Segregation, Assortment, and
Dominance Relationships
A.
B.
C.
D.
E.
Genes and alleles
Random segregation
Independent assortment
Assortment vs. Linkage
Dominance relationships
A. Genes and Alleles
Gene
 Classical definition:
• A unit of inheritance
• A factor transmitted during reproduction and responsible
for the appearance of a given trait
 Contemporary understanding:
• A segment on a DNA molecule
• Usually at a specific location (locus) on a chromosome
• Characterized by its nucleotide sequence
A. Genes and Alleles
Genes play three notable roles:
 To encode the amino acid sequences of proteins
 To encode the nucleotide sequences of tRNA or
rRNA
 To regulate the expression of other genes
A. Genes and Alleles
Alleles:
 Variant forms of a gene found within a population
 Alleles
of a gene usually have small differences in
their nucleotide sequences
 The differences can affect the trait for which the
gene is responsible
 Most genes have more than one allele
A. Genes and Alleles
Homozygous and heterozygous:
 In a diploid species, each individual carries two
copies of each gene (with some exceptions)
 The two copies are located on different members of
a homologous chromosome pair
 If the two copies of the gene are identical alleles,
then the individual is homozygous for the gene
 If the two copies are different alleles, then the
individual is heterozygous for the gene
A. Genes and Alleles
Genotype:
 The genetic makeup of an individual with reference
to one or more specific traits
 A genotype is designated by using symbols to
represent the alleles of the gene
A. Genes and Alleles
Example:
 Consider a gene for plant height in the pea plant with
two alleles, “D” and “d”
 Each individual pea plant will carry two copies of the
plant height gene, on a homologous chromosome
pair
 An individual pea plant will be one of three possible
genotypes:
• Homozygous “DD”
• Homozygous “dd”
• Heterozygous “Dd”
A. Genes and Alleles
Dominant and recessive:





A dominant allele is expressed over a recessive allele in a
heterozygous individual
This means that a heterozygous individual and a
homozygous dominant individual have identical phenotypes
Often, a dominant allele encodes a functional protein, such
as an enzyme
The recessive allele is a mutation that no longer has the
information for the correct amino acid sequence; Therefore,
its protein product in nonfunctional
In the heterozygote, the dominant allele encodes sufficient
production of the protein to produce the dominant phenotype.
This is also called complete dominance
A. Genes and Alleles
Phenotype:
 The appearance or discernible characteristics of a
trait in an individual
 Phenotypes can be determined by a combination of
genetic and environmental factors
A. Genes and Alleles
Example:
 In the pea plant height gene, the dominant allele “D”
encodes a hormone that promotes tall growth
 The recessive allele “d” is a mutation that does not
produce functional hormone
 If an individual pea plant has at least “one good
copy” of the “D” allele, then it makes enough
hormone to grow tall
 Otherwise, the plant is dwarf in size
A. Genes and Alleles
Example (continued):
 Therefore, there are two possible phenotypes for
plant height in peas:
• Genotype “DD” produces tall plants
• Genotype “Dd” produces tall plants
• Genotype “dd” produces dwarf plants
 Note that “D” is completely dominant over “d”
 There is no observable difference in phenotype
between “DD” (homozygous dominant) and “Dd”
(heterozygous) plants
B. Random Segregation
Mendel’s law of random segregation:
 Diploid germ-line cells of sexually reproducing
species contain two copies of almost every
chromosomal gene
 The two copies are located on members of a
homologous chromosome pair
 During meiosis, the two copies separate, so that a
gamete receives only one copy of each gene
B. Random Segregation
