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Introduction to
Genetics and
Polygenic Traits
EVERYBODY – CROSS YOUR ARMS
IS THIS GENETIC? YES NO
BEND YOUR THUMB BACKWARD
IS THIS GENETIC? YES NO
Sort Of!
DO YOU HAVE DIMPLES?
IS THIS GENETIC? YES NO
ROLL YOUR TONGUE (LIKE A HOT DOG! OR A CLOVER IF YOU’RE REALLY COOL)
IS THIS GENETIC? YES NO
Sort Of!
DO YOU HAVE A CLEFT CHIN?
IS THIS GENETIC? YES NO
IS YOUR SECOND TOE LONGER THAN YOUR BIG TOE?
IS THIS GENETIC? YES NO
Sort Of!
IS YOUR EAR LOBE CONNECTED TO YOUR HEAD?
IS THIS GENETIC? YES NO
DARWIN’S TUBERCLE (EAR BUMP)
IS THIS GENETIC? YES NO
ARE LEMONS SOUR?
IS THIS GENETIC? YES NO (MOSTLY)
WHAT COLOR ARE YOUR EYES? YOUR PARENTS?
IS THIS GENETIC? YES NO
HOW MANY GENES CONTRIBUTE TO EYE COLOR?
• Brown iris: abundance of
melanocytes and melanin in the
anterior border layer and
stroma
• Blue iris: very little melanin,
consequence of structure, not
chemical compositionLight traverses these relatively
melanin-free layers, collagen
fibrils of the iris scatter the short
blue wavelengths to the surface
Shades of blue, shades of
grey, green and hazel
determined by:
• thickness and density of
the iris itself
• accumulation of white
collagen fibers
• patches of tissue loss in
the anterior border
layer and stroma
10 Genes? Traits with many genes
involved are called polygenic
WHAT ARE OTHER TRAITS YOU THINK ARE POLYGENIC?
• Talk amongst yourselves and come up
with some traits or even some diseases
that are polygenic
Height
Combined size of all
body parts
determines height: an
additive effect. Sizes
of all body parts in
turn, determined by
numerous genes
Obesity
Leptin - 7q31.3
Leptin receptor - 1p31
Proopiomelanocortin - 2p23.3
Melanocortin-4 receptor - 18q22
Protein convertase subtilisin/kexin type 1 - 5q15-q21
Cardiovascular
Disease
Multifactorial disease
Lipoprotein response to dietary
manipulation has a strong genetic
component
Many stops in various pathways can
mutate causing abnormal lipid
metabolism
Skin color, hair color, autism spectrum disorders, hypertension, and more
are all considered to be polygenic.
For many
traits and
diseases, even
if there is a
genetic
component,
they still may
be influenced
by
environment
(diet, womb,
geographic
location, etc).
TO BETTER UNDERSTAND GENES AND HOW THEY IMPACT OUR PHENOTYPE, WE HAVE TO DIG DEEPER.
DNA EXTRACTION
CENTRAL DOGMA
DNA
RNA
Protein
Transcription
Translation
DNA
Deoxyribonucleic Acid
Double helix
Twisted ladder
The sides of the ladder are
deoxyribose sugar and phosphate units
The rungs of the ladder are
complementary pairs of chemical bases
Sugar + Phosphate + One Base =
Nucleotide
RNA
Single stranded
Smaller than DNA
DNA base thymine
replaced by RNA base
uracil
Sugar component is
different - ribose
ONLY 1.2-1.5% OF THE 2.9 BILLION BASE
PAIRS IN HUMAN DNA ACTUALLY CODE FOR
GENES.
WHAT’S IN OUR DNA?
