GENE SEGREGATION AND
INTERACTION
Phenotype

Referred to the appearance of an organism:
 its
morphology, (color, height, shape, etc.)
 Physiology,
 Behavior


This was pointed out by Wilhelm Johannsen in
(1857-1927)
Phenotype may change as results from complex
networks of interactions between different genes
and between the genes of the environment
Genotype



Is the genetic constitution that an individual inherits
Or set of DNA variants or alelles found at one
particular or more loci in an individual
It remains constant
 E.g.
BB, Bb, bb
Laws of Heredity
Gregor Johann Mendel = 1865 between
crosses between peas (Pisum satixum) &
discovered the laws of heredity transmission
Test Cross and Punnett Square
Test cross
 First
introduced by Gregor Mendel
 To determine the genotype of an individual which has a
dominant phenotype.
Test Cross and Punnett Square
Punnett Square
 Named
after Reginald Punnett
 A chart used by geneticists to show all possible allelic
combinations of gametes in a cross of parents with
known genotypes.
 A diagram that is used to predict an outcome of a
particular cross or breeding experiment.
 Tabular that shows the summary of every possible
combination of one maternal allele with one paternal
allele for each gene being studied in the cross
Test Cross and Punnett Square
Example of Punnett Square
Laws of Heredity
Gregor Johann Mendel (1865)
 Studied cross between sweet peas (Pisum sativum)
and discovered the laws of heredity
 He selected several varieties of sweet peas that
have pairs of differential or contrasting
characteristics
 E.g.
plants with white & red flowers
 smooth & rough seeds
 Yellow and green seeds
 Long and short stems
Laws of Heredity

After crossing the parental generation = P1, he
observes resulting hybrids of the first filial
generation = F1 and studied the result in the second
filial generation = F2
Useful genetic vocabularies





An organism with two identical alleles for a character is
said to be homozygous for the gene controlling that
character (e.g. BB, bb)
An organism that has two different alleles for a gene is
said to be heterozygous for the gene controlling that
character (e.g. Bb)
Unlike homozygotes, heterozygotes are not truebreeding
Dominant – a character that is shown and represented
by a capital letter (e.g. B)
Recessive – a character that is hidden, represented by
a small letter (e.g. b)
Useful genetic vocabularies



Because of the different effects of dominant and
recessive alleles, an organism’s traits do not always
reveal its genetic composition
Therefore, we distinguish between an organism’s
phenotype, or physical appearance, and its
genotype, or genetic makeup
In the example of flower color in pea plants, PP and
Pp plants have the same phenotype (purple) but
different genotypes
Useful genetic vocabularies



P = parental generation
F1 = first filial generation
F2 = second filial generation
Law of Segregation
The gene could be segregated in the hybrid into
different gametes to be distributed in the
offspring of the hybrid.
For this reason, this is called the Law or Principle of
Segregation of the genes.
Law of Segregation



When Mendel crossed contrasting, true-breeding
white- and purple-flowered pea plants, all of the F1
hybrids were purple
When Mendel crossed the F1 hybrids, many of the
F2 plants had purple flowers, but some had white
Mendel discovered a ratio of about three to one,
purple to white flowers, in the F2 generation
Figure 14.3-3
EXPERIMENT
P Generation
(true-breeding
parents)
Purple
flowers
White
flowers
F1 Generation
(hybrids)
All plants had purple flowers
Self- or cross-pollination
F2 Generation
705 purpleflowered
plants
224 white
flowered
plants
Law of Segregation





Mendel reasoned that only the purple flower factor
was affecting flower color in the F1 hybrids
Mendel called the purple flower color a dominant trait
and the white flower color a recessive trait
The factor for white flowers was not diluted or
destroyed because it reappeared in the F2 generation
Mendel observed the same pattern of inheritance in six
other pea plant characters, each represented by two
traits
What Mendel called a “heritable factor” is what we
now call a gene
Law of Segregation (Monohybrid Cross)

