What is heredity?



Download 41,71 Kb.
Date conversion10.04.2017
Size41,71 Kb.

UNIT 3-GENETICS

What is heredity?

  • What is heredity?
  • The passing of characteristics from parents to offspring. The fittest survive to reproduce and pass on traits that improve survivability
  • What are Characteristics?
  • Are traits that are inherited from the parents (i.e. flower color, plant height, eye color, hair color )

Gregor Mendel a Monk in Austria

  • Who is the founder of Genetics ?

Mendel

  • Modern genetics had its beginnings in an abbey garden in the 1850’s, where a monk named Gregor Mendel documented a mechanism of inheritance.
  • He discovered the basic principles of heredity by breeding garden peas in carefully planned experiments.
  • His approach to science had been influenced at the University of Vienna by one of his professors: the physicist Doppler.

He was the first person to succeed in predicting how traits are transferred from one generation to the next.

  • He was the first person to succeed in predicting how traits are transferred from one generation to the next.

Why did Mendel Succeed?

  • In order to study inheritance, Mendel chose to use peas.
  • His use of plants also allowed strict control over the crosses/mating.
  • He chose to study only characteristics that varied in an ‘either-or’ rather than a ‘more-or-less’ manner. (i.e.white or purple, tall or short)

He chose his subject carefully. Mendel chose the garden pea for his experiments for several reasons:

  • He chose his subject carefully. Mendel chose the garden pea for his experiments for several reasons:
  • -Garden peas reproduce sexually-they
  • produce male and female sex cells
  • called GAMETES
  • -Male gametes form in the pollen
  • produced in male reproductive organ
  • -The female gametes form in the
  • female reproductive organ

He chose his subject carefully. Mendel chose the garden pea for his experiments for several reasons:

  • He chose his subject carefully. Mendel chose the garden pea for his experiments for several reasons:
  • Garden peas reproduce sexually-they produce male and female sex cells called GAMETES
  •  Male gametes form in the pollen which is produced in male reproductive organ
  • The female gametes form in the female reproductive organ

Perfect Flowers have both male and female parts. Imperfect flowers have one or the other.

  • Male
  • Parts
  • Female
  • Parts

The male and female gametes unite in a process called fertilization

  • The male and female gametes unite in a process called fertilization
  • The resulting fertilized cell is called a zygote and develops into a seed.
  • The transfer of pollen grains from a male reproductive organ to a female reproductive organ is called Pollination.

What was his procedure?

  • To hybridise 2 varieties of pea plants, Mendel used an artist’s brush.
  • He transferred pollen from a true breeding white flower to the carpel of a true breeding purple flower.

When he wanted to breed, or cross, one plant with another, Mendel opened the petals of a flower and removed the male organs called Stamens.

  • When he wanted to breed, or cross, one plant with another, Mendel opened the petals of a flower and removed the male organs called Stamens.
  • He then dusted the female organs
  • with pollen from the plant he
  • wished to cross it with.

The process Mendel used is called cross-pollination.

  • The process Mendel used is called cross-pollination.
  • By using this technique, Mendel could be sure of the parents in his cross.
  • Each seed was a single offspring
  • He studied only one trait at a time to control variables, and he analyzed his data mathematically. (Remember the scientific method?)
  • He used plants that breed true. This means if the plants are allowed to self-pollinate, they show characteristics that never vary from the parent. (white flower parents-white flower offspring)
  • The tall pea plants he worked with were from populations of plants that had been tall for many generations and had always produced tall offspring.

The first parent generation is called the P1

  • The first parent generation is called the P1
  • The offspring of the first mating is called the F1 generation (first filial which means son in Latin)
  • Mendel let the F1 Generation to self-pollinate to produce the F2 generation
  • Mendel’s Experiments

Tracking heritable characteristics

  • Mendel tracked heritable characters for 3 generations.
  • When F1 hybrids were allowed to self-pollinate a 3:1 ratio of the 2 varieties occurred in the F2 generation.

WHAT TOOL CAN WE USE TO MAKE PREDICTIONS?