Random segregation can be demonstrated with a
monohybrid cross experiment
Monohybrid cross:
A parental cross between two individuals that differ in the
genotype of one gene
 The offspring of the parental generation is called the F1 (first
filial) generation
 The F1 generation can be allowed to interbreed or self-fertilize
(inter se cross, or “selfing”) to produce the F2 (second filial)
generation

B. Random Segregation
Example of a monohybrid cross:
P generation:
F1 generation:
F2 generation:
Homozygous tall pea plants (pollen)
X
Homozygous dwarf pea plants (ovules)
All tall pea plants
F1 tall X F1 tall
About ¾ of the F2 plants will be tall
About ¼ of the F2 plants will be dwarf
B. Random Segregation
Genotypic explanation of the monohybrid cross:
 Parental generation:
Pollen from a DD plant X ovules from a dd plant
Pollen genotype: D
Ovule genotype: d
 Therefore, in the F1 generation:
Genotype of all F1 plants: Dd
F1 pollen: ½ D and ½ d
F1 ovules: ½ D and ½ d
B. Random Segregation
Genotypic explanation (continued):
 When the F1 plants self-fertilize:
F1 pollen X F1 ovule  F2 genotype
F2 phenotype
½D
½D
½ x ½ = ¼ DD
½D
½d
(½ x ½ )
or
+
½d
½D
(½ x ½) = ½ Dd ¼ DD+ ½ Dd = ¾ Tall
½d
½d
½ x ½ = ¼ dd
= ¼ Dwarf
B. Random Segregation
Random segregation can also be demonstrated
with a testcross
Testcross:
 Cross heterozygous F1 individuals
with homozygous
recessive
Pollen from Dd X Ovules from dd  Testcross progeny
½D
All d
½ x 1 = ½ Dd Tall
½d
All d
½ x 1 = ½ dd Dwarf
C. Independent Assortment
Mendel’s law of independent assortment
 When the alleles
of two different genes separate
during meiosis
 They do so independently of one another
 Unless the genes are located on the same
chromosome (linked)
C. Independent Assortment
Independent assortment is demonstrated by a
dihybrid cross
Dihybrid cross:
 A parental cross between two individuals
in the genotype of two different genes
that differ
C. Independent Assortment
Example: Consider genes for vestigial wing
shape and ebony body color in Drosophila
melanogaster
 Vestigial
wing shape gene:
vg+ allele: normal “wild type” wing shape; dominant
vg allele: vestigial wing; recessive
 Ebony body color gene:
e+ allele: tan-colored “wild type” body; dominant
e allele: ebony body; recessive
C. Independent Assortment
As usual with complete dominance, there are
three possible genotypes for wing shape, and
three for body color:
vg+ vg+ = homozygous wild type wing
vg+ vg = heterozygous wild type wing
vg vg = vestigial wing
e+ e+ = homozygous wild type body color
e+ e = heterozygous wild type body color
ee
= ebony body color
C. Independent Assortment
P:
Homozygous wild type males X Vestigial ebony females
F1:
All wild type phenotypes, males & females
F1 X F 1
F2:
9/16 wild type phenotypes
3/16 wild type wings, ebony body
3/16 vestigial wings, wild type body
1/16 vestigial ebony
C. Independent Assortment
Genotypic explanation for the dihybrid cross
 P generation:
vg+ vg+ e+ e+ males X vg vg e e females
 F1 generation:
All heterozygous vg+ vg e+ e , males and females
F1 sperm
F1 ova
¼ vg+ e+
¼ vg+ e+
¼ vg+ e
¼ vg+ e
¼ vg e+
¼ vg e+
¼ vg e
¼ vg e
C. Independent Assortment
How many different ways can we make
wild type wing, wild type body color in the F2?
F1 sperm
¼ vg+ e+
¼ vg+ e
¼ vg e+
¼ vg e
Answer: 9 different ways
F1 ova
¼ vg+ e+
¼ vg+ e
¼ vg e+
¼ vg e
C. Independent Assortment
How many different ways can we make
wild type wing, ebony body color in the F2?
F1 sperm
¼ vg+ e+
¼ vg+ e
¼ vg e+
¼ vg e
Answer: 3 different ways
F1 ova
¼ vg+ e+
¼ vg+ e
¼ vg e+
¼ vg e
C. Independent Assortment
How many different ways can we make
vestigial wing, wild type body color in the F2?
F1 sperm
¼ vg+ e+
¼ vg+ e
¼ vg e+
¼ vg e
Answer: 3 different ways
F1 ova
¼ vg+ e+
¼ vg+ e
¼ vg e+
¼ vg e
C. Independent Assortment
How many different ways can we make
vestigial wing, ebony body color in the F2?
