patterns of inheritance
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Patterns of Inheritance. Inheritance Hypotheses. Blending Hypothesis – parental contributions combined Particulate Hypothesis – parents pass along discrete heritable units. Some Important Vocab. - PowerPoint PPT PresentationTRANSCRIPT
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Patterns of Inheritance
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Inheritance Hypotheses
• Blending Hypothesis – parental contributions combined
• Particulate Hypothesis – parents pass along discrete heritable units
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Some Important Vocab• Allele= an alternative form of a gene (one
member of a pair) that is located at a specific position on a specific chromosome.
• Homozygous = two identical alleles (same)• Heterozygous = two different alleles Genotype
– The two genes that an organism inherits for a certain trait (Example Tt, TT, or tt)
• Phenotype – Physical characteristics (example: tall
and short)
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Analyzing Patterns
• Genetic Cross – controlled experiment to determine the inheritance of a trait– P – parental generation– F1 – first generation
– F2 – second generation
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General Patterns of Inheritance
• Alternative versions of genes cause variation
• Offspring inherit one copy from each parent
• Dominant alleles are expressed in phenotype
• Alleles for genes separate during meiosis
• Gametes fuse randomly
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Mendel’s Laws• Father of modern genetics• Researched with pea plants • Developed ideas of
dominance and trait segregation– Allelic Interactions– Pleiotropy– Epistasis – Environment– Polygenics
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Each gene (allele) separates from the other so that the offspring get only one gene from each parent for a given trait.Let’s cross a dominant tall plant (TT) with a short plant (tt).Each plant will give only one of its’ two genes to the offspring or F1 generation.
TT x tt
T T t t
Law of Segregation
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Law of Independent Assortment • Hypothesis: Inheritance of seed
shape is independent of inheritance seed color.
• Medal crossed two genetic traits:– Yellow vs. Green – Round vs. Wrinkled
• Results: Yes, inheritance of seed shape is independent of seed color.
• Conclusion: Chromosomes assort independently during meiosis
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Developed ideas of dominance and trait segregation
• Allelic Interactions– Complete dominance– Incomplete dominance– Codominance
• Pleiotropy• Epistasis • Environment• Polygenics
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Punnett Squares A square which can be used to show the random combinations of genes which are possible when a sperm
fertilizes an egg.
Tt Tt
Tt Tt
The genes from one parent go here.
The genes from the other parent go here.
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Allelic Interactions
• Complete dominance – dominant allele fully expressed
• Incomplete dominance – neither allele fully expressed
• Codominance – both alleles fully expressed
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Allelic Interactions: Complete Dominance
– Dominant vs. Recessive Gene– Dominant Gene: A gene that always
expresses itself. It is symbolized by a CAPITAL letter
– Recessive Gene: a gene that expresses itself only when a dominant form of the gene is NOT present. It is symbolized by a lower case letter
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Allelic Interactions: Complete Dominance
• Practice Problem: – Let's say that in seals, the gene for the length of the whiskers has two
alleles. The dominant allele (W) codes long whiskers & the recessive allele (w) codes for short whiskers.
a) What percentage of offspring would be expected to have short whiskers from the cross of two long-whiskered seals, one that is homozygous dominant and one that is heterozygous?
b) If one parent seal is pure long-whiskered and the other is short-whiskered, what percent of offspring would have short whiskers?
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Allelic Interactions: Incomplete Dominance • Neither allele fully expressed• the phenotype of the heterozygote will
be intermediate between the phenotypes of the two homozygotes
• Example: RR = Red snapdragon flower Rr = pink snapdragon flower rr = white snapdragon flower
Practice Problem: Cross a red snapdragon with a pink snapdragon flower. What will the genotype and phenotype % be for the offspring.
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Co-Dominance
• If two alleles have a co dominant relationship, in the heterozygote both alleles will be completely expressed.
• Example: blood type
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Example: Blood Types
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Co-DominanceWhat would happen if you crossed a plant with red flowers with a plant with blue flowers?(a) Give the genotypes and phenotypes for the offspring.(b) How many of the plants would have red flowers? _____%(c) How many of the plants would have purple flowers? _____ %(d) How many of the plants would have blue flowers? _____ %
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Sex Linked (X-Linked) Traits
• The genes that are located on the chromosomes are called sex-linked traits
• Many traits that are carried on the X chromosome do not have a corresponding spot on the Y chromosome. This causes for some unique possibilities for the offspring.
• Ex. Colorblindness in humans, fruit fly eye color, hemophilia.
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Pleiotropy
• Condition where a single gene influences multiple traits– Example: Marfan’s Syndrome
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Environment
• Phenotype = Genotype + Environment
• Epigenetics – study of changes in phenotype due to mechanisms other than changes in DNA sequence
Epigenetics
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Polygenics
• Traits that are determined by many genes – Examples – eye color,
skin color, height, etc.
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Analyzing Patterns
• Pedigree – illustration of relationships among family members over multiple generations
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Inheritance and Genetic Disorders
• Hereditary disorders– Autosomal or sex-linked– Recessive or dominant
• Genetic disorders are often polygenic and influenced by environment
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Autosomal Recessive Disorders• Carriers – heterozygotes that have one allele but not
the disorder– Examples – Cystic Fibrosis, Tay Sachs
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Autosomal Dominant Disorders• Disorders that are expressed
with only one copy of the allele– Example – polydactyly – Example – Huntington’s
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Sex-linked Recessive Disorders• Disorders that are on the sex chromosome
– Example – Hemophilia
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Sex-linked Dominant Disorders