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Transcript
Biology 1-3a & 1-3b Mendelian Genetics 1-3: Mendelian Genetics • Genetics is the study of genes, variation in genes throughout generations, and heredity (the ways in which genes are past on from generation to generation) in living things. 1-3: Mendelian Genetics • Gregor Mendel (1822-1884) was an Austrian Monk with scientific training in mathematics, physics and biology. • His experiments laid the foundation for everything we know about genetics today. • He is often called the Father of Genetics 1-3: Mendelian Genetics • Before we take a closer look at exactly what Mendel did to receive such a major title, we need a little more background… 1-3: Mendelian Genetics • We inherit 23 chromosomes from our mother and 23 chromosomes from our father. • These chromosomes contain genes for the same type of information, so essentially we have two versions of every gene in our DNA. • Each version of the gene we inherit is called an allele. – So you inherit one allele from your mother and one from your father 1-3: Mendelian Genetics 1-3: Mendelian Genetics Mendel’s Experiments: • In order to understand what Mendel did in his experiments, we need a basic understanding of plant reproduction: • The male reproductive part of the flower is called the stamen. – From the anther located at the top of the stamen, pollen (plant sperm) is released. • The female reproductive part of the flower is called the pistil. – The sticky top of the pistil, called the stigma, collects the pollen which moves down into the ovary to create the next generation of pea plants. • Now, because pea plants have BOTH male and female reproductive parts, it is possible for them to self-pollinate themselves. • Also, because the pollen grains are usually distributed by either the wind or pollinators, such as bees, it is very difficult to know which plants are reproducing with each other • For his experiments, Mendel needed to control exactly what plants were reproducing with each other… • In order to control the breeding, Mendel cut the stamen off of half of the flowers • He then brushed the pistil of these stamenless flowers with pollen that he had collected from a chosen plant • Finally, the flowers were covered with a bag to prevent stray pollen grains from finding their way in. 1-3: Mendelian Genetics Mendel’s Experiments – Phase One • The first thing Mendel did was to isolate true breeding strains of peas with distinctive traits. – To do this he crossed pea plants with, for example, purple flowers with other purple flower pea plants until all of their offspring were consistently purple. 1-3: Mendelian Genetics Mendel’s Experiments – Phase Two • Once he had isolated his true breeding strains, he began to cross-breed them and analyze what their offspring looked like. • The first cross was between a true breeding white flowering plant and a true breeding purple flowering plant – ALL of their offspring (the F1 generation) were purple! 1-3: Mendelian Genetics Mendel’s Experiments – Phase Two • Next, he took two of the offspring from this first breeding test and bred them to each other – Yes, it’s technically incest, but they’re flowers! • 75% of the new offspring were purple, but 25% were white again! • Now keep in mind that the prevalent thought of the day was that if a purple flower and a white flower were bred, they should create a light purple flower • For Mendel to have produced white flowering plants by breeding two purple flowering plants….well, these were very strange results indeed! 1-3: Mendelian Genetics Mendel’s Experiments – Phase Two • Mendel’s next step was to take plants from the F2 generation and breed them with each other to try and determine if they were “true breeding plants” or not – He discovered that the white plants WERE true breeding, but only 1/3 of the purple plants were. – The other 2/3 of the purple plants would product a mix of purple and white offspring • Mendel repeated this type of experiment with six other traits, examining thousands of pea plants – In all cases, one trait disappeared in the F1 generation and then reappeared in the F2 generation. What in the world does this all mean? 1-3: Mendelian Genetics Mendel’s Logic: • If Mendel crossed purple peas with white peas he got all purple peas • When these purple peas were crossed with each other he got 3 purple to 1 white • He decided to use symbols to represent what happened. He used a P for purple and a W for white. He represented the cross as follows: • In the parent generation PxW • In the F1 generation all W • In the F2 generation 3P to 1W 1-3: Mendelian Genetics Mendel’s Logic: • Mendel reasoned that if there were no “W” traits present in the F1 generation, they must have died out and should never reappear • When the W trait reappeared in the next generation, Mendel reasoned that it must have been present but not seen in the previous generation. – He concluded that there must be two factors for every trait present in each individual, but that one factor could be masked by another • • • • His new theory was represented in the cross as follows: In the parent generation PP x PW In the F1 generation all PW In the F2 generation 1PP to 2PW to 1WW • Ok, confusion about all the letters aside, what Mendel had discovered was the existence of two “factors” (what we now know as genes or alleles) for each genetic trait!! • Even Mendel realized that all the lettering was confusing, so he developed some new terminology for genetics… 1-3: Mendelian Genetics Genetics Terminology: • The appearance of an organism is its phenotype – i.e. purple or white • The genetic makeup of an organism is its genotype – i.e. WW or PP or PW • The original, true-breeding strain is called the parental (P) generation • The first generation of offspring are the F1 generation • The second generation of offspring are the F2 generation 1-3: Mendelian Genetics Genetics Terminology: • The ability of one factor to mask the appearance of another makes it dominant. • The ability of a factor to be masked by another makes it recessive. • To reduce the amount of letters used, when choosing letters for the genotype use the letter of the dominant characteristic – True breeding purple = PP – True breeding white = pp • Now that we know the terminology, how do we predict the genetic makeup of future generations? • By using a PUNNET SQUARE 1-3: Mendelian Genetics The Punnet Square: • Maternal and paternal genotypes are written on the outsides • Fill in the boxes based on the alleles present 1-3: Mendelian Genetics The Punnet Square: • Show the P generation cross between a true breeding purple and a true breeding white plant – What would the phenotypes of the offspring be? – What would the genotypes of the offspring be? 1-3: Mendelian Genetics The Punnet Square: • Show an F1 generation cross from the previous question – What would the phenotypes of the offspring be? – What would the genotypes of the offspring be? A bit more vocabulary… • If an individual has two copies of the same allele, they are said to be homozygous – Eg. PP or pp • If an individual has two different copies of an allele, they are said to be heterozygous – Eg. Pp A bit more vocabulary… • If an individual has two copies of the same DOMINANT allele, they are said to be homozygous dominant – Eg. PP • If an individual has two copies of the same RECESSIVE allele, they are said to be homozygous recessive – Eg. pp