Genetics. Instructor: Dr. Jihad Abdallah Topic 7: Mendelian Genetics

Transcription

1 Genetics Instructor: Dr. Jihad Abdallah Topic 7: Mendelian Genetics 1

2 In 1866, Gregor Mendel published the results of a series of experiments that formed the basis of the modern day genetics. He performed his experiments on the garden pea (Pisum sativum) simple genetic crosses between strains of pea. Garden pea is self-fertilizing in nature, but easy to cross-breed experimentally. He studied seven characters involving seed shape and color, pod shape and color, flower color and position, and plant height. 2

3 The Monohybrid Cross Involves a single character with two contrasting forms ( e.g., seed color: yellow, green). The monohybrid cross is made by mating individuals from two parent strains, each strain exhibiting one of the two contrasting forms of the character under study. P1(White flowers) x P2 (Purple flowers) F1 F2 Selfing (First filial generation) All purple (Second filial generation) 705 Purple and 224 White (3 : 1 ratio) 3

4 3 1 4

5 5

6 Mendel s first three postulates 1. Unit factors in pairs: genetic characters are controlled by unit factors that exist in pairs in individual organisms (Diploid). These unit factors are now called genes or alleles. Allele: an alternative form of the same gene. Because the factors occur in pairs, three combinations are possible: - two factors for the purple color (PP) Homozygous - two factors for the white color (pp) - one factor for each (Pp) Heterozygous 6

7 2. Dominant/recessive factors: when two unlike unit factors responsible for a single character are present in a single individual, one unit factor is dominant over the other which is said to be recessive. - PP purple - Pp purple - pp white P (the purple color allele) is the dominant and p (the white color allele) is the recessive. 7

8 3. Segregation: during the formation of gametes, the paired unit factors (alleles) separate or segregate randomly so that each gamete receives one factor (allele) or the other with equal probability (chance). This third postulate is called Mendel s first law Law of Segregation Pp x Pp (½ P, ½ p) x (½ P, ½ p) ¼ PP, ½ Pp, ¼ pp ¾ purple, ¼ white 1: 2 : 1 Genotypic ratios 3: 1 Phenotypic ratios 8

9 Punnett Square Female gametes ½ P ½ p Male gametes ½ P ¼ PP ¼ Pp ½ p ¼ pp ¼ pp Genotypic ratios: ¼ PP : ½Pp : ¼ pp 1 : 2 : 1 Phenotypic ratios: ¾ Purple : ¼ white 3 : 1 9

10 10

11 The Test Cross (single character) A test designed to reveal the genotype of an individual which shows the dominant phenotype. In this cross, the individual with unknown genotype but showing the dominant phenotype is crossed to a homozygous recessive individual (an individual showing the recessive phenotype). Example: Tall plant (DD or Dd) x Dwarf plant (dd) - If the geotype of the tall plant is DD we expect all offspring plants to be tall (have Dd genotype) - If the genotype is Dd we expect half the offspring to be tall (Dd) and the other half to be dwarf (dd). 11

12 The Dihybrid Cross A cross which involves two characters Mendel crossed two pure strains of pea, one with round yellow seeds and one with wrinkled green seeds all F1 offspring are dihybrids having round yellow seeds. - Round seeds allele (R) is dominant over wrinkled seed allele (r) - Yellow seed allele (Y) is dominant over the green seed allele (y) He then allowed the F1 plants to self-fertilize and got the following results: - 9/16 round, yellow - 3/16 round, green - 3/16 wrinkled, yellow - 1/16 wrinkled, green 12

13 The dihybrid cross P1 x P2 (round, yellow) (wrinkled, green) F1 dihybrids (all have round, yellow seeds) Self-fertilization F2 9/16 round, yellow 3/16 round, green 3/16 wrinkled, yellow 1/16 wrinkled, green 9 : 3 : 3 : 1 phenotypic ratio 13

14 Mendel s Fourth Postulate Independent assortment: during gamete formation, segregating pairs of unit factors assort independent of each other (the alleles at one locus segregate into the gametes independently of the pair of alleles found at a different locus). This is Mendel s second law and is called the Law of Independent Assortment RrYy 4 gamete types with equal proportions: RY, Ry, ry, ry 14

15 15

16 Easy way to obtain phenotypic and genotypic ratios Phenotypic ratios First character Second character (3/4 round: 1/4 wrinkled) x (3/4 yellow:1/4 green) ¾ round x ¾ yellow 9/16 round, yellow ¾ round x ¼ green 3/16 round, green ¼ wrinkled x ¾ yellow 3/16 wrinkled, yellow ¼ wrinkled x ¼ green 1/16 wrinkled, green Genotypic ratios (1/4 RR: 2/4 Rr: 1/4 rr ) x (1/4 YY : 2/4Yy : 1/4 yy) 1/16 RRYY, 2/16RRYy, 1/16 RRyy 2/16 RrYY, 4/16 RrYy, 2/16 Rryy 1/16 rryy, 2/16 rryy, 1/16 rryy 16

17 n = number of characters (loci) monohybrid dihybrid trihybrid n -hybrid F 1 gametic genotypes n Proportion of homozygous recessives in F 2 1/4 1/16 1/64 (1/2 n ) 2 Number of different F 2 phenotypes given complete dominance Number of different F2 genotypes n n 17