  • PUNNETT SQUARES
  • Do you remember how to set one up?
  • Two columns and Two rows

P-Purple Dominant p- White Recessive F1 Generation

  • PP
  • Pp
  • Pp
  • pp
  • P p
  • p P

Mendel’s Results

  • Mendel’s Results
  • In the P1, he crossed true breeding plants that had purple flower with other plants that had white flowers.
  • All of the F1 plants had purple flowers. There were not any plants with white flowers
  • He let the F1 plants self-pollinate to produce the F2 Generation. The F1 had 705 purple flowers and 224 white flowers (ratio 3:1)
  • Mendel repeated his experiment many times with the same results

Punnett Squares

  • Capital letters are used for the dominant trait
  • Lower case letters are used for a recessive trait.
  • The presence of a Dominant allele cancels out the expression of Recessive allele
  • For a recessive allele to show itself offspring must get one recessive allele from each parent

Two alleles together determine how a trait will be expressed

  • Two alleles together determine how a trait will be expressed
  • One allele comes from each parent
  • Homozygous for a trait- Both alleles are the same for a trait (BB or bb)
  • Heterozygous for a trait- Both alleles are different for a trait (Bb or Ww)

Mendel derived three main laws of Genetics

  • Mendel derived three main laws of Genetics
  • It is amazing that he came up with the right conclusions in the 1850s even though he knew nothing about chromosomes, genes or DNA
  • Chromosome
  • Genes
  • DNA

DNA was not discovered until 100 years later by Watson and Crick

  • DNA was not discovered until 100 years later by Watson and Crick
  • Each trait or characteristic exists in two versions. These two versions of a gene are called alleles (one allele for purple flowers and one for white flowers) 

1. Law of Dominance- one allele shows itself

  • For each trait or characteristic, an individual inherits two alleles…..one from each parent.
  • True breeding plants had two identical alleles
  • This is called homozygous or pure (PP or pp)
  • PP could also be called homozygous dominant
  • What would pp be called
  • homozygous recessive

Plants in the F1 generation had two different alleles for flower color (one purple and one white). Organisms with two different alleles are said to be hybrid or heterozygous (Pp)

  • Plants in the F1 generation had two different alleles for flower color (one purple and one white). Organisms with two different alleles are said to be hybrid or heterozygous (Pp)
  • Genotype-The two letters representing the two alleles (Pp, PP, Ww, Tt)
  • -The genotype for purple flowers would be
        • -PP or Pp
        • -The genotype for white flowers would be
        • It is recessive so the ONLY possibility is pp
        • -The genotype for brown hair could be
        • BB or Bb
        • -The genotype for blond hair would be
        • Only possibility is bb

Phenotype-The actual physical expression of the two alleles. (What we see with our eyes)

  • Phenotype-The actual physical expression of the two alleles. (What we see with our eyes)
  • -The phenotype of Pp would be
  • purple
  • -The phenotype of PP would be
  • purple
  • -The phenotype of pp would be
  • white
  • -The phenotype of BB for hair color would be
  • brown hair
  • -The phenotype of bb would be
  • blond hair

The allele that shows itself is called the dominant allele. In the F1 generation all flowers were purple (purple was dominant (Big P) and white was recessive (Little p) )

  • The allele that shows itself is called the dominant allele. In the F1 generation all flowers were purple (purple was dominant (Big P) and white was recessive (Little p) )
  • The allele that doesn’t show itself when the dominant allele is present is called the recessive allele
  •  

2. Law of Segregation- one allele comes from each parent

  • The two alleles for each trait separate when the organism makes gametes (sex cells-sperm, eggs or pollen).
  • When an organism makes gametes the chromosome number is reduced by half (results in one allele from each parent)
  • How many chromosomes do humans have?
        • 46 Chromosomes

By the law of segregation, the two alleles for a characters are packaged into separate gametes

  • If the blending model were correct, the F1 hybrids from a cross between purple-flowered and white-flowered pea plants would have pale purple flowers.
  • Instead, the F1 hybrids all have purple flowers, just a purple as the purple-flowered parents.
  • Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
  • Fig. 14.2

Law of Segregation - the two alleles for each character segregate during gamete production

If humans have 46 chromosomes, how many chromosomes are in the sex cells (eggs and sperm)?

