Transcript Sex linked inheritance, sex linkage in Drosophila and man, XO, XY

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