F1 sperm
¼ vg+ e+
¼ vg+ e
¼ vg e+
¼ vg e
Answer: 1 way
F1 ova
¼ vg+ e+
¼ vg+ e
¼ vg e+
¼ vg e
Summary of All Possible F2 genotypes
¼ x ¼ = 1/16 vg+ vg+ e+ e+
(¼ x ¼) + (¼ x ¼ ) = 2/16 vg+ vg e+ e+
(¼ x ¼) + (¼ x ¼ ) = 2/16 vg+ vg+ e+ e
(¼ x ¼) + (¼ x ¼ ) + (¼ x ¼) + (¼ x ¼ ) = 4/16 vg+ vg e+ e
¼ x ¼ = 1/16 vg+ vg+ e e
(¼ x ¼) + (¼ x ¼ ) = 2/16 vg+ vg e e
¼ x ¼ = 1/16 vg vg e+ e+
(¼ x ¼) + (¼ x ¼ ) = 2/16 vg vg e+ e
¼ x ¼ = 1/16 vg vg e e
9/16
Wild Wing,
Wild Body
3/16
Wild Wing,
Ebony
3/16
Vestigial,
Wild Body
1/16
Vestigial,
Ebony
C. Independent Assortment
Here is a “shortcut” for dihybrid cross ratios: combine
the monohybrid cross ratios!
F2 wing phenotypes:
F2 body phenotypes:
¾ wild type wings
¾ wild type body
¼ vestigial wings
¼ ebony body

¾ x ¾ = 9/16 wild wings, wild body
¾ x ¼ = 3/16 wild wings, ebony body
¼ x ¾ = 3/16 vestigial wings, wild body
¼ x ¼ = 1/16 vestigial wings, ebony body
C. Independent Assortment
The testcross can also be applied to
independent assortment:
vg+ vg e+ e X vg vg e e

¼ vg+ vg e+ e (wild wing, wild body)
¼ vg+ vg e e (wild wing, ebony body)
¼ vg vg e+ e (vestigial wing, wild body)
¼ vg vg e e (vestigial wing, ebony body)
D. Assortment vs. Linkage
Independent assortment works because the two genes
are located on separate homologous chromosomes
pairs
Their alleles assort independently during meiosis
D. Assortment vs. Linkage
D. Assortment vs. Linkage
If two genes are located on the same chromosome,
their alleles can recombine only when there is crossing
over during meiosis
The probability that crossover will occur is proportional
to the distance between the genes
Typically, there are fewer recombinant (crossover)
gametes than nonrecombinant gametes
D. Assortment vs. Linkage
E. Dominance Relationships
Codominance
 Two alleles are codominant if each encodes a
different but functional protein product
 In the heterozygote, the presence of two different
functional proteins means that the phenotype of the
heterozygote is different from either homozygous
dominant or homozygous recessive
 Example: M-N blood groups
E. Dominance Relationships
Example of codiminance: M-N blood group gene
in humans
 Two alleles, LM & LN
 Each produces a “functional” blood cell antigen
(capable of causing an immunological reaction)
 Three possible genotypes & phenotypes
• LM LM: Produces group “M” blood
• LM LN: Produces group “MN” blood
• LN LN: Produces group “N” blood
E. Dominance Relationships
Incomplete dominance
 An incompletely
dominant allele produces a
functional protein product
 However, in the heterozygote, there is insufficient
protein production from the allele to produce the
same phenotype as homozygous dominant
 Therefore, the phenotype of the heterozygote is
different from either homozygous dominant or
homozygous recessive
 Example: snapdragon flower color
E. Dominance Relationships
Example of incomplete dominance: snapdragon
flower color
 Two alleles, “R” and “r”
 “R” produces red pigment; “r” produces no pigment
 Three possible genotypes & phenotypes
• RR: Red flowers
• Rr: Pink flowers (One copy of “R” produces less red
pigment than two copies of “R”)
• rr: White flowers
E. Dominance Relationships
Because each genotype has a unique
phenotype, the F2 phenotypic ratio in
codominance or incomplete dominance is 1:2:1