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