Example 1
Parent Generation = RR (round seeds) X rr (wrinkled seeds)
Gametes =
First Filial Gen./
F1
R
Female Gamete
x
r
Male Gamete
Rr (round seeds)
Law of Segregation
Punnett Square
Gametes
Second filial
generation
F2
Female Gamete
Male
Gamete
R
r
R
RR (round)
Rr (round)
r
Rr (round)
Rr (wrinkled)
Genotypic ratio: 1RR: 2 Rr: 1Rr
Phenotypic ratio: 3 round (R_) : 1 wrinkled (rr)
Law of Segregation
• Mendel derived the law of segregation by
following a single character
• The F1 offspring produced in this cross were
monohybrids, individuals that are
heterozygous for one character
• A cross between such heterozygotes is called a
monohybrid cross
Law of Segregation

1.
2.
The ff. pattern was established from summarizing
the results of Mendel’s hybridization work:
For any character, the F1 showed only one of the alternative traits such
character that was shown was Dominant (represented by big letter) and
Recessive – the character that was hidden (small letter)
The trait that did not appear in the F1 reappeared in the F2 but in a
frequency of ¼ of the total number
• From this observation it was deduced that each parent must have
contributed equally to the progeny (offspring)
Law of Segregation



This was supported by the fact that in the F2 a 3:1 segregation of
the dominant; recessive trait was observed. This means that the F1
contains the two alternative factors or is Heterozygous (Rr).
These two factors or alleles separate or segregate from each other
during gamete formation in the F1, so that some gametes carry R &
other r.
These two types of gametes occur at equal frequencies in the ova
and in the pollen grains. The random combination of these gametes
to form zygotes will then account for 3:1 ratio.
Law of Segregation




Based on the result, Mendel proposed his law of segregation
It states that unit hereditary characters (e.g. round vs. wrinkled)
occur in pairs, and that in the formation of gametes these
segregate so that only one member of the pair goes into a
particular gamete.
It is a matter of chance whether a particular gamete gets the
dominant or the recessive allele.
When the male and female gametes fuse during fertilization to
form zygote, the diploid member is restored
Sample Problem

Consider blue eyes in man as recessive to brown
eyes. Show the expected children.
Sample Problem

In chickens, black eye color is dominant over blue
eye color. If a purebred black-eyed rooster mated
with a purebred blue-eyed hen, what is most likely
for the next generation’s eye color characteristics?
Show your P1, F1, and F2.
solution

P1 =
Law of Independent Assortment
-
-
Describes the simultaneous behavior of two or
more pairs of genes located in the different
pairs of chromosomes
States that genes for different characters are
inherited independently of one another or that the
members of one pair of alleles segregate
independently of the other pairs
Law of Independent Assortment

Mendel based his second law on the results of his dihybrid
crosses, as follows:
P1 & P2:
Gametes
Genotype (phenotype)
Genotype (phenotype)
RRYY (Round, Yellow)
rryy (wrinkled, green)
RY
Female gametes
Fertilization
ry
Male gametes
RrYy
Round, Yellow
F1
Selfing/Inbreeding: RrYy
x
Crossing of same genotype
RrYy
Law of Independent Assortment
Punnett Square
Female
gametes
Male gametes
RY
Ry
rY
ry
RY
RRYY
Round yellow
RRYy
Round yellow
RrYY
Round yellow
RrYy
Round yellow
Ry
RRYy
Round yellow
RRyy
Round green
RrYy
Round yellow
Rryy
Round green
rY
RrYY
Round yellow
RrYy
Round yellow
rrYY
Wrinkled yellow
rrYy
Wrinkled yellow
ry
RrYy
Round yellow
Rryy
Round green
rrYy
Wrinkled yellow
rryy
Wrinkled green
Law of Independent Assortment
F2: Genotypic ratio:
Phenotypic ratio:
1:2:1:2:4:2:1:2:1
1 RRyy)
2 RRyy)
1 RRyy)
2 RrYY)
4 RrYy)
2 Rryy)
1 rrYY)
2 rrYy)
1 rryy)
9:3:3:1
(9
(3
(3
(1
R_Y_ round yellow
R_yy round green
r_Y_ wrinkled yellow
rryy wrinkled green
Law of Independent Assortment

