskan satu gambarajah untuk menunjukkan kedudukan alel-alel pada kromosom nombor 9. (d) Alel O disebut juga sebagai..................................................................................... (e) Jadual dibawah menunjukkan enam pasangan kacukan antara induk-iduk yang mempunyai kumpulan darah tertentu dan peratusan anak yang diperolehi mengikut kumpulan darah. Nyatakan genotip induk bagi keenam-enam pasangan kacukan didalam jadual yang disediakan. Pasangan Fenotip induk Fenotip anak (%) Genotip induk A B AB O 1 B x O 100 x 2 B x AB 25 50 25 x 3 B x A 25 25 25 25 x 4 B x AB 50 50 x 5 B x B 75 25 x 6 B x O 50 50 x 3/2002 Sistem kumpulan darah ABO pada manusia merupakan satu contoh saling tindakan alel yang dilustrasikan olel alel berbilang. (a) Apakah yang dimaksudkan dengan alel berbilang?................................................................................................................................................................................................................................................................................................ (b) Nyatakan gabungan –gabungan alel yang mungkin dalam sistem kumpulan darah ABO................................................................................................................................................................................................................................................................................................ (c) Jadual di bawah menunjukkan tiga pasangan suami isteri dengan kumpulan darah masing-masing dan peratusan bilangan anak yang diperoleh mengikut kumpulan darah. Pasangan Kumpulan (%) bilangan anak mengikut kumpulan Genotip induk darah induk darah A B AB O Pertama O x A 50 50 x Kedua B x A 25 25 25 25 x Ketiga A x AB 50 25 25 x Nyatakan genotip induk bagi setiap pasangan kacukan dalam jadual diatas. (d) Sistem kumpulan darah ABO menunjukkan fenomena alel kodominan. (i) Takrifkan alel kodominan................................................................................................................................................................................................................................................................................................. (ii) Nyatakan alel kodominan dalam sistem kumpulan darah ini................................................................................................................................................. 16.2.6 Linked genes o Are genes that are situated on the same chromosome o All genes on a single chromosome form a linkage group and usually pass into the same gamete and inherited together o Genes belonging to the same linkage group do not show independent assortment. o Since these genes do not conform to Mendel’s principle of independent assortment they fail to produce the expected ………………ratio in a breeding situation involving dihybrid inheritance. o Example: when pure breeding grey bodied long winged Drosophila are crossed with black bodied vestigial winged Drosophila, the F2 generation shows an approximately 3:1 ratio of parenral phenotypes  o In practice , though this 3:1 ratio is never achieved and four phenotypes are invariably produced o This is because total linkage is rare o Most breeding experiments involving linkage produce approximately equal numbers of parental phenotypes and a significantly smaller number of phenotypes showing new combinations of characteristics (recombinants) o Definition of linkage: “two or more genes are said to be linked when phenotypes with new gene combinations (recombinants) occur less frequently than the parental phenotypes.” o Discovery of linkage by Thomas H.Morgan o A cross between a male homozygous grey-bodied long winged Drosophila and a female homozygous black bodied vestigial winged Drosophila , produced heterozygous grey bodied long winged F1 offspring. o Test crossing the F1 generation flies with homozygous double recessive flies produced the following results: Parental phenotypes : grey body, long wing 965 black body, vestigial wing 944 Recombinant phenotypes: black body, long wing 206 grey body, vestigial wing 185 o These results indicate that the genes for body colour and wing length are linked. o The appearance of recombinant genotypes are due to crossing over which occurs during prophase 1 of meiosis. o Crossing over is a process of exchange between homologous chromosome, which give rise to new combination of traits and it is the only way in which linked genes are separated o When crossing over occurs between the two loci, two of the four haploid gametes are parental while the other two are recombinants o Importance of crossing over: results in recombination of linked genes.As many chiasmata can occur randomly at different parts along the homologous chromosomes, this can produce large amount of genetic variety in the gametes. Nov 85/K2/6 Biji jagung (Zea mays) boleh mempunyai pelbagai warna dan bentuk.Warna merah dan bentuk tidak kecut masing-masing adalah dominant keatas warna putih dan bentuk kecut. Satu pengacukkan telah dilakukan antara baka homozigus merah kecut dengan baka homozigus putih tidak kecut.Generasi F1 kemudiannya telah dikacukujikan dengan baka homozigus putih kecut dan progeny berikut dihasilkan. Fenotip bilangan pokok Merah, kecut 21379 Putih, tidak kecut 21096 Merah, tidak kecut 638 Putih, kecut 672 (a) Adakah progeny kacukan uji ini menepati Nisbah Mendel? ……………………………………………………………………………………………… (b) Apakah yang telah berlaku dalam kacukan uji ini? ……………………………………………………………………………………………… ……………………………………………………………………………………………… (c) Nyatakan dua fakta asas yang boleh menjelaskan mengapa nisbah fenotip sedemikian didapati ……………………………………………………………………………………………… ……………………………………………………………………………………………… (d) Berikan genotip bagi setiap progeny dan buat satu gambarajah bagi pindah silang yang telah berlaku. 