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Introduction to Geneticsjaentschypantsbiology.weebly.com/.../2/6/26264482/...to_genetics_pa… ·...
Transcript of Introduction to Geneticsjaentschypantsbiology.weebly.com/.../2/6/26264482/...to_genetics_pa… ·...
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Introduction to GeneticsChapter 11
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What is genetics?• Genetics – the scientific study of heredity.– * Heredity is the passing of traits from parents to
offspring.
• A trait is a specific characteristic that varies from one individual to another.
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The Work of Gregor Mendel• Gregor Mendel – the father of genetics.
– Born in Austria in 1822
• He was a priest, a teacher, and a gardener
– He made many observations of pea plants in his monastery garden which revolutionized the study of inheritance.
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The Work of Gregor Mendel• Mendel’s Experiments
– Mendel had several different “true-breeding” lines of pea plants, each with different traits.
– Mendel cross-pollinated the different lines of pea plants and studied the inheritance of traits in the offspring.
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The Work of Gregor Mendel• Mendel’s F1 Results
Seed
Shape
Seed
Color
Seed
Coat
Color
Pod
Shape
Pod
Color
Flower
PositionPlant
Height
P
Round Yellow Gray Smooth Green Axial Tall
Wrinkled Green White Wrinkled Yellow Terminal Short
F1
100%
Round
100%
Yellow
100%
Gray
100%
Smooth
100%
Green
100%
Axial
100%
Tall
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The Work of Gregor Mendel
• Mendel’s Early Conclusions
1) Inheritance is determined by “factors”(genes) that are passed from parents to offspring.
- We now know that a gene is a segment of DNA that can be thousands to millions of base pairs long and codes for a trait.
2) Each of these “factors” (genes) has different versions (alleles)
3) The principle of dominance: Some alleles are dominant and block out others that are recessive.
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The Work of Gregor Mendel
• Mendel’s Next Experiment
– Mendel wanted to find out if the recessive alleles still existed in the F1 plants.
– To determine this, he allowed the F1 plants to self-pollinate.
– He then studied the inheritance of the offspring (F2 plants)
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The Work of Gregor Mendel• Results of the F1 Cross
– The recessive alleles reappeared!
• About 25% of the resulting F2 plants had the recessive trait.
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The Work of Gregor Mendel• Explaining the F1 Cross
– Each F1 plant had one dominant and one recessive allele for the trait being studied.
– The principle of segregation: The alleles from each parent are separated during the formation of gametes (sperm and egg).
• In the F1 cross, ½ of the gametes received the dominant allele and the other ½ received the recessive allele.
– When the plants self-pollinated, two gametes came together (fertilization).
• If both gametes contained the recessive allele, the resulting plant showed the recessive trait. (25% chance)
• If one or both of the gametes contained the dominant allele, the resulting plant showed the dominant trait. (75% chance)
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Punnett Squares• Punnett Square – a diagram used to
predict the outcomes of a genetic cross.
– The types of gametes produced by both parents go on the outside of the square.
– The possible gene combinations of the offspring appear in the four boxes that make up the square.
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Punnett Squares• An organism is said to be homozygous
for a trait if both alleles for that trait are the same. (RR or rr)
• An organism is said to be heterozygousfor a trait if both alleles for that trait are different. (Rr)
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Punnett Squares
• Phenotype – the physical characteristics of an organism. (Tall or short)
• Genotype – the genetic makeup of an organism. (TT or Tt or tt)
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• Practice problem #1:
– In pea plants, the allele for tall plants (T) is
dominant to the allele for short plants (t).
Perform the following cross:
• A heterozygous tall plant X a short plant.
Genotype Ratio = 2/4 Tt : 2/4 tt
Phenotype Ratio = 2/4 tall plants : 2/4 short plants
T t
t
t
Tt
Tt
tt
tt
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• Practice problem #2:
– In a breed of dog, the allele for long tails (L) is
dominant over the allele for short tails (l). If
you cross two heterozygous long-tailed dogs,
what is the chance of a puppy having a short
tail?
Genotype Ratio = 1/4 LL : 2/4 Ll : 1/4 ll
Phenotype Ratio = 3/4 long tails : 1/4 short tails
25% chance
L
L
l
l
LL
Ll
Ll
ll