THE DIHYBRID CROSS Studying the inheritance of two
THE DIHYBRID CROSS Studying the inheritance of two characters simultaneously © 2007 Paul Billiet ODWS
Mendel’s peas Character Seed shape Cotyledon colour © 2007 Paul Billiet ODWS Trait Allele Round R Wrinkled r Yellow Y Green y
Combinations Genotype RRYY RRYy Rr. YY Rr. Yy RRyy Rryy rr. YY rr. Yy rryy © 2007 Paul Billiet ODWS Phenotype Round Yellow Round Green Wrinkled Yellow Wrinkled Green
THE LAW OF INDEPENDENT ASSORTMENT It appears that the inheritance of seed shape has no influence over the inheritance of seed colour ¢ The two characters are inherited INDEPENDENTLY ¢ The pairs of alleles that control these two characters assort themselves independently ¢ © 2007 Paul Billiet ODWS
Dihybrid cross genetic diagram P Phenotypes Round Yellow seed Genotypes RRYY rryy meiosis RY ry Gametes X Wrinkled Green seed fertilisation F 1 Phenotypes Rr. Yy Genotypes Round Yellow Proportions 100% © 2007 Paul Billiet ODWS (Pure bred)
Dihybrid cross genetic diagram F 1 Phenotypes Rr. Yy Genotypes Round Yellow Proportions 100% (Selfed) meiosis Gametes © 2007 Paul Billiet ODWS Y y R RY Ry r r. Y ry
Dihybrid cross genetic diagram Gametes Y y R RY Ry r r. Y ry fertilisation F 2 Genotypes © 2007 Paul Billiet ODWS RY Ry r. Y ry RY RRYy Rr. YY Rr. Yy Ry RRYy RRyy Rr. Yy Rryy r. Y Rr. Yy rr. YY rr. Yy ry Rr. Yy Rryy rr. Yy rryy
Dihybrid cross proportions Phenotypes Round Yellow Round Green Wrinkled Yellow Wrinkled Green © 2007 Paul Billiet ODWS Proportions 9/16 or 56. 25%
Dihybrid cross proportions Phenotypes Round Yellow Proportions 9/16 or 56. 25% Round Green 3/16 or 18. 75% Wrinkled Yellow 3/16 or 18. 75% Wrinkled Green 1/16 or 6. 25% © 2007 Paul Billiet ODWS
Dihybrid test cross In monohybrid crosses, to know if a dominant trait is homozygous (RR) or heterozygous (Rr) it is necessary to carry out a test cross ¢ This is done with a homozygous recessive (rr) individual ¢ The same is true for a dihybrid cross where the test cross is made with an individual which is homozygous recessive for both characters (rryy) ¢ © 2007 Paul Billiet ODWS
Dihybrid test cross Phenotypes Genotypes Round Yellow Rr. Yy Gametes RY, Ry, r. Y, ry Genotypes ry Phenotypes Proportions X Wrinkled Green rryy ry RY Ry r. Y ry Rr. Yy Rryy rr. Yy rryy Round Wrinkled Yellow Green 1/4 or 25% © 2007 Paul Billiet ODWS
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