Transcript Document

Gregor Mendel
Pea Plants and Inheritance Patterns
Who is Gregor Mendel?
• Mendel was born in 1822 in Austria
• His father was a peasant farmer,
tenanted to a local aristocrat who was
very interested in scientific crop
improvement
• The family was very poor
• Mendel’s early education was with local
priest and teacher
• Showed considerable academic
capability and so was sent to larger
town with more opportunities
• Financial problems plagued him,
affecting his health, so Mendel decided
to enter monastery as means of
support
At the Monastery…
Mendel
At the Monastery…
• Mendel taught 6th
and 7th grade age
children
• He also had access
to the monastery’s
library
• Fr. Napp, head of
the monastery, had
a shared interest in
botany and
agriculture
Mendel’s Garden
Mendel’s Model
Organism –
The Garden Pea
1.
2.
3.
Self-Fertilizing
Matured Quickly
Several Easily
Identifiable Traits
Used Consistent Methods:
Opened flower & placed
pollen from one type onto
the stigma
Pea Plant
Characteristics
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Seed Shape
Seed Color
Pod Shape
Pod Color
Flower Color
Flower Position
Stem Length
Mendel’s First Experiment
• Crossed Pure Tall Pea Plant (TT) x Pure Short (Dwarf) Pea Plant (tt)
• Hypothesis:
– The offspring would be:
• All tall
• All short
• All intermediate
• Some would be talls and some short
The Results?
• 1st Experiment:
– Crossed Pure Tall x Pure
Short
– All offspring (F1) tall
• 2nd Experiment:
– Bred F1
– Ratio of 787 tall to 277
short (3:1)
• Similar to chance events
from flipping 2 pairs of coins
Monohybrid Crosses and the Law of Segregation
A cross between individuals differing in
single character is a monohybrid cross.
The analysis of monohybrid crosses
allowed Mendel to deduce the Law of
Segregation…
Genes come in pairs that separate in
the formation of sex cells (and these
sex cells unite randomly at fertilization).
Staying the Course –
Crosses to the F2
(The Grandchildren)
• Crossed one of the F1 tall
plants with its dwarf parent:
F1 Tall x Dwarf
• Possible Outcomes:
– All would be tall
– Mixture of Tall & Dwarf
– All would be intermediate
• Experimental results 
The reappearance of the recessive trait in ¼ of the F2, suggests
genes come in pairs that separate in the formation of sex cells.
Monohybrid Crosses are Consistent!
Therefore, the Law of Segregation indeed is a general principle of genetics.
Mendel’s Experiments – The Next Generation
• Mendel recognized that it is
not always possible to tell
what offspring will be like by
inspecting the parent
• Mendel could test if tall
plants were pure-breeds
(homozygotes) or hybrid
(heterozygotes) by the “backcross” or “test-cross”
• What % would you predict
for each genotype?
Tt
Tt
tt
tt
Mendel’s Hypothesis
Mendel’s Hypothesis
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There are alternative forms for genes, the units that
determine inheritable characteristics (AA or Aa or aa)
For each inherited characteristic, an organism has two
alleles, one inherited from each parent.
A sperm or egg carries only one allele (A or a)for each
inherited characteristic, because allele pairs separate from
each other during meiosis. At fertilization, the sperm and
egg unite and restore the gene to the paired condition.
When the two alleles of a pair are different, one is fully
expressed (dominant) and the other is completely masked
(recessive). The members of the pair may be identical
(homozygous) or non-identical (heterozygous).
Revisiting Meiosis
Principle of Independent Assortment:
The assortment of one pair of genes into
gametes is independent of the
assortment of another pair of genes.
The alignment of one pair of homologs
is independent of any other.
Incomplete
Dominance
• Incomplete
dominance is a
blending of colors
• Dominance
relationships may
differ, but the
Principle of
Segregation is the
same
Polygenic Inheritance:
When a Single Trait is Influenced by Many Genes
Height is a
polygenic trait
Multiple Alleles
• Many genes are present
in three or more versions
(alleles) – this is multiple
alleles
• The human ABO blood
group is determined by
three alleles (IA, IB, and I)
of a single gene
Codominance
• The human ABO blood
group also exhibits
codominance – another
genetic phenomenon
• Codominance occurs when
the phenotype associated
with each allele is
expressed in the
heterozygote
The AB
phenotype
(genotype IA
IB) is an
example of
codominance
Genetics of Blood Types
Phenotype
A
B
AB
O
Genotype
Antigen
on RBC
Antibodies
in Blood
Donation
Status
A A or A i
type A antigens
on surface
of RBC
anti-B antibodies
__
BB or B i
type B antigens
on surface
of RBC
anti-A antibodies
__
AB
both type A &
type B antigens
on surface
of RBC
no antibodies
universal
recipient
ii
no antigens
on surface
of RBC
anti-A & anti-B
antibodies
universal
donor
Sex-Linked Traits
2 normal parents,
but mother is carrier
XH Y x XH Xh
male / sperm
XH
XH
Y
XH
XH XH
XH Y
Xh
XH Xh
XhY
Y
XH
XH Xh
Xh
female / eggs
XH Y
Sex-Linked Traits
• Sex chromosomes have other genes on them,
especially the X chromosome
– Hemophilia in humans
• Blood doesn’t clot
– Duchenne muscular dystrophy in humans
• Loss of muscle control
– Red-green color blindness
• See green & red as shades of grey
Polydactyly
• Individuals are born
with extra fingers and
toes
• The allele for 6+
fingers and toes is
dominant,while the
allele for 5 digits is
recessive
• Recessive is far more
common! (1:500 have
polydactyly)
Are Different Characters
Like Color and Shape
Inherited Together or
Inherited Independently?
Mendel performed dihybrid
crosses to find out.
Dihybrid Crosses
Note that we’re
simultaneously
applying the
Principles of
Segregations and
Independent
Assortment.