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

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Mendels 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

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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)

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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.

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Mendel's 2nd Law the Law of Independent
Assortment

• Two types of crosses
• Dihybrid Self crosses
• Dihybrid Test crosses

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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

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Dihybrid Self crosses

• Parental Phenotypes Yellow Round X Green
  Wrinkled
• Parental genotypes YYRR X yyrr
• Gametes YR
  yr
• F1 YyRr ( All Yellow
  Round)
• Self cross(F2) F1 X F1
• Parental Phenotypes Yellow Round X Yellow
  Round Parental genotypes YyRr X
  YyRr

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Gametes
No. Parent1 Parent2
1 YR YR
2 Yr Yr
3 yR yR
4 yr yr
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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)

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By multiplying segregation ratios
No Gametes Segregation ratios Total ratios Phenotypes
1 YR ¾ X ¾ 9/16 Yellow Round
2 Yr ¾ X ¼ 3/16 Yellow Wrinkled
3 yR ¼ X ¾ 3/16 Green Round
4 yr ¼ X ¼ 1/16 Green Wrinkled
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By checker board (Punnet square)

• Parents F1 X F1
• Parental Phenotypes Yellow Round X Yellow
  Round
• Parental genotypes YyRr X YyRr
• Gametes YR , Yr, yR, yr X YR , Yr, yR,
  yr

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Dihybrid Crosses Test Cross

• F1 (Dihybrid Crosses) offspring is crossed with
  recessive parent
• Parental Phenotypes
• Yellow Round X Green Wrinkled
• Parental genotypes
• YyRr X yyrr
• Gametes YR , Yr, yR, yr all yr

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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
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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

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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

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Punnett Square (Checkered board)

• 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 1111
• Ratios of self cross 9331
• Hence proved non-homologous chromosome assort
  independently.

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Sex DeterminationSex linked inheritance

• Lecture 3
• Dr. Attya Bhatti

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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

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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.)

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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

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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)

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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

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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

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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

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The X and Y chromosomes in humans differ in size
and genetic content
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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

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Haplodiploidy

• Insects possess haplodiploid sex determination
• Males develop from unfertilized eggs and are
  haploid
• Females develop from fertilized eggs and are
  diploid

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In insects with haplodiploidy, males develop from
unfertilized eggs and are haploid females
develop from fertilized eggs and are diploid
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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

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Life cycle of Drosophila melanogaster, the common
fruit fly.
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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 XA ratio)
The chromosomes of Drosophila melanogaster
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Sex Determination in Humans
From Sex chromosomes and sex-linked in

• 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

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Chromosomal Determination of Sex in Drosophila
and Humans
SEX CHROMOSOMES SEX CHROMOSOMES SEX CHROMOSOMES SEX CHROMOSOMES
Species XX XY XXY XO
Drosophila ? ? ? ?
Human ? ? ? ?
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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
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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

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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

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The SRY gene is on the Y chromosome and causes
the development of male characteristics