Transcript Early Concepts in Genetics

GENETICS
A Study of Heredity and Inheritance
Early Concepts in Genetics
Medieval Age 1600s
The Blending Hypothesis
Aristotle, 384-322 B.C.
and his Pangenic Theory
Pangenes
in
blood
contained memories of each
structure in the body.
•Blood carried these pangenes
in the blood towards the
reproductive organs.
• Hence, terms such as blood
relative, bloodline and blood
kin are still used today.
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Traits from parents passed to
their offspring thru sexual
union.
These traits blended in their
offspring producing
intermediate forms of the
parents.
The Prevailing 18th Century
•A chip of an old block
•Any traits can be passed
• Modern Genetics begun in 1860s
when an Austrian monk and scientist,
Johann Gregor Mendel worked with
a species of garden pea plants.
•He had studied biology, mathematics
and physics that enabled him to
interpret his observations.
•In his paper "Versuche über
Pflanzenhybriden" ("Experiments in
Plant Hybridization"), presented in
1865 to the Brunn Natural History
Society, Gregor Mendel traced the
inheritance patterns of certain traits in
pea plants and showed that they
could be described mathematically.
•However, his works were not fully
recognized until 50 years later.
Rediscovery of Mendel's work
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It was not until the early 20th century that the
importance of his ideas was realized. In 1900,
his work was rediscovered by Hugo de Vries,
Carl Correns, and Erich von Tschermak.
His results were quickly replicated, and
genetic linkage quickly worked out.
Mendel used the garden pea plants for
several reasons:
1. The plants have several characters,
each with two distinct forms.
2. The garden pea plants reproduce at a
fast rate. This gave him the results in
just about 90 days.
3. They are a hardy plants; they do not
need much caring or cultivating.
Kingdom:
Plantae
Division:
Magnoliophyta
Class:
Magnoliopsida
Order:
Fabales
Family:
Fabaceae
Subfamily:
Faboideae
Tribe:
Vicieae
Genus:
Pisum
Species:
P. sativum
4. The plants are also structurally adapted
for self pollination. Hence, pure breds
are possible.
5. At the same time, cross-pollination is
manually possible. Control and
variation in results can be obtained.
Mendelian Experimental Designs
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Mendel cross pollinated two
pure breeding plants with
contrasting expressions for
the same character. For ex.,
yellow and green seed coat
color.
The parents were referred to
as the first parental (P1)
generation. The offspring
produced from the cross is
referred as the first filial (F1)
generation.
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Only one character was
expressed in all offspring.
Mendel concluded that a
trait was being masked or
dominated by another.
This
trait
that
was
expressed, he called as
dominant and the trait that
was not expressed as
recessive.
For seed coat color, yellow
is dominant to green.
Mendelian Experimental Designs
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Mendel allowed the F1
plants (now P2) to self
pollinate. He counted the
number of offspring (F2)
that expressed each of the
two contrasting traits for all
seven characters studied.
The ratio of the plants with
dominant traits to the plants
with recessive traits is
almost 3:1.
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Mendel hypothesized that each
trait is controlled by a pair of
unit determiners.
In 1903, W. Sutton called
these factors as genes.
He knew each species must
have two factors for a trait that
remain separate and distinct
during gametogenesis.
As a result, some trait will
disappear and reappear in
succeeding generations.
Mendel concluded that factors
do not blend
Mendelian Principles of Inheritance
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First parental (P1) generation
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Gametes
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First filial (F1) generation
Genotype
Phenotype
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GG
G
Gg
Gg
x
G
gg
g
Gg
Gg
Gg
Gg
all yellow seeded
g
Gg
Gg
• Yellow and green are the physical color and are called phenotypes. GG and gg are
genotypes, which describes the genes of the individual. Also G and g are called
alleles or different forms of the gene for seed color.
• Gene pairs are also said to be homozygous if they are made up of the same alleles,
GG or gg (pure breds). If they have dissimilar alleles, then it is called heterozygous.
Mendelian Principles of Inheritance
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Second parental (P2) generation
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Gametes
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Second filial (F2) generation
 Genotype
 Phenotype
 Genotypic ratio
 Phenotypic ratio
Gg
G
GG
Gg
GG
Gg
yellow yellow
1GG : 2 Gg : 1 gg
3 yellow : 1 green
x
g
Gg
G
Gg
Gg
yellow
g
gg
gg
green
When only one character is considered for inheritance, this is called monohybrid
crosses.
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Probability and Genetics
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Probability is very useful in
biology. Geneticists use
probability in determining
phenotypic and genotypic
ratio of offspring in
breeding experiments.
A Punnet square is used to
grid and organize genetic
informations.
Male
G
g
Female
G
GG
Gg
g
Gg
gg
Mendel's Laws of Heredity
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1) The Law of Segregation: Each inherited trait is defined by a
gene pair. Parental genes are randomly separated to the sex
cells so that sex cells contain only one gene of the pair.
Offspring therefore inherit one genetic allele from each parent
when sex cells unite in fertilization.
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2) The Law of Independent Assortment: Genes for different
traits are sorted separately from one another so that the
inheritance of one trait is not dependent on the inheritance of
another.
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3) The Law of Dominance: An organism with alternate forms
of a gene will express the form that is dominant.
Self - Check
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5.
Widow’s peak (S) is dominant over straight hairline (s). If the
mother is heterozygous for the gene pair and the father has
straight hairline.
What is the genotype of the mother? Father?
What are the kinds of gametes produced by the mother?
Father?
Perform a Punnet Square.
Give the genotypic ratio of the cross.
Give the phenotypic ratio of the cross.
Assignment/Homework
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What are Non-Mendelian Crosses?
What are multiple allelic traits?
What are polygenes or multiple genes?
What is Incomplete Dominance?
What is Codominance? Penetrance?
Heritability?
What is Pedigree-making in family trait
studies?