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DIHYBRID CROSS

DIHYBRID CROSS. Mendel also investigated the inheritance of two pairs of contrasting characters in the pea plant. This is referred to as the dihybrid cross. Pure breeding X pure breeding round seeds wrinkled seeds yellow cotyledons green cotyledons F1 progeny all round

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DIHYBRID CROSS

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  1. DIHYBRID CROSS

  2. Mendel also investigated the inheritance of two pairs of contrasting characters in the pea plant. • This is referred to as the dihybrid cross.

  3. Pure breeding X pure breeding round seeds wrinkled seeds yellow cotyledons green cotyledons F1 progeny all round yellow peas

  4. The F1 progeny were selfed:- • F2 progeny Round round wrinkled wrinkled Yellow green yellow green 315 108 101 32

  5. The ratio between the numbers of different seeds reduces to: 9.8 : 3.38 : 3.15 : 1.0 Or about 9 : 3 : 3 : 1

  6. Mendel also noticed that two new combinations not represented in the parents appeared in the progeny • Round or wrinkled seeds can turn up with either green or yellow cotyledons. • The two pairs of factors were inherited independently.

  7. Either one of the contrasting characters could be passed on to the offspring • Therefore the heterozygote plant (F1 – round and yellow) must produce four types of gamete in equal numbers

  8. F1 phenotype RrYy Produces 4 different gametes RY Ry rY ry

  9. Use this information to draw the initial cross and the F1 cross on your worksheet. • How many different phenotypes are there and what will they be?

  10. Answers • CROSS 1 • Parents Round yellow X wrinkled green genotype RRYY rryy gametes RY ry

  11. F1 phenotype – all round and yellow • F1 genotype – all RrYy (heterozygous for both characters)

  12. F1 generation crossed • F1 phenotype – round yellow X round yellow • Genotype - RrYy X RrYy • Gametes RY Ry rY ry

  13. F2 generation Phenotypes – Round and yellow – 9 Round and green – 3 Wrinkled and yellow - 3 Wrinkled and green - 1

  14. The Dihybrid Test Cross • As with monohybrids the appearance of the heterozygous dominant offspring will be identical to that of the homozygous dominant organism. • How could they be distinguished?

  15. If the organism in question is heterozygous dominant and is crossed with a homozygous recessive individual then the resulting progeny would show all possible combinations of characters in the ratio • 1 : 1 : 1 : 1

  16. This is described as a test cross in which the genotype of the F1 offspring can be determined. • Perform a genetic cross to show the possible genotypes resulting from a test cross for a homozygous dominant individual (RRYY) and a heterozygote (RrYy)

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