Extensions of Mendelian Genetics
Phenomena that are not explained by basic Mendelian genetics
Bio 3600: Genetics
Dr. G Cornwall
The classification of dominance relationships is dependent upon the
level at which phenotype is examined. (cystic fibrosis)
•Simple Dominance: the phenotype of the heterozygote …
•Incomplete Dominance: the phenotype of the heterozygote is …
•Codominance: the phenotype of the heterozygote …
•When there are more than two possible alleles for a particular character it is
said to be a multiple allele locus.
–ABO blood grouping
–Duck feather patterns
•An allele that causes death prior to birth is termed a lethal allele
–A recessive lethal allele almost always produces …
–Lethal alleles can also be dominant
•The effects of genes at one locus depend on the presence of genes at
another locus (non-allelic genes)
–The products of genes at different loci combine to produce new
•Gene Interaction Producing novel phenotypes
–In peppers, the R allele provides red color
–the C allele causes decomposition of chlorophyll
•Gene interaction with epistasis:
–Fur coloration in Labrador retrievers is a great example of recessive epistasis
•B = black, b = brown (simple dominance) –
•E = melanin, e = no melanin in hair & skin cells (simple dominance) – if a lab is ee it has no (very
little) melanin in its skin & fur
• Gene interaction with dominant epistasis:
– Fruit color in summer squash
• Yellow, white, or green
– The allele for fruit color is at one locus
• Yellow (Y-) is dominant over green (yy)
– The allele that codes for the enzyme that produces the pigment is at another locus
• The dominant allele W at one locus prevents the production of pigment
– What ratios result from a YyWw and YyWw?
• Gene interaction with duplicate recessive epistasis:
– Albinism in freshwater snails
– Need two enzymes for the production of pigment (A and B)
• aa = nonfunctional enzyme produced
• bb = nonfunctional enzyme
– Alleles at both loci are epistatic to each other
•A complementation test is used to determine whether a mutation that affects a characteristic is at the
same locus as that characteristic (is allelic) or at a different locus (non-allelic)
–White is a mutation of the Red eye locus
–Apricot is another eye color mutation
–Are these mutations at the same locus, or at different loci?
If the alleles are at the different loci, the
parents will have wildtype genes at the other
If the alleles are at the same locus,
the heterozygous offspring will …and the heterozygous offspring
exhibit only mutant phenotype will exhibit only wildtype
Interactions Between Sex & Heredity
•Sex influenced characters …
–Autosomal gene Bb for beards in goats is dominant in males &
recessive in females.
–Pattern Baldness exhibits the same pattern (not X-linked as is
•Also expressed weakly in females due to hormone interaction.
•Sex-Limited characteristics are expressed in ….
–Hen feathering is an autosomal recessive sex limited trait
–Precocious puberty is an autosomal dominant trait limited to males
•cytoplasmic inheritance: Not all characteristics are coded for by genes in
– DNA in mitochondria (mtDNA) & chloroplasts (cpDNA).
–Thus characteristics encoded on extranuclear genetic material …
–Variegation in four o’ clock plants
•Maternal effect: the phenotype of the offspring is determined by …
–Sinistral vs. dextral coiling in snails
–Contrast to cytoplasmic inheritance?
•Genomic imprinting: the differential expression of genes dependent upon
which parent they were inherited from.
•Anticipation: when a genetic character is expressed earlier, or is more
strongly expressed as it is passed from generation to generation
–Unstable regions of DNA that increase / decrease in size from generation to generation
Genes & Environment
Expression of many genes is modified by the environment
•Norm of reaction:
•The products of temperature sensitive alleles are only functional at certain temperatures
–Himalayan allele in rabbit
–PKU and phenylalanine
•Polygenic Inheritance: several genes influence one characteristic,
resulting in a continuous range of phenotypes.
–A single (2 allele) locus gives 3 possible phenotypes
–Number of possible genotypes = 3n
•Pleiotropy: one gene affects multiple characters
•Many characters are Multifactorial
–Perhaps only 3 genotypes, but because each has a broad norm of reaction the character displays a
•These modifications & extensions of gene expression do not alter the way in which the genes are
inherited. They are simply modifications of the way in which genes determine the phenotype.
–Interaction between genes
–Interactions between genes & sex
–Interaction between genes & environment