Modification of Mendelian Ratios
The effect of alleles on
a given phenotype.
of a single phenotype by more than one set of genes.
• Alternate forms of the same gene.
• Wild type(normal)-occurs most frequently in a
• Derived from mutations-deleterious
• Recessive Allele- d (dwarf)
• Dominant Allele- D (tall)
• D. melanogaster
-wild-type trait= superscript
a slash (/) between letters=same
Factors that modify the mendelian patterns of inheritance
Neither allele is
Heterozygotes= Intermediate (blended) phenotype (Red X White
Phenotypic ratio =
Genotypic ratio (1:2:1)
disease-Enzymatic disorder (hexosaminidase)
recessive = die
50% of enzyme activity.
Both alleles of a single
gene are expressed.
MN blood group
on the surface of RBCs.
-Two forms = M and N
• An Individual may exhibit on or both.
• Three or more alleles of the same gene.
(e.g.) (a) The
ABO blood group
(b) The Rh antigens
ABO blood group
One of four possible
phenotypes resulting from 2 antigens (A and B).
Determined by mixing
blood sample with antiserum containing A or B antibodies.
Designation- IA, IB IO
Testing compatibility of
A individual= IAIA or IAIO
B individual= IBIB or IBIO
AB individual =
O individual= IOIO
Expression of ABO
influenced by gene at the secretor locus.
Se/Se or Se/se=
(form of anemia)
Rh Antigens may result from
(1) Multiple alleles at a single locus, or
(2) Three or more linked genes, each with 2 alleles
• Recessive lethal allele-Homozygous (lethal)
-Heterozygous (mutant phenotype)
• Dominant lethal allele
copy of allele results in death -Huntington’s Disease
of Two Gene Pairs
• 9:3:3:1 ratio modified
• Two modes of inheritance- 3:6:3:1:2:1
• E.g. = 2 individuals heterozygous for albinism and both have AB blood type.
• Epistasis- Several genes control one phenotype
• A ratio expressed in 16 parts (9:3:4) when studying
inheritance of a single character = two gene pairs are interacting.
• A genotype mask /suppresses expression of an allele.
• A dominant allele at one locus mask expression of
alleles at a second locus.
• The homozygous condition of either recessive allele
mask the expression of the dominant allele at the other locus.
• New phenotype in the F2 generation.
• Transmission of genes located on the x-chromosome.
• Crisscross pattern of inheritance.
• Males are hemizygous for all alleles on the x-chromosome.
Inheritance-DNA from the mitochondria (chloroplast) determines offspring
Effect- Phenotypic effects on the offspring produced by factors transmitted
through the egg cytoplasm.
established during early development.