  • If humans have 46 chromosomes, how many chromosomes are in the sex cells (eggs and sperm)?
  • The egg and sperm
  • combine to make 46
  • chromosomes. So the
  • egg has 23 and
  • the sperm has 23.
  • What alleles could a heterozygous parent (Pp) contribute ?
  • Either a “Big P” or a “Little p”

What alleles could a homozygous

  • What alleles could a homozygous
  • parent (PP or pp) contribute ?
  • PP only “Big P”
  • pp only “little p”

3. Law of Independent Assortment

  • More than one characteristic or trait is inherited simultaneously with each fertilization.(flower color, tall or short, eye color, curly vs straight hair etc.)
  • The inheritance of one trait has nothing to do with the inheritance of another trait.
  • Human eye color from your parent is a totally separate event regarding whether you get the allele for straight or curly hair.

Mendel’s terminology

  • True breeding: When the plants self-pollinate, all their offspring are of the same variety.
  • Hybridisation: Mating, or crossing, of two varieties.
  • Monohybrid cross: A cross that tracks the inheritance of a single character.
  • P generation: True breeding parents.
  • F1 generation: (first filial) Hybrid offspring of the P generation.
  • F2 generation: (second filial) Offspring from the self-fertilization of the F1 hybrids.

Mendel’s impact

  • Mendel’s theories of inheritance, first discovered in garden peas, are equally valid for figs, flies, fish, birds and human beings.
  • Mendel’s impact endures, not only on genetics, but on all of science, as a case study of the power of hypothesis/deductive thinking.

Scientists found that Mendel’s principles applied not only to peas but other organisms as well.

  • Scientists found that Mendel’s principles applied not only to peas but other organisms as well.
  • Much of the genetic research
  • since Mendel’s work has been
  • done with the common fruit fly (Dropsophilia melanogaster).
  • Why is the fruit fly a good organism to study genetics?

Why is the fruit fly a good organism to study genetics?

  • Why is the fruit fly a good organism to study genetics?
  • -new generation can be reproduced every 14 days
  • -two flies can produce 100 offspring
  • every two weeks

Why aren’t humans good organisms to study genetics?

  • Why aren’t humans good organisms to study genetics?
  • -new generation can be reproduced about every 20 years.
  • -gestation is long (9 months)

Genetics Vocabulary

  • Genetics Vocabulary
  • Allele- a version of a particular gene (one trait or characteristic)
  • Phenotype- Physical expression of an inherited trait in an individual (What you can see or easily test)
  • Genotype-the combination of alleles in an individual ( TT, Tt). Not easy to determine except in the case of individuals exhibiting recessive traits. Can require test crosses or other tests to determine the actual genotype.

Gene-The segment of DNA that contains the genetic information for a given trait or characteristic. Located at specific points on the chromosomes in the nucleus and made of two alleles.

  • Gene-The segment of DNA that contains the genetic information for a given trait or characteristic. Located at specific points on the chromosomes in the nucleus and made of two alleles.
  • Chromosome-a structure composed of DNA that contains the genetic material and is located in the nucleus of the cell. Arranged in homologous pairs

Incomplete Dominance-when the heterozygous offspring has a phenotype that is a mixture of the homozygous parent’s phenotype.

  • Incomplete Dominance-when the heterozygous offspring has a phenotype that is a mixture of the homozygous parent’s phenotype.

Co- dominance

  • Occurs when alternative alleles are present in the genotype and fully observed in the phenotype
  • ABO blood grouping system, where a single gene locus features multiple alleles- IA, IB, and i. Individuals carrying alleles for both A and B express both in the blood phenotype AB.

Co- dominance

  • Co- dominance
  • Roan Cattle- both Red and White alleles are expressed resulting in cattle that are red, white or roan.
  • Multiple Alleles-many genes
  • have more than two alleles.This
  • means that more than two possibilities exist.
  •  

Multiple Alleles

  • Antigens found in blood given in a transfusion that is foreign to the body produces antibodies which causes the blood to clump up and cause serious problems.
  • Look at the right side of the chart on the previous page and see if
  • you can answer the following questions.
  • What blood type would be a universal
  • donor? (could give a transfusion to any other blood type)
  • A person with Type O blood can donate blood to individuals of any ABO blood group (ie A, B, O or AB). Blood group O individuals do not have either A or B antigens on the surface of their RBCs, but their blood serum contains IgM anti-A antibodies and anti-B antibodies against the A and B blood group antigens.
  • What blood type would be a universal
  • recipient? (could get a transfusion of any type of blood)
  • Therefore, an individual with type AB blood can receive blood from any group (with AB being preferable), but can donate blood only to another type AB individual. Blood group AB individuals have both A and B antigens on the surface of their RBCs, and their blood serum does not contain any antibodies against either A or B antigen.
  • Is there anything else that needs to be considered before blood transfusions?
  • Rh Factor

