mendelian genetics
DESCRIPTION
Mendelian Genetics. Genetics and heredity. For a long time, general ideas of inheritance were known. =. +. +. What was really lacking was a quantitative understanding of how particular traits were passed down to their offspring. +. =. Gregor Mendel (1822-1884). German monk born in 1822 - PowerPoint PPT PresentationTRANSCRIPT
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Mendelian Genetics
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Genetics and heredityFor a long time, general ideas of inheritance were known
++ =
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What was really lacking was a quantitative understanding of how particular traits were passed down to their offspring.
+ =
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Gregor Mendel (1822-1884)
• German monk born in 1822
• Lived in what is now the Czech Republic
• Tended the garden at his monastery, conducted experiments with pea plants
• Studied peas for 7 years, published results 1866; ignored until 1900
• Now considered the “Father of Genetics”
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What is a trait?• A trait is a characteristic that can vary from one
individual to the next (e.g., eye color)• Mendel chose 7 pea plant traits to study:
– seed shape, seed color, flower color, pod shape, pod color, flower position, plant height
• He was lucky that each trait happened to be located on different chromosomes (people didn’t know about chromosomes back then)
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Mendel's experiments• First, he created
true (pure) breeding pea plants
• Then he experimented with different traits (one at a time)– He cross-
pollinated plants with different traits, and observed the offspring
(Normally done by bees… Mendel used scissors and a paint brush)
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Important vocabulary• True (pure) breeding = offspring always
have same trait as parent (every time, no matter what!)
• Self-pollination = plant fertilizes itself• Cross-pollination = one plant fertilizes
another• P generation = Parent generation• F1 generation = 1st generation offspring
(“filius” is Latin for “son”; offspring of P generation)
• F2 generation = 2nd generation offspring (offspring of F1 generation)
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Mendel’s crosses• When Mendel crossed true-breeding tall plants with
true-breeding dwarf plants (P generation)…
…what size were the offspring (F1 generation)?
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• The F1 generation were all tall
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• Mendel had expected the traits to blend: tall + dwarf = medium
• Instead, the tallness of parent had masked the shortness of the other parent
tall + short = tall????
Mendel was surprised!
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• Mendel then let the F1 generation self-pollinate to create the F2 generation…what size were the
offspring (F2 generation)?
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• Some of the F2 offspring were tall, and some were dwarf
• The ratio was always 3 tall to 1 dwarf (3:1)
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Mendel's findings• When Mendel crossed true-breeding plants
with different traits, all of the F1 plants showed only one trait (e.g., all tall); the F2 plants showed a 3:1 ratio
• He did not get “medium” plants! • He called the traits dominant (showed in
F1 generation) or recessive (didn’t show up in the F1 generation)
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Mendel’s conclusionsMendel realized that:1.Inheritance of traits is controlled by genes
contributed by each parent (e.g., flower color)
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Mendel’s conclusions2. These genes come in different forms,
called alleles (e.g., purple or white flowers)
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Mendel’s conclusions3. Some alleles are
dominant while others are recessive – When both types of
alleles are present, dominant alleles mask (cover up) recessive alleles
Mendel’s Discoveries & Results
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Mendel’s Law of Dominance:
• Generally, one allele is dominant to another
• Dominant allele always expressed, even if combined with recessive allele– written as uppercase letter of the trait– e.g., tall = dominant: TT, tT, or Tt
• Recessive allele expressed only if dominant allele is not present.– written as a lowercase letter of the dominant
trait– e.g., dwarf = recessive: tt
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More vocabulary…• Genotype = genetic combination of alleles
(e.g., TT, Tt, tt)• Phenotype = physical characteristic or trait
(e.g., tall, short)• Homozygous = both alleles are the same
(e.g., TT or tt) → “true breeding”• Heterozygous = alleles are different (e.g.,
Tt)• Hybrid = offspring of crosses of parents with
different traits (e.g., offspring of TT and tt)
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Mendel’s Law of Segregation
• When gametes form, the alleles from each parent are segregated (separated) from each other
• Each gamete carries a single allele for each gene
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Circle Circle ## GenotypeGenotype Hetero/Homo-Hetero/Homo-zygouszygous PhenotypePhenotype GenerationGeneration Gamete?Gamete?
(y / n)(y / n)
11
33
77
1414
TT homozygous tall P noT
heterozygous
homozygous
n/a n/a n/a yesTt tall F1 nott short F2 no
Parents (P)
gametes
F1
gametes
F2
Probability of F2 generation being TT?
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Punnett Squares• Instead of doing all of that hard work for each
of the possible outcomes under the Law of Segregation, we can do a Punnett Square (geneticist Reginald Punnett created it)
Mother’s Gametes
Fath
er's
Gam
etes
T t
T
t
TT
Tt
Tt
Tt
There is a 1 in 4 chance of the offspring being TTThere is a 2 in 4 chance of the offspring being TtThere is a 1 in 4 chance of the offspring being tt
3 out of 4 plants = tall1 out of 4 plants = dwarf
Tall to dwarf = 3:1
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Mendel’s Law of Independent Assortment
• Genes for two different traits are inherited independently
• There is no connection between them (e.g., plant height and seed color)
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