3/2003 Corn (Zea mays) seeds may have various colours and shapes. Red (R) and non-shrunken (S) are dominant over white (r) and shrunken (s) respectively. A cross was done between a homozygous red shrunken line and a homozygous white non-shrunken line. The F 1 generation was then crossed with a homozygous white shrunken line, and the following progeny were produced. Phenotype Number of plants Red, shrunken 48 White, non-shrunken 46 Red, non-shrunken 13 White, shrunken 12 (a) State the type of cross between the F 1 generation and the homozygous white shrunken line above. ……………………………………………………………………………………………… (b) Does the above cross conform to the Mendelian ratio? Give reasons for your answer. ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… (c) Explain how the situation in (b) can occur. ……………………………………………………………………………………………… ……………………………………………………………………………………………… …………………………………………………………………………………………… (d) State the genotypes of the F2 progeny. Sex-linked genes  Genes located on the sex chromosome and are transmitted along with those determining sex.  In humans , most sex-linked are carried on the X chromosome  Female of sex-linked traits are haemophilia and red-green colour blindness in humans and eye colour in fruit fly Drosophila i. Haemophilia  or “ bleeder’s disease” is a sex-linked recessive condition which prevents the formation of factor VIII, an important factor in increasing the rate of blood clotting.  The gene for substance VIII is carried on the non-homologous portion of the X chromosome and can appear in two allelomorphicforms; normal (dominant) and mutant (recessive) Let: H represent normal allele for blood clotting (dominant) h represent allele for haemophilia XX represent female chromosome XY represent male chromosome  The following possible genotypes and phenotypes can occur: genotype phenotype XHXH normal female XHXh normal female (carrier) XHY normal male XhY haemophiliac male Mechanism of inheritance of the sex-linked allele for haemophilia Parental phenotypes normal female (carrier) x normal male Parental genotypes Meiosis Gametes Random fertilization Offspring genotypes Offspring phenotypes ii. the inheritance of white eye colour in Drosophila  this is the first sex-linked trait discovered in Drosophila by TH Morgan  example : a homozygous redeyed female is crossed with white-eyed male Let: R represent normal (wild-type) allele for red colour r represent recessive allele for white eye colour Parental phenotypes red -eyed female x white-eyed male Parental genotypes Meiosis Gametes Random fertilization Offspring genotypes Offspring phenotypes F1 genotypes F1 phenotypes Crossing of the F1 flies Parental phenotypes Parental genotypes Meiosis Gametes Random fertilization F2 genotypes F2 phenotypes Morgan’s reciprocal experimental crosses between red-eyed and white-eyed Drosophila Parental phenotypes red -eyed male x white-eyed female Parental genotypes Meiosis Gametes Random fertilization F1 genotypes F1 phenotypes Parental phenotypes white -eyed male x red-eyed female Parental genotypes Meiosis Gametes Random fertilization F1 genotypes F1 phenotypes Pedigree analysis  A pedigree is a systematiclisting of the anxestors of a give individual or the construction of family tree in order to study the inheritance of genes in humans.  A series of symbols are used to represent different aspects of a pedigree.  Female as ○  Male as  Mating as horizontal lines  Offspring are connected by a vertical line to the mating line  Phenotypes different shadws or colours  Individuals within a generation with Arabic numerals from left to right Nov 95 Gambar rajah di bawah menunjukkan salasilah pewarisan satu cirri pada haiwan akibat gen autosom dominant. Bentuk segi empat hitam dan bulatan hitam mewakili individu yang mewarisi ciri yang sama. (a) Nyatakan progeny sebagai hasil kacukan induk I, dan berikan genotip bagi setiap progeny tersebut. Progeni Genotip (b) Lukis gambar rajah kacukan antara individu II2 dengan III5. Berikan nisbah fenotip dan nisbah genotip progeni yang terhasil. (c) Dengan bantuan gambar rajah kacukan , hitung kebarangkalian untuk sesuatu progeni mendapatkan ciri gen autosom dominan sebagai kacukan II2 dengan III3 Sex determination a. sex chromosomes  In many organisms such as humans and other mammals, the sex of an individual is determined by specific chromosomes known as sex chromosomes, the X and Y chromosomes.  Example: every human cell contains 23 pairs of chromosomes of which 22 pairs are autosomes and one pair are sex chromosomes  Females have two large X chromosomes (XX) and they are the homogametic sex.  