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Genetics
Ms Morin
May 2008
 TRAITS
: Physical characteristics
of an organism inherited from
parents and passed on from one
generation to the next.
 HEREDITY:
The passing of the
traits from parents to offspring.
 GENETICS:
Study of heredity.
Common Traits





Eye color
Eye shape
Hair color
Height
Coat color
seed color
petal color
seed texture
blood type
Seed shape

There are also MANY biochemical traits that are
unseen (having certain enzymes, for example)
The genetic diversity of James Bond
http://www.athro.com/
evo/gen/geframe.htm
Chromosomes & Genes

Our body cells have
46 chromosomes
found in 23 pairs
 Egg and sperm
cells have 23
chromosomes
each- no pairs.
 There are a total of
between 20,000
and 30,000 genes
on our 23 pairs of
chromosomes.

Traits are controlled by
genes.
 Genes are segments of
DNA on a chromosome
that code for a specific
trait
 Each gene holds the
DNA “code” to make
one protein.
http://www.ncbi.nlm.nih.gov/bo
oks/bv.fcgi?call=bv.View..Sho
wTOC&rid=gnd.TOC&depth=2

Different forms of the same gene are called
alleles.

Simple traits are controlled by 2 alleles of the
same gene: we inherit 2 alleles of each gene
for every simple trait.

1 allele is inherited from female sex cell (the
egg from the mom)

1 allele is inherited from male sex cells (the
sperm from the dad)
Different forms of a gene

Different forms of genes are called alleles.
Alleles can be dominant or recessive.

DOMINANT= trait is always expressed no
matter what the other allele is..

RECESSIVE = trait is only expressed if the
other allele for that gene is also recessive.

We use a capital letter to represent a
dominant allele and a lower case for
recessive alleles
Purebred or Hybrid?
PUREBRED: two identical alleles for a
trait
 HYBRID= two different alleles for a trait

ExamplePurebred
Dominant
BB
Purebred
Recessive
bb
Hybrid
Bb

PHENOTYPE- visible traits or physical
appearance of an organism
Examples- blue eyes, short, black fur

GENOTYPE- genetic makeup of an
organism- its allele combination
Examples- bb, Tt, GG
Punnett Squares

What is it? A chart showing all the
possible combinations of alleles.
Geneticists use them to predict
outcomes.
(male)
(female)
1.
2.
3.
Trait- Hitchhikers Thumb
Hitchhiker’s Thumb is dominant (H).
No hitchhiker’s Thumb is recessive (h)
Cross- purebred Hitchhiker’s thumb
female X non-Hitchhiker’s thumb male
4. Offspring predictions
Genotype: ______%=4/4 = Hh
Phenotype: _____%= 4/4= Hitchhiker’s Thumb
Gregory Mendel
Father of genetics
 Univ. of Vienna monk 1851
 Wondered why different pea plants had
different characteristics
 He observed that many pea plants’
traits were similar to their parents

Mendel’s Experiment
First generation F1
Second generation F2
purebred tall X purebred short
TT x tt
Phenotype=
Genotype=
Tt X Tt
Phenotype=
Genotype=
Mendel’s Conclusions

When you cross 2 purebred plants
– 1st generation only saw the dominant trait
(Tall; Purple flowers- next slide)
– 2nd generation showed purebred and
hybrid. (Tall & short; Purple &White)

1866- he presented his work but it
wasn’t looked at by the scientific world
until 1900!
Probability and Genetics

PROBABILITY: A number that describes
how likely it is that an event will occur
# times event occurs
# possible outcomes
= probability
*Results do not affect future results

HOMOZYGOUS- 2 identical alleles: tt, TT
same thing as purebred

HETEROZYGOUS- 2 different alleles:
Example: Tt
same thing as hybrid
Incomplete Dominance
alleles are not dominant nor recessive
 the phenotype is a blend of the two alleles.
(Notice the use of all capital letters.)
Codominance* neither allele is dominant or recessive
* the two different traits appear together in
the organism
* example: ABO blood type