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Transcript
The Dawn of Genetics Genetics – the study of heredity which is the transmission of characteristics from parents to offspring Gregor Johann Mendel • born on July 22, 1822 • to peasant parents in a small agrarian town in Czechoslovakia • In 1843 he entered an Augustinian monastery in Czechoslovakia • He was later sent to the University of Vienna to study The above photo is from http://www.open.cz/project/tourist/person/photo.htm. The garden of the Augustinian Convent in Brno. part of the foundations of the greenhouse that Mendel used Mendel studied Pea Plants Why? • The pea flower Pistil Stamen • The male part of the flower (stamen) was easily removed to prevent self-pollination. • Pea flowers are easy to cross-pollinate • They were easy to grow. Mendel Isolated 7 pairs of contrasting traits. • Dominant Recessive Round wrinkled Yellow seed Green seed Purple flower White flower Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates In each case, one characteristic was dominant and one was recessive Inflated pod Constricted pod Green pod Yellow pod And the last 2 traits he selected: Tall is dominant over short. Flower postion: Axial is dominant over terminal • Mendel tested all 34 varieties of peas available to him through seed dealers. The garden peas were planted and studied for eight years. • Mendel's experiments used some 28,000 pea plants. • So just imagine what life would have been like living in the same monastery as Brother Gregor?? Genetic Terminology you will need to know: • Genotype - The alleles that an organism has for a particular trait. Tt is hybrid tall, TT is pure tall • Phenotype - The observable traits of an organism. Tall and short are phenotypes. • Allele – Alternate forms of genes. Tall (T) and short (t) are the alleles for height in pea plants. • Dominant –Tall is stronger over short so the tall allele will always be expressed if it is present. We use capital letters to show dominant alleles. • Recessive – Short is the weaker allele and won’t show unless both alleles for short are present. We use lower case for recessive alleles. Chromosomes, Genes and Alleles….Oh My! More terms: • Homozygous – a genotype in which both alleles are the same…pure • Heterozygous – a genotype in which the alleles are different…hybrid So what did Mendel find out about his pea plants? • Mendel first selected certain traits and bred those plants so that they would be purebred for that trait. • He chose plants that were tall and bred them together. He then took only the tall offspring and bred those together. He did this until only tall offspring were produced. • Mendel produced pure breeding varieties of all six traits. What next? • Mendel called his purebred plants the parent generation or P generation. • He then crossed a tall plant (TT) with a short plant (tt). • This is called a monohybrid cross because only one trait, plant height, is being tested. • The offspring from this cross are called the first filial generation or the F1 generation What did the F1’s look like? • Mendel found that 100% percent of the F1’s were tall. • This is where the terms dominant and recessive came from. Mendel concluded that tall was dominant over short. • Mendel concluded that heredity is not just a blending of traits. Instead one trait will be dominant over another. • This is called the Law of Dominance. Law of Dominance • When individuals that are pure for contrasting traits are crossed, the offspring will express only the dominant trait • So, when a purebred tall plant is crossed with a purebred short plant, all the offspring will be tall • This also means that when an organism is heterozygous (Tt) the dominant trait will always be expressed. The Next Step • Mendel’s next experiment crossed the F1 generation with itself to produce the F2 generation • This is just like our fast plant experiment • What happened when we crossed the F1 generation with itself to produce the F2 generation? How our findings compare to Mendel’s What happened? • Mendel repeated this experiment many times and each time found that the F2 generation had phenotypes like the dominant trait 75% of the time and phenotypes like the recessive trait 25% of the time • This 3:1 ratio is known as the Mendelian ratio Why did this happen? • During Meiosis the alleles for a trait separate into different gametes • Offspring inherit