Topic 4, Extensions of Mendelian Genetics

1. Symbolism for alleles (page 78)
   A. In peas and most other organisms
       1) A capital letter designates a dominant allele and
       2) A small letter designates a recessive allele
   B. In Drosophila melanogaster:
       1) + designates the "wild-type" allele, the allele found most frequently in nature
       2) An allele that is recessive to a wild-type allele is designated with a small letter
       3) An allele that is dominant to a wild-type allele is designated with a capital letter
   
   
2. Incomplete dominance (neither allele is dominant)
   A. Flower color in snapdragon
   
   
3. Codominance (both alleles produce a detectable gene product in the heterozygote)
   A. MN blood groups in humans
   
   
4. Multiple alleles (more than two alleles are present at a locus)
   A. Coat color in rabbits
   B. ABO blood groups (multiple alleles and codominance)
       1) Antigens and antibodies
       2) Alleles
       3) Blood transfusions (compatible and incompatible combinations)
       4) Bombay phenotype
   C. Rh blood groups
   D. White eye in Drosophila melanogaster
   
   
5. Lethal alleles (alleles that are lethal to an organism)
   A. Recessive lethal alleles (alleles that are lethal in the homozygous condition)
       1) Albino corn
       2) Yellow coat color in mice
   B. Dominant lethal alleles (alleles that are lethal in the heterozygous condition)
       1) Huntington's disease
   
   
6. Combinations of two gene pairs with different modes of inheritance give a modified 9:3:3:1 ratio in the F2
   A. Inheritance of albinism and ABO blood groups in humans (page 86 in text)
   
   
7. Phenotypes are often controlled by more than one gene
   A. Definitions of epistasis and hypostasis
       1) ABO blood groups and the Bombay phenotype
           a. The h gene is epistatic to the I gene and
           b. The I gene is hypostatic to the h gene
       2) Kernel colors in Zea mays (corn)
   B. Novel phenotypes (two pairs of alleles that affect the same character interact to give
        a 9:3:3:1 F2 ratio but unexpected F1 and/or F2 phenotypes (page 91)
       1) Comb shape in chickens
   
   
8. Complementation analysis (used to determine if two independently isolated recessive mutations
   that produce similar mutant phenotypes are at the same or different genetic loci, page 92)).
   Individuals that are homozygous recessive for one mutation are crossed by individuals that are homozygous recessive for a different mutation.
   A. Case 1, if  the F1s have the non-mutant phenotype, we conclude that the two recessive mutations complement each other (are not allelic)
   B. Case 2, if the F1s have the mutant phenotype, we concede that the two recessive mutations do not complement each other (are allelic)
   
   
9. X-linked genes (genes on the X chromosome)
   A. X-linked recessive inheritance
       1) Red vs. white eye color in Drosophila melanogaster
       2) Red-green colorblindness (deutan type) in humans
           a. Allele frequencies
   B. X-linked dominant inheritance
       1) Hypophosphatemia in humans
   
   
10. Y-linked genes (genes on the Y chromosome)
    A. Gene for maleness are on the Y chromosome in humans (testes determining factor, TDF)
    B. Genes for fertility in males are on the Y chromosome in Drosophila melanogaster
    
    
11. Sex-limited inheritance, the locus produces a phenotype that is expressed in only one sex
    A. Milk production in cattle
    B. Beard density in humans
    C. Hen feathers vas cock feathers in chickens
    D. BRCA-1 and BRCA-2 mutations in humans
    
    
12. Sex-influenced inheritance, the dominance of a gene is influenced by the sex of an individual
    A. Pattern baldness in humans
    
    
13. Penetrance, not every individual with the mutant genotype expresses the mutant phenotype
    A. Interrupted wing veins in Drosophila melanogaster
    B. Polydactyly
    
    
14. Expressivity, the phenotypic expression of a gene is different in different individuals
    A. Eyeless mutation in Drosophila melanogaster
    
    
15. Suppressor mutations, one gene modifies the expression of a non-allelic mutation
    A. Suppressor of sable in Drosophila melanogaster
    
    
16. Environmental effects on gene expression
    A. Corn hybrids
    B. Himalayan rabbits and Siamese cats
    C. Skin color in humans
    D. Nature-nurture controversy
    
    
17. Onset of gene expression, each gene has a characteristic time of expression
    
    
18. Phenocopy, an environmentally induced phenotype that resembles the phenotype caused by a known mutant gene
    A. Yellow-green leaf color in Zea mays
    B. Phocomelia in humans
    
    
19. Pleiotrophic genes, genes that have more than one phenotypic effect
    A. In cats, a single recessive mutation causes blue eyes, white coat color, and deafness
    B. Yellow coat color in mice (discussed previously)

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        Updated 9/11/00