Di- & Trihybrid Crosses

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1. Genetic Inheritance
2. • Genotype: the 2 alleles of an organism for a given gene • Phenotype: an organism’s traits • Alleles: variations of a gene • Homozygous: twice the same allele for a gene (PP, pp) • Heterozygous : two different alleles for a gene (Pp) Genetics Terminology
3. • Dominant allele: An allele that is expressed in the phenotype in the homozygous ANDthe heterozygous states in the genotype • Dominant phenotype: Phenotype dictated by the dominant allele • Recessive allele: An allele that is expressed in the phenotype only if in the homozygous state in the genotype • Recessive phenotype: Phenotype dictatedby the recessive allele Genetics Terminology
4. • Characteristic: heritable feature (i.e., fur color) • Trait: variant for a character (i.e.brown) • True-bred: all offspring are of same variety • Hybridization: crossing of 2 different true-breds We label the different generations of a cross as: • P generation (Parents) • F1 generation (1st Filial generation) • F2 generation (2nd Filial generation) Genetics Terminology
5. Genetic Inheritance Mendelian inheritance Di- and Polyhybrid crosses
6. Dihybrid cross: Follow inheritance of two characteristics 2 genes, 4 alleles R = round r = wrinkled Y = Yellow y = green
7. Dihybrid cross
8. Dihybrid cross
9. Dihybrid cross
10. Dihybrid cross Mendel’s 4th Postulate Results of Mendel’s dihybrid crosses canbe understood by considering the probabilities separately – COLOR: ¾ are yellow, ¼ aregreen – SHAPE: ¾ are round, ¼ arewrinkled – Use the product law ofprobability — the combined probabilityof the twooutcomes is equal to the product of their individual probabilities
11. Dihybrid cross Probabilities
12. Dihybrid cross (4)Independent Assortment • During gamete formation, segregating pairs of unit factors assort independentlyof each other • This means that all possible combinations of gametes will be formed with equal frequency • Final dihybrid ratio (assumes independent assortment and random fertilization) is 9:3:3:1 Mendel’s 4th Postulate
13. Test crosses = Back-crosses Dihybrid inheritance Ø Smooth = W Dominant over Wrinkled = w Recessive WW Smooth Ww Smooth ww Wrinkled Ø Yellow = G Dominant over Green = g Recessive GG Yellow Gg Yellow gg Green Ø Smooth and yellow can be: WWGG or WwGG or WWGg or WwGg Cross them with double homozygous recessive wwgg to find out what is the actual genotype wwgg only wwGG or wwGg WWGG or WWGg or WwGG or WwGg Wwgg or WWgg
14. Mendelian inheritance Test crosses = Back-crosses Dihybrid inheritance Ø Smooth = W Dominant over Wrinkled = w Recessive WW Smooth Ww Smooth ww Wrinkled Ø Yellow = G Dominant over Green = g Recessive GG Yellow Gg Yellow gg Green Ø Smooth and yellow can be: WWGG or WwGG or WWGg or WwGg Cross them with double recessive wwgg wrinkled and green to find out what is the actual genotype wrinkled green: wwgg only wrinkled Yellow: wwGx (wwGG or wwGg) Smooth Yellow: WxGx (WWGG or WWGg or WwGG or WwGg) Smooth green: Wxgg (Wwgg or WWgg)
15. Mendelian inheritance Test crosses = Back-crosses Dihybrid inheritance 1. WWGG X wwgg Gametes WG 100% WWGG Offspring WwGg100% Smooth and Yellow Gametes wg100% wwgg wrinkled green
16. Mendelian inheritance Test crosses = Back-crosses Dihybrid inheritance 1. WwGG X wwgg Gametes WG 50% wG 50% WwGG WwGg 50% Smooth Yellow wwGg 50% wrinkled Yellow Gametes wg100% wwgg wrinkled green
17. Mendelian inheritance Test crosses = Back-crosses Dihybrid inheritance 1. WWGg X wwgg Gametes WG 50% Wg 50% WWGg WwGg 50% Smooth Yellow Wwgg 50% Smooth green Gametes wg100% wwgg wrinkled green
18. Mendelian inheritance Dihybrid inheritance WG 25% Wg 25% wG 25% wg 25% Test crosses = Back-crosses 1. WwGg X wgg WwGg Gametes Gametes wg100% WwGg 25% Smooth Yellow Wwgg 25% Smooth green wwGg 25% wrinkledYellow wwgg 25% wrinkled green wwgg wrinkled green
19. Mendelian inheritance Test crosses = Back-crosses Dihybrid inheritance 100% 50% 50% 1:1 50% 50% 1:1 25% 25% 25% 25% 1:1:1:1
20. Trihybrid Cross • Punnett square has 64 boxes… • Demonstrates that Mendel’s principles apply to inheritance of multiple traits Follow inheritance of three characteristics
21. Trihybrid cross Probabilities
22. Trihybrid cross Useful Rules Examples: 1. Aaà [A, a] à [AA, Aa, aa] à [A ora] 2.AaBb à [AB, Ab, aB, ab] à [AABB, AaBB, aaBB,AABb, AaBb, aaBb, aaBB, aaBb, aabb] à [AB, Ab, aB,BB]
23. Trihybrid cross Example: Tall, pinched pod, axial X dwarf, inflated, terminal TT pp AA tt PP aa F1: Tall, inflated, axial Tt Pp Aa F1 X F1 --> ?
24. Trihybrid cross F1 X F1 --> ? Ø Figure out the gametes produced by each parent (use 2n) Parents: Tt Pp Aa; 23 = 8 Eight possible gametes from each parent: TPA, Tpa, TpA, tPA, Tpa, tPa, tpA, tpa
25. Trihybrid cross F1 X F1 --> ? Ø Breaks down multi-hybrid crosses into a series of monohybrid crosses Tt Pp Aa X Tt Pp Aa Break into: Tt X Tt; Pp X Pp; AaX Aa • Tt X Tt --> 3/4 T: 1/4 t • Pp X Pp --> 3/4 P: 1/4 p • Aa X Aa --> 3/4 A: 1/4 a
26. Trihybrid cross F1 X F1 --> ? Ø Combine the individual ratios (multiply) to get the final ratio F 2 offspring: 3/4 T 1/4 t < 3/4 P 1/4 p < 3/4 P 1/4 p < < < 3/4 A 1/4 a 3/4 A 1/4 a 3/4 A 1/4 a 3/4 A 1/4 a --> 27/64 TPA --> 9 /64 TPa --> 9 /64 TpA --> 3 /64 Tpa --> 9 /64 tPA --> 3 /64 tPa --> 3 /64 tpA --> 1 /64 tpa
27. Trihybrid cross Example:
28. Trihybrid cross Example: An other way of solving problems is using the Punnett square Three genes in fruit flies affect a particular trait, and one dominant allele of each gene is necessary to get a wild-type phenotype. What phenotypic ratios would you predict among the progeny if you crossed triply heterozygous flies? The crossing between three triploid heterozygous genes (AaBbCc) of fruit flies results in the production of 27 allele combinations with 64 genotypes. Here is the Punnett square diagram of the triploid heterozygous genes that shows all allele combinations
29. Correlation of Mendel’s Postulates with the Behavior of Chromosomes • Formed the foundationof modern transmission genetics • Unit factors,genes • Pairs, homologous chromosomes