Simple Genetic Crosses

1. Simple Genetic Crosses THE MONOHYBRID CROSS These involve a cross between parents who differ in a single characteristic. e.g. eye colour

2. Example : Flies can havewhite or grey eyes. Grey is DOMINANT over white White is RECESSIVE to grey

3. Each individual has 2 genes for each characteristic. Use : G for the dominant grey gene g for the recessive white gene

4. Pure breeding (HOMOZYGOUS) flies have 2 identical genes Pure breeding grey fly GG Pure breeding white fly gg

5. A pure breeding white eyed fly A pure breeding grey eyed fly Parents gg x GG g gamete G gamete Offspring all grey eyed. g G F I

6. HETEROZYGOUS flies have 1 of each type of gene. Gg They will have GREY eyes because they carry one DOMINANT gene.

7. What happens when we cross two of the grey eyed offspring? x Gg Gg x

8. Parents Grey eyed offspring Grey eyed offspring x Gg Gg Gametes G g G g GG Gg Gg gg : F II 3 1 Grey eyed. White eyed.

9. In this second generation(F II) we get : 3 Grey eyed flies to 1 White eyed fly

10. In genetics we do not draw pictures for a genetic cross. We use : Punnett Square diagrams.

11. How to draw Punnet Square Diagrams for a Genetic Cross. GG x gg

12. Step 1 • Work out the gametes (eggs or sperm) that each parent will produce.

13. Parents GG x gg G G g g gametes gametes

14. Step 2 Draw the Punnet square outline

15. GG x gg Step 3 FI g g Now fill in the letters for the gametes of the cross. Gg Gg G grey grey Gg Gg G grey grey

16. All the offspring of the FIRST GENERATION are Gg They are all grey.

17. What happens in the SECOND GENERATION Gg x Gg

18. Parents Gg x Gg G g G g gametes gametes

19. Gg x Gg FII Now fill in the letters for the gametes of the cross.

20. Gg x Gg FII Now fill in the letters for the gametes of the cross.

21. Gg x Gg FII Now fill in the letters for the gametes of the cross.

22. Gg x Gg FII Now fill in the letters for the gametes of the cross.

23. Gg x Gg FII Now fill in the letters for the gametes of the cross.

24. Gg x Gg FII Now fill in the letters for the gametes of the cross.

25. Gg x Gg FII Now fill in the squares with the possible offspring

26. Gg x Gg FII Now fill in the squares with the possible offspring

27. Gg x Gg FII Now fill in the squares with the possible offspring

28. Gg x Gg FII Now fill in the squares with the possible offspring Grey Grey Grey White

29. F II Genotype ratio i.e. The ratio of the different gene combinations GG Gg gg : : 1 2 1 : :

30. F II Phenotype ratio i.e. The ratio of the different types of offspring Grey White : 3 1 :