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 (P 1 ) was made up of true breeding tall and true breeding dwarf plants all offspring were tall

P 1 phenotype: Tall XDwarf genotype:TT X tt gametes:Tt F 1 phenotype: Tall XTall genotype:Tt XTt gametes:T t T t F 2 phenotype:3 Tall : 1 dwarf genotype:1 TT : 2 Tt : 1 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 T t T Tt t T T t tt TT t

Possible Crosses D D d Dd dd DD d E e e Ee ee Ee e C c C Cc c A A A AA A B B B Bb BB Bb BB b f f f ff f

Monohybrid CrossCross

Test Cross When an individual showing the dominant phenotype is crossed with a homozygous recessive individual to determine the genotype t T t ?t Tt ?t Tt ? 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 P 1 : genotype YYRR X yyrr gametes YR yr F 1 : genotype YyRr X YyRr gametes YR Yr yR yr F 2 : YR YryR yr yR Yr

Dihybrid Crosses YR YryR yr yR Yr YYRR YYRr YYrr YyRR YyRr yyrryyRr yyRRYyRr Yyrr YyRr

Mendel’s First & Second Law The Law of Segregation & The Law of Independent Assortment

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 CRCR CRCR CWCW CRCWCRCW CRCWCRCW CWCWCWCW CRCRCRCR CWCW E2E2 E1E1 E3E3 E2E4E2E4 E1E3E1E3 E3E4E3E4 E1E2E1E2 E4E4 HrHr HrHr HwHw HrHwHrHw HrHwHrHw HwHwHwHw HrHrHrHr HwHw

Polygenic Inheritance –Gene interaction: Example Chicken Comb Single (rrpp) Rose (R-pp) Pea (rrP-)Walnut (R-P-)

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 1 Clip 2

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