genetics
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Genetics. Prehistoric Pooches. Dogs and humans have been companions for thousands of years. The domestication of dogs may have been one of the earliest human experiences with genetics. Genetics - scientific study of how traits are passed from parent to offspring - PowerPoint PPT PresentationTRANSCRIPT
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Genetics
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Prehistoric Pooches
• Dogs and humans have been companions for thousands of years. The domestication of dogs may have been one of the earliest human experiences with genetics.
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• Genetics - scientific study of how traits are passed from parent to offspring
• Millions of years ago there were no dogs. Today’s domestic dogs are descended from a wild, wolfish ancestor.
• Certain wolves were selected to mate which affected the traits that were passed from the parents to the pups.
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• Trait – a characteristic that can be passed on
• With each generation, the traits of the wolves living with humans became more distinct from the traits of the wild wolves. Eventually, this led to a new breed – the domestic dog.
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• We’ve known for centuries that traits are passed from parent to offspring, but we haven’t always known how those traits are determined.
• Blending Hypothesis – traits of parents were blended in the offspring
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Chromosomes and cell division
• The chromosomes make copies of themselves and then pass those on to the daughter cells.
• We know that traits are passed on in these chromosomes but the relationship between chromosomes and traits was not always understood.
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Gregor Mendel
• Austrian monk in 1860’s• Experimented with pea plants
- Structure of pea flowers- Traits showed obvious differences that were easy to see- Reproduced quickly
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Mendel’s Experiments
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Structure of pea plants
• the relatively closed structure of the pea flower petals makes it very easy for pollen from the anther to fertilize the pistil of the same flower
Self-fertilization
What’s the difference between self fertilization and asexual reproduction? Are they the same?
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Structure of pea plants
• Purebred – receives same genetic traits from both parents
• Hybrid – receives different forms of a trait from each parent
Cross-pollination
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Traits are easy to see
• Mendel studied 7 traits that only come in 2 distinct forms
• Example: pea pods were either yellow or green (there were no in-between colors)
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Short reproductive cycle - 90 days
• Quick results• Could repeat the experiments many times
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Mendel’s Observations
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• Mendel began his experiments using two different purebred plants
• P – the parental generation or first group to fertilize
• F1 – the filial generation or offspring• F2 – the offspring of the F1 generation
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Findings:
• When crossing 2 different purebred parents, F1 generation were only one form of the trait
• F2 generation showed 75% of F1 trait, and 25% of other trait
P
F1
F2What should have happened according to the blending hypothesis?
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Repeated his experiments with the other 6 traits
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Mendel’s Conclusions
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• Blending hypothesis was wrong – forms of a trait must remain separate in offspring
P
F1
F2
P
F1
F2
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• Genes – sections of a chromosome that code for a trait
• Allele – distinct form of a gene
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• Dominant allele – form of a gene that is fully expressed when two different alleles are present
• Recessive allele – form of a gene that is not expressed when paired with a dominant allele
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Unnoticed
• Mendel performed these experiments for 8 years and published his work. Unfortunately, at the time scientists were focused on the work of Charles Darwin and so Mendel’s work went unnoticed for the next 37 years.
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Walter S. Sutton
• Observed stained cells under a microscope and noticed that the chromosomes acted like Mendel’s factors.
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Chromosome Theory of Heredity
• material of inheritance is carried by the genes in chromosomes
To help explain the new theory, scientists came up with some new terms and symbols
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• Gene – section of chromosome that code for traits
• Allele – different forms of that trait
• Pea color is controlled by a single gene with two alleles – one for green and one for yellow
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Letters are used…
• Dominant allele – capital letter- Example: Yellow is the dominant pea color and would be represented by Y
• Recessive allele – lowercase letter- Example: green is the recessive pea
color and would be represented by y
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• What letters would be used for other traits?
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Genotype
• the genetic makeup of an organism (YY, Yy, yy)
• It includes both genes in a homologous pair of chromosomes
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Phenotype
• the physical appearance (yellow, green)
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Homozygous
• the two alleles in a gene are identical (YY or yy)
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Heterozygous
• the two alleles in a gene are different (Yy)
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Mendel’s Laws
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Law of Segregation
• each pair of genes separates during meiosis
F1 generation Meiosis 1 Meiosis 2
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Law of Independent Assortment
• gene pairs separate into gametes randomly and independently of each other
PPpp
PPpp
PP
Pp
Pp
pp
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Law of Dominance
• the dominant allele is expressed and the recessive allele is hidden. The recessive allele is only expressed when no dominant allele is present.