Download Genetics and Mendel

Genetics and Mendel
Genetics • the study of heredity and variation • heredity ­ the basic genetic blueprint of the parents is passed on to their offspring. • variation ­ the genetic difference between parents and offspring Genes ­ distinct packets that pass on hereditary information from generation to generation. • may not be expressed for several generations 1
17th and 18th centuries • controlled experiments on inheritance were restricted • quick breeding and prolific animals (rats/mice) were not acceptable • plant were not thought of as “having sex” • religious restrictions on experiments and beliefs, yet some experimentation began 2
1800's • thought that some information in the blood of both parents were mixed at conception (hence the term “bloodline”)
­not based on careful scientific observation and experiment 1860's: Two notions of inheritance: blending theory of inheritance ­“seeds” that controlled hereditary traits were blended together from generation to generation pangenesis ­ hereditary traits could be modified throughout a person's lifetime Pangenesis theory of long giraffe neck
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Gregor Mendel (mid century) • Moravian Monk • demonstrated the basis of heredity used science and math to establish patterns in how traits were inherited • observed crosses in pea plants (Pisum sativum) The pea plant was ideal for these studies:
­ self pollinating so it was easy to control parental crosses ­ easily grown, matured quickly, producing many seeds
­ show several pairs of contrasting traits 4
Controlling the pollination process
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Mendel's Experiments Phenotypes ­ observable characteristics or traits (smooth or wrinkled peas, tall or dwarf height, purple or white flowers....) Viability ­ different lines of plants could be expected to produce approximately the same number of plants for every 100 seeds planted P generation ­ (P for Parental) cross between pure­breeding plants that were different for only one contrasting pair of traits Pure­breeding ­ Mendel began his experiments with plants that produced seeds that grew into plants identical to the parents for several generations
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Hybrids ­ resulting plants from the cross between two pure­breeding plants with only one contrasting pair of traits
Pure­breeding tall plant
Pure­breeding dwarf plant
The F1 generation are the result of a tall and a dwarf parent cross. Although they are all tall, they have inherited contrasting height information from both parents and so must be a hybrid or combination.
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Then the hybrid plants from the F1 generation were crossed.
Pure­breeding tall plant
F1 Hybrid (tall)
Pure­breeding dwarf plant
F1 Hybrid (tall)
The F2 generation that resulted from crossing the
F1 hybrids revealed a 3:1 phenotypic ratio
tall
tall
tall
dwarf
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The 3:1 ratio results from which of the original P generation parent traits is expressed or "turned on"
Dominant Trait ­ expressed trait, use uppercase letters (eg. T = Tall) Recessive Trait ­ not expressed trait, use lowercase letters (eg. t = Dwarf) Label the plants with the trait they are T
expressing.
t
T
F1 Hybrid (tall)
T
T
tall
t
T
F1 Hybrid (tall)
T
T
Pure­breeding dwarf plant
Pure­breeding tall plant
tall
t
tall
dwarf
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Mendel’s Law of Segregation Factors ­ units of inheritance (genes) ­ for any given characteristic, there were several different forms of these genes ­ called alleles Homologous Chromosomes
Ex: Gene for plant height has two different alleles, a dominant tall allele (T) and a recessive dwarf allele (t).
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Label the plants again, knowing that each plant carries two alleles for the height gene.
T
tt
TT
Pure­breeding tall plant
Tt
Pure­breeding dwarf plant
Tt
F1 Hybrid (tall)
F1 Hybrid (tall)
Tt
TT
t
Tt
tt
tall
tall
tall
dwarf
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To illustrate a cross more graphically, a Punnet Square can be used. This works like a multiplication chart.
Consider the P generation:
Pure breeding tall plant (TT)
Pure breeding dwarf plant (tt)
can only give a t gamete
T
t
Tt
can only give a T gamete
Only possible result is a hybrid for both alleles.
Since T is dominant, all plants have a tall phenotype.
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The hybrids from the F1 generation are then crossed:
Hybrid (Tt)
can give a T or a t gamete
T
t
T
TT
Tt
t
Tt
tt
Hybrid (Tt)
can give a T or a t gamete
Any F2 offsring with the T allele has a tall phenotype
If no T allele is present, the dwarf allele t can be expressed
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Each plant's phenotype was determined by a pair of alleles that could be identical or different. • The allele expressed in the F1 is dominant (all hybrids were tall)
• The allele not expressed in the F1 is recessive (the recessive dwarf allele could not be expressed until the F2 generation)
Law of Segregation: Members of a pair of alleles for a given trait are segregated (separated) when gametes are formed. A heterozygous plant that is Tt forms gametes that are T and t in equal numbers. The gametes are not a blend of the two traits. Genotype ­ gene makeup (the alleles an organism contains) Homozygous ­ genotypes with identical alleles
(example: TT or tt) Heterozygous ­ genotypes with different alleles (example: Tt) How'd you like them peas?
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Try a cross for round peas R and wrinkled peas, r.
Smooth Allele
r
P
R
Wrinkled Allele
R R r
F1
r
r
R
R
R R
R r
r
R r
r r
F2 Phenotypic Ratio
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Mendel’s Law of Independent Assortment Consider more than one trait in a cross:
Law of Independent Assortment: when two or more pairs of characteristics are considered at one time, each pair shows dominance and segregation independently of the other. A plant that is heterozygous for two pairs of alleles,
for example TtRr, (for height and pea shape) can form four types of gametes: TR, Tr, tR and tr. These can be identified using the FOIL method from expanding binomials from math!
T t R r
Dihybrid ­ an individual who is heterozygous for two traits: represented by the genotype AaBb for example. Dihybrid cross ­ mating of two individuals both heterozygous for two particular traits (AaBb x AaBb). 16
Try a dihybrid cross for height Tt pea shape Rr.
gametes from F1 dihybrids
TR
Tr
tR
t r
F2 Phenotypic ratio
TtRr x TtRr
T R Tr
TT
RR
rT T
R
T t
R R
T t
R r
tall
round
tR
tr
tT t T
T T
R r RR r R
r r T t r r
T T R r t T
t t
T t t t
R r R R R r
T t t t t t
r r R r
r r
tall
wrinkled
dwarf round
Tall Allele
T
Dwarf Allele
t
Smooth Allele
R
Wrinkled Allele
r
dwarf
wrinkled
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