Gene Segregation and Interaction

BIO 31 (Principles of Genetics) Lecture 4 Gene Segregation and Interaction TERMINOLOGIES 1.Factor: A particle or unit in the organism which is responsible for the inheritance and expression of a particular character. 2.Gene: Mendel’s factor is now known as gene. A gene is a particular segment of a DNA molecule which determines the inheritance and expression of a particular character. 3.Alleles or Allelomorphs: Two or more alternative forms of a gene. E.g. In pea, the gene for seed shape may occur in two alternative forms: round (R) and wrinkled (r). Round and wrinkled forms of the gene are alleles of each other. Alleles occupy same locus on homologous chromosomes 2 TERMINOLOGIES 4. Dominant: The trait which appears in the F1 (first filial generation) hybrid is called the dominant trait (Dominant Allele). 5. Recessive: The trait which is suppressed (does not appear) in the F1 hybrid is called the recessive trait (recessive allele). 6.Genotype: The genetic make-up or genic constitution of an individual (which he/she inherits from the parents) is called the genotype e.g., the genotype of pure round- seeded parent 3 TERMINOLOGIES 7. Phenotype: The appearance (morphology, physiology, and behavior) of an organism for any trait or traits is called the phenotype, e.g. for seeds, round shape or wrinkled shape is the phenotype. 8. Homozygous: An individual possessing (receiving from parents) identical alleles for a trait is said to be homozygous or pure for that trait, e.g. plant with RR alleles is homozygous for the seed shape. A homozygous always breeds true for that trait. 9. Heterozygous: An individual receiving dissimilar alleles for a trait is said to be heterozygous or impure for that trait, e.g. a plant with Rr alleles is heterozygous for the 4 TERMINOLOGIES 10.Parent generation: The parents used for the first cross represent the parent (or P) generation. 11. F1 generation: The progeny produced from a cross between two parents (P) is called First Filial or F1 generation. 12. Inbreeding: When the individuals of a progeny (e.g. F1 generation) are allowed to cross with each other, it is called inbreeding.  Mating of closely related individuals. 13. F2 generation: The progeny resulting from self hybridization or inbreeding of F individuals 5 TERMINOLOGIES 15.Monohybrid cross: The cross between two parents differing in a single pair of contrasting characters is called monohybrid cross and the F1offspring as the monohybrid(heterozygous for one trait only). 16. Monohybrid ratio: The phenotypic ratio of 3 dominants : 1 recessive obtained in the F2 generation from the monohybrid cross is called monohybrid ratio. 17. Dihybrid cross: The cross between two parents in which two pairs of contrasting characters are studied simultaneously for the inheritance pattern. The F1 offspring is described as dihybrid or double 6 Mendel’s Hybridization Experiments Involved testing 7 characters individually by hybridizing 2 varieties showing alternative traits E.g. tall x short; green-seeded x yellow-seeded The parental plants (P generation) were from pure breeding lines (homozygous) The F1 generation produces all round seeds When F1 plants were selffertilized, round and wrinkled Results of All Mendel’s Crosses in Which Parents Differed in One Character Parental F2 F1 F2 phenotype percentage 74.74; 25.26 1. All round 5474 round; 1850 Round×wrinkled wrinkled seeds 75.06; 24.94 2. Yellow×green All yellow 6022 yellow; 2001 seeds green 3. Purple×white All purple 705 purple; 224 white 75.89; 24.11 petals 74.68; 25.32 4. All inflated 882 inflated; 299 Inflated×pinched pinched pods 5. Green×yellow All green 428 green; 152 yellow 73.79; 26.21 pods 6. Axial×terminal All axial 651 axial; 207 terminal 75.87; 24.11 flowers 7. Long×short All long 787 long; 277 short 73.97; 26.03 F2 ratio 2.96:1 3.01:1 3.15:1 2.95:1 2.82:1 3.14:1 2.84:1 Mendel’s Hybridization Experiments The following pattern was established from summarizing the results: 1. For any character, F1 showed one of the alternative traits. Such character that was shown was dominant and the character that was hidden was recessive. 