chapter 4: modification of mendelian ratios allele *wild-type allele *mutant allele conventional...
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Chapter 4: Modification of Mendelian RatiosAllele
*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!
Incomplete or Partial DominanceCross between parents
with contrasting traits: Red flowers or white
flowers
Offspring with an intermediate
phenotype: pink flowers
Codominance:
Example:
MN Blood group- red blood cells contain a transmembrane glycoprotein (glycophorin); two different forms of this protein exist, M and N
Examples:
*Table 4.1: over 100 alleles at a given locus in Drosophila
*ABO Blood group in humans
Multiple Alleles:
*Characterized by the presence of glycoprotein antigens on the surface of red blood cells
*Distinct from the M and N antigens
*Also exhibits codomiance
Lethal Alleles:Example: Coat color in mice
*A = agouti = wild-type allele
*AY = yellow = mutant allele
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)
*Novel phenotypes due to gene interactionExample:
disc-shaped fruit (AABB) X long fruit (aabb)
pepper color
Genes on the X Chromosome:
*Sex chromosomes*Autosomes
Example: In Drosophila and all mammals
sex chromosomes designated as X and Y
XX=female
XY=male
*1909 Thomas Hunt Morgan
II III IV
XX
XY
or
Sex-limited Inheritance:*Sex-limited trait
*holandric genes: genes on the Y chromosome
*autosomal genes
Example: milk production in mammals; L=lots, l=little
*Sex-influenced trait
Sex-influenced Inheritance:
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