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Section 5 Mendelian Genetics (The Inheritance of Many Traits) Independent Assortment (Mendel second law) The law of independent assortment states that.

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Presentation on theme: "Section 5 Mendelian Genetics (The Inheritance of Many Traits) Independent Assortment (Mendel second law) The law of independent assortment states that."— Presentation transcript:

1 Section 5 Mendelian Genetics (The Inheritance of Many Traits) Independent Assortment (Mendel second law) The law of independent assortment states that each pair of alleles segregates independently of the other pairs and all possible combinations of alleles can occur in the resulting gametes. This law is dependent on the random arrangement of homologous pairs at metaphase. Mendel Law of segregation: 1- One trait is controlled by one gene. 2- Each gene is found in two alleles 3- During Gamete formation each allele is segregated in one gamete A)Dominancy (Co-dominance (Complete Dominance), Incomplete Dominance) B)Multiple Alleles C)Lethal genes (Dominant Lethal, Recessive Lethal) Exceptions to Mendel 1:

2 In two-trait crosses, genotypes of the parents require four letters because there is an allelic pair for each trait. Gametes will contain one letter of each kind in every possible combination. (why?) Crossing individuals who are heterozygous for two traits can produces four phenotypes. The ratio of these four phenotypes will be in a 9:3:3:1. Two-Trait Crosses Slide Key: 2 trait -- > 4 letters? Gamete -- > 1 letter/Comb. Cross. 2 indiv. Heter. --> 9:3:3:1

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5 Segregation and independent assortment Independent Assortment (Mendel second law) The law of independent assortment states that each pair of alleles segregates independently of the other pairs and all possible combinations of alleles can occur in the resulting gametes. This law is dependent on the random arrangement of homologous pairs at metaphase.

6 Repeated! Independent Assortment (Mendel second law) The law of independent assortment states that each pair of alleles segregates independently of the other pairs and all possible combinations of alleles can occur in the resulting gametes. This law is dependent on the random arrangement of homologous pairs at metaphase.

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8 Modifying the Dihybrid Ratio Combinations of two gene pairs involving two modes of inheritance modify the 9:3:3:1 ratio Mendels principle of independent assortment applies to situations in which two modes of inheritance occur simultaneously, provided that the genes controlling each character are not linked on the same chromosome. The probability of each phenotype arising in a cross can be determined by the forked-line method or by Punnett square assuming that the genes under consideration undergo independent assortment.

9 Consider a cross involving two independently assorting genes with recessive alleles: Aa Bb x Aa Bb F2 ratios: 9/16 A_B_ 3/16 A_ bb 3/16 aa Bb 1/16 aa bb What happens if both A and B affect the same trait? (Gene Interactions) If both genes affect the same trait, segregation of genes is the same, but can get new phenotypic patterns

10 Modifying the Dihybrid Ratio Exception to Mendel second law A)Gene interaction. B)Complementary Genes. C)Epistasis (dominant – Recessive). D)Recessive phenotype. E)Novel phenotype.

11 A.Four phenotypes, all affecting the same structure or process Ex.- Inheritance of Comb color in check Rose, pea, walnut, and single combs

12 Try this! RRWW x rrww P1: G1: F1: P2 (F1) x P2 (F1) P2 G2: F2

13 B. Complementary Genes (Recessive allele of either of two genes causes phenotype) 9/16 A_ B_ (Dominant Phenotype) 3/16 A_ bb 7/16 3/16 aa B _ (Recessive Phenotype) 1/16 aa bb Example: Inheritance of flowers color in pea

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15 C- Epistasis Occurs when one gene masks (hide) the effect of another gene or when two gene pairs complement each other such that one dominant allele is required at each locus to express a certain phenotype.

16 Dominant Epistasis Dominant allele at one genetic locus masks the expression of the alleles of a second locus Occurs when the dominant allele at one genetic locus masks or hide the effect of the other allele at the other locus. EX: Summer squash color White yellow Green 12 : 3 : 1 WWYY x wwyy G1 F1 WwYy x WwYy G2 F2 Punnett square Analysis? W-Y- W-yy Ww-Y- wwyy

17 Recessive Epistasis Occurs when the homozygous recessive genotype masks or suppresses expression of another gene Ex: Inheritance of coat color in mouse Agouti Black albino 9 : 3 : 4 If You know that (A) is a recessive allele that masks the expression of allele (B) in the trait describing the coast color of mice, and (A) is responsible for Agouti colour, while (B) will is responsible for the black coat colour. Find the expected ratio of the resulting phenotypes in the second filial generation

18 If you know that one dominant allele is enough for a triangular seedling shape in the capsule of sepherds purse. Find the expected ratio of the resulting phenotypes in the second filial generation D- Recessive Phenotype

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20 Ex: Seed Capsule in Shepherds purse 15 (Triangular ) : 1 (Ovoid)

21 E-Novel Phenotypes In addition to modifying the dihybrid ratio, gene interaction may introduce new phenotypes which will be observed among heterozygotes Ex: Squash fruit shape Disc (9) Sphere (6) Long (1) If you know that the shapes of the squash fruit are: disc, sphere, and long, known that interaction between dominant and recessive alleles of the two genes results in a novel phenotype which is the sphere shape fruit Find the expected ratio of the resulting phenotypes in the second filial generation

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