Genetics
SPI.4.4 Determine the probability of a particular trait in an offspring based on the
genotype of the parents and the particular mode of inheritance.
SPI.M.2 Predict the outcome of a cross between parents of known genotype.
Genetics
The study of heredity
Gregor Mendel
• Worked with pea plants
Studied various traits
Trait – genetically determined
characteristic or condition
Alleles
Different forms of the same gene
Example: Hair color
Generations
• P(parental) = original pair
• F1(1st filial) = P offspring
– Filius = Latin for “son”
• F2 (2nd filial) = F1 offspring
First Experiment
P
(parental)
F1
Second Experiment
F1
F2
Gregor Mendel
Four major conclusions:
1. Inheritance factors
2. Principle of Dominance
3. Law of Segregation
4. Law of Independent Assortment
Mendel’s Conclusion
1. Inheritance is determine by factors
passed down by parents
*factors = genes
Mendel’s Conclusions
2. Some alleles are dominant to others
Recessive
*Appears in absence
of dominant allele
*Lower case letter
Dominant
*Always appears
when present
*Capital letter
Genotype
Phenotype
Allele combination
Physical appearance
Trait = Flower Color
Purple (P) color is dominant to white (p) color
Genotype?
Phenotype?
PP or Pp
Purple
pp
White
Homozygous
Heterozygous
Same alleles
Different alleles
Mendel’s Conclusions
3. Law of Segregation
• A pair of alleles is separated in the
formation of gametes
Meiosis
Mendel’s Conclusions
4. Law of Independent Assortment
Alleles on non homologous chromosomes
will be distributed randomly into gametes
Punnett Square
• Monohybrid
1 trait
• Dihybrid
2 traits
Punnett Squares
Parent 2 (aa)
a
a
A
Parent 1 (AA)
Aa
A
Aa
Offspring genotype
Aa
Aa
Gamete
Example
•
•
•
•
Tall is dominant to Short in pea plants
Tall – T
Short – t
Cross a heterozygous tall plant with a short
plant
• Parent 1 Tt
• Parent 2 tt
Example Cross
t
t
T
t
Tt
tt
Tt
tt
Probability
• Predict the chance of each phenotype’s
occurrence
• Think of each little square as a quarter
• Think of each large square as a dollar
25
25
25
25
= 100
Example Cross
t
t
T
t
Tt
50%
Tt
tt
50%
tt
Monohybrid Cross Rules
• 100% of the offspring – dominant phenotype
– At least 1 parent is homozygous dominant
• 100% of the offspring – recessive phenotype
– Both parents are homozygous recessive
• 75% dom. pheno. & 25% rec. pheno.
– Both parents are heterozygous
• 50% dom. Pheno. & 50% rec. pheno.
– 1 parent is heterozygous &
1 parent is homozygous recessive
Dihybrid
Examines two traits
Shows independent assortment
Pea Plants
•
•
•
•
•
Round seeds (R) are dominant to wrinkled (r)
Yellow seeds (Y) are dominant to green (y)
How many traits are we looking at? 2
So how many alleles per parent? 4
So how many alleles per gamete? 2
Finding Gametes?
•
•
•
•
•
FOIL
F – first
O – outer
I – inner
L – last
RrYy
•
•
•
•
F – RY
O – Ry
I – rY
L – ry
Lethal Inheritance
• Inherited allele may cause death
• Prolonged
–Huntington’s disease
• Immediate
–Humans – Tay Sacs
–Plants - Albino
Incomplete Dominance
occurs when a heterozygous genotype results in an
intermediate trait
• Trait 1 – 2 capital letters
• Example: red flowers = RR
Trait 2 – 2 capital letters
Example: white flowers = WW
Heterozygous = Mix
Example – Pink RW
Incomplete Domiance
• White Seahorse
–WW
• Black Seahorse
–BB
• Gray Seahorse
–BW
Incomplete Dominance
• What color are the parents?
– Gray
• What color is the top left
seahorse?
– Black
• What color is the bottom right
seahorse?
– White
• What color are the BW?
– Gray
Codominance
Share dominance
Use a capital letter for trait with a different
exponent letter to represent each allele
CBCW
CBCB
CWCW
Codominance Punnett
• Cross a speckled chicken with a
black chicken.
CB
CB
CB
CW
CBCB
CBCW
CBCB
CBCW
Multiple Alleles
Traits having >2 allele options
• Example: Rabbit Coat Color
Multiple Alleles &
Codominance
• Human example: blood types
• Blood types are A, B, AB, O.
• A & B are dominant alleles (IA or
B
I ) and O is recessive (i).
Codominance
A
Example: Blood types
B
AB
Genotype- IAIA
or IAi
Fights B or AB
Genotype- IBIB
Or IBi
Fights A or AB
Genotype- IAIB
Fights None
Universal
Recipient
O
Genotype- ii
Fights all but O
Universal
Donor
How common is your blood type?
46.1%
38.8%
11.1%
3.9%
Blood Type Punnett
• Cross a man that is heterozygous for
B blood with a woman who has AB
blood.
Blood types
Polygenic Traits
• Traits produced by the interaction of
several genes
• Example: Human Skin Color
– 3-6 genes
– Amount of melanin
– Incomplete dominance
Eye Color
•
•
•
•
Amount of melanin in iris
Chromos. 15 – Brown from Blue
Chromos. 19 – green & blue
Iris
–Anterior – front
–Posterior – back
Environment Affects
• Nutrition
• Exercise
• Heart disease
• Epigenome
Autosomal Versus Sex Chromosomes
Autosomes
All chromosomes
except X or Y
Sex Chromosomes
Female XX
Male XY
Inheritance of Sex Chromsomes
Mother always
give s an X to the
offspring
Father gives an
X or Y to the
offspring
Most sex-linked traits are on the X chromosome
Sex Linked Traits
Males are more likely to show
–Males only have one X
Examples:
Male Pattern
Baldness
Colorblindness
Hemophilia
Sex Linked Punnetts
Gender must be included for sex linked punnetts.
Hemophilia is a sex linked recessive trait. Cross a
normal male with a female who is a carrier for
hemophilia.
Types of Inheritance
Inheritance
Complete
Incomplete
Letters
# of
Dominant
Alleles
Other
1
Default
A&B
phenotypes
will mix
Either both
show up or if
Ay a – dom.
0
Remember
to blend
the 2
Hetero can
be both or
the dom.
Only female
can be hetero
1
Heterozygous
A-dominant Dominant covers
recessive
a- recessive
A – trait 1
B- trait 2
CoDominance Ay – Dom 1
Az – Dom 2
a-recessive
Sex Linked
XaXa
XaY
2
Males
either have
trait or not