1. 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!

2. Offspring with an intermediate phenotype: pink flowers
Incomplete or Partial Dominance
Cross between parents with contrasting traits: Red flowers or white flowers
Offspring with an intermediate phenotype: pink flowers

3. Codominance: Example:
MN Blood group- red blood cells contain a transmembrane glycoprotein (glycophorin); two different forms of this protein exist, M and N

4. 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

5. Lethal Alleles: Example: Coat color in mice
*A = agouti = wild-type allele
*AY = yellow = mutant allele

6. Combining modified modes of inheritance:

7. 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 hair
C = presence, c = absence (colorless)

8. *Novel phenotypes due to gene interaction
Example:
disc-shaped fruit (AABB) X long fruit (aabb)
pepper color

9. Genes on the X Chromosome:
*1909 Thomas Hunt Morgan II
III IV XX XY or
*Sex chromosomes
*Autosomes
Example: In Drosophila and all mammals
sex chromosomes designated as X and Y
XX=female
XY=male

10. Genes on the X Chromosome con’t:
*X-linkage
X-linkage in Drosophila:
white mutation (eyes)

11. Sex-limited Inheritance:
*Sex-limited trait
*holandric genes: genes on the Y chromosome
*autosomal genes
Example: milk production in mammals; L=lots, l=little

12. 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

13. Phenotypic Expression:
Gene expression often governed by genotype and environment
*Penetrance
If 9/10 of individuals carrying an allele express the trait, the trait is said to be 90% penetrant
*Expressivity
*Temperature
*Onset of genetic expression