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Transcript
And Probability of Inheritance Considered the father of genetics He was a monk who enjoyed caring for a garden of pea plants These pea plants can self-pollinate (fertilize one of its own eggs with its own pollen) or cross-pollinate (allow pollen from another pea plant to fertilize its eggs) Mendel created stocks of true-breeding pea plants True-breeding means that if these plants selfpollinate they will always create offspring with the same traits as the parent plant Mendel looked at several traits in his pea plants: Tall or short Yellow or green seeds Green or yellow pods Round or wrinkled seeds Gray or white seed coat Smooth or constricted pod shape He noticed when he crossed true-breeds of each there was always the same outcome. So, every time he fertilized a true-breeding tall plant with pollen from a true-breeding short plant, the offspring were tall (no matter which way the fertilization occurred) Mendel realized that something was being passed on from generation to generation, today we recognized that as genes Each gene has different alleles, or different genetic make-up that leads to a different expression of that gene We each get two alleles, one from each parent This lead Mendel to create the law of dominance: Some alleles are dominant and some are recessive Tall = dominant pea plants Short = recessive pea plants F1 Generation Mendel was still curious about whether or not the recessive alleles had disappeared in the offspring So, Mendel took the offspring (F1 generation) of the true tall and true short and bred them After breeding several F1 plants with other F1 plants Mendel noticed that the recessive allele reappeared in about ¼ of the F1’s offspring (F2 generation) F2 Generation Mendel concluded that the dominant allele in the F1 generation masked the recessive allele, but these alleles could be separated again in the next generation Mendel presumed that the alleles separated from one another during formation of the gametes (sex cells) Mendel came up with the Law of Segregation: When each organism produces sex cells, the two alleles separate so that each gamete only carries only one single copy of each gene Therefore, the F1 generation were all heterozygous Heterozygotes carry two different alleles for the same gene Homozygotes carry the two of the same allele True breeding tall plants A True breeding short plants Hybrid tall plants capital letter always represents the dominant allele, and a lower-case of the same letter is the recessive The fact that ¼ of the pea plants in the F2 generation were short was important, because probability plays a role in genetics Probability is the likelihood that a certain event will occur (like getting a child with blue eyes) The easiest way to figure out genetic probability is using Punnett squares Heterozygous – having two different alleles for a gene Ex: Homozygous – having the same allele for a gene Ex: Genotype – the genetic make-up or two alleles that are present Ex: Phenotype alleles Ex: – the physical expression of a set of A punnett square is simply a way to visualize Mendel’s Law of Segregation One parent goes on top and one on the right, then just fill in the two alleles they have in the boxes and check your outcome. T t t T Now Now try it with the F1 generation plants (hybrids) you can see why ¼ of the F2 generation of pea plants ended up short Now Mendel was curious to see what would happen if he crossed a plant with two true-breeding traits and a plant with the two recessive true-breeding alleles of that gene. So, he took a true-breeding round seed, yellow seed and crossed it with a true-breeding wrinkled green seed pea plant. We’ll need a bigger punnett square! Mendel found that when he crossed the F1 generation F2 plants developed with traits that were not there in either parent (example: wrinkled yellow seeds) This lead Mendel to come up with the Law of Independent Assortment: Genes for different traits can segregate independently during the formation of gametes. What this means is that the inheritance of one gene does not necessarily affect other genes because they may be located on separate chromosomes. Law of Dominance – Some alleles are dominant and some are recessive Law of Segregation - When each organism produces sex cells, the two alleles separate so that each gamete only carries only one single copy of each gene Law of Independent Assortment - Genes for different traits can segregate independently during the formation of gametes