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Biology 12. Unit 3: Heredity Mendelian Genetics. Genes and Heredity. Mendel’s Experiments Alleles Punnett Square Monohybrid & Dihybrid Crosses. Mendelian Genetics. Traits are controlled by genes Law of Dominance: dominant and recessive Law of Segregation: genes separate during meiosis
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Biology 12 Unit 3: Heredity Mendelian Genetics
Genes and Heredity • Mendel’s Experiments • Alleles • Punnett Square • Monohybrid & Dihybrid Crosses
Mendelian Genetics • Traits are controlled by genes • Law of Dominance: dominant and recessive • Law of Segregation: genes separate during meiosis • Law of Independent Assortment: genes for one trait will separate independently of how other traits separate
More Mendelian • Mendel worked with pea plants • True breeding tall plants are homozygous for the tall (T) allele • True breeding dwarf plants are homozygous for the dwarf (t) allele • When the parental generation (P1) was made up of true breeding tall and true breeding dwarf plants all offspring were tall
P1 phenotype: Tall X Dwarf genotype: TT X tt gametes: T t F1 phenotype: Tall X Tall genotype: Tt X Tt gametes: T t T t F2 phenotype: 3 Tall : 1 dwarf genotype: 1 TT : 2 Tt : 1 tt
T T T t t T Tt Tt TT Tt t t Tt Tt Tt tt Punnett Square • This F1 generation of tall plants were all heterozygous: one tall allele and one dwarf allele • When F1 individuals were crossed Mendel found a 3:1 ratio of tall to short plants
f C B E A D f B d e A C c D A f B e Cc BB Dd ee AA ff DD ff BB Ee AA Cc b c f e d A Dd ff Ee Cc AA Bb ff AA Cc Bb ee dd Possible Crosses
t t T Tt Tt ? ?t ?t Test Cross • When an individual showing the dominant phenotype is crossed with a homozygous recessive individual to determine the genotype If dwarf individuals are observed in the offspring, then the unknown genotype is heterozygous
Dihybrid Crosses • Mendel looked at two traits at a time to determine the Law of Independent assortment • True breeding Yellow Round Seed cross with true breeding green wrinkled seed
Let Y be yellow, y is green Let R be round, r is wrinkled P1: genotype YYRR X yyrr gametes YR yr F1: genotype YyRr X YyRr gametes YR Yr yR yr F2: YR Yr yR yr YR Yr yR yr
YR Yr yR yr YR YYRR YYRr YyRR YyRr Yr YYRr YYrr YyRr Yyrr yR YyRR YyRr yyRR yyRr yr Yyrr YyRr yyRr yyrr Dihybrid Crosses
Mendel’s First & Second LawThe Law of Segregation &The Law of Independent Assortment
Hr Hw E2 E3 CR CW Hr HrHw E1 E1E3 CR CRCW HrHr E1E2 CRCR Hw HrHw HwHw E4 E2E4 E3E4 CW CRCW CWCW Non-Mendelian Inheritance • Multiple Alleles • Example: Eye colour in Drosophelia • Wild Type > Apricot > Honey > White • Incomplete Dominance • Example: Snap dragons, red X white => pink • Co-Dominance • Example: Shorthorn cattle, red X white => roan
Polygenic Inheritance • Gene interaction: Example Chicken Comb • Rose (R-pp) Single (rrpp) • Walnut (R-P-) Pea (rrP-)
Pleiotropic genes • One gene with multiple effects • Example: Sickle-cell anemia • Lethal Genes • When a specific genotype is not viable • eg. if AA is lethal, a heterozygous cross would give a 2:1 ratio (2 Aa : 1 aa – the AA is dead)
Non-Mendelian Genetics Clip 2 Clip 1
Linkage • Mendel demonstrated his Law of Independent Assortment using simple traits. • Some traits do not follow this Law. • When genes are on the same chromosome they do not separate independently