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Genetics
101
Before we knew anything
about DNA, there was only the
examination of traits.
The grandfather of this was…
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Mendel’s experiment

Mendel looked at 7
characteristics of pea plants,
each had 1 alternate form that
did not “blend” when crossbred
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Mendel’s experiment:
P = parental
Generations
are
F1= results of PxP
F2= results of F1 x F1
named
What were Mendel’s
conclusions?
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Essential Genetics Vocab
homozygous – 2 identical alleles for a trait, ex. DD, dd
 heterozygous – 2 different alleles for a trait, carrier, ex. Dd
 phenotype – organism’s expressed traits, ex. color, height
 genotype – organism’s genetic makeup, letters, ex. PP, Pp

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If an
offspring
received one
of these
traits from a
parent, did
they always
get the
other?
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Conclusion:
Law of
Segregation
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So… if you have two genes, in
typical (somatic) cells, do your
gametes have both, too?
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So…
Law of Segregation

When do alleles segregate?
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So…

When do alleles segregate? Anaphase I
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
Punnett square used to
predict the results
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
So now we can go back and apply
Mendel’s ideas to the knowledge of DNA
and chromosomes
Chromosome sets

You have 23 pairs of
identical chromosomes,
1 from each parent
◦ 22 autosomes &
1 sex chromosome
 (see karyotype to right)

Other organisms have
different amounts
Examples:
◦ Chicken: 49 pairs
◦ Mosquito: 3 pairs
◦ Lettuce: 9 pairs



Each chromosome
is composed of a
super long strand
of DNA, and each
section of the
chromosome
codes for a specific
gene
you have about
20,000 genes
98% of your DNA
does not code for
genes, but is
“empty space”
Chromosomes and sex cells
(a.k.a. gametes)

Your sex cells (sperm and
eggs) only contain 1 set of
chromosomes, totaling 23
(HAPLOID or N) , as opposed
to body cells containing 46
(DIPLOID or 2N)
◦ Non-sex cells are called somatic
cells

Which parent determines
gender?
Traits
A trait is a physical expression of a gene (hair color,
height, etc.), and your phenotype is your physical
characteristics
 Your genetic make-up is your genotype


Genes come in different versions called alleles
◦ e.g. eyes come in brown, blue, green, hazel

Some traits are polygenic, meaning their
expression is controlled by more than one gene
◦ Like blood type
Gene Expression

You have 2 of each of your 20,000 genes, 1 from
mom and 1 from dad.
but
◦ Some genes are dominant and some recessive,
◦ alleles can also be codominate, where both alleles show in
the phenotype (ex. black and white cows) or
◦ incompletely dominant , where one allele is not completely
dominant over another (ex. gray kittens from black and
white parents)
 See section 11-2

What’s happening here?

What’s happening here?
Incomplete dominance

What’s happening here?

What’s happening here?
codominance

A better example of a codominant trait
 Punnett squares show probablity of
traits being passed down to offspring.
•Since one allele is
from each parent, the
Punnett Square shows
all of the possible
options that two gene
combinations can
make
•This is just like the
drop your genes
activity, only all
options are show
Carrying alleles

Heterozygous: 2 different alleles for a given trait
(Bb)

Homozygous: 2 identical alleles for a given trait
(can be homozygous dominant or recessive, i.e. BB
or bb)

Predicting offspring traits with Punnett squares

Predicting offspring traits with Punnett squares
What would the pups look like?

Predicting offspring traits with Punnett squares
You can also look at 2 traits in the
same Punnett square
Independent assortment

The principle of independent assortment states
that genes for different traits can segregate
independently during the formation of gametes
◦ Some genes are inherited together (linked)
 What is an example?
Independent assortment

Worked for Mendel’s 7 traits in peas, but not
always the case.
◦ Some genes are inherited together (linked)
 What is an example?
 How is this possible that genes get often passed on together?
Independent assortment

Worked for Mendel’s 7 traits in peas, but not
always the case.
◦ Some genes are inherited together (linked)
 What is an example?
 How is this possible that genes get often passed on together?
 How can two genes on the same chromosome still separate?
Independent assortment

The principle of independent assortment states
that genes for different traits can segregate
independently during the formation of gametes
◦ Some genes are inherited together (linked)
 What is an example?
 How can two genes on the same chromosome still separate?

Crossing-over during tetrad formation
◦ The farther apart genes are on a chromosome, the more
likely that they will be able to separate during meiosis.
Pedigree charts show how traits are passed within families
Human Genome Project

The human genome is made up of 3 billion base pairs,
totaling about 6 feet in each cell. (There are about 30 trillion
cells in your body, which means that all your
DNA could stretch to the sun and back
300 times)
For more info:

See chapter 11 & 14 in your text