Mendel’s Second Law
(Law of independent assortment)
It states that gens coding for different characteristics
separate independently of one another when gametes are
formed owing to independent separation of homologous
pairs of chromosomes during meiosis
This principle states that alleles at different loci
separate independently of one other
Mendel's 2nd Law – the Law of Independent Assortment
This law stats that:
When two pairs of contrasting traits are brought in the
same cross, they together in offspring of F1 generation but
assort independently at meiosis (in F2 generation)
This means that
Homologous chromosomes and alleles segregate at meiosis
it one to one ratio.
Non-homologous chromosomes along with their genes
separate and recombine again in new combinations at
meiosis independently.
Mendel's 2nd Law – the Law of Independent Assortment
Two types of crosses:
Dihybrid Self crosses
Dihybrid Test crosses
Dihybrid Self crosses
Dihybrid cross - a cross between two parents that differ by two pairs of alleles
(AABB x aabb)
Parental Cross: Yellow, Round Seed x Green, Wrinkled
F1 Generation: All yellow, round
F2 Generation: 9 Yellow, Round, 3 Yellow, Wrinkled, 3 Green, Round, 1 Green,
Wrinkled
Seed Color: Yellow = G; Green = g
Seed Shape: Round = W; Wrinkled = w
Dihybrid Self crosses
Parental Phenotypes: Yellow Round X Green Wrinkled
Parental genotypes:
Gametes:
F1:
Self cross(F2):
Parental Phenotypes: Yellow Round
Parental genotypes:
YyRr
YYRR
X
YR
yyrr
yr
YyRr ( All Yellow Round)
F1 X
F1
X
X
Yellow Round
YyRr
Gametes:
Parent:1
Parent:2
1
YR
YR
2
Yr
Yr
3
yR
yR
4
yr
yr
No.
F2 ratios of Independent assortment are calculated by two methods:
By multiplying segregation ratios (9: 3: 3: 1)
By checker board (Punnet square) (9: 3: 3: 1)
By multiplying segregation ratios
N Gametes
o
Segregation ratios Total
ratios
1
YR
¾ X
2
Yr
3
4
Phenotypes
¾
9/16
Yellow Round
¾
X ¼
3/16
Yellow Wrinkled
yR
¼
X ¾
3/16
Green Round
yr
¼
X ¼
1/16
Green Wrinkled
By checker board (Punnet square)
Parents:
F1
X
Parental Phenotypes: Yellow Round
Parental genotypes:
Gametes:
YyRr
YR , Yr, yR, yr
X
F1
X Yellow Round
X
YyRr
YR , Yr, yR, yr
Dihybrid Crosses
Test Cross:
F1 (Dihybrid Crosses) offspring is crossed with recessive
parent:
Parental Phenotypes:
Yellow Round X Green Wrinkled
Parental genotypes:
YyRr
Gametes:
X
yyrr
YR , Yr, yR, yr
all yr
This test cross ratio tell that non-homologous chromosomes assort
independently.
No
Gametes
Genotypes
Phenotypic ratio
1
YR X yr
YyR r
Yellow Round: 1
2
Yr X yr
Yyrr
Yellow Wrinkled: 1
3
yR X yr
yyRr
Green Round: 1
4
yr X yr
yyrr
Green Wrinkled: 1
Backcross
Mendel crossed two varieties of peas that differed in
height, He established that tall (T) was dominant over
short (t)
He tested his theory concerning the inheritance of
dominant traits by crossing an F1 tall plant that was
heterozygous (Tt) with the short homozygous parental
variety (tt)
This type of cross between an F1 genotype and either of
the parental genotype is called backcross
Punnet Square
It is constructed by drawing a grid putting the gametes
produced by one parent along the upper edge and the
gametes produced by the other parent down the left side
Punnett Square (Checkered board)
1.
2.
Why it is used?
Help to predict the results of experimental crosses.
To determine the kind of gametes each parent
produces.
For this purpose,
One of the two axes of a square is designated for each
parent, and the different kinds of gametes, each parent
produces are listed along the appropriate axis.
Combining the gametes in the interior of the square
shows the results of random fertilization.
Ratios for test cross: 1:1:1:1
Ratios of self cross : 9:3:3:1
Hence proved non-homologous chromosome assort independently.
