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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?
Terms to Know and Use
• Genetics – the study of heredity
• Heredity – the passing of traits from the
parents to their offspring
• 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)
• 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
• 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)
• Genotype - the type of alleles on a
chromosome: (gene makeup)
• Phenotype - The way a genotype is
expressed: (physical appearance)
Gregor Johann Mendel
“Father of Genetics”
• 1822- 1884
• Austrian monk
• Experimented with
pea plants
• Trained in math and
science
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
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).
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).
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.
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
Mother contributes:
P Generation
P
or
P
p
Pp
Pp
p
Pp
Pp
Mother contributes:
F1 Generation
P
or
p
P
PP
Pp
p
Pp
pp
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)
Pea Characteristics
Trait on the left is dominant. Trait on the right is recessive.
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)
• Law of Independent Assortment - states
that different alleles (traits) separate
independently. Thus, color, height, pod
shape, etc. are not connected together.
(dihybrid cross)
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.
Mother contributes:
OPTION 1:
Test Cross
a
or
a
A
Aa
Aa
A
Aa
Aa
Mother contributes:
OPTION 2:
Test Cross
a
or
a
A
Aa
Aa
a
aa
aa
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)
•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
Mother contributes:
SB Sb
SB
SSBB
sB
sb
SSBb
SsBB
SsBb
Sb
SSbB
SSbb
SsbB
Ssbb
sB
sSBB
sSBb
ssBB
ssBb
sb
sSbB
sSbb
ssbB
ssbb
Dihybrid Cross
Smooth Tall
Smooth
Tall
Smooth wrinkled
short
Tall
wrinkled
short
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)
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
Chromosomes = DNA
Meiosis I
One diploid sex cell divides……
Meiosis II
Result: One diploid cell = four haploid cells
Crossing Over in Meiosis I
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/Lecture5/515%20Meiosis.JPG
Monohybrid, dihybrid, test cross & pea flower: ntri.tamuk.edu/homepagentri/lectures/ biology/test-cross.gi
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