ii) An example of co-dominance: (2 marks) Scenario: (1) Bloodtypes OR (2) Roan cattle Explanation: When both alleles IA and IB when inherited together are seen in the phenotype as they are expressed together/equally , giving the blood type AB (1). This is co-dominance where neither A or B phenotype is recessive to the other (1). OR When a cross between two pure breeding individuals such as the white (homozygous) cow and red (homozygous) bull are mated producing offspring that are heterozygous Roan –the cow - which expresses both phenotypes (1) having both red and white hairs displaying codominance where neither trait is recessive to the other, but seen individually in the heterozygous form (1). iii) An example of (2 marks) multiple alleles: Scenario: (1) Blood types Explanation: There are three alleles, IA, IB or i , and not two alleles that can be expressed to determine blood type (1) so that instead of 2 possible phenotypes being produced there are four possible phenotypes – Type A blood can be an expression of the genotypes IAIA, IAi Type B blood can be an expression of the genotypes IBIB, IBi Type AB blood is expressed by the genotype IAIB Type O blood is expressed by the genotype ii COMMENTS: Most students identified the correct scenario, however, did not explain in relation to that scenario. Some students made up their own scenario to explain and many just used the information straight from the information sheet. If the latter was the case, a maximum of 1 mark could be achieved. If the scenario was not used in the explanation, maximum marks could not be obtained. Question 5 Using information from the pedigree, analyse and describe the likely mode of transmission for retinosa pigementosa. (3 marks)
ii) An example of co-dominance: (2 marks) Scenario: (1) Bloodtypes OR (2) Roan cattle Explanation: When both alleles IA and IB when inherited together are seen in the phenotype as they are expressed together/equally , giving the blood type AB (1). This is co-dominance where neither A or B phenotype is recessive to the other (1). OR When a cross between two pure breeding individuals such as the white (homozygous) cow and red (homozygous) bull are mated producing offspring that are heterozygous –the Roan cow - which expresses both phenotypes (1) having both red and white hairs displaying codominance where neither trait is recessive to the other, but seen individually in the heterozygous form (1). iii) An example of multiple alleles: (2 marks) Scenario: (1) Blood types Explanation: There are three alleles, IA, IB or i , and not two alleles that can be expressed to determine blood type (1) so that instead of 2 possible phenotypes being produced there are four possible phenotypes – Type A blood can be an expression of the genotypes IAIA, IAi Type B blood can be an expression of the genotypes IBIB, IBi Type AB blood is expressed by the genotype IAIB Type O blood is expressed by the genotype ii COMMENTS: Most students identified the correct scenario, however, did not explain in relation to that scenario. Some students made up their own scenario to explain and many just used the information straight from the information sheet. If the latter was the case, a maximum of 1 mark could be achieved. If the scenario was not used in the explanation, maximum marks could not be obtained. Question 5 Using information from the pedigree, analyse and describe the likely mode of transmission for retinosa pigementosa. (3 marks)
Biology-Answers-Cr8-moderation.docx