Fundamentals of Genetics
I. Introduction to Genetics
1. Gregor Mendel - “Father of Genetics”
• Austrian monk, teacher, scientist, gardener
• Formulated basic laws of heredity in the early 1860s
2. Worked with garden peas because:
• Easy to grow and had a short
generation time
• Could be self-pollinated
3. Chose varieties that were true-breeding
• No genetic variation for a trait
4. Studied 7 simple traits
5. Mendel cross-pollinated plants
• P = parental generation
• F1 = first-generation
(sons/daughters)
• F2 =second-generation
6. Principle of Dominance and Recessiveness
•
•
•
•
F1 plants resembled only one of the parents
F1 hybrids contained two different factors (alleles) for
each trait
one dominant; (masks recessive);
one recessive; (seems to disappear)
Which are you?
7. Principle of Segregation
•
•
•
Organism contains two factors for each trait.
Alleles separate from one another during formation
of gametes.
Each gamete contains one allele for each trait.
8. Principle of Independent Assortment
•
•
•
Each trait is independent of another
Genes of one pair of traits assort
independently
All combinations of genes occur in
gametes
II. Chromosomes, Genes, & Genetic Crosses
1. Homologous
Chromosomes contain
genes (locations on
DNA for traits) for the
same traits
2. Traits are controlled by
alleles (alternative
forms of a gene).
3. Genotype refers to the
alleles an individual
receives at fertilization
4. Phenotype refers to
the physical
appearance
of the individual.
5. Homozygous dominant genotypes =
two dominant alleles for a trait. (BB)
6. Homozygous recessive genotypes =
possess two recessive alleles for a trait (bb)
7. Heterozygous genotypes =
one of each allele for a particular trait (Bb)
Review:
True-breeding organisms
would be homozygous
dominant OR recessive
individuals.
Hybrids would be
heterozygous
8. Punnett Squares
• Show probabilities of future offspring.
• A monohybrid cross = cross between
individuals that involve 1 contrasting trait.
9. Monohybrid Crosses Examples
Ex. 1: One is heterozygous for round seeds x one with
wrinkled seeds
R
r
Phenotype/Genotype Chart
Phenotype Genotype(s)
Round
RR & Rr
Wrinkled
rr
r
Rr
rr
Plant #1’s genotype: Rr
r
Rr
rr
Plant #2’s genotype: rr
Genotype probability:______________
Genotype ratio:___________________
Phenotype probability:_____________
Phenotype ratio:__________________
B. Example 2 & 3: P & F1 cross for true-breed tall x true breed short
Phenotype Genotype(s)
Tall
TT, Tt
Short
tt
P cross
T
t
t
Tt
Tt
F1 cross
T
t
T
TT
Tt
t
Tt
tt
T
Tt
Tt
100% Tall
100% Heterozygous
(Homozygous x Homozygous)
25% pure tall
50% hybrid tall
25% pure short
(Heterozygous x Heterozygous)
End for today
Complete and diagram the 3 types of
monohybrid crosses below.
1. Round peas are dominant to wrinkled
peas. Cross a homozygous dominant x
homozygous recessive.
2. Short plants are recessive to tall plants.
Cross a homozygous recessive x
heterozygous
3. Green peas are dominant to yellow peas
cross a heterozygous x heterozygous
1. Round peas are dominant to wrinkled peas. Cross a
homozygous dominant x homozygous recessive.
Phenotype Genotype(s)
2. Short plants are recessive to tall plants. Cross a
homozygous recessive x heterozygous
Phenotype Genotype(s)
3. Green peas are dominant to yellow peas cross a
heterozygous x heterozygous
Phenotype Genotype(s)
10. Test Crosses
• A cross of an individual of unknown genotype with an
individual of known genotype.(has to be homozygous
recessive)
• Results tells if unknown individual is heterozygous or
homozygous
• Very important to breeders
I. Beyond Mendel
1. Incomplete dominance
• Offspring are intermediate
between two parental
phenotypes
• Neither allele is completely
dominant over the other
• Both alleles influence
phenotype
• 3 phenotypes
• Ex: Japanese Four o’clocks
RR = red RR’ = pink R’R’ = white
R
R
R’
RR’
R’
RR’
RR’
RR’
All Pink
R
R’
R
RR
RR’
R’
RR’
R’R’
1 red : 2 pinks : 1 white
2. Codominance
• Both alleles of a
gene are expressed.