2. Reciprocal crosses gave the same result 3. The trait that did not appear in the F1 reappeared in the F2 but in a frequency of ¼ of the total number Mendel’s Hybridization Experiments Q: What can be deduced from these observations? Each parent must have contributed equally to the progeny F1 contains the two alternative factors or is heterozygous These two factors or alleles separate or segregate from each other during gamete formation in the F1 some gametes carry R and others r. https://images.app.goo.gl/UJPEXt8fzWqEUTAa7 Mendel’s Hybridization Experiments These two type of gametes occur at equal frequences in the ova and in the pollen grains Self fertilization of the F1 causes the random combination of the male and female gametes to form the F2 embryos This accounts for the 1:2:1 genotypic ratio and 3:1 phenotypic ratio The inheritance of seedThe homozygous dominant (RR) and heterozygous (Rr) exhibited the same phenotype (Round) characteristics follows https://images.app.goo.gl/UJPEXt8fzWqEUTAa7 Law of Dominance • In a hybrid union, the allele which expresses itself phenotypically is the dominant allele while the other allele which fails to express itself phenotypically is the recessive allele. The hybrid individual shows phenotypically only the dominant character. • The law of dominance is often described as Mendel’s first law of inheritance. Pea traits studied by Mendel Dominant and recessive characters in some plants and animals Example Dominant s PLANTS Sorghum (i) Pearly grain (ii) Awnless Maize Full Endosperm Rice Starchy endosperm ANIMAL Rabbit Black coat S Mice Normal body size Man (i) Brown eyes Recessive Appearance of F1 hybrids Chalky Pearly grain Awned Awnless Shrunken Full endosperm Glutinous Starchy endosperm white coat Black dwarf Normal blue eyes Brown Law of Segregation • On the basis of the results obtained for the monohybrid crosses, Mendel formulated the law of segregation, also called Mendel’s second law of heredity. • The law states that unit of hereditary characters (e.g. round vs. wrinkled) occur in pairs and that in the formation of gametes during meiosis, these separate from each other so that only one member of the pair goes into a particular gamete. • It is a matter of chance whether a particular gamete gets the dominant or recessive allele. • Fusion of male and female gametes during fertilization to form the embryo will restore the diploid chromosome number Law of Segregation when a pair of alleles is brought together in hybrid union, the members of the allelic pair remain together without mixing, diluting or altering each other and separate or segregate from each other A Punnett square can be used to predict genotyp es (allele combinations) and phenotypes (observable traits) of offspring from Results of All Mendel’s Crosses in Which Parents Differed in One Character Parental F2 F1 F2 phenotype percentage 74.74; 25.26 1. All round 5474 round; 1850 Round×wrinkled wrinkled seeds 75.06; 24.94 2. Yellow×green All yellow 6022 yellow; 2001 seeds green 3. Purple×white All purple 705 purple; 224 white 75.89; 24.11 petals 74.68; 25.32 4. All inflated 882 inflated; 299 Inflated×pinched pinched pods 5. Green×yellow All green 428 green; 152 yellow 73.79; 26.21 pods 6. Axial×terminal All axial 651 axial; 207 terminal 75.87; 24.11 flowers 73.97; 26.03 7. Long×short All long 787 long; 277 short F2 ratio 2.96:1 3.01:1 3.15:1 2.95:1 2.82:1 3.14:1 2.84:1 Law of Independent Assortment • Mendel’s Law of Segregation applies to the behavior of a single pair of alleles or a single gene. • When two or more genes are considered simultaneously, the Law of Independent Assortment applies • The law states that genes for different characteristics are inherited independently of one another or alleles of different gene pairs separate independently from each other and randomly combine during meiosis • Mendel based this law on the results of his dihybrid crosses Law of Independent Assortment • "When a dihybrid (or a polyhybrid ) forms gametes, (i) each gamete receives one allele from each allelic pair and (ii) the assortment of the alleles of different traits during the gamete formation is totally independent of their original combinations in the parents. • In other words, each allele of any one pair is free to combine with any allele from each of the remaining pairs during the formation for the gametes • This is known as the Law of Independent Assortment of characters. It is also referred to as Mendel’s third law of heredity Law of Independent Assortment • Considering the genotypes of the parents (RRYY and rryy), the alleles of the two gene pairs (R vs. r and Y vs. y) will separate independently from each other and randomly combine during meiosis RRYY R----Y  RY R----Y  RY rryy r---y  ry r---y  ry The gametes from the parents will be RY and ry, respectively. When the gametes of the parents are combined, the genotype of the F1 will be RrYy Law of Independent Assortment • To obtain the F2 generation, selfing or sibmating can be done ⮾ Selfing occurs when the pollen of the the same plant pollinates its own stigma  Sib-mating is considered as brother-sister