Males have one X chromosome and one smaller Y chromosome (XY) and therefore the heterogametic sex.  Both X and Y chromosomes contain homologous segments with incompletely sex-linked genes  They also contain non-homologous segments which are completely sex-linked  Genes on the non-homologous segment of the Y chromosome are known as holandric genes, which will only be transmitted to the males. b. sex determining mechanism i. sex chromosomes XY system  characteristics of all mammals  the female is homogametic sex, which produces one type of egg , all with the X chromosomes.  The male is heterogametic sex, producing two types of sperms, ½ with the X chromosomes and ½ with the Y chromosomes   The sex of an individual is determined at the moment of fertilization , depending on which type of sperm fuses with the egg. In man, the presence of Y chromosome determines maleness. (sex determination by chromosome number) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 XO system 27 28 29 30 31 32 33 34 35 36  Found in some insects, example from the orde Hemiptera (true bugs) and Orthoptera (grasshoppers ad cockroaches).  The males are heterogametic : ZW system  Examples: butterflies, moths, silkworms, some birds and fishes have the opposite system  The females are heterogametic (ii) Genetic balance  The presence of the Y chromosome in Drosophila, though it is essential for fertility, apparently has nothing to do with sex  Instead the factors for maleness are found on the autosomes and it takes 2X chromosomes to mask their effects and produce a female. Nov.1987 Jadual dibawah menunjukkan kesan nisbah bilangan kromosom X kepada bilangan set autosom terhadap penentuan seks pada Drosophila sp. Bilangan kromosom X Bilangan set autosom Nisbah m/n Fenotip seks (m) (n) 3 2 1.50 Superbetina 2 2 1.00 Betina normal 2 3 0.67 Antara seks 1 2 0.50 Jantan normal 1 3 0.33 superjantan (a) Nyatakan tiga kesimpulan yang boleh dibuat daripada maklumat diatas. ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… (b) Pada Drosophila sp, kromosom seks yang manakah, X atau Y yang memainkan peranan yang lebih penting didalam penentuan seks? Jelaskan jawapan anda. ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… ……………………………………………………………………………………………… (c) Bina satu gambarajah untuk menunjukkan hasil kacukan diantara seekor Drosophila sp jantan normal dengan seekor Drosophila sp. betina normal. Gene complex  Two or more genes situated at different loci interacting together, controlling a single characteristics.  Example: inheritance of the shape of the comb in domestic fowl.  There are genes at two loci situated on different chromosomes, which interact to give rise to four distinct phenotypes known as pea, rose, walnut and single comb.  The appearance of pea comb and rose comb are each determined by the presence of their respective dominant allele (P or R) and the absence of the other dominant allele (i.e pea: P_rr and rose: ppR_ ) 16.3 Genetic mapping o To determine: (i) the relative positions and sequences of genes on a chromosome (ii) the relative distances between genes o Chromosome maps are constructed by directly converting crossover frequency or value (COV) into hypothetical distances along the chromosome. o A crossover frequency or value of 4% between genes A and B means that those genes are situated 4 units apart on the same chromosome.  o The crossover frequency (COV) or recombinant frequency can be calculated using the formula : o Consider the results obtained from a test cross between F1 generation flies (grey body, long wing) with homozygous recessive flies (black body, vestigial wing) Parental phenotypes : grey body, long wing 965 black body, vestigial wing 944 Recombinant phenotypes: black body, long wing 206 grey body, vestigial wing 185 o Therefore the recombinant frequency or COV (show your working) = o This means that the distance between gene G and L are................... map units. (COV of 1% = distance of 1 map unit on a chromosome) o The further apart linked genes are on the chromosome , the greater the possibility of......................................... occurring between them. Exercises: 1. In maize the genes for coloured seed and full seed are dominant to the genes for colourless seed and shrunken seed. Pure breeding strains of the double dominant variety were crossed with the double recessive variety and a test cross of the F 1 generation produced the following results. Coloured, full seed 380 Colourless, shrunken seed 396 Coloured, shrunken seed 14 Colourless, full seed 10 Calculate the distance in units between the genes for coloured seed and seed shape on the chromosomes.  Recombination frequency or crossing over value (COV) will not only indicate the distance between two genes, it can also indicate the linear sequence of the genes.  