Sex Determination
Sex linked inheritance
Lecture 3
Dr. Attya Bhatti
Sex Determination
Sex refers to sexual phenotype
Two sexual phenotypes: male and female
Difference between males and females is
gamete size:
◦ males produce small gametes;
◦ females produce relatively
large gametes
Mechanism by which sex is
established is termed
sex determination
Sex Determination
Cells of female humans have two X chromosomes
Cells of males have one X chromosome and one Y
chromosome
Ways in which sex differences arise:
◦ Hermaphroditism ( that has only bisexual reproductive units)
◦ Monoecious (an individual that has both male and female
reproductive units)
◦ Dioecious (refers to a plant population having separate male and
female plants.)
Chromosomal Sex-Determining Systems
Sex chromosomes: differ between males and females
Autosomes: nonsex chromosomes which are the same for
males and females
XX-XO sex determination
XX-XY sex determination
ZZ-ZW sex determination
XX-XO sex determination
Sex determination in the grasshoppers studied by
McClung
In this system
◦ Females have two X chromosomes (XX)
◦ Males possess a single X chromosome (XO)
◦ No O chromosome (O signifies the absence of a sex
chromosome)
XX-XO sex determination
In females: the two X chromosomes pair and then
separate with one X chromosome entering each haploid
egg
In males: the single X chromosome segregates in meiosis
to half the sperm cells, the other half receive no sex
chromosome
XX-XY Sex Determination
Cells of males and females have the same number of
chromosomes
Cells of females have two X chromosomes (XX)
Cells of males have a single X chromosome and a smaller
sex chromosome called the Y chromosome (XY)
Male is the heterogametic sex
Female is the homogametic sex
XX-XY Sex Determination
X and Y chromosomes are not generally homologous do
pair and segregate into different cells in meiosis
Pseudoautosomal Regions
◦ In humans there are pseudoautosomal regions at both
tips of the X and Y chromosomes
The X and Y chromosomes in humans differ in size and genetic
content
ZZ-ZW Sex Determination
Female is heterogametic
Male is homogametic
Sex chromosomes are labeled Z and W
Females in this system are ZW
Males are ZZ
ZZ-ZW system is found in:
◦ Birds,moths, some amphibians, and some fishes
Haplodiploidy
Insects possess haplodiploid sex determination
Males develop from unfertilized eggs and are haploid
Females develop from fertilized eggs and are diploid
In insects with haplodiploidy, males develop from unfertilized eggs and are haploid;
females develop from fertilized eggs and are diploid
Sex Determination in
Drosophila
Fruit fly Drosophila melanogaster has eight chromosomes
◦ Three pairs of autosomes
◦ One pair of sex chromosomes
Females have two X chromosomes
Males have an X chromosome and a Y chromosome
Life cycle of Drosophila melanogaster,
the common fruit fly.
The sexual phenotype of a fruit
fly is determined by the ratio of
the number of X chromosomes to
the number of haploid sets of
autosomal chromosomes (the X:A
ratio)
The chromosomes of Drosophila
melanogaster
Sex Determination in Humans
XX-XY sex determination
Presence of a gene on the Y chromosome
determines maleness
Which arise when the sex chromosomes do not segregate
properly in meiosis or mitosis?
◦ Turner syndrome
◦ Klinefelter syndrome
◦ Poly-X females
Chromosomal Determination of Sex in Drosophila
and Humans
SEX CHROMOSOMES
Species
XX
XY
XXY
XO
Drosophila
♀
♂
♀
♂
Human
♀
♂
♂
♀
Persons with Turner
syndrome have a single X
chromosome in their cells
Persons with Klinefelter syndrome
have a Y chromosome and two or
more X Chromosomes in their cells
The Role of Sex Chromosomes in
Humans
X chromosome contains genetic information
Male-determining gene is located on the Y
chromosome
Absence of the Y chromosome results in a female
phenotype
Genes affecting fertility are located on the X and Y
chromosomes
Additional copies of the X chromosome may upset normal
development
The Male-Determining Gene in Humans
Sex-determining region Y (SRY) gene
Found in XX males
Missing from all XY females
SRY gene on the Y chromosome causes a human embryo to
develop as a male
Absence of this gene a human embryo develops as a
female
The SRY gene is on the Y chromosome and causes the
development of male characteristics
