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I. Introductory DefinitionsA.Heredity: passing traits from
parents to offspringB. Genetics: study of heredityC. Chromosomes:rod-shaped, coiled DNA; transmits hereditary info
D.Genes: units of heredity located on chromosomes
E.Allele: one form of a gene
II. Mendel’s Pea PlantsA. 1850sB. Why pea plants?
1. Lots of offspring2. Grow quickly3. Self-pollinate (offspring
identical to themselves)4. Cross-pollinate (2 parents)
C. Studied 7 traits (e.g. tall/short) by controlling pollination
D.Mendel’s experiments1.P (parental) generation
a)14 pure strains (ex: pure tall)2.F1 generation
a)Hybrids: offspring from crossing opposite pure strains
b)Ex: breed tall with short3.F2 generation
1.Offspring from self-pollinating F1 generation
E.Mendel’s results1.F1 generation
a)One trait always disappeared
b)Ex: tall X short --> all tall
2.F2 generationa)“invisible” trait reappearedb)Ex: F1 tall X F1 tall -->
3/4 tall & 1/4 short
F.Mendel’s conclusions1.Traits controlled by pairs of genes (FACTORS)2.Principle of Dominance: one
gene in a pair may mask the other (tall masks short gene)a)Dominant allele = capital
letter (T = tall)b)Recessive allele = small
letter (t = short)
c)Possible pair combinations:• Homozygous (purebred): both
genes in pair identical Homozygous dominant: TT Homozygous recessive: tt
• Heterozygous (hybrid): one dominant & one recessive: Tt
3.Principle of segregation: each pair of genes (alleles) separates when sex cells are formed (MEIOSIS)• each F1 plant produces two
different gametes (Tt --> T gamete & t gamete)
4.Principle of Independent Assortment: gene pairs separate independent of other gene pairs
III.Punnett SquareA. Diagram that shows the gene
combinations that might result from a cross
B. Genotype: the gene combination of an individual
• Ex: TT, Tt, ttC. Phenotype: physical traits as
determined by the genotype• Ex: TT & Tt= tall, tt = short
D.Monohybrid cross1.single genetic trait2.Each parent will pass on
one allele to the offspring3.Each offspring will have 2
alleles (one from each parent)
E.Dihybrid cross1.Two genetic traits2.Each parent will pass on
one allele from each gene3.Each offspring will have 2
pairs of alleles (2 alleles from each parent)
4.Since the parents in a dihybrid cross are heterozygous, there are 4 different combinations of alleles (gametes) they can pass on to their offspring:
RY RrYy Ry
rYry
IV.Beyond Dominant & RecessiveA. Incomplete dominance
-One allele is not completely dominant
1. Heterozygous phenotype is between the two parent phenotypes
2. Ex: red X white --> pink
B.Codominance1.both alleles are expressed
in the phenotype2.Ex: red X white --> roan
(red & white both present)
C.Multiple alleles: gene with more than 2 possible alleles
1.Each individual inherits only 2
2.Examples: Blood types (A
B o) + Rabbit Coats (C
ch h c )
D.Polygenetic traits: some characteristics are controlled by two or more gene pairs
1.Wide range of phenotypes
2.Ex: skin color, eye color, Human Height
“Labrador Coat Colors –Epistasis”