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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 Second Law Mendel’s Second Law (Law of independent assortment)(Law of independent assortment)
Mendel's 2Mendel's 2ndnd Law – the Law of Independent Assortment 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 thatThis 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 Mendel's 2nd Law – the Law of Independent Assortment Assortment
Two types of crosses:
Dihybrid Self crosses Dihybrid Test crosses
Dihybrid Self crossesDihybrid Self crosses
Dihybrid crossDihybrid cross - a cross between two parents that differ by two pairs of
alleles (AABB x aabb)
Parental CrossParental Cross: Yellow, Round Seed x Green, Wrinkled
F1 GenerationF1 Generation: All yellow, round
F2 GenerationF2 Generation: 9 Yellow, Round, 3 Yellow, Wrinkled, 3 Green, Round, 1
Green, Wrinkled
Seed ColorSeed Color: Yellow = G; Green = g
Seed ShapeSeed Shape: Round = W; Wrinkled = w
Dihybrid Self crossesDihybrid Self crosses
Parental Phenotypes: Yellow Round XX Green Wrinkled
Parental genotypes: YYRR X X yyrr
Gametes: YR yr F1: YyRr ( All Yellow Round)
Self cross(F2): F1 XX F1
Parental Phenotypes: Yellow Round XX Yellow Round Parental genotypes: YyRr X X YyRr
Gametes:Gametes:
No.No.
Parent:1Parent:1 Parent:2Parent:2
11 YRYR YRYR
22 YrYr YrYr
33 yRyR yRyR
44 yryr yryr
F2 ratios of Independent assortment are calculated by two methods: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)
NNoo
GametesGametes Segregation Segregation ratiosratios
Total Total ratiosratios
PhenotypesPhenotypes
11 YRYR ¾ X ¾¾ X ¾ 9/169/16 Yellow RoundYellow Round
22 YrYr ¾ X ¼¾ X ¼ 3/163/16 Yellow WrinkledYellow Wrinkled
33 yRyR ¼ X ¾¼ X ¾ 3/163/16 Green RoundGreen Round
44 yryr ¼ X ¼¼ X ¼ 1/161/16 Green WrinkledGreen Wrinkled
By multiplying segregation ratios
By checker board (Punnet square)By checker board (Punnet square)
Parents: F1 XX F1
Parental Phenotypes: Yellow Round X X Yellow Round
Parental genotypes: YyRr X X YyRr
Gametes: YR , Yr, yR, yr X X YR , Yr, yR, yr
Dihybrid Crosses Dihybrid Crosses Test Cross:Test Cross:
F1 (Dihybrid Crosses) offspring is crossed with recessive parent:
Parental Phenotypes:
Yellow Round XX Green Wrinkled Parental genotypes:
YyRr XX yyrr
Gametes: YR , Yr, yR, yr all yr
This test cross ratio tell that non-homologous chromosomes This test cross ratio tell that non-homologous chromosomes assort independently.assort independently.
NoNo GametesGametes GenotypesGenotypes Phenotypic ratioPhenotypic ratio
11 YR YR XX yr yr YyR rYyR r Yellow Round: 1Yellow Round: 1
22 Yr Yr XX yr yr YyrrYyrr Yellow Wrinkled: 1Yellow Wrinkled: 1
33 yR yR XX yr yr yyRryyRr Green Round: 1Green Round: 1
44 yr yr XX yr yr yyrryyrr Green Wrinkled: 1Green Wrinkled: 1
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
BackcrossBackcross
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
Punnet SquarePunnet Square
Punnett Square (Checkered board)Punnett Square (Checkered board)
Why it is used?1. Help to predict the results of experimental crosses. 2. 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 DeterminationSex DeterminationSex linked inheritanceSex linked inheritance
Lecture 3Dr. Attya Bhatti
Sex DeterminationSex 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 DeterminationSex 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 Chromosomal Sex-Determining SystemsSystems
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 determinationXX-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 determinationXX-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 DeterminationXX-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 DeterminationXX-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 DeterminationZZ-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
HaplodiploidyHaplodiploidy
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 Sex Determination in DrosophilaDrosophila
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 chromosomes of Drosophila melanogaster
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)
Sex Determination in HumansSex 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
From: Sex chromosomes and sex-linked in
Chromosomal Determination of Sex in Chromosomal Determination of Sex in Drosophila and HumansDrosophila and Humans
SEX CHROMOSOMES
Species XX XY XXY XO
Drosophila
♀ ♂ ♀ ♂
Human ♀ ♂ ♂ ♀
Persons with Klinefelter syndrome have a Y chromosome and two or more X Chromosomes in their cells
Persons with Turner syndrome have a single X chromosome in their cells
The Role of Sex Chromosomes in The Role of Sex Chromosomes in HumansHumans
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 The Male-Determining Gene in HumansHumans
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