Rh Factor

  • The Rh system was named after rhesus monkeys , since they were initially used in the research to make the antiserum for typing blood samples.
  • The inheritance of this trait usually can be predicted using a punnett square in which there are two alleles, D and d. 
  • Individuals who are homozygous dominant (DD) or heterozygous (Dd) are Rh+. 
  • Those who are homozygous recessive (dd) are Rh- (i.e., they do not have the key Rh antigens).

Mother-fetus incompatibility

  • The greatest problem with the Rh group is not so much incompatibilities following transfusions as those between a mother and her developing fetus.
  • Mother-fetus incompatibility occurs when the mother is Rh- (dd) and the father is Rh+ (DD or Dd).  Maternal antibodies can cross the placenta and destroy fetal red blood cells. 
  • The risk increases with each pregnancy as the level of antibodies increases

Rh type mother-fetus incompatibility occurs only when an Rh+ man fathers a child with an Rh- mother.  Since an Rh+ father can have either a DD or Dd genotype, there are 2 mating combinations possible:

  • Rh type mother-fetus incompatibility occurs only when an Rh+ man fathers a child with an Rh- mother.  Since an Rh+ father can have either a DD or Dd genotype, there are 2 mating combinations possible:
  • The fetus can be protected by giving Rh- women with Rh+ mates a serum (Rho-GAM ) containing anti-Rh+ antibodies.

Polygenic Traits –involves the interaction of MORE than one gene. In humans this results in a wide range of phenotypes for skin color.

  • Polygenic Traits –involves the interaction of MORE than one gene. In humans this results in a wide range of phenotypes for skin color.

Polygenetic Inheritance

  • Qualitative variation usually indicates polygenic inheritance.
  • This occurs when there is an additive effect from two or more genes.
  • Pigmentation (amount of melanin) in humans is controlled by at least three (3) separately inherited genes.

There are some exceptions to Mendel’s law. Some alleles do blend (white and red snapdragons do produce pink flowers)

  • There are some exceptions to Mendel’s law. Some alleles do blend (white and red snapdragons do produce pink flowers)
  • 1. Eye color
  • (brown, blue,
  • hazel,green etc)

A closer look at chromosomes

  • A closer look at chromosomes
  • Humans have 23 pairs of Chromosomes
  • Autosomes include the 22 pairs that are not the sex chromosomes
  • Karyotypes are prepared
  • to look for chromosome
  • abnormalities
  • We say each chromosome
  • Pair is homologous
  • Is this Karyotype for a male or Female? Male? (look at Chromosome #23)
          • MALE
          • Sex Chromosome on #23

Autosome VS Sex Chromosomes

  • Some Genetics Disorders are associated with Chromosomes 1-22
  • Other Genetic disorders are associated with the sex chromosomes #23
  • Sex linked disorders behave a bit differently. So it is useful to look at them separately.

Genetic Disorders

  • Disorder
  • Mutation
  • Chromosome
  • Angelman Syndrome
  • DCP
  • 22
  • Color Blindness
  • P
  • 23 (X)
  • Cystic Fibrosis
  • P
  • 7
  • Down Syndrome
  • C
  • 21 (extra copy)
  • Duchenne Muscular D.
  • D
  • 23 (X)
  • Haemophilia
  • P
  • 23 (X)
  • Klinefelter’s Syndrome
  • C
  • X (male XXY)
  • Phenylketonuria
  • P
  • 12
  • Sickle Cell Disease
  • P
  • 11
  • Turner Syndrome
  • C
  • X (female XO)
  • P-Point Mutation
  • D-Deletion of Gene
  • C-Whole Chromosome extra, missing or both
  • Syndrome-a number of symptoms occurring together and
  • characterizing a specific disease or condition
  • Technology
  • And
  • Genetic testing
  • Carrier Recognition
  • Newborn screening
  • Fetal testing
  • Amniocentesis
  • Chorionic villus sampling (CVS)
  • Ultrasound
  • Fetoscopy

Image of the karyotype of a child with Down syndrome or Trisomy 21:

  • Do you see anything that looks abnormal?
  • -In 1866 a physician named John Langdon Down published
  • an essay in England describing the symptoms .
  • -Medical research has shown that age of mother is a factor
  • Under age 30-1:900 chance and Over age 40 1:100 chance.

