Mendel and the gene idea - Instructure
Transcript of Mendel and the gene idea - Instructure
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Mendel and the gene ideaChapter 11
Good to know
• Concepts of Mendelian genetics• Laws segregation and independent assortment
• Multiplication and addition rules for predicting genetic outcomes
• Extensions of Mendelian genetics
• Pedigrees
• Dominant and recessive disorders
What’s in your genes?
• Earlobes
• Dimples
• Tongue Rolling
• Cleft Chin
• Hair Line
• Freckles
• Hair Appearance
• Hand Clasping
• Second Toe
• PTC Tasting
• Thumb
• Allergies
• Color blindness
• Vulcan Sign
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Early concepts of inheritance
• Blending hypothesis• Parental traits blended together in the offspring
• Over time, what would happen?
• Particulate hypothesis• Parents passed on discrete heritable units
• Genes like cards in a deck
Enter Gregor Mendel (1822‐1884)
• Father of Genetics• Published findings in 1866
• Research ignored until 20 years after death
Exemplary use of scientific method
• Hypothesis based on observation
• Meticulous experiments and data collection• 28,000 pea plants
• Developed conclusions based on the data
• Research supported the particulate hypothesis
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Pea: character and trait
• Character – heritable feature that varies among individuals
• Trait – variant for a character
Genetic terminology
• True breeding• If self pollinated – always produced trait
• Hybridization• results of crossing 2 true breeding plants with different traits
• P generation
• F1 generation (filial)
• F2 generation
Quick look at Mendel’s experimental model
Concept of dominant and recessive traits
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Mendel's Model and deductions
• Alternate versions of a ‘heritably factor’ (gene) account for variations in inherited characteristics
• An organism inherits two copies of a gene, one from each parent
• If the two alleles at a locus differ, then the dominant gene will determine the organisms appearance
• The two alleles of a heritable character separate from each other during gamete formation
Support for law of segregation
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More genetic terminology
• Allele
• Homozygous
• Heterozygous
• Dominant
• Recessive
• Phenotype
• Genotype
Group activity
• I have a organism that displays the dominant phenotype. How can I determine its’ genotype?
What am it…….I really wannaknow
Law of independent assortment
• Different characters (genes) can be inherited independently of one another (caveat – only if they are on different chromosomes or far apart on same chromosome)
• Mendel’s experiment • Dihybrid cross
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Mendelian inheritance governed by probability
• Probability – the odds that a specific event, or set of events, will occur
• Multiplication rule• Answers question – what are the odds that two or more independent events will occur together in a specific combination?
• What are the odds that if I flip two coins, both will be heads?
• Multiply odds of 1 event occurring by the odds of the second event occurring.
Probablility Problem
• What are the odds that the offspring of a cross between two heterozygous individuals will express a recessive trait?
• What are the odds that the offspring of a cross between two heterozygous individuals will be heterozygous?
• Set up a Punnett Square ‐ does it verify your answers?
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Probability for heterozygous combinations
• Addition Rule – get the desired outcome in more than one way• Probability that two mutually exclusive events will occur
• Adding the individual probabilities together
• What are the odds that the offspring of a cross between two heterozygous individuals will be heterozygous?
• PpYyRr x Ppyyrr
• What fraction from this cross is predicted to exhibit the recessive phenotype for at least two of the three characteristics?
• Step 1 – list all possible genotypes that meet the criteria.
PpYyRr x Ppyyrr
Possible genotypes Probability of each event(multiplication rule)
ppyyRr ¼ x ½ x ½ 1/16
ppYyrr ¼ x ½ x ½ 1/16
Ppyyrr ½ x ½ x ½ 2/16 (1/8)
PPyyrr ¼ x ½ x ½ 1/16
ppyyrr ¼ x ½ x ½ 1/16
Chance of at least two recessive traits (addition rule) 6/16 (3/8)
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Beyond Mendelian genetics
• When alleles are not completely dominant or recessive
• When a particular gene has more than two alleles
• When a single gene produces multiple phenotypes
Degrees of dominance
• Complete (Mendelian rules apply)
• Incomplete dominance• Neither allele is completely dominant
• Phenotype of offspring ‘blending’ of parental phenotypes
• Codominance• Both alleles are dominant
• Phenotype of offspring displays both parental phenotypes
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Incomplete dominance
Codominance
Dominance May depend on the level
NTSAD link
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Are dominant alleles more prevalent in populations?
Genes with Multiple alleles
Pleiotropy
• One gene can effect multiple phenotypes
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When Multiple Genes are involved
• Epistasis• A gene at one location masks or modifies the phenotype of a gene at another location
• Polygenic inheritance• The additive effect of 2 or more genes
• Cumulative effect
B – blackb – brownE – pigment depositede – pigment not deposited
Epistasis
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Polygenic inheritance
Quantitative characters ‐ gradations along a continuumOther examples?
Tracking Mendelian traits in humans
• Pedigree analysis
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Tracking disease through pedigree
Recessive inherited disorders
• Mutation• Malfunctioning protein
• No protein at all
• Examples• Albinism
• Cystic fibrosis
• Sickle‐cell anemia
• Tay‐Sachs
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Population genetics
• Genetic disorders not evenly distributed among all groups of people• Tay‐Sachs
• 100x greater among Ashkenazic Jews
• Sickle‐cell• 73/1000 Blacks, 3/1000 Caucasians
• Cystic fibrosis• 1/2500 Caucasians European descent, 1/15,000 Blacks
Population genetics
• When recessive allele rare – unlikely two carriers will mate
• Individuals with recent common ancestors more likely to carry same recessive allele
• Effect on isolated populations?
• Consanguineous relationships – increase frequency of recessive traits• Laws prohibiting close relative marriage
• inbreeding
Recessive disorders
• Can two normal parents have a child with a disorder?
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Sickle‐cell disease
Well…………that depends
Dominant genetic disorders
• Achondroplasia (dwarfism)
• Can two normal parents pass this trait to their offspring?
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Huntington’s disease
• Degenerative disease of nervous system
• Irreversible and fatal
• Asymptomatic until 35‐45 years old
• Afflicts 1:10,000 people
• Gene located on chromosome 4 and was sequenced in 1993
• Genetic testing now available
Would you want to know?
It isn’t all about genetics
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Multifactoral disorders
• Genetic component + significant environmental influence• Heart disease
• Alcoholism
• Schizophrenia
• Bipolar disorder
• Typically polygenic