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CH 9: Patterns of Inheritance (p.154)
Date:
Mendel’s Expts (p.156)
 Genetics is the study of heredity, the transmission of traits from parents to offspring…which
plants did he use? Why?
o Used pea plants that self-fertilize – true-breeding
o Crossed dif. true-breeding plants– produced hybrids
o P (parental) generation: original true-breeding plants
o F1 (1st filial) generation: offspring of P gen.
o On your bellringer, predict the results in the offspring when Mendel crossed the following:
 Purple flowers X White flowers
 Tall plants X Short plants
 Results: F1 plants were all the same
o Mendel allowed the F1 plants to self-fertilize & produce F2 offspring
On your bellringer, predict the results!
 Results: ¼ of the plants showed recessive traits (3:1 ratio)
 Alleles are dif forms of a gene (dominant/recessive)
- Dominant alleles mask the expression of recessive
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Homozygous: having 2 of the SAME alleles for a trait
o Ex: TT, tt
Heterozygous: having 2 DIFFERENT alleles for a trait
o Ex: Tt
Genotype: genetic makeup…the allele combo (Ex. TT, Tt, tt)
Phenotype: physical appearance/characteristic (Ex. tall, short)
Date:
Mendel’s Theory
 Principle of segregation: alleles segregate during gamete formation (meiosis)
o each gamete carries ONE allele for each trait
o each homologous chromosome has ONE allele for each trait
 Principle of independent assortment: alleles segregate independently during meiosis
o …unless they are on the same chromosome!
o chromosomes actually assort independently, not individual alleles
o crossing-over can separate genes on the same chr.
Punnett Squares
 Monohybrid cross: parents differ in 1 trait
 Dihybrid cross: parents differ in 2 traits
Date:
Non-Mendelian Genetics (p.168)
 Incomplete dominance: heterozygous phenotype is a blend of 2 homozygous phenotypes
o Ex. red(R) and white (W)  pink (RW)
 Codominance: both alleles appear in phenotype at same time
o Ex. RW horses are “roan” colored (have both red and white hairs)
 Multiple alleles: most genes have > 2 alleles
o Ex. ABO blood type has 3: IA, IB, & i
 IA & IB are codominant
 ii is type O
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Sex-linked genes: recessive alleles located on the X (p.178)
o ALWAYS expressed in males…why?
o b/c they only have 1 X
o Women can be “carriers” (heterozygous) & show dominant phenotype
o Ex: hemophilia
Date:
Other Forms of Inheritance
 Pleiotropy: when 1 gene has multiple effects (p.170)
o Ex: sickle-cell disease
 Polygenic traits: traits controlled by > 1 gene (p.172)
o Ex. human skin tone & height have a continuous range of phenotypes
 Epistasis: 1 gene influences the expression of another
o Ex: in mice, black is dominant over brown
 There’s another gene that codes for the ability to produce ANY pigment (C =
pigment, c = no pigment / albino)
 Bbcc  no color (albino)
 BbCc  black, bbCc  brown
 Environmental influence: an organism’s phenotype may be influenced by its environment
o Ex: the color of the arctic fox changes from brown during the summer time to white during
the winter
Genetic Disorders (p.164)
 Genes code for proteins w/ a specific fxn
o Many genes have only 1 allele
o Any ∆ in that allele is a mutation that can be passed on
 Usually harmful
 Disorders may be caused by recessive, dominant, or codominant mutations
o Recessive
 Cystic fibrosis – gene mutation causes improper folding of membrane protein CFTR
 absence of CFTR in cell’s membrane
- chloride ions (Cl-) unable to cross cell membranes  mucus in lungs, digestive
tract
 PKU (phenylketonuria) – mutation in gene coding for an enzyme that breaks down
the AA phenylalanine  phenyl. accumulation in tissues may cause mental
retardation  early detection can allow for survival w/ dietary restrictions
 Tay-Sachs disease – mutation in gene that codes for enzyme that breaks down
gangliosides in neurons  results in nervous system break down & death
o Dominant (less common because…?)
 Huntington disease - loss of muscle control & mental function  starts in 30s or
older
 achondroplasia – form of dwarfism
o Codominant
 Sickle-cell disease –mutation ∆’s 1 AA in hemoglobin
- must be homozygous to have disease
- heterozygotes are resistance to malaria
- abnormal hemoglobin causes sickle-shaped RBCs  get caught in capillaries &
cause weakness & damage brain, heart, & spleen