Patterns of Inheritance
Chapter 12
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Mystery of heredity
• Before the 20th century, 2 concepts were the basis for ideas about heredity– Heredity occurs within species– Traits are transmitted directly from parent to
offspring
• Thought traits were borne through fluid and blended in offspring
• Paradox – if blending occurs why don’t all individuals look alike?
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Early work
• Josef Kolreuter – 1760 – crossed tobacco strains to produce hybrids that differed from both parents– Additional variation observed in 2nd generation
offspring contradicts direct transmission
• T.A. Knight – 1823 – crossed 2 varieties of garden pea, Pisum sativa– Crossed 2 true-breeding strains– 1st generation resembled only 1 parent strain– 2nd generation resembled both
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Gregor Mendel
• Chose to study pea plants because:
1.Other research showed that pea hybrids could be produced
2.Many pea varieties were available
3.Peas are small plants and easy to grow
4.Peas can self-fertilize or be cross-fertilized
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Mendel’s experimental method
• Usually 3 stages1.Produce true-breeding strains for each trait
he was studying2.Cross-fertilize true-breeding strains having
alternate forms of a trait – Also perform reciprocal crosses
3.Allow the hybrid offspring to self-fertilize for several generations and count the number of offspring showing each form of the trait
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Monohybrid crosses
• Cross to study only 2 variations of a single trait
• Mendel produced true-breeding pea strains for 7 different traits– Each trait had 2 variants
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Conclusions
• His plants did not show intermediate traits– Each trait is intact, discrete
• For each pair, one trait was dominant, the other recessive
• Pairs of alternative traits examined were segregated among the progeny of a particular cross
• Alternative traits were expressed in the F2 generation in the ratio of ¾ dominant to ¼ recessive
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5 element model
1. Parents transmit discrete factors (genes)
2. Each individual receives one copy of a gene from each parent
3. Not all copies of a gene are identical– Allele – alternative form of a gene– Homozygous – 2 of the same allele– Heterozygous – different alleles
4. Alleles remain discrete – no blending
5. Presence of allele does not guarantee expression
– Dominant allele – expressed– Recessive allele – hidden by dominant allele
• Genotype – total set of alleles an individual contains
• Phenotype – physical appearance
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Principle of Segregation
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Human traits
• Some human traits are controlled by a single gene– Some of these exhibit dominant and recessive
inheritance
• Pedigree analysis is used to track inheritance patterns in families
• Dominant pedigree – juvenile glaucoma– Disease causes degeneration of optic nerve
leading to blindness– Dominant trait appears in every generation
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– In 1909, British physician Archibald Garrod first suggested that genes dictate phenotypes through enzymes that catalyze specific chemical reactions
– Inborn errors of metabolism– alkaptonuria (alkapton)
Linking genes to enzymes : a metabolic pathway
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• Recessive pedigree – albinism– Condition in which the pigment melanin is not
produced– Pedigree for form of albinism due to a
nonfunctional allele of the enzyme tyrosinase– Males and females affected equally– Most affected individuals have unaffected
parents
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Dihybrid crosses
• Examination of 2 separate traits in a single cross
• Produced true-breeding lines for 2 traits
• RR YY x rryy
• The F1 generation of a dihybrid cross (RrYy) shows only the dominant phenotypes for each trait
• Allow F1 to self-fertilize to produce F2
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Principle of independent assortment
• In a dihybrid cross, the alleles of each gene assort independently
• The segregation of different allele pairs is independent
• Independent alignment of different homologous chromosome pairs during metaphase I leads to the independent segregation of the different allele pairs
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Testcross
• Cross used to determine the genotype of an individual with dominant phenotype
• Cross the individual with unknown genotype (e.g. P_) with a homozygous recessive (pp)
• Phenotypic ratios among offspring are different, depending on the genotype of the unknown parent
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Extensions to Mendel
• Mendel’s model of inheritance assumes that– Each trait is controlled by a single gene– Each gene has only 2 alleles– There is a clear dominant-recessive
relationship between the alleles
• Most genes do not meet these criteria
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Polygenic inheritance
• Occurs when multiple genes are involved in controlling the phenotype of a trait
• The phenotype is an accumulation of contributions by multiple genes
• These traits show continuous variation and are referred to as quantitative traits– For example – human height– Histogram shows normal distribution
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Pleiotropy
• Refers to an allele which has more than one effect on the phenotype
• Pleiotropic effects are difficult to predict, because a gene that affects one trait often performs other, unknown functions
• This can be seen in human diseases such as cystic fibrosis or sickle cell anemia– Multiple symptoms can be traced back to one
defective allele
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Multiple alleles
• May be more than 2 alleles for a gene in a population
• ABO blood types in humans– 3 alleles
• Each individual can only have 2 alleles• Number of alleles possible for any gene is
constrained, but usually more than two alleles exist for any gene in an outbreeding population
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• Incomplete dominance– Heterozygote is intermediate in phenotype
between the 2 homozygotes– Red flowers x white flowers = pink flowers
• Codominance– Heterozygote shows some aspect of the
phenotypes of both homozygotes– Type AB blood
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Human ABO blood group
• The system demonstrates both– Multiple alleles
• 3 alleles of the I gene (IA, IB, and i)
– Codominance• IA and IB are dominant to i but codominant to each
other
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Environmental influence• Coat color in
Himalayan rabbits and Siamese cats– Allele produces
an enzyme that allows pigment production only at temperatures below 30oC
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Epistasis
• Behavior of gene products can change the ratio expected by independent assortment, even if the genes are on different chromosomes that do exhibit independent assortment
• R.A. Emerson crossed 2 white varieties of corn– F1 was all purple
– F2 was 9 purple:7 white – not expected
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