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Fundamentals of Genetics Mendel and Crosses
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Gregor Mendel father of genetics
performed plant breeding experiments using garden pea plants
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Garden Pea Plant
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Who was this Mendel and what the heck is he doing in a monastery?
born in 1822 trained himself to be a naturalist early in life worked as a substitute science teacher failed the qualifying exams to be a regular high school teacher! joined a monastery in Brunn, Austria sent to Vienna U. to study science and math
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Why was Mendel successful?
used garden pea plant (readily available) cultivated quickly large number of offspring only 7 traits self-pollinating logical experimental methods kept very careful records
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Self Pollination
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Cross Pollination
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TECHNIQUE 1 2 Parental generation (P) Stamens 3 Carpel 4
Figure 14.2 Research Method: Crossing Pea Plants Stamens 3 Carpel 4 14
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RESULTS First filial generation offspring (F1) 5
Figure 14.2 Research Method: Crossing Pea Plants 15
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EXPERIMENT P Generation (true-breeding parents) Purple flowers
Figure EXPERIMENT P Generation (true-breeding parents) Purple flowers White flowers F1 Generation (hybrids) All plants had purple flowers Self- or cross-pollination Figure 14.3 Inquiry: When F1 hybrid pea plants self- or cross-pollinate, which traits appear in the F2 generation? F2 Generation 224 white flowered plants 705 purple- flowered plants 16
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EXPERIMENT P Generation (true-breeding parents) Purple flowers
Figure EXPERIMENT P Generation (true-breeding parents) Purple flowers White flowers F1 Generation (hybrids) All plants had purple flowers Self- or cross-pollination Figure 14.3 Inquiry: When F1 hybrid pea plants self- or cross-pollinate, which traits appear in the F2 generation? F2 Generation 224 white flowered plants 705 purple- flowered plants 17
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EXPERIMENT P Generation (true-breeding parents) Purple flowers
Figure EXPERIMENT P Generation (true-breeding parents) Purple flowers White flowers F1 Generation (hybrids) All plants had purple flowers Self- or cross-pollination Figure 14.3 Inquiry: When F1 hybrid pea plants self- or cross-pollinate, which traits appear in the F2 generation? F2 Generation 224 white flowered plants 705 purple- flowered plants 18
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Mendel’s Laws
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1. Law of Unit Characters a hereditary trait is carried by a pair of genes (Mendel’s factors) one gene is on one chromosome and the other is on the other chromosome ex: TT, Tt, tt
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2. The Principle of Dominance
dominant gene covers up (masks) the recessive gene if both are present ex: plant color R-red (dominant) r- white (recessive) RR- red plant Rr- red plant rr- white plant
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3. The Principle of Segregation
gene pairs separate into different gametes during meiosis each gamete has only one gene from the pair ex: Tt separates into T and t T t Meiosis T t Gametes
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4. The Principle of Independent Assortment
Parents Genotype applies to gene pairs on different chromosomes separation (segregation) of a gene pair is not affected by gene pairs on other chromosomes separate independently Yy Tt Possible Gametes yt YT yT Yt
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CROSSES
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Monohybrid Crosses one pair of traits are considered in a cross
use a special chart called a Punnett square to predict the genotype of the offspring phenotype ratio - appearance genotype ratio - genes in square
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How to Work a Genetics Problem
Follow these steps!
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Read the problem. What information is provided in the problem?
Cross a pure white flowered pea plant with a pure purple flowered plant. What is the trait? What is dominant? What letter will we use to represent this trait? Flower Color P Dominant – purple - What is recessive? Recessive – white - p What letter will we use to represent this trait?
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Determine the genotypes of the parents.
Cross a pure white flowered pea plant with a pure purple flowered plant. Pure purple P – purple p - white Pure white X pp PP X Pure = homozygous
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Law of Segregation: Determine the gametes
Cross a pure white flowered pea plant with a pure purple flowered plant. P – purple p - white Pure white X Pure purple PP pp X Meiosis Meiosis P P p p Gametes Gametes
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Punnett Square: place the gametes across the top and down one side
Cross a pure white flowered pea plant with a pure purple flowered plant. p p P – purple p - white pp X PP P p p P P P
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Punnett Square: join the gametes
Cross a pure white flowered pea plant with a pure purple flowered plant. p p P p P p P – purple p - white pp X PP P p p P P P P p p P
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Determine the genotype and phenotype of the offspring.
Cross a pure white flowered pea plant with a pure purple flowered plant. p p P – purple p - white pp X PP P p P p P Genotype - Pp purple purple purple P P Phenotype - p p P purple purple
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What is the genotype ratio? The phenotype ratio?
Cross a pure white flowered pea plant with a pure purple flowered plant. p p P – purple p - white pp X PP P p P p P Genotype Pp to PP or pp 4 : 0 P P Phenotype p p P Purple to White 4 : 0
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What percentage of the offspring are homozygous? Heterozygous?
Cross a pure white flowered pea plant with a pure purple flowered plant. p p P – purple p - white pp X PP P p P p P Genotype Homozygous - 0% Heterozygous - 100% P P p p P
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What percentage of the offspring are purple? White?
Cross a pure white flowered pea plant with a pure purple flowered plant. p p P – purple p - white pp X PP P p P p P Phenotype Purple - 100% White - 0% P P p p P
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Cross 2 heterozygous purple flowered pea plants.
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