1) Genetics is more complex than just dominant and recessive alleles. Traits can be controlled by incomplete dominance or codominance where heterozygotes express an intermediate or combined phenotype rather than one allele dominating the other. 2) Incomplete dominance is seen in snapdragons where a cross of a brown and white flower produces tan offspring. Codominance is seen in cattle that are both red and white. 3) Multiple alleles, like the three alleles that determine blood type, add another layer of complexity with multiple genotypes producing different phenotypes. Polygenic traits are influenced by two or more genes.

1. Mendelian Genetics, part IIFrom Section 11-3, 11-4, 14-1

2. Beyond simple dominant and recessive alleles (other possibilities out there in the real world).

3. The real world of geneticsSome alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes.

4. Incomplete dominanceIncomplete dominance – neither allele is completely dominant over the other. Therefore, in this case, the heterozygote’s phenotype is in between the two homozygous phenotypes (intermediate phenotype).

5. Incomplete dominance - snapdragons

6. Incomplete dominance?Allow these to self-pollinate?

8. Incomplete dominance, cont.A brown horse bred with a white horse produces…tan-colored foals

9. Incomplete dominanceMake a Punnett square and give the genotypic and phenotypic ratios for a cross between two heterozygotes for a trait with incomplete dominance. (B allele for black fur, b allele for white fur)

10. Example: incomplete dominance in fur colorBB = Black FurBb = Gray FurBb = White FurParents: Bb x BbBoth grayThe heterozygous phenotype is in between the homozygous phenotypes. The alleles are incompletely dominant.

11. CodominanceIn codominance, both alleles contribute to the phenotype (neither allele is completely dominant nor completely hidden by the other-it’s a combination).

12. Codominance in Shorthorn cattleredwhite×roan

13. Codominance in Rhododendrons

14. CodominanceMake a Punnett square and give the genotypic and phenotypic ratios for a cross between two heterozygotes for a trait with Codominance. G allele for green leaves, g allele for yellow leaves)

15. Example: Codominance in leaf colorParents: Gg x GgBoth green and yellow leavesGG = GreenGg = BOTH green and yellowgg = yellowThe heterozygous phenotype Expresses both the green and yellow leaf color of the homozygotes. The alleles are codominant.

16. Multiple AllelesWhen there are more than two alleles for a gene the trait is said to have multiple alleles.

17. Multiple AllelesHuman blood type is controlled by multiple alleles.There are 3 alleles:IA IB iTwo different things are going on—codominance and dominance.codominantRecessive to

18. What are the possible combinations of the three alleles and what blood types result from each combination?IA IB i

19. Genotypes and Phenotypes for ABO blood groups (alleles IA, IB, i)GenotypesPhenotypesIA IAIA iIB IBIB iIA IBiiType A bloodType B bloodType AB bloodType O blood

20. Polygenic TraitsPolygenic traits are those traits that are controlled by two or more genes (poly=many, genic=genes).Polygenic traits often show a wide range of phenotypes.

21. Multiple Gene (Polygenic) Inheritance of skin colorParent 1 genotype: AaBbCcParent 2 genotype: AaBbCcEach capital letter represents an allele for dark skin, each lower-case letter represents an allele for light skin. Count the number of capital letters each possible child has to determine the how dark or light their skin would be.

22. Sex ChromosomesTwo of the 46 chromosomes in humans are known as the sex chromosomes.The remaining 44 chromosomes (22 pairs) are known as autosomes.

23. X and Y chromosomes

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