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Fundamentals of Genetics
I. Gregor Mendel – “ The Father of Genetics”
A. History
1. he was a monk who tended a garden in the 1850’s
2. he was educated in math and statistics
3. he observed pea plants and noticed trends in the
inheritance of certain traits
a. looked at 7 traits (seed color and shape, flower color and position, pod color and shape, plant height)
b. based on his observations he conducted experiments
with certain plants.
c. Self-fertilization – fusion of a sperm (pollen) and egg
made on the same plant
d. Cross-fertilization – fusion of a sperm (pollen) and egg
made on different plants
Experimental design:
• Worked with plants
until he knew he had
true breeding
•Terms:
•Cross
•Hybrid
•Hybridization
•P generation
•F1 generation
•F2 generation
B. Conclusions Mendel made from his experiments
1. Principle of Dominance and Recessiveness
a. genes are in pairs (think homologous chromosomes)
b. Dominant gene – masks over the other gene in the pair
c. Recessive gene – gene in pair that is “hidden”
2. Principle of Segregation
a. the paired genes separate during the formation of egg
and sperm (think meiosis…Metaphase 1 to Anaphase 1)
3. Principle of Independent Assortment
a. genes for different traits are distributed to egg and
sperm independent from each other. (think how chromosomes line
up in Metaphase 1 and 2…independent from one another)
II. Terms
A. Gene – segment of DNA that codes for a protein
B. Allele – variety of a gene
1. Dominant Allele – use CAPITAL letters to symbolize
2. Recessive Allele – use lower case letters to symbolize
C. Phenotype – Physical appearance of a trait
D. Genotype – Genetic makeup of the phenotype
1. Homozygous Dominant: TT
2. Heterozygous: Tt
3. Homozygous Recessive: tt
E. Homologous chromosomes – chromosomes that carry
genes coding for the same characteristics
1. You get 1 from your mom and 1 from your dad
F. Autosomes – chromosomes that do not code for gender (44)
G. Sex Chromosomes – chromosomes that code for gender
1. Female – XX
2. Male -- XY
III. Genetic Crosses: Punnett Squares – used to find the
probability of types of offspring certain parents may obtain
A. Monohybrid cross: Crossing of one trait
1. write down cross
2. write possible egg and sperm combinations (if there are two
letters, then egg and sperm will have 1 letter each)
3. make Punnett square (4 boxes)
a. egg possibilities go on top
b. sperm possibilities go along the side
4. complete the cross
5. write the genotypic ratio (the chance that one of the
genotypes would be present in an offspring)
a. homozygous dominant : heterozygous : homozygous recessive
6. write the phenotypic ratio (the chance that one of the
phenotypes would be present in an offspring)
b. dominant phenotype : recessive phenotype
t
t
T
T
T
t
T
T
T
T
t
t
t
t
T
t
B. Dihybrid cross: crossing of two traits
1. write down cross
2. write all possible egg and sperm combinations
(think meiosis…principle of segregation and principle of independent assortment)
(if there are 4 letters, then egg and sperm will have 2 letters…one of each kind)
3. make Punnett square (16 boxes)
4. complete the cross
5. write the genotypic ratio:
TTRR
TTRr
TTrr
TtRR
TtRr
Ttrr
ttRR
ttRr
ttrr
6. write the phenotypic ratio:
Dom/Dom : Dom/Rec : Rec/Dom : Rec/Rec
C. Testcross – mating between an individual of unknown
genotype and a homozygous recessive individual.
1. Mrs. Barton has brown eyes (BB or Bb) ?
2. Mr. Barton has blue eyes (bb)
B__
I would be BB if my kids were
brown eyed
bb
I would be Bb if my kids were
blue eyed
Tricks
• Rule of Multiplication (aka Product Rule)
– how do we determine the chance that two or
more independent events will occur together
in a specific combination?
Example: TtRr X TtRr
What is the probability of getting a short smooth
offspring?
Short?
Smooth?
Example: TtRr X TtRr
What is the probability of getting a tall wrinkled offspring?
• The probability of getting a tall offspring
is _______
• The probability of getting a wrinkled
offspring is _______
• The probability of getting a tall wrinkled
offspring is:
Example:TtrrGG X TtRrgg
What is the probability that you would get a Tall plant with
Wrinkled Green seeds?
“T’s” = Tt X Tt =
“R’s” = rr X Rr =
“G’s” = GG x gg =
Product is:
Another Trick
• Rule of Addition
– used to find the probability of an event that
can occur in two or more different ways is the
sum of the separate probabilities of those
ways
• Use the Product Rule and Rule of Addition
to calculate the results of complex crosses
rather than work out the Punnett Squares.
What is the probability that offspring will
exhibit recessive phenotypes for at
least 2 of the 3 traits?
TtRrGg X Ttrrgg
Write the possibilities
Solve for probability for each possibility (rule of multip)
Add probabilities together to get answer.
IV. Exceptions to Mendel’s Principles
A. Sex Linked traits – there are traits (genes) that are on the
sex chromosomes and are therefore linked to gender.
1. X chromosome carries most of these genes
a. colorblindness
b. hemophilia
c. muscular dystrophy
d. baldness
2. most of these are recessive traits
3. Genotypes you would need to have to express these
traits are: XhXh and XhY
4. A carrier of these traits (has the gene, but doesn’t
express it) would be: XHXh (this can also be written XhX)
B. Codominance – both alleles are expressed when a
heterozygous genotype is present
1. “Co” means “with, together, jointly”
2. Black chicken (BB) x White chicken (bb)
100% Black and white checkered chicken (Bb)
(some feathers are black and some are white)
C. Incomplete dominance – the dominant allele doesn’t
completely mask the recessive allele, so you get a
“blending” appearance when a heterozygous genotype is
present.
1. Red flowers (RR) x White flowers (rr)
100% Pink flowers (Rr)
D. Multiple Alleles
1. when a trait has more than 2 alleles
2. each organism still ends up with 2 alleles, but there are
more alleles to choose from
3. Blood type – there are three alleles: iO IA IB
Phenotype
Genotype
Type O
iOiO
Type A
IAIA or IAiO
Type B
IBIB or IBiO
Type AB
IA IB
a. this is an example of codominance as well
E. Pleiotropy
1. When a single gene controls more than one phenotypic
characteristic
2. Cystic fibrosis and sickle cell disease.
F. Polygenic inheritance
1. The additive effects of two or more genes on a single
phenotypic character.
2. human skin color and height
V. Pedigrees – a record that tracks the inheritance of a trait
through several generations of a family
A. Helps us to determine how and why certain traits are
inherited.
B. Used as a form of genetic counseling
1. keeping records of an individual’s relatives with a genetic
disorder enables a genetic counselor to determine an
individual’s chances of being a carrier for that disorder
and therfore make family planning decisions.
C. Common symbols you should know: (go to next page)
I.
Male
Female
II.
Mating between
individuals
X-linked
carrier
or
Affected individual
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