Download Extensions and Exceptions to Mendel*s Laws

Extensions and Exceptions to Mendel’s Laws
• Gene expression appears to alter Mendelian ratios
• Lethal Genotype: causes death before the individual can
reproduce; prevent transfer of his/her genes to the next
generation; presence of just 1 copy of the allele results in the
death of an individual
 Humans: miscarriages (spontaneous abortions)/Huntington Disease
• Incomplete Dominance: Heterozygous phenotype is
intermediate between that of either homozygote
 Snapdragons: Red (RR) Pink (Rr) White (rr)
 Andalusian Fowl: Black feathers (RR) Blue (Rr) White (rr)
 Roan Cattle: Brown (RR) Roan (Rr) White (rr)
• Codominance: when different alleles are both expressed in a
heterozygote; ABO blood group (determined by presence of
antigens) on RBC (on chromosome 9)
• 1900’s Karl Landsteiner
Blood Type
A
Antigens
A
Genotypes
IAIA
A
A
IAi
B
B
IBIB
B
B
IBi
AB
A and B
IAIB
O
None
ii
• Epistasis: 1 gene masks or affects the expression of a different
gene; % of individuals show some degree of expression of the
mutant
 Bombay Phenotype: the presence of a mutant form masks the expression of
antigens A and B
 A and B antigens = carbohydrates bound to lipid protruding from RBC membrane; A
and B antigens derived from H substance (2 sugars added); Bombay Phenotype: H
substance incompletely formed and not a good substrate for the enzyme that adds a
sugar (Type O blood)
• Penetrance: all or none expression of a genotype
 Huntington Disease (completely penetrant)
 Expressivity: refers to severity or extent
• Pleiotropy: gene controls several functions or has more than 1
effect
 Porphyria: King George III with an “episode”
relapse 13 years later
relapse 3 years later (a single protein affects different parts of the body or
participates in more than 1 type of biochemical reaction); problems with the
breakdown of hemoglobin; deep red urine, abdominal pain, headaches,
vision problems, delirium, muscular weakness, convulsions
 Marfan Syndrome: defect in elastic connective tissue
bones of limbs and ribs; mutation encoding fibrillin
lens, aorta,
 Alkaptonuria: 1 metabolic abnormality
blackened urine, problems
with muscles and joints, kidney stones, hearing loss
• Phenocopy: a characteristic that appears to be inherited but is
environmentally caused
 Limb loss from thalidomide; infections
• Genetic Heterogeneity : when different genes produce the
same genotype
 Hearing loss; clotting disorders
• Maternal Inheritance and Mitochondrial Genes:
 Mitochondria with “mini chromosome” of 37 genes
 Maternally inherited
 No crossing over; mutates faster (lacks DNA repair enzymes); high number
of free radicals in a confined space
 Encode proteins used in protein synthesis and energy production
 Mutations cause great fatigue
 Myoclonal Epilepsy and Ragged Red Fiber Disease: only affects child of
affected mother, not affected father; blotchy red patches, deafness, seizures,
brain function problems
 Leber’s Hereditary Optic Neuropathy: sudden bilateral blindness; vision
loss at 27 years of age (average)
 Heteroplasmy: rare condition in which a particular mutation may be
present in some mitochondrial chromosomes but not others
 Expressivity varies among siblings; very severe
• Linkage:
 On the same chromosome; No recombination of genes
 Drosophila melanogastor (Thomas Hunt Morgan): the farther apart 2
genes are on a chromosome, the more likely they are to cross over
 Led to construction of linkage maps: show order of genes on chromosomes
and relative distances between them