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Mendel’s genetics Mendel and his experiments Gregor Mendel Austrian monk Mid 1850’s experimented with pea plants Interested in various forms found in plant’s seeds, flowers, and height The garden pea Why did he choose peas? Easy to grow They reproduce sexually (male and female parts on same flower) Petals tightly enclose reproductive parts (so pollination can be controlled) Seed to crop in a short time Many characteristics that can be studied (flower color, color of seeds, height, etc.) Mendel’s experiment – step 1 Mendel carefully selected a group of pea plants that came from a line that produced only tall pea plants. Step 1 continued Another group of pea plants was selected that came from a line that produced only short plants. Step 2 Mendel cross-pollinated the pollen from a tall pea plant onto the female part of a short plant. This pollination would later produce pea seeds that he planted and grew. The tall plant and the short plant that were crossed were called the P1 generation. (P for parents) Step 3 Mendel took the seeds produced from the tall plant-short plant cross, planted them, and grew them into mature pea plants. Results of first experiment The seeds that grew from the tall-short cross all grew into tall plants No short plants were produced These plants were called the F1 generation (F for filial, meaning “offspring”) Experiment 2 Mendel allowed the F1 generation plants to self-pollinate. He collected the seeds from this cross, planted them (about 1000 seeds), and grew them to maturity These plants were known as the F2 generation 75% turned out tall, 25% turned out short Ratio The F2 generation had 3 tall plants for every 1 short plant Mathematically, this is known as a “ratio” We say that the F2 generation had a ratio of 3 tall to 1 short, written as 3:1 Other pea crosses Mendel repeated his experiments using 6 other traits (illustrated on p. 262) This type of cross is known as a monohybrid cross, because only 1 trait at a time is being studied Mendel’s conclusions Each plant has 2 “factors” that control each of it’s traits (height, seed color, etc.) One “factor” comes from one parent, one “factor” comes from the other The factor that disappears in a generation is called recessive, and the factor that shows up in each generation is called dominant What Mendel didn’t know Today we use the word gene instead of factor Genes are found on chromosomes (humans have 46; 23 from each parent) Each different form of a gene that can be expressed is called an allele Each copy of an organism’s alleles comes from it’s parents Alleles We use letters to stand for each allele In Mendel’s pea plants, we use T (capital) for tall and t (lower-case) for short So Mendel’s P1 generation tall plants would be TT, his P1 short plants would be tt, and the hybrid F1 generation would be Tt. Mendel’s experiment (using letters to stand for alleles) P1 TT x tt tall x short F1 all Tt all tall F2 ¼ TT, ½ Tt, ¼ tt (3/4 tall, ¼ short) 3 tall:1 short Other important terms Heredity – the passing of characteristics from parents to offspring Genetics – the branch of biology that studies heredity Traits – inherited characteristics Gametes – sex cells Hybrid – organism with 2 different forms of an allele Pollination – transfer of male pollen to female part of plant Fertilization – when male and female gametes unite Monohybrid crosses Heterozygous means both alleles are different ex: Tt, Bb, Rr Homozygous means both alleles are alike ex: ee, HH, LL, or ff phenotype The physical expression of a gene ex. Green peas, purple flowers, tall plant, albino skin Phenotype is always a description Phenotype is always an adjective genotype Genotype is always a combination of letters to describe an organism ex: Pp, Rr, NN, mm Monohybrid crosses In peas, purple flowers are dominant to white. Predict the potential offspring of a cross between 2 heterozygous purpleflowered plants. Monohybrid cross In summer squash, white fruit color is dominant to colored fruit. Predict the offspring of a cross between a homozygous white-fruited plant and a heterozygous squash plant. Monohybrid cross In humans, colored skin is dominant to skin with no pigment. Describe the cross between 2 heterozygous people.