It states that gens coding for different characteristics separate independently of one another when gametes are formed owing to independent separation.

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