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Chapter 4: Modification of Mendelian Ratios

Chapter 4: Modification of Mendelian Ratios. Allele. *Wild-type allele. *Mutant allele. Conventional symbols for alleles: recessive allele- initial letter of the name of the recessive trait, lowercased and italicized dominant allele- same letter in uppercase .

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Chapter 4: Modification of Mendelian Ratios

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  1. Chapter 4: Modification of Mendelian Ratios Allele *Wild-type allele *Mutant allele Conventional symbols for alleles: recessive allele- initial letter of the name of the recessive trait, lowercased and italicized dominant allele- same letter in uppercase Genetic nomenclature is extremely diverse!

  2. Cross between parents with contrasting traits:Red flowersor white flowers Offspring with an intermediate phenotype: pink flowers Incomplete or Partial Dominance

  3. Codominance: Example: MN Blood group- red blood cells contain a transmembrane glycoprotein (glycophorin); two different forms of this protein exist, M and N

  4. Multiple Alleles: Examples: *Table 4.1: over 100 alleles at a given locus in Drosophila *ABO Blood group in humans *Characterized by the presence of glycoprotein antigens on the surface of red blood cells *Distinct from the M and N antigens *Also exhibits codomiance

  5. Lethal Alleles: Example: Coat color in mice *A = agouti = wild-type allele *AY = yellow = mutant allele

  6. Combining modified modes of inheritance:

  7. Gene interaction: *Epistasis Example: *In Drosophila, the recessive gene eyeless (when homozygous) prevents the expression of eye color genes present in genome *Coat color in mice *Black (B) is dominant to brown (b) *Second gene responsible for allowing pigment to be deposited in hair C = presence, c = absence (colorless)

  8. *Novel phenotypes due to gene interaction Example: disc-shaped fruit (AABB) X long fruit (aabb) pepper color

  9. II III XY IV or XX Genes on the X Chromosome: *1909 Thomas Hunt Morgan *Sex chromosomes *Autosomes Example: In Drosophila and all mammals sex chromosomes designated as X and Y XX=female XY=male

  10. Genes on the X Chromosome con’t: *X-linkage X-linkage in Drosophila: white mutation (eyes)

  11. Sex-limited Inheritance: *Sex-limited trait *holandric genes: genes on the Y chromosome *autosomal genes Example: milk production in mammals; L=lots, l=little

  12. Sex-influenced Inheritance: *Sex-influenced trait Examples: *cleft palate in humans *horns in sheep *pattern baldness in humans Summary: Sex-linked on X or Y sex-chromosome Sex-limited all or none expression by sex Sex-influenced genotype + sex determines phenotype

  13. Phenotypic Expression: Gene expression often governed by genotype and environment *Penetrance If 9/10 of individuals carrying an allele express the trait, the trait is said to be 90% penetrant *Expressivity *Temperature *Onset of genetic expression

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