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Mendelian Genetics Introduction to Genetics
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Gregor Mendel Father of Genetics GENETICS: study of heredity HEREDITY: passing of traits from parent to offspring TRAITS: characteristics Gregor learned about these patterns by studying PEAS
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Mendel’s Pea Plants Pea plants are normally self- pollinating (fertilizes itself; producing offspring from a single parent) Self-pollinating plants produce offspring identical to the parent. Mendel eliminated the self- pollinating and allowed them to cross-breed so he could observe the offspring of 2 different parents.
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Medel’s Pea Plants Mendel Studied 7 different traits and looked at 3 generations. P1= the original parent generation F1= first generation of offspring F2= second generation of offspring
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Mendel’s Conclusions Inheritance is determined by genes that determine the traits AND there are 2 different forms of a gene, called alleles. Some alleles are DOMINANT and some are RECESSIVE
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Review: How do traits get passed from parent to offspring? Through GAMETES: sex cells Male gamete is the SPERM Female gamete is the EGG FERTILIZATION: union of egg & sperm ZYGOTE: fertilized egg
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GENES VS. ALLELES Genes Segment of DNA that codes for one protein/trait Most traits are determined by TWO genes. Remember chromosomes have a homologous match…two chromosomes with similar genetic information EXAMPLE: Hair color, eye color, height in pea plants Alleles Each form of a gene is called an ALLELE EXAMPLE: brown or blond blue, hazel, brown tall, short
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Law of Dominance DOMINANT (allele) Always expressed if present Only need ONE dominant allele to be expressed EXAMPLES: dark hair, brown eyes, tall peas RECESSIVE (allele) May be present but will not be expressed if paired w/ dominant allele Requires TWO recessive alleles for recessive trait to be expressed EXAMPLES: light hair, blue eyes, short peas
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Law of Segregation Gene pairs separate during formation of gametes (meiosis) Each gamete gets ONE allele from a pair Each parent passes down one allele to their offspring Offspring get 2 alleles (one from each parent)
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Preparing for Genetic Crosses Letters will represent alleles Use the first letter of the dominant trait Capital letter = dominant trait Lower case letter = recessive trait EXAMPLE: height in pea plants TALL is dominant over short, so… T = tallt = short
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Genotype vs. Phenotype GENOTYPE: actual genes in the pair; can’t see…they’re on chromosomes…but they determine the individual’s traits Use terms: homozygous: both genes the same (TT, tt) heterozygous: genes different; 1 dominant, 1 recessive (Tt, tT) All possible combinations using T, t: TT, tt, Tt, tT
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Genotype vs. Penotype PHENOTYPE: the trait that is expressed; the ‘physical’ appearance Phenotype Genotype TT…tall (homozygous dom) tt….short (homozygous rec) Tt….tall (heterozygous) tT….tall (heterozygous)
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Mendelian Traits Traits that have 2 alleles EXAMPLE: cleft chin If a person inherits two dominant alleles (CC) or one dominant & one recessive (Cc), they will have a cleft chin (top) If a person inherits 2 recessive alleles (cc), they will not have a cleft chin (bottom)
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Solving Genetic Crosses Use PUNNETT SQUARES Diagram used to predict the offspring given genotypes of parents. The parents genotypes go on outside of square (one parents’ across top; other parents’ down left side
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Monohybrid Cross These involve just ONE trait; therefore, you will see just ONE letter (but it can vary in size). Ex. Aa x Aa or AA x aa
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Test Cross Just because you know the phenotype, does not mean you know the genotype for a particular trait! To identify the genotype of an organism with a dominant trait, you must perform a test cross. This means you mate the organism of the unknown genotype with a homozygous recessive organism. Ex. You need to find the genotype of a brown rat. Brown is dominant to white. The genotype of the brown rat could be _____ or _______.
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