The document discusses Mendel's experiments on monohybrid crosses using pea plants. It explains that a monohybrid cross involves a single trait determined by one gene, such as tall vs dwarf plants. Mendel observed that the ratio of dominant to recessive phenotypes in the F2 generation is 3:1, while the ratio of genotypes is 1:2:1. His experiments supported his laws of segregation and independent assortment.

1. LECTURE 2

2. OBJECTIVES Explain the Mendel’s experiments on monohybrid cross. Explain monohybrid cross State Mendel’s first law/law of segregation. Calculate the genotypic ratio (1:2:1) and phenotypic ratio (3:1) in F 2 generation MONOHYBRID

3. Monohybrid Breeding that involves a single characteristic determined by one gene . eg. : tall and dwarf plants The ratio of dominant to recessive phenotypes is 3 : 1.

4. Mendel’s Law of Segregation Two alleles for heritable character separate (segregate) during gamete formation and end up in different gametes.

5. Mendel’s cross of pea plants differing in flower color Page 251

6. Cross of pea plants differing in flower color All of the F 1 generation are Pp heterozygotes (purple flowers) When two heterozygotes F 1 are crossed, 3 kinds of F 2 offspring are possible: PP homozygotes (purple flowers) Pp heterozygotes (also purple flowers) pp homozygotes (white flowers)

7. So, in the F 2 generation, the ratio of dominant to recessive phenotypes is 3: 1 But the ratio of genotypes is 1:2:1 (1 PP : 2 Pp : 1pp)

8. Mendel Experiment P : Tall x Dwarf F1: Tall (self cross) F2: Tall(787) : Dwarf (277) 3 : 1

9. TT tt P : Tall x Dwarf G : all T all t F1: Tall x Tall Tt Tt G : ½ T ½ t ½ T ½ t F2 : ¼ TT ¼ Tt ¼ tT ¼ tt Tall Dwarf segregation

10. P G Progeny VV vv Vv vv V v Vv Vv vv ½ V ½ v v

11. Monohybrid inheritance in fruit fly, Drosophila melanogaster P : normal wing x vestigial wing G : all V all v F1: normal wing x normal wing ½ V ½ v ½ V ½ v F2 : ¼ VV ¼ Vv ¼ vV ¾ vv

12. LAW OF SEGREGATION vs. MEIOSIS

13. LAW OF SEGREGATION- The two alleles for each gene separate during gamate formation. 1) The R and r alleles segregate at anaphase I, yielding two types of daughter cells for this locus. 2) Each gamete gets one long chromosome with either the R or r allele 3) Fertilization recombines the R and r alleles at random

14. A test of Mendel’s hypothesis using test cross for monohybrid Crossed heterozygous with homozygous recessive E;g cross round peas (Rr) with wrinkled peas (rr). - Result : phenotypic ratio 1:1