Consider the following crossover values as determined by a series of breeding experiments involving four genes. P,Q, R and S. P ─ Q = 24%. R─ P = 14%. R─ S = 8%, S ─ P = 6%  To calculate the sequence and distances apart of the genes, a line is drawn representing the chromosome and the following procedure carried out:  i. insert the positions of the genes with the least COV in the middle of the chromosome, that is S ─ P = 6% xiii. examine the largest COV, that is R─ S = 8%, and insert both possible positions of R on the chromosome, relative to S. iii. repeat the procedure for the next largest COV, that is R─ P = 14%.This indicates that the right hand position of R is incorrect. iv. repeat the procedure for the COV for P ─ Q = 24%.The position of Q cannot be ascertained without additional information. If for example, the COV for Q─R = 10% , this would confirm the left hand position for gene Q. Exercises. 1. Jadual dibawah menyenaraikan nilai pindah silang (NPS) diantara gen-genP,Q, R dan S pada satu kromosom. Pindah silang diantara gen NPS P dengan Q 20% P dengan R 30% S dengan R 20% S dengan Q 10% Jujukan gen pada kromosom itu ialah Topic : Transmission Genetics (MCQ) 1. In humans , gene r is responsible for blonde hair and recessive to gene R for black hair. What are the probable genotypes for P and Q? A RR x RR only B Rr x Rr or RR x Rr C RR x Rr or rr x rr D Rr x Rr or RR x RR 2. The shape and colour of the seed coat in a plant species X is controlled by a pair of different alleles. The round (B) is dominant to the wrinkled (b), and the yellow (K) is dominant to the green(k). A cross between the round , yellow and the wrinkled, yellow seed plants produces the following result. What is the parental genotype that will produce a round and yellow seed? A BBKK B BbKK C BbKk D BBKk 3. The inheritance of the disease is controlled by A autosomal recessive gene B autosomal dominant gene C X-linked recessive gene D X-linked dominant gene 4. What is the phenotype ratio for the crossing of F 1 individuals which are produced from the crossing of AABB x aabb?  A 3:1 B 1:2:1 C 1:1:1:1 D 9:3:3:1 5. Puan Aishah has type B blood group and Puan Minah has type A. Both of them give birth on the same day at the same hospital.Puan Aishah named her child Abu while Puan Minah named her child Yahya. After being discharged from the hospital, Puan Minah suspected that her child had been changed with that of Puan Aishah’s. A blood test found that Abu has type O blood group while Yahya has type B blood group. Which of the following statements is/are true about the above case if their husbands have type a blood group. I Puan Minah’s assumption is not correct II Yahya is Puan Aishah’s child III their children have been changed at the hospital IV Yahya can have type B blood group A II only B I and II C I and III D II,III and IV 6. In a species of flowering plant, C RCR genotype produces red flowers, CWCW genotype produces white flowers and CRCW genotype produces pink flowers.What is the percentage of the progeny that have pink flowers if a cross is made between C RCW and CRCW? A 0% B 25% C 50% D 75% 7. In an insect, the normal wings (vg+) and wild-type body (b+) are dominant to the vestigial wings (vg) and the black body (b). a test cross produces 500 F 1 progeny as follows. Normal wings, black body 46 Vestigial wings, wild-type body 48 Normal wings, wild-type body 199 Vestigial wings, black body 207 Which of the following conclusions about the test-cross are correct? I The distance between the loci is 18.8 map units II the genes for the wing shape and the body colours are linked III the genotype of the insect used in the test-cross is vg+ vg+ b+ b+ A I and II B I and III C II and III D I,II and III 8. Gregor Mendel crossed a purple flowered plant with pollen of a white flowered plant. The resulting progeny are all purple flowered plants. He then carried out a series of crosses as follows. I F1 plant with any white flowered plant II F1 plant with purple flowered plant III pollen of homozygous purple flowered plant with white flowered plant Which of the following are true of the above crosses? I II III A test-cross back cross reciprocal cross B back cross test-cross reciprocal cross C back cross reciprocal cross test-cross D reciprocal cross test-cross back cross 9. Which is used to produce a pure breed variety? A selfing B test cross C back cross D reciprocal cross 10. In plants , a monohybrid cross between two F 1 individuals produces F2 progeny with a phenotypic ratio 1:2:1. Which conclusion cannot be drawn from the cross? A F1 individuals are heterozygous B both parents in the P generation are homozygous C the cross is an example of multiple alleles D the cross is an example of incomplete dominance 11. In humans, colour blindness is a recessive sex-linked characteristics which is controlled by allele B or b. What genotypes of the individuals I-1, II-5, and III-4 could be predicted ? I-1 II-5 III-4 A XbXb XBXb XBXB B XbXb XBXB XBXB C XXB b XX B B XBXb D XBXB XBXb XbXb 12. A test cross between individuals of genotypes below is carried out as follows: The genotype and number of progeny obtained are as shown below. What is the map distance , in map units, between the two genes? A 9 B 11 C 20 D 40. 13. A cross between red-flowered and white flowered snapdragons produces all pink- flowered progeny. This type of interaction is called A epistasis B codominance C complete dominance D incomplete dominance