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## THE DIHYBRID CROSS

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**THE DIHYBRID CROSS**Studying the inheritance of two characters simultaneously © 2007 Paul Billiet ODWS**Mendel’s peas**© 2007 Paul Billiet ODWS**Combinations**© 2007 Paul Billiet ODWS**Is the inheritance of one character affected by the**inheritance of another? © 2007 Paul Billiet ODWS**Is the inheritance of one character affected by the**inheritance of another? © 2007 Paul Billiet ODWS**A dihybrid cross can be treated as two separate monohybrid**crosses The expected probability of each type of seed can be calculated: • Probability of an F2 seed being round = 75% or ¾ • Probability of an F2 seed being wrinkled = • Probability of an F2 seed being yellow = • Probability of an F2 seed being green = © 2007 Paul Billiet ODWS**A dihybrid cross can be treated as two separate monohybrid**crosses The expected probability of each type of seed can be calculated: • Probability of an F2 seed being round = 75% or ¾ • Probability of an F2 seed being wrinkled = 25% or ¼ • Probability of an F2 seed being yellow = 75% or ¾ • Probability of an F2 seed being green = 25% or ¼ © 2007 Paul Billiet ODWS**Therefore**• Probability of an F2 seed being round and yellow = ¾ x ¾ = 9/16 = 56.25% • Probability of an F2 seed being round and green = • Probability of an F2 seed being wrinkled and yellow = • Probability of an F2 seed being wrinkled and green = © 2007 Paul Billiet ODWS**Therefore**• Probability of an F2 seed being round and yellow = ¾ x ¾ = 9/16 = 56.25% • Probability of an F2 seed being round and green = ¾ x ¼ = 3/16 = 18.75% • Probability of an F2 seed being wrinkled and yellow = ¼ x ¾ = 3/16 = 18.75% • Probability of an F2 seed being wrinkled and green = ¼ x ¼ = 1/16= 6.25% © 2007 Paul Billiet ODWS**Predicting how many seeds we could expect to get in a sample**© 2007 Paul Billiet ODWS**Predicting how many seeds we could expect to get in a sample**© 2007 Paul Billiet ODWS**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**Mendel & Meiosis**• The pairs of chromosomes could orientate in different ways at Anaphase 1 © 2007 Paul Billiet ODWS**Dihybrid cross genetic diagram**© 2007 Paul Billiet ODWS**Dihybrid cross genetic diagram**© 2007 Paul Billiet ODWS**Dihybrid cross genetic diagram**© 2007 Paul Billiet ODWS**Dihybrid cross proportions**© 2007 Paul Billiet ODWS**Dihybrid cross proportions**© 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**© 2007 Paul Billiet ODWS