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Biology
Chapter 9
Fundamentals of Genetics
What is Genetics?
a. Study of heredity
b. Transmission of traits from parent to
offspring
Who is the father of genetics?
• Gregor Mendel
• 1822-1884
Why was Mendel Successful?
• 2 reasons
1. He used a garden pea as his test subject
• Why would he use the pea?
• 6 reasons
a. Small
b. Easy to grow
c. Produce many offspring
d. Mature quickly
e. Many varieties
f. Easy to fertilize
» Self fertilization – within same plant
» Cross fertilization – involved two plants
Second reason?
2. He used a quantitative approach
Mendel’s Experiment
• Step 1
– He produced a parent generation (P)
• He allowed pea plants to self fertilize for many
generations
• This made sure that he had pure parents that were
true breeding or pure
• Example: He has 1 pure purple pea plant and 1
pure white pea plant for his P generation
Mendel’s Experiment Cont.
• Step 2
– He produced the 1st generation (F1)
– He cross fertilized two of the P generation pea
plants
– What was his results?
• 100% Purple plants, no white
Mendel’s Experiment Cont.
• Step 3
– He produced the 2nd generation (F2)
– He took 2 of the F1 generation pea plants and
self fertilized them.
– What were the results?
• 75% purple, and 25% white
Mendel’s Conclusions
• Parents transmit information about traits to
their offspring
• Each individual has 2 factors (genes) for
each trait, 1 from each parent
• Factors (genes) are represented by letters
or alleles.
Alleles
• If both alleles are the same, the individual
is homozygous for that trait
• If both alleles are different, the individual is
heterozygous for that trait
Traits can be described in 2 ways:
• Genotype
– Alleles that represent the trait
– Example: PP, Pp, pp
• Phenotype
– Expression of the trait
– Example: purple, white
More about traits
• Only some traits are seen, others are
masked
– Dominant – only need 1 letter to be expressed
» Represented by capital letters
– Recessive – need both letters or its masked
» Represented by lowercase letters
Do you understand?
Genotype Dom/Rec Homo/Hetero
Pheno
Mendel’s Law of segregation
• Members of each
pair of alleles
separate when
gametes are formed.
A gamete will receive
1 allele of the other.
This occurs in
meiosis.
Principle of Independent
Assortment
• Two or more pairs of chromosomes
separate independently of one another
during the formation of gametes. This is
random.
Why did Mendel’s results repeat?
1.
-
Chance and probability
Leads to predictions
1. coins
- flipping a head?
2. cards
- diamond?
- nine?
- nine of diamonds?
3. sex of children
- having a boy child?
- having a girl after having 4 boys in a row?
Monohybrid Crosses
•
•
•
•
Involves 1 trait
Crosses 2 alleles on the same locus
Uses a 4-boxed Punnett Square
Example: Cross a white flowered pea
plant with a heterozygous purple flowered
pea plant
Monohybrid Example
• pp x Pp
p
p
P
p
Genotype %
50% Pp
50% pp
Phenotype %
50% purple
50% white
How do you find out whether an
individual is BB or Bb?
• To a testcross
– Technique that takes the unknown genotype and cross it
with a recessive individual and then look at the results.
Dihybrid Crosses
•
•
•
•
Involves 2 traits
Crosses individuals with 4 alleles at 2 loci
Uses a 16 box punnett square
When both genotypes for both individuals are
heterozygous (BbTt x BbTt), the phenotype
percentage will be 9:3:3:1
• Example: Cross a homozygous purple flowered,
heterozygous green pod pea plant with a white
flowered, yellow pod pea plant. (Green is
dominant over yellow)
Dihybrid Example
• PPGg x ppgg
• Must do Foil to get the gametes!
– First, Outer, Inner, Last
•
•
•
•
•
PPGg
F = PG
O = Pg
I = PG
L = Pg
– FOIL for ppgg are pg, pg, pg and pg
– So now take these gametes and place them
in the Punnett Square
Dihybrid Example
Genotype: 50% PpGg, 50% Ppgg
Phenotype: 50% Purple Green
50% Purple Yellow
Dominant Recessive relationships
1. Lethal recessive – homozygous
recessive organisms cannot survive (ex.
Tay Sachs, Cystic Fibrosis)
2. Incomplete Dominance
3. Codominance
Incomplete Dominance
• Heterozygote is an intermediate between
phenotypes of two homozygotes
• Blending occurs!
• Occurs in Japanese 4:00 plants and
snapdragons
– RR = red
– WW = white
– So RW = pink!
• Example: Cross a red flowered Japanese 4:00
plant with a white flowered 4:00 plant
Incomplete Dominance Example
Genotype:
100% RW
Phenotype:
100% pink
Codominance
• Two traits share dominance (Ex. Human
Blood Types)
– Must use special notations when doing these
problems
•
•
•
•
IAIA and IAi…………bloodtype A
IBIB and IBi…………bloodtype B
IAIB………………….blood type AB
ii……………………..bloodtype O
• Cross a person with bloodtype AB with a
person with bloodtype O
Codominance Example
IAIB x ii
IA
IB
Genotype %
i
i
A
I i
IBi
A
I i
IBi
50% IAi
50% IBi
Phenotype %
50% bloodtype A
50% bloodtype B
X linked Genes
• Also called Sex linked genes
• Genes that follow the transmission of the
X chromosome
• Always expressed in males, and is
considered to be dominant
• Females may be expressed
• Hemophilia is an example – special
notation for these types of problems
Sex linked notations
• For hemophilia
– Females
• XHXH normal
• XHXh carrier/heterozygous
• XhXh has hemophilia
– Males
• XHY normal
• XhY has hemophilia
Example: Cross a hemophiliac man with
a female carrier
Sex Linked Example
XHXh x
XhY
Xh
Y
Genotype
XH
XHX
h
XHY
25% XHXh
25% XHY
Xh
XhXh
Phenotype: 50%
normal, 50%
hemophiliac
XhY
25%XhXh
25% XhY