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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 . - PowerPoint PPT Presentation
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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 italicizeddominant 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
Combining modified modes of inheritance:
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 hairC = 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 mammalssex chromosomes designated as X and YXX=femaleXY=male
*1909 Thomas Hunt Morgan
II III IV
XX
XY
or
*X-linkage
X-linkage in Drosophila: white mutation (eyes)
Genes on the X Chromosome con’t:
Sex-limited Inheritance:*Sex-limited trait
*holandric genes: genes on the Y chromosome
*autosomal genesExample: 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-chromosomeSex-limited all or none expression by sexSex-influenced genotype + sex determines phenotype
Phenotypic Expression:
*Penetrance
*Expressivity
Gene expression often governed by genotype and environment
If 9/10 of individuals carrying an allele express the trait, the trait is said to be 90% penetrant
*Onset of genetic expression
*Temperature