• A person with AB
blood has both A and
B antigens on their
red blood cells.
• Neither allele is
dominant or
recessive
P1 generation cross
Brown
WHITE
Both Brown
and white
expressed
at the same
time.
ROAN
• Ex: Red Coat - RR
White Coat – WW
White & Red Mixed – RW (“roan”)
W
W
R
R
RW
RW
RW
RW
All Roan – both red and white
hair
R
W
R
W
RR
RW
RW
WW
1 red : 2 roan : 1 white
3. Multiple Allele Traits are traits
with 3 or more alleles; only get 2
1. ABO Blood group system; types – A, B, AB, O
2. IAIA =type A IAi = type A
ii = type O
IBIB= type B IBi = type B
IAIB= type AB
• *A,B codominant
*Both dominant to O
What are the possible blood types when a
Person with AB blood and a person that is
homozygous for B blood reproduce?
Phenotype Genotype(s)
Type A
I AI A , I Ai
Type B
I BI B , I Bi
Type AB
I AI B
Type O
ii
IA
IB
IB
IAIB
IBIB
IB
IAIB
IBIB
Type AB & B are possible
What are the possible blood types when a
Person with homozygous A blood and a
person that is homozygous for B blood
Phenotype Genotype(s)
reproduce?
Type A
I AI A , I Ai
Type B
I BI B , I Bi
Type AB
I AI B
Type O
ii
IA
IA
IB
IAIB
IAIB
IB
IAIB
IAIB
Type AB only
What are the possible blood types when a
Person with B blood and a person with O
blood reproduce?
Phenotype Genotype(s)
Type A
I AI A , I Ai
Type B
I BI B , I Bi
Type AB
I AI B
Type O
ii
IB
i
i
IBi
ii
i
IBi
ii
End of the notes today!
• Any questions?
RrYy X RrYy
Law of Independent Assortment:
- Genes for different traits are inherited
independently
- R, r, Y, y go to sperm/egg independently of
each other
- Can recombine in four ways:
RrYy
3. Dihybrid Cross: A cross involving two pairs of
A = feathers
contrasting traits
C = crest
Phenotypes
Genotypes
Blue feathers
Red feathers
aa
AA, Aa
Phenotypes Genotypes
w/ Crest
w/o Crest
CC, Cc
cc
Phenotypes
Genotypes
Red feathers, w/ Crest
Red feathers, w/o Crest
Blue feathers, w/ Crest
Blue feathers, w/o Crest
AACC, AACc, AaCC, AaCc
AAcc, Aacc
aaCC, aaCc
aacc
Example Cross
RRYY
P1:
rryy
x
yellow, round
(homozygous)
green, wrinkled
(homozygous)
RY
RY
RY
RY
ry
RrYy
RrYy
RrYy
RrYy
ry
RrYy
RrYy
RrYy
RrYy
ry
RrYy
RrYy
RrYy
RrYy
ry
RrYy
RrYy
RrYy
RrYy
Offspring are
all
heterozygous
round, yellow:9
round, green : 3
wrinkled, yellow: 3
wrinkled ,green: 1
Round seeds are dominant to
wrinkled and yellow is dominant to
green. What are all the possible
phenotypes?
RrYy X RrYy
Phenotypic Ratio is 9:3:3:1
RY
Ry
rY
ry
RY
RRYY
RRYy
RrYY
RrYy
Ry
RRYy
RRyy
RrYy
Rryy
rY
RrYY
RrYy
rrYY
rrYy
ry
RrYy
Rryy
rrYy
rryy
End of the notes today!
• Any questions?