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Transcript
Faculty of Sciences Classroom Activity 10 Big Question: What is life? Cattle genetics Agriculturalists have long been using breeding, and therefore genetics, to develop cattle that better suit their needs. While some genetic traits are complex, others are quite simple to identify and map through generations. In this activity, we will assume a number of cattle traits follow Mendelian genetics the classic gene expression model which utilises the ‘Punnett Square’ method of mapping the offspring’s genetics. Before you begin, you will need a basic understanding of the principles and terms used in genetics, so start by researching the following terms: Allele Dominant vs recessive genes Mendel’s genetics or Mendelian genetics (also Gregor Mendel) Homozygous and heterozygous Incomplete dominance Monohybrid cross and dihybrid cross Punnett square Genotype and phenotype Breeding scenarios Assume that you have several different herds of cattle that you wish to breed from. Plot a simple Punnett square for each of the following situations: a. You wish to breed from a bull carrying two copies of the dominant gene (homozygous) for a black coat (B) and a cow carrying two copies of the gene for the recessive brown coat (b). Genotypic ratio: Phenotypic ratio: This cross is (circle the correct answer): Monohybrid/dihybrid b. All your cows and bulls are heterozygous for the coat colour gene and have one copy of the dominant gene for a black coat (B) and one for the recessive brown coat (b). Genotypic ratio: Phenotypic ratio: This cross is (circle the correct answer): Monohybrid/dihybrid The University of Adelaide 2 c. All your cows and bulls are heterozygous for the long tail (L) and the short tail (S) genes which have incomplete dominance. A LS offspring will have a medium length tail. Genotypic ratio: Phenotypic ratio: This cross is (circle the correct answer): Monohybrid/dihybrid The University of Adelaide 3 d. All your cows and bulls are RrHh where R is the dominant rough horn gene and r is the recessive smooth horn gene, and H is the dominant straight horn gene and h is the recessive curved horn gene. (Hint: you will need to use the expanded Punnett square and think carefully about your choices.) Genotypic ratio: Phenotypic ratio: This cross is (circle the correct answer): Monohybrid/dihybrid The University of Adelaide 4 Answers a. You wish to breed from a bull carrying two copies of the dominant gene (homozygous) for a black coat (B) and a cow carrying two copies of the gene for the recessive brown coat (b). B B b Bb Bb b Bb Bb Genotypic ratio: All are heterozygous Bb Phenotypic ratio: All will have the black coat This cross is: Monohybrid b. All your cows and bulls are heterozygous for the coat colour gene and have one copy of the dominant gene for a black coat (B) and one for the recessive brown coat (b). B b B BB Bb b Bb bb Genotypic ratio: 1BB:2Bb:1bb Phenotypic ratio: 3 will have black coats to every brown (3:1) This cross is: Monohybrid c. All your cows and bulls are heterozygous for the long tail (L) and the short tail (S) genes which have incomplete dominance. A LS offspring will have a medium length tail. L S L LL LS S LS SS Genotypic ratio: 1LL:2LS:1SS Phenotypic ratio: 1 long and 2 medium to every short (1:2:1) The University of Adelaide 5 This cross is: Monohybrid d. All your cows and bulls are RrHh where R is the dominant rough horn gene and r is the recessive smooth horn gene, and H is the dominant straight horn gene and h is the recessive curved horn gene. (Hint: you will need to use the expanded Punnett square and think carefully about your choices.) Each of the parental gametes is going to carry one of each the alleles for each gene, so either an R or an r, and either an H or an h. The resulting possible combinations each parent might contribute to the offspring are RH, Rh, rH, rh. But as it takes two gametes to make an offspring, these can be combined in the offspring in the following ways: RH Rh rH rh RH RRHH RRHh RrHH RrHh Rh RRHh RRhh RrHh Rrhh rH RrHH RrHh rrHH rrHh rh RrHh Rrhh rrHh rrhh Genotypic ratio: 1RRHH:2RRHh:1RRhh:2RrHH:4RrHh:2Rrhh:1rrHH:2rrHh:1rrhh Phenotypic ratio: There will be nine offspring that have rough straight horns, three that have rough curved horns, and three that have smooth straight horns to every one with a smooth curved horns (9:3:3:1). This cross is: Dihybrid The University of Adelaide 6
