Mendelian patterns of inheritance
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Gregor Mendel performed experiments with pea plants that helped establish the laws of inheritance. Through monohybrid and dihybrid crosses, Mendel discovered that traits are inherited through discrete factors (now known as genes) that segregate and assort independently during the formation of gametes. His work showed that dominant traits are expressed when only one gene is present, while recessive traits require two recessive genes to be expressed. The ratios of traits seen in subsequent generations provided evidence of his principles of inheritance and independent assortment.
Mendelian patterns of inheritance
- 1. Why do we look like family members or not? By: Peter T. Andres Reporter
- 3. At the time, most breeders believed parents of contrasting appearance always produce offspring of intermediate appearance. Mendel’s experiments helped him formulate the particulate theory of inheritance. Inheritance involves reshuffling of genes from generation to generation.
- 4. Figure 13-2 Trait Phenotypes Seed shape Round Wrinkled Yellow Green Inflated Constricted Green Yellow Seed color Pod shape Pod color Flower color Purple White Flower and pod position Axial (on stem) Terminal (at tip) Stem length Tall Dwarf short
- 5. One-Trait Inheritance Mendel performed cross-breeding experiments between true-breeding plants. Chose varieties that differed in only one trait (monohybrid cross) Performed reciprocal crosses Parental generation = P First generation offspring = F 1 Second generation offspring = F2
- 6. Figure 13-1 Self-pollination SELFPOLLINATION Female organ (receives pollen) Eggs Male organs (produce pollen grains, which produce male gametes) Cross-pollination CROSSPOLLINATION 1. Remove male organs 2. Collect pollen from a 3. Transfer pollen to the from one individual. different individual. female organs of the individual whose male organs have been removed.
- 7. Mendel Studied a Single Trait Mendel cross-fertilized two plants, one with white flowers with one with purple flowers. The hybrids, F1 generation, all had purple flowers. Studying one trait through cross-fertilization is termed a monohybrid cross.
- 8. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Pollen transferred from white flower to stigma of purple flower Anthers removed All purple flowers as a result
- 9. Mendel Studied a Single Trait Mendel’s experiments cont’d Mendel allowed F1 generation plants to self- fertilize. Their offspring, the F2 generation, expressed (demonstrated) both purple and white flowers. The ratio of plants with purple to white flowers was always 3:1. Where did these white flowered plants come from?
- 10. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. P (parental) generation Crossfertilize Purple F1 generation White Self-fertilize F2 generation Purple Purple 3 Purple White : 1
- 11. Mendel Studied a Single Trait Mendel cont’d The F1 generation plants all resembled only parent plant; i.e. one variation of the trait is dominant. The F2 generation showed plants with both variations of the character, purple and white. The variation of the trait that was only seen in the F2 generation (white flowers) is recessive.
- 12. Mendel Studied a Single Trait Mendel cont’d The F2 generations were allowed to self- fertilize. Looking at the F3 generation, Mendel discovered that the F2 generation actually consisted of 3 different types of plants: Pure breeding purple Not pure breeding purple (produced both purple and white flowered plants. Pure breeding white. The ratio was actually 1:2:1.
- 13. Monohybrid Cross
- 14. Monohybrid Cross
- 16. Genes and Mendel’s Findings Traits are carried by genes. An individual has 2 genes or alleles for each trait, 1 on each homologous chromosome. Meiosis results in separation of the homologous chromosomes and the alleles so that each is carried by a different gamete.
- 17. Genes and Mendel’s Findings An individual with 2 identical alleles is said to be homozygous, while an individual with 2 different alleles is said to be heterozygous. The genetic make-up of an individual is its genotype. The appearance or expression of the genotype is called its phenotype.
- 18. Genes and Mendel’s Findings Mendel’s results can be predicted using Punnett squares. Dominant genes are represented by uppercase letters, ex. round peas (R) . Expressed when there is 1 or 2 dominant alleles present. Recessive genes are represented by lowercase letters, ex. wrinkled peas (r). Only expressed when there are 2 recessive alleles present.
- 19. A cross between two homozygotes Homozygous mother Meiosis Homozygous father Meiosis Male gametes Female gametes Offspring genotypes: All Rr (heterozygous) Offspring phenotypes: All round seeds A cross between two heterozygotes Heterozygous mother Female gametes Heterozygous father Male gametes Figure 13-4 Offspring genotypes: 1/4 RR : 1/2 Rr : 1/4 rr Offspring phenotypes: 3/4 round : 1/4 wrinkled
- 20. Monohybrid Cross
- 21. Monohybrid Cross
- 22. Genes and Mendel’s Findings Mendels’ Principle of Segregation Each individual has two factors for each trait. The factors segregate during gamete formation. Each gamete contains only one factor from each pair of factors. Fertilization gives each new individual two factors for each trait.
- 23. Figure 13-7 Rr parent Dominant allele for seed shape Recessive allele for seed shape Chromosomes replicate Meiosis I Alleles segregate Gametes Meiosis II Principle of segregation: Each gamete carries only one allele for seed shape, because the alleles have segregated during meiosis.
- 24. Two-Trait Inheritance Mendel performed cross using true- breeding plants differing in two traits. Dihybrid Cross Observed phenotypes among F plants. 2 Formulated assortment law of independent
- 25. Mendel Studied 2 Traits Mendel then looked at two traits simultaneously – dihybrid cross. Ex. plants that produced round (R), yellow (Y) peas and plants that produced wrinkled (r), green (y) peas. The pure breeding parents’ genotypes were RRYY and rryy, fig 13.5. What is the genotype and phenotype of the F1 generation? The F2 generation?
- 26. Hypothesis of independent assortment: Alleles of different genes don’t stay together when gametes form. Female parent F1 PUNNET SQUARE Male gametes Male parent Female gametes F1 offspring all RrYy F2 female parent Alleles at R gene and Y gene go to gametes independently of each other F2 PUNNET SQUARE Female gametes F2 male parent Male gametes Figure 13-5a F2 offspring genotypes: 9/16 R–Y– : 3/16 R–yy : 3/16 rrY– : 1/16 rryy F2 offspring phenotypes: 9/16 : 3/16 : 3/16 : 1/16
- 27. Dihybrid Cross
- 29. Genes and Mendel’s Findings Mendel’s Principle of Independent Assortment Each pair of factors segregates independently of the other pairs. All possible combinations of factors can occur in the gametes.
- 30. Figure 13-8 R y y R r r Y R Rr Replicated chromosomes prior to meiosis Y r R R r Alleles for seed shape Alleles for seed color r Chromosomes can line up in two ways during meiosis I Y Y y y R Meiosis I R Y y yY Y r r R y y Y Meiosis I R Gametes Y Y 1/4 RY Y Meiosis II r R r Y y y Meiosis II R r r y y 1/4 ry r R R y y 1/4 Ry r Y 1/4 rY Principle of independent assortment: The genes for seed shape and seed color assort independently, because they are located on different chromosomes. Y