EXONS
• Meaningful code sequences
• Will make a protein
INTRONS
• Remaining 98+% of DNA bp
• Can act as subtle enhancers or
suppressors of genes
• Absorb the negative effects of viruses
to protect the gene-coding regions for
changes
• Help determine the shape of
chromosomes
Protein 1Gene 1 Coding Region
Gene 2 Coding Region Protein 2
Noncoding Region
Noncoding region
Exon = Coding Region
Intron = Noncoding Region
TRANSCRIPTION
• The coding region of DNA is
transcribed, or copied, into RNA
• DNA untwists and unzips with the
help of several enzymes
• Bases separate from one
another
TRANSCRIPTION
• Nucleotides that are freely roaming in the nucleus of the cell are
attracted to the unwound DNA – find a complementary bp match
• The new strand that is made is an messenger RNA (mRNA) molecule
• mRNA is a specific section of nuclear DNA that corresponds to a gene
TRANSLOCATION
Messenger RNA molecules
move from the nucleus to the
cytoplasm
The protein that the mRNA is
coded for is assembled here
TRANSLATION
Protein synthesis begins as
ribosomes move along the
messenger RNA strand
Transfer RNA (tRNA)
anticodons (each with 3
bases) are attached to
triplets of complementary
bases on the mRNA
HOW DO CELLS KNOW WHEN A GENE NEEDS TO BE TURNED ON?
Transcription Translation = Gene Expression
Promoter – region of the DNA (sequence of DNA) that receives signals from other compounds that say “HEY WE NEED PROTEINS!!”
NUTRIENTS AFFECT GENE EXPRESSION
…Which is why you need to eat your broccoli and go
outside!
Vitamin D – binds to receptor that increases transcription
of genes and translation of proteins that enhance the
absorption of calcium
Vitamin A - binds to receptor which initiates synthesis of
various proteins which are involved with skin health
and/or eyesight
Of the 3.2 billion bases,
roughly 99.9% are the
same between any two
people.
It is the variation in the
remaining tiny fraction of
the genome, 0.1% --
roughly several million
bases -- that makes a
person unique.
This small amount of variation
determines attributes such as
how a person looks or the
diseases he or she develops.
Occasionally, an error is made in DNA replication.
Incorrect base pair may be included = mutation
If mutation during meiosis inherited
Such errors in replication are the ultimate sources of all new
genes and are essential for the evolution of new species.
POINT MUTATIONS
• most common type of mutation
• also called a base substitution
• single nucleotide replaced with a
different nucleotide
Silent Mutation –
Change in codon
makes no difference
in amino acid
outcome
Missense Mutation
– Change in codon
sequence changes
the amino acid
produced
Nonsense Mutation
– Change in
sequence leads to a
premature stop
codon (cuts the
protein short)
FRAMESHIFTMUTATION
• caused by the insertion or a deletion of a
base pair
• inserted or deleted nucleotide alters the
triplet grouping of nucleotides into codons
and shifts the reading frame so that all
nucleotides downstream from the mutation will
be improperly grouped
• result is protein with extensive missense
ending sooner or later in nonsense
INHERITANCE & POLYGENIC TRAITS
MENDEL’S EXPERIMENTS
PHENOTYPE GENOTYPE
PRINCIPLE OF SEGREGATION
• For any trait, the pair of alleles of each parent separate and only one allele
passes from each parent on to an offspring.
Which allele = chance
Now know segregation of alleles occurs during meiosis
• Different pairs of alleles are passed to offspring independently of each other
Result = new combinations of genes
PRINCIPLE OF INDEPENDENT ASSORTMENT
MENDEL’S OBSERVATIONS GAVE US TWO PRINCIPLES:
PUNNETT SQUAREGraphical way of discovering potential combinations of genotypes in
children, given genotypes of parents
Odds of each genotype
EXCEPTIONS TO SIMPLE INHERITANCE
INHERITING ONE ALLELE CAN, AT TIMES, INCREASE THE CHANCE OF INHERITING ANOTHER
OR CAN AFFECT HOW AND WHEN A TRAIT IS EXPRESSED IN AN INDIVIDUAL'S PHENOTYPE
THERE ARE DEGREES OF DOMINANCE AND RECESSIVENESSWITH SOME TRAITS
SOME THINGS ARE A COMBINED EFFECT OF MANY GENES
• These are called polygenic traits or disorders
• Include:
• Height
• Obesity
• Risk of Cardiovascular Disease
• Fat Absorption and Delivery
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