Multiple Alleles & Gene Interaction - Chapter 4

Normally, one individual has two copies of each gene.  So they 
have one gene & two alleles.  When Mendel looked at his pea 
plants, each trait had only two alleles.  However, it's possible 
for one gene to have many different possible alleles.  This is 
called an allelic series.  However, this does not change the 
fact that each individual has only two alleles- no matter how 
many are available in the population.  
Be sure you understand the difference!

One gene, many alleles (unlike Mendel) = allelic series
	interactions between alleles within an individual:
		complete dominance
			(ex: pea color)
			3:1
		incomplete (partial) dominance
			intermediate phenotype
			(ex: palomino horses = C/Cr)
				do not breed true
			1:2:1
		overdominance
			phenotype more extreme than either homozygote
			(ex: greater survivorship of malaria in 
			 heterozygotes)
			1:2:1
		codominance
			both alleles expressed
			(ex: MN & ABO blood groups)
			1:2:1
	one allele can be both recessive and dominant
		depending on the phenotype described
			e.g., sickle-cell anemia
				2 alleles:  HbA (normal) and HbS (sickle)
				-HbA is completely dominant 
					physical manifestation of disease
				-HbA is incompletely dominant
					when blood subjected to low oxygen
				-HbA/HbS shows overdominance with regard 
				 to malaria resistance (host survival)
				-HbA/HbS are codominant for presence of 
				 the two hemoglobin molecules
Departures from expected Mendelian ratios:
	can be due to a variety of causes:
		mutant allele at one gene masks expression of another 
		 gene (two genes interact to affect one phenotype)
			known as epistasis
				ex:  mouse coat color (Fig. 4.14)
				epistatic alleles can be dominant or recessive
				what is dihybrid ratio of AAbb x aaBB? 
					(9:4:3) agouti, white, black

				it is also possible for one gene to affect two or 
				more phenotypes
					known as pleiotropy
					ex: seed coat color & flower color in 
					 Mendel's peas
		penetrance
			def: the frequency that a dominant phenotype is 
			 manifested in individuals with one or more alleles
			incomplete penetrance is when there are some 
			 individuals with one or more copies of a dominant 
			 allele who do NOT exhibit the dominant 
			 phenotype
				due to a gene x environment interaction
				all of Mendel's traits showed complete 
				 penetrance
				ex. of incomplete penetrance:
					retinoblastoma (eye tumor) 
						autosomal dominant allele
					polydactyly
						autosomal dominant allele
		expressivity
			def: the degree to which a dominant allele is 
			 manifested
			two individuals with same genotype can show 
			 different degrees of the phenotype due to variable 
			 expressivity
				due to a gene x environment interaction
				ex: blackpatch in Drosophila
				ex: osteogenesis imperfecta
					autosomal dominant disease with 
					 variable expressivity
		lethal alleles
			an allele whose presence causes death
				ex:  Manx tailessness in cats
					causes fetal death
					autosomal recessive
				ex: Huntington's chorea in humans
					death after reproduction
					autosomal dominant
		
		the influence of sex on expression of traits [this section 
		 is not found in your text]:
			sex-limited traits
				traits that appear in one sex but not the other, 
				 regardless of genotype
					due to the role of hormones
				ex:  feathering in chickens
			sex-influenced traits
				these traits appear in both sexes
				the frequency is different or the relationship 
				 between genotype and phenotype is different	
					due to the role of hormones
				ex: pattern baldness in humans
					same allele is recessive in females but 
					 dominant in males