2. OUTLINE
INTRODUCTION
GENETICS BEFORE MENDEL
MENDEL EXPERIMENTS
MENDEL’S PRINCIPLES OF INHERITANCE
APPLICATION OF MENEDL’S PRINCIPLES TO
HUMAN GENETICS
MEDELIAN PRINCIPLES IN HUMAN GENETICS
CONCLUSION
REFERENCES
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3. INTRODUCTION
Mendelism simply put, refers to the laws of
inheritance postulated by Gregor Mendel.
Gregor Mendel was an Austrian Monk who lived
between 1822-84.
He experimented on various plants species and animal
He was the first to state the universal laws governing
inheritance of traits.
Gregor Mendel is known as father of modern genetics
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4. GENETICS BEFORE MENDEL
Before Mendel Heredity was a mystery
“The laws governing inheritance are quite
unknown; no one can say why the same peculiarity …
is sometimes inherited and sometimes not so; why the
child often reverts in certain characters to its
grandfather or grandmother or other much more
remote ancestor…”
- Charles Darwin, Origin of Species 1872
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5. GENETICS BEFORE MENDEL
The concept of inheritance of physical ‘units’ (later
called genes) was accepted, and scientists had
reported on many hybridization experiments in both
animals and plants.
Yet no one had set forth principles of inheritance
which could be used as a universal theory to explain
how traits in offspring can be predicted from traits in
the parents.
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6. MENDEL EXPERIMENTS
Gregor experimented with pea
plants, by crossing various
strains and observing the
characteristics of their offspring.
He also experiment on
drosophila, but much success
was recorded on pea plants
Between 1856 and 1863,
Mendel cultivated and tested
some 28,000 pea plants.
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7. MENDEL EXPERIMENTS
The choice of Garden pea (Pisum sativum) was
due to the following reasons:
Can be grown in a small area
Produce lots of offspring
Produce pure plants when allowed to self-
pollinate several generations
Can be artificially cross-pollinated
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8. MENDEL EXPERIMENTS
One peculiarity of pea reproduction is that the petals of
the flower close down tightly, preventing pollen grains
from entering or leaving. This enforces a system of self-
fertilization, in which the male and female gametes from
the same flower unite with each other to produce seeds.
As a result, individual pea strains are highly inbred,
displaying little if any genetic variation from one
generation to the next.
Because of this uniformity, we say that such strains are
true-breeding
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9. MENDEL EXPERIMENTS
The true breed traits of the pea plant are:
Seed shape --- Round (R) or Wrinkled (r)
Seed Color ---- Yellow (Y) or Green (y)
Pod Shape --- Smooth (S) or wrinkled (s)
Pod Color --- Green (G) or Yellow (g)
Seed Coat Color ---Gray (G) or White (g)
Flower position---Axial (A) or Terminal (a)
Plant Height --- Tall (T) or Short (t)
Flower color --- Purple (P) or white (p)
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10. Mendel focused on the study of inheritance of
one trait at a time.
In an experiment, Mendel crossed tall and dwarf
pea plants to investigate how height was
inherited.
This type of crossing between parents differing in
only one trait or in which only one trait is being
considered is termed monohybrid cross.
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12. MENDEL EXPERIMENTS
All the offspring from the cross (First filialgeneration- F1)
were all tall regardless of the way he performed the cross
(tall male with dwarf female or dwarf male with tall
female).
Mendel noted that the dwarf characteristic seemed to
have disappeared in the progeny of the cross.
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13. MENDEL EXPERIMENTS
Mendel decided to cross the F1 generation with each other
to see if the dwarf trait would reappear in the next
generation.
When he examined the progeny (F2 generation), he
found that they consisted of both tall and dwarf
plants in a ratio of approximately 3:1.
Mendel inferred that these hybrids carried a latent
genetic factor for dwarfness, one that was masked
by the expression of another factor for tallness.
He said that the latent factor was recessive and
that the expressed factor was dominant.
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14. MENDEL EXPERIMENTS
Mendel repeated the experiment examining the
transmission of other traits such as flower colour, pod
colour.
The same pattern and results was observed
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16. Dihybrid Crosses: The Principle of Independent
Assortment
Mendel also performed experiments with plants
that differed in two traits.
The purpose of the experiments was to see if the
two seed traits, color and texture, were inherited
independently.
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17. He crossed plants that produced yellow, round
seeds with plants that produced green, wrinkled
seeds.
F1 seeds were all yellow and round, the alleles
for these two characteristics were dominant.
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20. MENDEL’S PRINCIPLES OF
INHERITANCE
The result of Mendel’s experiments led to the principles
of inheritance.
It was published in 1865 through a single research paper,
‘Experiments in Plant Hybridization’
Unfortunately, this paper languished in obscurity until
1900, when it was rediscovered by three botanists—Hugo
de Vries in Holland, Carl Correns in Germany, and Eric
von Tschermak-Seysenegg in Austria.
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21. Genes In Pairs
Genes In Pairs: Genetic characters are controlled by
factors (gene) that exist in pairs of alleles in individual
organisms and are passed from parents to their
offspring. When two organisms produce offspring,
each parent gives the offspring one of the alleles from
each pair
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22. Dominance and Recessiveness
Dominance and Recessiveness: When two unlike
alleles responsible for a single character are present in
a single individual, one allele can mask the expression
of another allele. That is, one allele is dominant to the
other. The latter is said to be recessive.
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23. The Law of Segregation
The Law of Segregation: During the formation of
gametes, the paired alleles separate (segregate)
randomly so that each gamete receives one allele or the
other.
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24. The Law of Independent
Assortment
The Law of Independent Assortment: During gamete
formation, segregating pairs of alleles assort
independently of each other. Example: genes on
different chromosomes will segregate independently.
Linked genes (close together on one chromosome) do
not follow this law.
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25. MEDELIAN PRINCIPLES IN HUMAN
GENETICS
Mendel’s principles can be applied to study the
inheritance of traits in humans.
The genetic analysis of human heredity depends
on family records, which are often incomplete.
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26. Pedigrees diagram the relationships among family
members where
squares designate males and circles designate females,
a horizontal line represents mating and
vertical lines beneath reflect offspring.
Birth order is left to right,
afflicted individuals are shaded or filled in
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27. Pedigrees : are used to identify
dominant and recessive
traits in human families
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28. Mendelian inheritance in humans
d) Earlobe attachmenta) Mid-digital
hair
b) Tongue
rolling
c) Widow's peak e)
Hitchhiker's
thumb
f) Relative
finger length
All these traits (and more) are transmitted in Mendelian fashion in humans
