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Genetics
From Mendel to DNA
Gregor Mendel
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Discovered modern
genetics through the
study of pea plants.
He watched the
passing of traits from
one generation of
plants to the next.
Over time, he formed
two laws that explained
how these traits were
being passed on.
Law 1 – Segregation of Alleles
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He stated that the alleles for a trait separated
in equal amounts.
He observed that some traits showed up
more than other and determined that those
were dominant traits.
He called the less frequent traits recessive.
Genes and Traits
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Mendel called the factors that control a trait
genes.
He observed how the alleles for each gene
interacted and devised that some plants were
pure-bred (meaning they had two of the same
allele) and some were hybrid (meaning they
had two different alleles).
Punnett Squares
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A Punnett square can
be used to determine
the probable
genotype (the set of
alleles) and
phenotype (the
physical appearance)
of an offspring based
on Mendel’s
observations.
Law 2 – Independent Assortment
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Mendel’s second observation was that the
different genes in the pea plants were
independent of each other.
This meant that alleles for one trait did not
affect the inheritance of alleles for another
trait.
Independent Assortment
Patterns of Inheritance
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There are several ways in which traits can be
passed down.
Complete dominance – One allele dominates
another allele.
Incomplete dominance – Neither allele is
expressed fully (a blending of the traits).
Codominance – Both alleles are expressed
fully (both are visible independently).
Patterns of Inheritance (cont.)
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Multiple alleles – more than two alleles for a
gene are found within a population.
Polygenic traits – many genes contribute to a
phenotype.
Human Genetics
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In humans, the sex of an offspring is
determined by special chromosomes.
These are called the X and Y chromosomes.
Males have XY and females have XX.
There are several genetic disorders
associated with these chromosomes.
Since males only receive one X
chromosome, they are more often affected by
X linked disorders.
DNA and RNA
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1.
2.
3.
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DNA and RNA are made up of three parts:
A phosphate group
A sugar (either deoxyribose or ribose)
A nitrogenous base
There are five types of nitrogenous bases.
Nitrogenous Bases
DNA
RNA
Adenine
A
Adenine
A
Thymine
T
Uracil
U
Guanine
G
Guanine
G
Cytosine
C
Cytosine
C
DNA Structure
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DNA is formed into a
Double Helix structure.
This structure looks like
a twisted ladder.
The phosphate groups
and sugars act as the
legs of the ladder and
the nitrogenous bases
act as the rungs
(steps).
Genes and DNA
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The hereditary information of genes are
found in the sequence of the nitrogenous
bases in DNA.
Each gene is a long section of DNA, coding
for the production of a particular protein,
which in turn controls the trait associated with
that gene.
DNA Replication
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DNA is copied from
other DNA.
The double helix
‘unzips’ and new
nitrogenous bases are
matched with the
original order.
The two new double
helix of DNA each have
one new and one
original strand.
Protein Synthesis
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Protein synthesis is the process of taking the
coded messages found on DNA and turning
them into proteins.
DNA is found inside the nucleus (in
eukaryotes) and ribosomes (which make
proteins) are found outside of the nucleus.
In order to complete the process RNA is used
as a messenger between the DNA and the
ribosomes.
Transcription and Translation
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The first step is for mRNA (m = messenger)
to copy the sequence of nitrogenous bases of
the DNA.
mRNA then carries this message out of the
nucleus to a ribosome.
At the ribosome, mRNA is paired with tRNA (t
= transfer) which carries amino acids.
The amino acids are put together in the
sequence determined by the DNA, thus
creating a protein.
Mutations
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A mutation is a change in the order of the
nitrogenous bases of DNA.
Some mutations are harmless, some are
damaging to the organism, and some are
fatal (causing the organism not to develop).