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Mendelian Laws(Principles)*Law of DominanceLaw of SegregationLaw of Independent Assortment *some books does not include this
Law of Dominance* “In a cross of parents that are pure for contrastingtraits, only one form of the trait will appear in the next generation. Offspring that are hybrid for a trait will have only the dominant trait in the phenotype.”
Law of Dominance* While Mendel was crossing (reproducing) his pea plants (over & over & over again), he noticed something interesting. When he crossed pure tall plants with pure short plants, all the new pea plants (referred to as the F1 generation) were tall. Similarly, crossing pure yellow seeded pea plants and pure green seeded pea plants produced an F1 generation of all yellow seeded pea plants. The same was true for other pea traits: Parent Pea Plants F1 Pea Plants tall stem x short stem all tall stems yellow seeds x green seeds all yellow seeds green pea pods x yellow pea all green pea pods pods round seeds x wrinkled all round seeds seeds axial flowers x terminal all axial flowers flowers
Law of Dominance* “There is a factor that makes pea plants tall, and another factor that makes pea plants short. Furthermore, when the factors are mixed, the tall factor seems to DOMINATE the short factor".
Law of Dominance* Now, in our modern wisdom, we use "allele" or "gene" instead of what Mendel called "factors". There is a gene in the DNA of pea plants that controls plant height (makes them either tall or short). One form of the gene (allele) codes for tall, and the other allele for plant height codes for short. For abbreviations, we use the capital "T" for the dominant tall allele, and the lowercase "t" for the recessive short allele.
Law of Dominance*Genotype Symbol Genotype Vocab Phenotype homozygous DOMINANTTT tall or pure tall heterozygousTt or tall hybrid homozygous RECESSIVEtt short or pure short
Law of Segregation “During the formation of gametes (eggs or sperm), the two alleles responsible for a traitseparate from each other. Alleles for a trait arethen "recombined" at fertilization, producing the genotype for the traits of the offspring.”
Law of Segregation Parent Pea Plants Offspring (Two Members of F1 (F2 Generation) Generation) Genotypes: Phenotypes:7 Genotypes: Phenotypes: 25% TT 5% tall Tt x Tt tall x tall 50% Tt 25% short 25% ttReminder!Any time two parents have the same phenotype for a traitbut some of their offspring look different with respect to that trait,the parents must be hybrid for that trait.
Law of IndependentAssortment “Alleles for different traits are distributed to sex cells (& offspring) independently of one another.”
Law of IndependentAssortment Mendel noticed during all his work that the height of the plant and the shape of the seeds and the color of the pods had no impact on one another. In other words, being tall didnt automatically mean the plants had to have green pods, nor did green pods have to be filled only with wrinkled seeds, the different traits seem to be inherited INDEPENDENTLY.
Summary of all the LawsLAW PARENT CROSS OFFSPRING TT x tt 100% TtDOMINANCE tall x short tall Tt x Tt 75% tallSEGREGATION tall x tall 25% short 9/16 round seeds & green pods 3/16 round seeds & RrGg x RrGgINDEPENDENT yellow pods round & green x round &ASSORTMENT 3/16 wrinkled seeds & green green pods 1/16 wrinkled seeds & yellow pods