1. Co-dominance and multiple alleles Text pages 121 -124

2. Symbols for codominant alleles Both alleles are expressed EQUALLY in the heterozygote both take a CAPITAL CASE letter An index letter identifies the allele E.g: T R = RED tomato T B = BLUE tomato

3. Co-dominance Sometimes neither allele is dominant and both show their effect in the Heterozygote We say the alleles are co-dominant E.g. flower colour in snapdragons shows THREE possible phenotypes 1. C R C R red flowers 2. C W C W white flowers 3. C R C W pink flowers

4. Cross between a red and a white snapdragon C R C R x C W C W CRCR CRCR CWCW CWCW What are the phenotypes of the F1 offspring? C R C R C W C W gametes: F1 offspring genotype:

5. Answer All are genotype C R C W Neither allele is dominant So all are phenotype pink A similar pattern is found in cattle where roans actually have a mixture of red and white hairs in their coats and appear pink from a distance

6. Cross between two pink snapdragons C R C W x C R C W CRCR CWCW CRCR CWCW C R & C W C R & C W gametes: F2 offspring genotypes: Fill in the F2 offspring genotypes

7. Answer: cross between two pink snapdragons C R C W x C R C W What are the phenotypes of the F2 offspring? gametes: F2 offspring genotypes: CRCRCRCR CRCWCRCW CRCWCRCW CWCWCWCW C R & C W CRCR CWCW CRCR CWCW

8. F2 Phenotypes 1 red (C R C R ) 2 pink (C R C W ) 1 white (C W C W )

9. Human example Sickle cell anaemia Some genes have alleles that are both fully expressed in the heterozygote individuals

10. Blood cells Blood smear (normal) Sickle cell anemia

11. The genetics of sickle cell anaemia The shape of the haemoglobin molecule is controlled by two alleles Normal Haemoglobin allele Sickle Cell Haemoglobin allele There are three phenotypes Normal Normal individuals have two normal haemoglobin alleles Sickle cell anaemia, a severe form where all the red blood cells are affected. Sickle cell anaemia patients have two sickle cell alleles in their genotype Sickle cell trait, a mild condition where 50% of the red blood cells are affected. Sickle cell trait individuals are heterozygotes, having one of each allele

12. Symbols for codominant alleles Both alleles are expressed in the heterozygote both take a CAPITAL CASE letter An index letter identifies the allele Therefore: Normal haemoglobin allele is Hb N Sickle cell allele is Hb S

13. Sickle cell anaemia genotypes and phenotypes GenotypesPhenotypes Hb N Normal haemoglobin Hb N Hb S Sickle cell trait Hb S Sickle cell anaemia

14. PhenotypeSickle cell trait x GenotypesHb N Hb S GametesHb N Hb S Hb N Hb S Hb N Hb S Hb N Hb N Hb S Hb S Hb N Hb S Hb S OffspringNorm al Sickle cell traitSickle cell anaemia Proportions25%50%25% Example cross 1:2:1

15. Questions Try Rowland questions on co-dominance

16. Multiple alleles Many genes have 3 or more alleles for a particular gene. There may be an hierarchy of dominance with some alleles dominant over several others E.g. gene for ABO blood groups in humans

17. ABO blood groups 4 different blood groups controlled by 3 different alleles I A I B I O I A and I B are co dominant I O is recessive to both

18. For each blood group list the possible genotypes Possible genotypes Blood group Group A Group B Group AB Group O

19. To do Try summary question 1 and 2 page 123 Rowland q 22.13 saq 5.7 and 5.8 Cambridge (difficult) Application questions p124