Amniocentesis

  • Amniocentesis is a diagnostic procedure performed by inserting a needle through the abdominal wall into
  • the uterus and withdrawing a small
  • amount of amniotic fluid surrounding
  • the fetus.
  • Amniocentesis can detect chromosomal
  • disorders, such as Down syndrome,
  • structural defects, such as spina bifida
  • (open spine, where the vertebrae fail
  • to close), anencephaly (a condition in
  • which the brain is incomplete or missing.

Ultrasound

  • The most well known application of ultrasound is its use in sonagraphy to produce pictures of fetuses in the human womb.

Ethical Questions?

  • Ethical motive: motivation based on ideas of right and wrong
  • How do we use ultrasound and amniocentesis as it develops further ?
  • Terminate a fetus if….
  • -Has Down Syndrome? Is that morally right or morally wrong? When is a fetus a human?
  • -It is a boy fetus and the parents want a
  • girl? Short and want tall?
  • You may have to make some ethical decisions using this technology during your life time.

Autosomal Dominant Inheritance

  • Autosomal Dominant Inheritance
  • Dominant gene located on 1 of the autosomes (all chromosomes except sex chromosomes)
  • Usually rare, Individual may not live to produce offspring
  • Affected individuals have to carry only 1 dominant gene (can be heterozygous or homozygous)
  • Passed onto both males and females
  • Every person affected must have
  • at least 1 parent with the trait
  • Does not skip generations
  • E.g. Huntington’s disease, Marfan syndrome
  • Do you know the symbols used to identify male and female?

Marfan syndrome is a heritable condition that affects the connective tissue which holds the body together and provides a framework for growth and development.

  • Marfan syndrome is a heritable condition that affects the connective tissue which holds the body together and provides a framework for growth and development.
  • In Marfan syndrome, the connective tissue is defective and does not act as it should.
  • Connective tissue is found throughout the body, Marfan syndrome can affect many body systems, including the skeleton, eyes, heart and blood vessels, nervous system, skin, and lungs. (Some Scientist’s think President Lincoln may have had it …tall and lanky)

Autosomal Dominant- Marfan syndrome

General Pedigree

Autosomal Dominant Pedigree

  • Look for:
  • Trait in every generation
  • Every person with the trait must have a parent with the trait
  • Males and females equally affected
  • Unaffected Male
  • Unaffected Female

Autosomal Recessive Inheritance

  • The recessive gene is located on 1 of the autosomes
  • Letters used are lower case ie bb
  • Unaffected parents (heterozygous) can produce affected offspring (if they get both recessive genes) We call the result homozygous recessive)
  • Inherited by both males and females
  • Can skip generations
  • If both parents have the trait then all offspring will also have the trait. The parents are both homozygous recessive.
  • E.g. Albinism, thalassemia, sickle cell anaemia and cystic fibrosis.

Autosomal Recessive Pedigree

  • Look for:
  • Skips in generation
  • Unaffected parents can have affected children
  • Affected person must be homozygous
  • Males and females affected equally
  • Unaffected Male
  • Affected Male

Autosomal recessive

  • Albinism- Albinism is an inherited condition present at birth, characterized by a lack of pigment that normally gives color to the skin, hair, and eyes.

Albinism

  • Thalassemia is an blood disorder passed down through families (inherited) in which the body makes an abnormal form of hemoglobin, the protein in red blood cells that carries oxygen.
  • Sickle cell anemia is a disease passed down through families in which red blood cells form an abnormal crescent shape. Sickle cells contain abnormal hemoglobin that causes the cells to have a sickle shape. Sickle-shaped cells don’t move easily through your blood vessels.

