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Revisions to Mendel

Revisions to Mendel. Some alleles are neither dominant or recessive, and many traits are controlled by multiple alleles or multiple genes. So special rules must be used for these cases. Incomplete dominance Codominance Multiple Alleles Lethal Alleles Sex Linked Inheritance Polygenic Traits.

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Revisions to Mendel

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  1. Revisions to Mendel • Some alleles are neither dominant or recessive, and many traits are controlled by multiple alleles or multiple genes. • So special rules must be used for these cases. • Incomplete dominance • Codominance • Multiple Alleles • Lethal Alleles • Sex Linked Inheritance • Polygenic Traits

  2. Incomplete Dominance • Neither allele is dominant. • The heterozygous phenotype is somewhere inbetween the two homozygous phenotypes. • Example: Four o’clock flowers

  3. Practice : Your Turn! • In Australia there is a creature known as wild dogs or dingoes. It comes in three colors, black, grey, and white. This trait is controlled by a single gene with incomplete dominance. A homozygous (BB) individual is black, a homozygous (bb) individual is white, and a heterozygous (Bb) individual is grey. • What would be the genotypes and phenotypes of the offspring if a black dingo were crossed with a white dingo?

  4. Check your work Genotypes: 100% or 4/4 Bb Phenotypes: 100% Grey dingo pups.

  5. Codominance • Both alleles are dominant. • Both alleles appear in the phenotype separately. • Each allele produces both types of protein, so they both appear. Example: A black chicken mating with a white chicken will create an erminette chicken (also known as checkerboard).

  6. Practice : Your Turn! • A white cow (WW) mates with a brown cow (RR) to create a roan calf (RW). This trait is controlled by a single gene with codominance. • Suppose this roan calf grows up to mate with another roan cow. • What would be the genotypes and phenotypes of the offspring?

  7. Check your work Genotypes: ¼ RR or 25% RR 2/4 RW or 50% RW ¼ WW or 25% WW Phenotypes: ¼ or 25% Brown calves ½ or 50% Roan calves ¼ or 25% White calves

  8. Multiple Alleles • In a population, there might be more than 2 alleles for one single gene. • Any individual cannot have more than 2 alleles, even if many more exist in their gene pool. • One from the mother • One from the father • Example: Laborador coat color, mouse coat color, human blood types

  9. Blood Typing • Human blood works of a system of Codominance with the AB blood type and Multiple Alleles due to three alleles, A, B and O . • Charles Drew is famous for his blood preservation methods. • We’ll talk more about blood later!

  10. Blood Typing

  11. Practice : Your Turn! • A man is concerned about if he is the father of a child. He has blood type B and the mother has blood type O. The child’s blood type is A. • Is he the father of this child?

  12. Check your work Since the mother is O, she is recessive and has the genotype of ii. Since the father is B, he could be homozygous (IBIB) or heterzygous (IBi) so you should complete two separate punnett squares for each possibility. Punnett #1: All children will have B blood type Punnett #2: ½ children can have B blood type, ½ can have O blood type. Result? If the child’s blood type is A, then he is not the father. 

  13. Lethal Alleles • This defies Mendel’s laws of 3:1 because one of the Genotypes results in a lethal (deadly) Phenotype. • Also called Pseudodominance • Example: Sickle Cell Anemia – the offspring that gets C1C1 will die immediately from the blood disorder. • Then how do lethal alleles still persist in the population if those with lethal alleles die before birth!?

  14. Sex linked inheritance varies the Mendel number of 3:1 by having males a 50/50 percent chance of inheriting the characteristic on the X chromosome. Females have XX and Males are XY. The Y carries little genetic information, mainly those that contribute to male characteristics. The X carries a lot more genetic information. Genes for certain traits are on the X chromosome, and boys only receive one X chromosome then they are more likely to inherit disorders passed to them from their mother. Women are affected when they get the disorder from their mother and paternal grandmother. Sex-Linked Inheritance

  15. Practice: Your Turn! • Hemophilia is a sex-linked trait where XH gives normal blood clotting and is dominant to the hemophilia allele Xh. • Identify the genotypes of… 1) a woman with normal blood clotting whose father had hemophilia 2) a normal man whose father had hemophilia. • What is the probability that a mating between these two individuals will produce a child, regardless of sex, that has hemophilia?

  16. Check your work 1) the woman has normal clotting so she has one XH but she got a Xh from her father, so she is XHXh 2) the man is XHY since he got the Y from his father and he is normal so must be XHY Genotypes: ¼ XHXH Phenotypes: ½ unaffected girls ¼ XHXh ¼ unaffected boy ¼ XHY ¼ affected boy ¼ XhY Notice how girls are “protected” from disorders and carry them.

  17. Polygenic Traits • Instead of a trait being controlled by a single pair of homologous chromosomes, some traits are controlled by several genes. • Poly = many • Genic = genes • You get more a bell shaped curve of diversity • Examples: Skin color, hair color, etc… • Example: AaBBCcddEe

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