Analysis of the data showed that each gene pair R vs.
r and Y vs. y segregated independently of each
other. That is, the chances for a plant to be round or
wrinkled do not interfere with or are independent of,
its chances to become yellow or green.
Gamete formation
Number of gene pairs
Kinds of gametes
Kinds of genotype
Kinds of phenotype
=n
= 2𝑛
= 3𝑛
= 2𝑛
One way of getting the gametes is using dichotomous method.
23 = 2 x 2 x 2 = 8
8 gametes
• Mendel identified his second law of inheritance
by following two characters at the same time
• Crossing two true-breeding parents differing in
two characters produces dihybrids in the F1
generation, heterozygous for both characters
• A dihybrid cross, a cross between F1
dihybrids, can determine whether two
characters are transmitted to offspring as a
package or independently
Law of independent assortment: Getting the genotypic ratio and
phenotypic ratio of the F2 using Dichotomous branching method
P1 & P2:
RRYY (Round, Yellow seed) X
rryy (wrinkled, green seed)
Steps
1.
Get the genotypic ratio of the monohybrid cross of both parental 1 (P1) and
parental 2 (P2).
Example:
P1=
Gametes =
First Filial Gen./
F1
RR (round seeds) X rr (wrinkled seeds)
R
Female Gamete
x
r
Male Gamete
Rr (round seeds)
Law of independent assortment: Getting the genotypic ratio of
P1 using Dichotomous branching method
Selfing/inbreeding = F1 x F1
= Rr x Rr
gametes
R
r
R
RR (round)
Rr (round)
r
Rr (round)
Rr (wrinkled)
Genotypic ratio of P1 = 1 RR : 2 Rr : 1 rr
Phenotypic ratio of P1 = 3R_ (round) : 1 rr (wrinkled)
Law of independent assortment: Getting the genotypic of P2
using Dichotomous branching method
P1 & P2:
RRYY (Round, Yellow seed) X
rryy (wrinkled, green seed)
Steps
1.
Get the genotypic ratio of the monohybrid cross of both parental 1 (P1) and
parental 2 (P2).
Example:
P2 =
Gametes =
First Filial Gen./
F1
YY (yellow seeds) X yy (green seeds)
Y
Female Gamete
x
y
Male Gamete
Yy (yellow seeds)
Law of independent assortment: Getting the genotypic ratio
and phenotypic ratio of the F2 using Dichotomous branching
method
Selfing/inbreeding = F1 x F1
= Yy x Yy
gametes
Y
y
Y
YY (yellow seed)
Yy (yellow seed)
y
Yy (yellow seed)
yy (green seed)
Genotypic ratio of P2 = 1 YY : 2 Yy : 1 yy
Phenotypic ratio of P2 = 3 Y_ (yellow seed) : 1 yy (green seed)
Law of independent assortment: Getting the genotypic ratio and
phenotypic ratio of the F2 using Dichotomous branching method
The genotypic ratio of the P1 and P2
The genotypic ratio of the F2
for dihybrid cross in the
Punnett square is the same
using Dichotomous Branching
Method
This means that 1/16 RRYY,
2/16 RRYy and so on….
Law of independent assortment: Getting the genotypic ratio and
phenotypic ratio of the F2 using Dichotomous branching method
The phenotypic ratio of the P1 and P2
The phenotypic ratio of the
F2 for dihybrid cross in the
Punnett square is the same
using Dichotomous Branching
Method
This means that 9/16 R_Y_ (round,
yellow), 3/16 R_yy (round, green),
3/16 rrY_ (wrinkled, yellow) and
1/16 rryy (round, wrinkled)