one allele from each parent • If the dominant allele is present it will be expressed (TT and Tt) • The recessive allele will only be expressed if both alleles are recessive (tt) Law of Segregation • Factors (alleles) that occur in pairs are separated from each other during gamete formation and recombined at fertilization. • The new combination consists of one allele from each parent, giving rise to new combinations of alleles and possibly new genotypes and phenotypes than the parents Punnett Square • Diagram that helps organize the results of a cross T between the gametes of two individuals • Used to predict phenotypes and genotypes of offspring t • Genotypic ratio - 1:2:1 • Phenotypic ratio - 3:1 T t TT Tt Tt tt • Since the genotypes TT and Tt both appear tall this type of inheritance is said to have complete dominance because both dominant homozygotes and heterozygotes have the same phenotype Homozygous Dominant or Heterozygous? • In order to determine the genotype of these individuals we must perform a test cross • Cross unknown (TT or Tt) with a homozygous recessive (tt) • If any offspring show the recessive trait then the unknown must be heterozygous. If all offspring show the dominant trait the unknown must be homozygous. Example Test Cross TT x tt Tt x tt t t T Tt Tt T Tt Tt Offspring: all dominant phenotype t t T Tt Tt t tt tt Offspring:1/2 dominant, ½ recessive Mendel’s Second Experiment • Does the inheritance of one trait affect the inheritance of a different trait? Ex: pea shape and pea color • Produced purebreds for both traits…round, yellow (dominant) and wrinkled green (recessive) • Dihybrid Cross - cross involving two traits • P generation was (RRYY) x (rryy) What was the F1 generation? • Phenotype? – 100% round yellow • Genotype? – RrYy RY ry RrYy Now, what about the F2 generation? • Phenotype? – 9 round yellow, 3 round green, 3 wrinkled yellow, and 1 wrinkled green….9:3:3:1 • Genotype? – Complicated! (RrYy) x (RrYy) What did Mendel do with this information? • Concluded that different traits were inherited independently of one another • Law of independent assortment: the inheritance of alleles of one trait do not affect the inheritance of alleles of another trait • Offspring may have new combinations of alleles that are not present in either parent Recap of Mendel’s Laws • Law of Dominance – When individuals that are pure for contrasting traits are crossed, the offspring will express only the dominant trait • Law of Segregation – Factors (alleles) that occur in pairs are separated from each other during gamete formation and recombined at fertilization. • Law of Independent Assortment – The inheritance of alleles of one trait do not affect the inheritance of alleles of another trait if they are on different chromosomes Beyond Mendel • Some traits do not show complete dominance…ie dominant and recessive • Some inheritance patterns we will discuss: – Incomplete dominance – Co-dominance – Multiple alleles Incomplete Dominance • Mendel thought that inherited traits were not simply blended but determined by dominant and recessive relationships. We now know that blending does occur in nature…it is called incomplete dominance Example: If red snapdragons (RR) are crossed with white snapdragons (R1 R1) the F1 generation is 100% pink (R R1) What about the F2 generation? • Phenotype? – 1 red, 2 pink and 1 white – 1:2:1 • Genotype? – 1 RR, 2 RR1 and 1 R1R1 Codominance • Sometimes both alleles for a trait are dominant and both dominant alleles can be expressed at the same time in heterozygotes • Example: Roan coat color in horses and cattle are codominant for the alleles R and r. Homozygous R individuals are red (or bay), homozygous r individuals are white, while heterozygous Rr individuals are roan and have both white and red hair, giving them a lightened appearance. What about the F2 generation? • Phenotype? – 1 red, 2 roan and 1 white – 1:2:1 • Genotype? – 1Hr Hr, 2 HrHw and 1Hw Hw Multiple Alleles • Many genes have more than two alleles • An individual may not have more than two alleles for each trait but different individuals can have different pairs of alleles when multiple alleles exist Example of Multiple Alleles • Human blood type is controlled by three alleles – IA, IB and i. Alleles IA and IB are dominant over I, but IA and IB are codominant Phenotype (blood type) Genotypes A B AB O IA IA or IAi IB IB or IBi IA IB ii