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Mendel’s Legacy
Genetics is everywhere these days – and it will continue to become even more important in decades to come.
So wouldn’t it be nice if people understood it better?
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Terms to Know and Use
• Genetics – the study of heredity
• Heredity – the passing of traits from the
parents to their offspring
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• Trait - variations of a gene: (i.e. black or brown hair) determined by alleles
• Allele – different forms a gene, often expressed as Y or y, X or x, Z or z
• Gene - a heritable feature on a chromosome:
(i.e. hair color)
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• Chromosome - strand of DNA that codes for genes
• Locus - location of a gene, or allele, on a chromosome
•Monohybrid cross – cross involving parents
differing in only 1 trait
•True-breeding - organisms that always pass
the same genes to their offspring
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• Dominant trait - expressed over a recessive trait when both are present
• Recessive trait - not expressed when the dominant trait is present
•Homozygous – when both alleles of a gene are
the same (i.e. pure, TT)
•Heterozygous – when the two alleles are not
the same (i.e. hybrid, Tt)
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• Genotype - the type of alleles on a chromosome: (gene makeup)
• Phenotype - The way a genotype is expressed: (physical appearance)
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Gregor Johann Mendel“Father of Genetics”
• 1822- 1884• Austrian monk• Experimented with
pea plants• Trained in math and
science
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Garden Pea Experiments 1856-64
Why pea plants? (Pisium sativum)
Because they…
1) Were easy to grow
2) Produced a large number of offspring
3) Matured quickly
4) Had both male and female parts
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The Problem
T.A. Knight, a scientist, saw that if he crossed
true bred true bred
purple pea plant (P) x white pea plant (P)
ALL the offspring would be purple (F1).
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The Problem (cont’d)
If he then crossed the purple (F1) offspring:
hybrid hybrid
purple pea plant (F1) x white pea plant (F1)
Most offspring are purple (F2) & few white (F2).
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Mendel’s Answer
Mendel used math with science to explain heredity.
He counted: 705 purple
224 white
total 929 pea plants
Thus he saw a 3:1 ratio.
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Mendel’s Answer (cont’d)
To explain this ratio he came up with Rules of Heredity
1) Parents transmit “genes” to offspring
2) Each individual has 2 genes (1 from each parent)
3) Some “genes” are dominant and others recessive
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P P
p
p
Pp
Pp
Pp
Pp
Fat
her
cont
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Mother contributes:
oror
P Generation
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P p
P
p
PP
Pp
Pp
pp
Fat
her
cont
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tes:
Mother contributes:
oror
F1 Generation
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Lucky or Right On?
Mendel repeated his experiments while observing other traits such as:
• height of plant (tall vs. short)• pod appearance (inflated vs. constricted)• pod color (green vs. yellow)• seed texture (round vs. wrinkled)• seed color (yellow vs. green)
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Pea Characteristics
Trait on the left is dominant. Trait on the right is recessive.
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Mendel's Laws of Inheritance
• Law of Segregation - states that the two alleles separate when gametes (sperm/egg) form. Thus, a gamete receives only one allele from each parent. (test cross)(test cross)
• Law of Independent Assortment - states that different alleles (traits) separate independently. Thus, color, height, pod shape, etc. are not connected together. (dihybrid cross)(dihybrid cross)
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Determining Unknown Genotypes
• How do you know if a dominant phenotype is homozygous or heterozygous?
• Scientists can perform a test cross, where they cross the unknown with a recessive (known) phenotype.
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a a
A
A
Aa
Aa
Aa
Aa
Fat
her
cont
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Mother contributes:
oror
OPTION 1:Test Cross
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a a
A
a
Aa
aa
Aa
aa
Fat
her
cont
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tes:
Mother contributes:
oror
OPTION 2:Test Cross
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Mendel's Laws of Inheritance
•Law of Segregation - states that the two alleles separate when gametes (sperm/egg) form. Thus, a gamete receives only one allele from each parent. (test cross)(test cross)
•Law of Independent Assortment - states that different alleles (traits) separate independently. Thus, color, height, pod shape, etc. are not connected together. (dihybrid cross)(dihybrid cross)
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SB
Sb
sB
SB
sb
sb
Fat
her
cont
r ibu
tes:
Mother contributes:Dihybrid Cross
Sb sBSSBB SSBb SsBB SsBb
SSbB SSbb SsbB Ssbb
sSBB sSBb ssBB ssBb
sSbB sSbb ssbB ssbb
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Dihybrid Cross
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Two chromosomes of one parent are represented on the left.Possible alleles passed on to the offspring are on the right.(Consider smooth or wrinkled peas AND tall or short plants)
Smooth Tall Smooth Smooth wrinkled wrinkledTall short Tall short
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DO ALL GENES ASSORT INDEPENDENTLY?
• Genes on the same chromosome tend to be inherited together = linked
• Sex-linked genes: color blindness, MD, hemophilia. X Y
• X
X
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Chromosomes = DNA
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Meiosis I
One diploid sex cell divides……
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Meiosis II
Result: One diploid cell = four haploid cells
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Crossing Over in Meiosis I
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Credits
• Mendel Image: pw1.netcom.com/~aguldo/ agga/bt/txt/bt1899.htm • Pea trellis: http://www.floridata.com/ref/p/images/pisu_sa1.jpg• Experiments in Plant Hybridization (1865)
by Gregor Mendel Read at the meetings of February 8th, and March 8th, 1865 to the the Natural History Society of Brünn (paper can be read at: http://www.mendelweb.org/home.html
• MendelWeb, edited by Roger B. Blumberg. (http://www.netspace.org/MendelWeb/, Edition 97.1 1997)
• The Biology Project: http://www.biology.arizona.edu/default.html• Meiosis Images: www.micro.utexas.edu/.../bio304/ genetics/meiosis.4.gif • Crossing Over image:
http://medlib.med.utah.edu/block2/biochem/Formosa/Figures/Lecture5/5-15%20Meiosis.JPG
• Monohybrid, dihybrid, test cross & pea flower: ntri.tamuk.edu/homepage-ntri/lectures/ biology/test-cross.gi
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Any Questions?
• “The problem with the gene pool is that there is no lifeguard.” —Unknown
• “Life can only be understood backwards; but it must be lived forwards.”
—Kierkegaard