That is, the chances for a plant
to be round or wrinkled do not
interfere with or are
independent of, its chances to
become yellow or green.
Dominance Relationships
A.
Incomplete Dominance or no dominance
-in this case dominance is absent and the progeny does not
resemble any of its parents.
-The F1s are intermediate between the two parents
- e.g. flower color in Mirabilis jalapa (4 o’clock plant)
-P1
and P2:
-F1:
-F2
:
RR(red)
x
rr(white)
Rr (Pink)
1RR Red: 2Rr (pink): 1rr(white)
• Heterozygotes are phenotypically intermediate between
the two homozygote types (1:2:1)
F2
A. Incomplete Dominance/no
Dominance

As more experiments were conducted – some
phenotypes and ratios could not be explained on
the basis of complete dominance. These exceptions
did not in any way disprove Mendel’s principles;
rather, they extended and developed them.
B. Complete Dominance

Heterozygotes are phenotypically identical to the
homozygous dominant

Phenotypic ratio
3R_:1rr

E.g. seed coat color in sweet peas
Overdominance


The heterozygote exceeds the phenotypic
measurements of the homozygous parents.
1RR: 2Rr : 1rr
Co-dominance

Co-dominance
 When
each allele of a gene is associated with specific
substance, co-dominance will occur when both
substances appear together in the heterozygote
 E.g.
M-N blood types in man. Landsteiner and Levine
(1927) were able to classify people into three general
types based on the agglutination characteristics of the
red blood cells.
Co-dominance
Blood type
genotype
Agglutinogen
Agglutinin
M
MM
M
Anti-N
N
NN
N
Anti-M
MN
MN
M:N
none
Analysis showed that the gene for M and N were alleles to
each other and that they were co-dominant.
E. Multiple Alleles

It is generally assumed that a gene pair has only
two alleles, this condition arises from the presence
of homologous pairs of chromosomes in the diploid
organism and each one contains one allele of the
gene pair.
Lethal Genes
A.
B.
Dominant lethal
Recessive lethal
Dominant lethal


Death of the affected individual (homozygote
dominant or heterozygous) occurs after reproduction
has taken place
0:1
Recessive lethal



Effects of recessive genes are sufficiently drastic to
kill the bearers of certain genotypes
2:1 – sickle cell anemia
3:0 – albinism in plants
Non-allelic interaction
-
Two genes controlling one trait
A. epistasis – an allele of a gene marks the effect
of the allele of the other gene.
1. Dominant epistasis
A. Complete dominance at both gene pairs but one
gene when dominant is epistatic to the other

(A dominant to a: B dominant to b: A epistatic to B or b)
B. Complete dominance at both gene pairs but the first
gene when dominant is epistatic to the second and the
second gene when homozygous recessive is epistatic to
the first.

A dominant to a; B dominant to b; A epistatic to B and b; bb
epistatic to A and aa
2. Recessive Epistasis

Complete dominance at both gene pairs, but one
gene, when homozygous recessive is epistatic or
marks the effect of the other gene.
A
dominant to a; B dominant to b; aa epistatic to B & b
3. Duplicate genes

Complete dominance at both gene pairs, but either
gene, when dominant is epistatic to the other gene
A
dominant a; B dominant to b; A epistatic to b; B
epistatic to a
4. Complementary genes

Complete dominance at both gene pairs, but either
gene when homozygous recessive is epistatic to the
effects of the other gene
A
dominant to a; aa epistatic to B; B dominant to b; bb
epistatic to A
D. Novel phenotypes

Complete dominance at both gene pairs; new
phenotypes are produced from interaction between
dominants, and between both homozygous
recessives.
A
dominant to a; B dominant to b; a interacts with B
producing new phenotype; aabb also produces a new
phenotypes.