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Heredity (part 3): Complete & Incomplete dominance, Multiple alleles & Sex determination

Students will be able to: <br>- use genetic diagrams to solve problems involving monohybrid inheritance (genetic diagrams <br>involving autosomal linkage or epistasis are not required) <br>- explain co-dominance and multiple alleles with reference to the inheritance of the ABO blood <br>group phenotypes (A, B, AB and O) and the gene alleles (IA, IB and Io) <br>- describe the determination of sex in humans u2013 XX and XY chromosomes

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Heredity (part 3): Complete & Incomplete dominance, Multiple alleles & Sex determination

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  1. Lesson Objectives (f) explain co-dominance and multiple alleles with reference to the inheritance of the ABO blood group phenotypes - A, B, AB, O, gene alleles IA, IBand IO (g) describe the determination of sex in humans - XX and XY chromosomes (h) describe mutation as a change in the structure of a gene such as in sickle cell anaemia, or in the chromosome number, such as the 47 chromosomes in a condition known as Down Syndrome (i) name radiation and chemicals as factors which may increase the rate of mutation

  2. Complete vs Incomplete dominance (Codominance)

  3. Is it possible? X White snapdragon Red snapdragon Pink snapdragon

  4. Incomplete dominance/Codominance ▪ A situation where neither allele dominates the other and both exercise an influence on the individual ➢ Example: Snapdragon cross ➢ RR = Red flowers ➢ rr = White flowers ➢ Rr = pink flowers

  5. Phenotypic ratio for Codominance

  6. Let R represent the alleles of the snapdragon with red flower. Let r represent the alleles of the snapdragon with white flower. Parental phenotype: Parental genotype: Gametes: Red flower x white flower RR x rr R R r r Random fertilisation: (using the Punnett Square) r r R Rr Rr R Rr Rr F1 genotype: F1 phenotype: All Rr All pink

  7. F1phenotype: F1genotype: Gametes: pink x Rr x R pink Rr r r R R RR r Random fertilisation: (using the Punnett Square) R Rr r rr Rr RR, Rr, Rr, rr red, pink, pink, white 1:2:1 (red:pink:white) F2 genotype: F2 phenotype: Phenotypic ratio:

  8. Another example X White cow Red cow roan

  9. Multiple alleles

  10. Multiple alleles ▪ Traits regulated by more than two alleles ▪ Example: Human Blood Types ▪ Three alleles: IA, IB, and Io ▪ IA, IBboth dominant over Io; Iois recessive ▪ A and B are codominant with each other ▪ Codominance = displaying two dominant phenotypes at the same time ➢Thus: ➢IAIAor IAIo= Type A ➢IBIBor IBIo= Type B ➢IAIB= Type AB ➢IoIo= Type O

  11. Question: The inheritance of the ABO blood groups in Man is controlled by three alleles (IA, IB, and Io), only two of which can be present in one individual. IAand IBare of equal dominance but both are dominant to Io. What are the possible blood groups of children born to a homozygous group A woman and heterozygous group B man?

  12. Parental phenotype: Parental genotype: Gametes: Blood group A x Blood group B IAIAx IBIo IA IA IB IO Random fertilisation: (using the Punnett Square) IA IA IAIB IB IAIB IO IAIOIAIO F1 genotype: F1 phenotype: IAIB, IAIB, IAIO, IAIO Blood groups AB, AB, A and A

  13. Am I legitimate? Project task: Collect information regarding the blood group of your parents and siblings. Based on this information, derive the possible genotypes of your parents.

  14. Sex Determination

  15. Sex determination • Humans have 23 pairs of chromosomes (46 chromosomes) i) 22 pairs are autosomal chromosomes ii) 1 pair of sex chromosomes • Males have the genotype XY • Females have the genotype XX

  16. Sex determination

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