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Funky Pigeons Revealing the biology of inheritance and selection Lesson 2 Genetics Picture courtesy John Ross: darwinspigeons.com Whose baby? Parents Offspring? Images © Genetics Society of America. From Genetics September 1, 1922 vol. 7 no. 5 466-507 STUDIES ON INHERITANCE IN PIGEONS. IV. CHECKS AND BARS AND OTHER MODIFICATIONS OF BLACK by Sarah Van Hoosen Jones. A Victorian Pigeon Club show (1853) © Mary Evans Picture Library / Illustrated London News Ltd Inheritance “The laws governing inheritance are for the most part unknown…” Charles Darwin On the Origin of Species (5th Ed.), 1869 Sarah Van Hoosen Jones: The Petticoat Farmer Courtesy Archives of Michigan Pigeon colour/pattern phenotypes Solid black Chequered Barred Barless © Genetics Society of America. From Genetics September 1, 1922 vol. 7 no. 5 466-507 STUDIES ON INHERITANCE IN PIGEONS. IV. CHECKS AND BARS AND OTHER MODIFICATIONS OF BLACK by Sarah Van Hoosen Jones. Albinism Type 1 occulocutaneous albinism in mice caused by the gene TYR (tyrosinase) Picture by Aaron Logan (commons.wikimedia.org) Human genetics and recessive epistasis Unaffected “Carrier” Father Unaffected “Carrier” Mother Unaffected 1 in 4 chance Affected 1 in 4 chance Unaffected “Carrier” 2 in 4 chance The MCR1 gene and red hair MCR1 acts as a switch controlling whether eumelanin (brown/black) or phaeomelanin (red) are produced The allele that causes red hair is recessive to the allele that causes brown and black hair. Picture by John Griffiths (commons.wikimedia.org) TYR is epistatic to MCR1 Sex linked inheritance in humans Sex linked inheritance in birds The female is the HETEROGAMETIC sex The male is the HOMOGAMETIC sex Summary Phenotypic ratios for crosses involving heterozygotes: One allele a monohybrid cross – 3:1 Two alleles a dihybrid cross – 9:3:3:1 Dihybrid cross with recessive epistasis – 9:3:4 Dihybrid cross with dominant epistasis – 12:3:1 Darwin on genetics “…it will now be seen that when two birds belonging to distinct races are crossed, neither of which have, nor probably have had during many generations, a trace of blue in their plumage, or a trace of wing-bars and the other characteristic marks, they very frequently produce mongrel offspring of a blue colour, sometimes chequered, with black wing-bars, etc.; or if not of a blue colour…” Charles Darwin The variation of animals and plants under domestication, 1868