mating since the F1s have common parents • To proceed to the F2 generation F1s are selfed or intercrossed RrYy x RrYy • The alleles of the two gene pairs will again separate independently from each other and randomly combine during meiosis R Y y RY Ry Y R RY y r Y y rY ry Y r rY y Ry ry Law of Independent Assortment • The gametes from the parents will be RY, Ry, rY and ry • These gametes randomly combine during fertilization as shown below using a Punnett Square: ♀ ♂ RY Ry rY ry RY RRYY RRYy RrYY RrYy Ry RRYy RRyy RrYy Rryy rY RrYY RrYy rrYY rrYy ry RrYy Rryy rrYy rryy Summary of genotypic ratio in the F2 1RRYY 2RrYY 1rrYY 2RRYy 4RrYy 2rrYy 1RRyy 2Rryy 1rryy Law of Independent Assortment Law of Independent Assortment Law of Independent Assortment Summary of genotypic ratio in the F2 1RRYY 2RrYY 1rrYY 2RRYy 4RrYy 2rrYy 1RRyy 2Rryy 1rryy • The phenotypes of the different genotypes will beWrinkled, as follows: Round, Round, Wrinkled, Yellow Green Yellow Green 1 RRYY 1 RRyy 1rrYY 1rryy 2 RRYy 2Rryy 2rrYy 3 3 2 RrYY 4 RrYy Total= 9 1 Law of Independent Assortment • The expected phenotypic ratio in the F2 will be 9:3:3:1 • Gregor Mendel in his experiment obtained the following in the F2: 315 round, yellow seeds 108 round, green seeds 101, wrinkled yellow seeds 32 wrinkled, green seeds • Analysis of the data showed that each gene pair R vs. r and Y vs. y segregated independently of each other.  The chances for a plant to have round or wrinkled seeds are independent of the chances of having yellow or green seeds Law of Independent Assortment • A simple mathematical procedure may be applied to obtain the F2 phenotypic and genotypic ratios. • This consists of calculating the probability of the genotype or phenotype for each gene pair and then multiplying these probabilities The F phenotypic ratio of this dihybrid may be computed using the forked-line or branching method as shown below: ¾ Yellow = 9/16 Round, Yellow ¾ Round ¼ Green = 3/16 Round, Green ¾ Yellow = 3/16 Wrinkled, Yellow ¼ Wrinkled ¼ Green= 1/16 Wrinkled, Green Law of Independent Assortment • The F genotypic ratio of this dihybrid may be computed as: ¼ YY = 1/16 RRYY ¼ RR ½ Yy = 2/16 RRYy ¼ yy = 1/16 RRyy ¼ YY = 2/16 RrYY ½ Rr ½ Yy = 4/16 RrYy ¼ yy = 2/16 Rryy ¼ YY = 1/16 rrYY ¼ rr ½ Yy = 2/16 rrYy ¼ yy = 1/16 rryy Law of Independent Assortment This can be extended to accommodate another gene pair in trihybrids Law of Independent Assortment Formulas NO. OF GENE PAIRS KINDS OF GAMETES KINDS OF GENOTYPE S KINDS OF PHENOTYP ES 1 2 3 4 n 2 4 8 16 2n 3 9 27 81 3n 2 4 8 16 2n Chi-Square test Q: How do we know if our data fit any of the Mendelian ratios? A statistical test that can test out ratios is the Chi-Square or Goodness of Fit test. • Chi-Square Formula for n>2 • Degrees of freedom (df) = n-1 where n is the number of classes Chi-Square test E.g. To test if the following data follows a 9:3:3:1 ratio: Observed Values Expected Values 315 Round, Yellow Seed Yellow Seed 108 Round, Green Seed Green Seed 101 Wrinkled, Yellow Seed Wrinkled, Yellow 32 Wrinkled, Green Wrinkled, Green 556 Total Seeds (9/16)(556) = 312.75 Round, (3/16)(556) = 104.25 Round, (3/16)(556) = 104.25 (1/16)(556) = 34.75 556.00 Total Seeds Chi-Square test E.g. To test if the following data follows a 9:3:3:1 ratio: Number of classes (n) = 4 df = n-1 = 4-1 = 3 Chi-square value = 0.47 Chi-Square test A Chi-Square Table Probability Degrees of Freedom 0.9 0.5 0.1 0.05 0.01 1 0.02 0.46 2.71 3.84 6.64 2 0.21 1.39 4.61 5.99 9.21 3 0.58 2.37 6.25 7.82 11.35 4 1.06 3.36 7.78 9.49 13.28 5 1.61 4.35 9.24 11.07 15.09 Chi-Square test Decision: Since the calculated ChiSquare value is lesser than the tabular Chi-Square value at 5% level of significance, we do not reject the null hypothesis that the observed values followed the Mendelian ratio of 9:3:3:1. https://images.app.goo.gl/cDBycvtugWLF1yCq5 Chromosomal Basis of Mendelian Laws • Walter Sutton and Theodor Boveri in 1900 found correlations between the behavior of the allele in a gene pair and homologous chromosomes during meiosis 3 correlations were noted: 1. If the allele exists in pair, the homologous chromosome also exists in pair; 2. If the alleles in the gene pair separate, the homologous chromosomes also separate at Anaphase I of meiosis; and 3. The alleles and chromosomes are in pairs immediately after fertilization They proposed the Chromosome Theory of Inheritance which states that the chromosomes are the carriers of the genes (i.e. genes have specific locations (loci) in the chromosomes) Chromoso mal Basis of Mendelian Laws Chromoso mal Basis of Mendelian Laws Chromosomal Basis of Mendelian Laws