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
Genetic nomenclature is extremely diverse!
Incomplete or Partial Dominance
Cross 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
Multiple Alleles:
Examples:
*Table 4.1: over 100 alleles at a given locus in Drosophila
*ABO Blood group in humans
*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
*Novel phenotypes due to gene interaction
Example:
disc-shaped fruit (AABB) X long fruit (aabb)
Genes on the X Chromosome:
*1909 Thomas Hunt Morgan
II
III
XY
IV
or
XX
*Sex chromosomes
*Autosomes
Example: In Drosophila and all mammals
sex chromosomes designated as X and Y
XX=female
XY=male
Genes on the X Chromosome con’t:
*X-linkage
X-linkage in Drosophila: white mutation (eyes)
½ red
½ white
Genes on the X Chromosome con’t:
½ red
Sex-limited Inheritance:
*Sex-limited trait
*holandric genes: genes on the Y chromsome
Example: hypertrichosis (ear hair)
*autosomal genes
Example: milk production in mammals; L=lots, l=little
Sex-influenced Inheritance:
*Sex-influenced trait
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