mendelian genetics – inheritance of traits. why peas?? many varieties (character, traits) easy to...
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Mendelian Genetics – inheritance of traits
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Why Peas??
Many varieties (character, traits) Easy to control pollination Could choose distinct characters
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First Experiments:
True-breeding parent generation
Why aren’t all the floweres light purple?
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Mendel’s Law of SegregationWhite “heritable factor” did not disappear in F1 generation but only purple “heritable factor” was acting.
Alternative version of genes (heritable factors) account for variations in inherited characters.
For each character, an organism inherits two alleles (versions of the gene) one from each parent.
If the two alleles differ, one is fully expressed – DOMINANT; the other does not affect the organism’s appearance - RECESSIVE trait.
The two alleles segregate during gamete production
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Mendelian Genetics Terms
• Homozygous• Heterozygous• Phenotype • Genotype• Punnett square• Monohybrid cross• Testcross
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Testcross
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What happens if looking at two traits simultaneously?
• Dihybrid cross
xTrue-breeding Parent (P)
F1
F2: 9:3:3:1
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Mendel’s Law of Independent Assortment
• Each character is independently inherited – segregate independently during gamete formation
• Practice dihybrid crosses• Use rules of probability to predict outcomes of
genetic crosses
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http://www.biology.arizona.edu/mendelian_genetics/problem_sets/monohybrid_cross/monohybrid_cross.html
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More Complicated Situations…
1. Incomplete dominance
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Multiple Alleles
IAIA = A; IAi = AIBIB=B; IBi = B
IAIB= AB ii = O A, B – codominant
i - recessive
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Other situations:• Pleiotropy – one gene affects several
phenotypes• Epistasis – one gene affects the phenotypic
expression of another gene• Polygenetic inheritance – additive effect of
two or more genes on a single phenotypic character
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Human Disorders that follow Mendelian Patterns of Inhertitance
• Recessively inherited disorders: albinism, cystic fibrosis, Tay-Sachs disease, sickle-cell anemia
• Dominantly inherited disorders: Achondroplasia, Huntington’s disease
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Cystic Fibrosis
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Tay-Sachs Disease
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Achondroplasia
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Huntington DiseaseLate Acting – middle age
Western Europeans
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Screening for inherited disorders
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Pedigrees
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Pedigree – Huntington disease
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Pedigree – Sickle cell anemia
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Chromosomal Theory of InheritanceThomas Morgan
Fruitflies
4 pairs of chromosomes
Wild type phenotype
Mutant phenotype
White-eyed males only
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Sex-linked traitsLocated on Sex chromosomes: X or Y
Females XX; Males XY
X-linked recessive
Hemophilia, color blindness