Mendelian Inheritance
• Known as single-gene
inheritance/unifactorial trait
• Gregor Mendel: “father of genetics”
 From 1857-1863 crossed and catalogued
24,034 pea plants
• Why pea plants?
 Easy to grow
 Develop quickly
 Traits with easily distinguishing forms
(green-yellow, tall-short, round-wrinkled)
continuous vs. discontinuous traits
Mendel’s Laws
• Law of Dominance: 1 gene (dominant) can mask the
appearance of another gene (recessive)
• Law of Segregation: during formation of gametes alleles
separate Aa
A /a
• Law of Independent Assortment: during formation of gametes
genes randomly arrange themselves
AB / Ab / aB / ab
 Inheritance of a gene on 1 chromosome does not influence inheritance of a
gene on a different chromosome
 Exception: Linkage (red hair-freckles)
Rules for Problem-Solving:
Write down dominant and recessive genes
Write down parents’ genes (P generation)
Get parents’ gametes
Do the cross on a punnett square (board for offspring
possibilities – F1 generation)
• Get results and label answer
Types of Problems:
• Simple 1 trait cross
• Monohybrid Cross (Ff X Ff)
 Genotype: 1:2:1
Phenotype: 3:1
• Simple 2 trait cross
• Dihybrid Cross (FfDd X FfDd)
 Phenotype: 9:3:3:1
• Incomplete Dominance (3 phenotypes present)
 Andalusian Fowl FF (black) ff (white) Ff (blue)
 Roan Cattle FF (brown/reddish) ff (white) Ff (brown with white)
 Four O’Clock Flowers FF (red) ff (white) Ff (pink)
Pedigree Analysis
• Displays familial relationships and depicts which have
specific phenotypes and genotypes
• Autosomal Dominant Trait: a trait can appear in either sex
because an autosome carries the disease; do not skip
 Porphyria variegata: red urine, abdominal pain, coma, death
 Huntington Disease: progressive uncontrollable movements and
personality change (begins in middle age)
 Polydactyly: extra fingers or toes
 Marfan Syndrome: long limbs, sunken
chest, lens dislocation, weakened aorta
• Autosomal Recessive Inheritance: can appear in either sex;
Affected individuals with homozygous recessive genotype; can
skip generations
Cystic Fibrosis: lung infections and congestion
Sickle Cell Anemia: joint pain, spleen damage
Tay Sachs Disease: NS degeneration
Phenylketonuria (PKU): fair skin, mental retardation
Albinism: lack of melanin in skin, eyes, hair
Pedigree Information:
• Female = circle Male = square
• Parents – connected by a single line
 Consanguineous: parents are related (shown by a double line)
Siblings’ ages read from left to right (older to younger)
Arabic numbers = siblings
Roman numerals = generations
Twins represented by diagonal lines
 Identical Twins (monozygotic): horizontal line
 Fraternal Twins (dizygotic): no line