Sex linked inheritance Dominant

  • Male sperm determines the
  • sex of the individual at
  • fertilization
  • -Sperm contains a Y- male offspring
  • -Sperm contains a X- female offspring
  • Dominant gene on X chromosome
  • Affected males pass to all daughters and none of their sons
    • Genotype= XAY

Sex linked inheritance

  • Genes are carried on the sex chromosomes (X or Y)
  • Sex-linked notation
    • XBXB normal female
    • XBXb carrier female
    • XbXb affected female
    • XBY normal male
    • XbY affected male

Sex linked dominant disorders

If the mother has an X- linked dominant trait and is homozygous (XAXA) all children will be affected

  • If the mother has an X- linked dominant trait and is homozygous (XAXA) all children will be affected
  • If Mother heterozygous (XAXa) 50% chance of each child being affected
  • E.g. dwarfism, rickets, brown teeth enamel.
  • Rickets can be genetic or from a vitamin D deficiency

Sex linked Inheritance Recessive

  • Gene located on the X chromosome
  • More males than females affected
  • Y does not contain a second allele
  • so Males have the trait if they get
  • ONE recessive allele from their mother.
  • Females can only inherit if the father is
  • affected and mother is a carrier (hetero)
  • or affected (homo)
  • Males cannot be carriers (only have 1 X so either affected or not)
  • Can skip generations
  • E.g. color blindness, hemophilia, Duchene muscular dystrophy

Sex linked recessive pedigrees

  • More males being affected
  • Affected female will pass onto all her sons
  • Affected male will pass to daughters who will be a carrier (unless mother also affected)
  • Unaffected father and carrier mother can produce affected sons

Sex linked recessive

Sex linked recessive problem

  • 8-10% of males and 1/200 females (0.5%) are born with red or green color deficiency.
  • XcY or XcXc results in color blindness
  • Females with XcX are carriers and have normal vision

Can a color blind father have normal sons?

  • XcX
  • XY
  • XcX
  • XY
  • Xc Y
  • X X
  • Yes, but all of his daughters will be carriers

Can a carrier mother have normal sons?

  • XcX
  • XcY
  • XX
  • XY
  • X Y
  • X Xc
  • Yes, 50% of her sons will have normal color vision and 50% of daughters will be carriers

Environmental Impact on Phenotype

  • Are these different species?
  • No. Increasing the pH of the soil will change the color of hydrangea flowers from blue to pink

The expression of genes in an organism can be influenced by the environment

  • The expression of genes in an organism can be influenced by the environment
  • Drugs, chemicals, temperature, and light are among the external environmental factors that can determine which genes are turned on and off
  • The environment can influence
  • the way an organism develops
  • and functions.

Effect of the Environment

  • Which color variation in hares is useful in winter?
  • Only difference is the soil
  • The pigment Chlorophyll makes grass green. What happens to grass’s color in winter?

Identical Twin and Environmental influences

  • When you study psychology in college you will study “Nature VS Nuture”
  • What is more important in making us who we are today?
  • Is it how we are brought up?
  • Is our genetic make-up more important?
  • Why do these identical twins separated
  • At birth look so different?

Epigenetics is the study of changes in phenotypes (appearance) or gene expression caused by mechanisms other than changes in the underlying DNA sequence, (Greek: επί- over, above) -genetics.

  • Epigenetics is the study of changes in phenotypes (appearance) or gene expression caused by mechanisms other than changes in the underlying DNA sequence, (Greek: επί- over, above) -genetics.

Allele

  • A different form of a particular gene, positioned in the same relative position (locus) on homologous chromosomes
  • Homologous chromosomes
  • Gene locus – position on the chromosome
  • Alleles (diff versions of the genes)

DNA nucleotides

  • Phosphate group
  • Pentose sugar
  • Nitrogenous base

Phosphodiester bond

  • Nucleotide
  • Polynucleotide
  • chain

4 different bases

  • C
  • G
  • A
  • T

Double helix

  • 5’end
  • 5’end
  • 3’end
  • 3’end

Organism No. chromosomes

  • Organism No. chromosomes
  • Human 46
  • Chimpanzee 48
  • Dog 78
  • Horse 64
  • Chicken 78
  • Goldfish 94
  • Fruit fly 8
  • Mosquito 6
  • Nematode 11(m), 12(f)
  • Horsetail 216
  • Sequoia 22
  • Round worm 2
  • This powerpoint was kindly donated to www.worldofteaching.com
  • http://www.worldofteaching.com is home to over a thousand powerpoints submitted by teachers. This is a completely free site and requires no registration. Please visit and I hope it will help in your teaching.


The database is protected by copyright ©sckool.org 2016
send message

    Main page