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Title: GENETICS AND HEREDITY


1
GENETICS AND HEREDITY
2
Heredity and Genetics
  • Heredity is the passing of physical
    characteristics from parents to offspring.
  • Genetics is the scientific study of heredity.

3
Mendel
  • Gregor Mendel, an Austrian monk of the nineteenth
    century, made the discoveries that is the
    foundation of our knowledge of genetics.

4
Mendels Experiments
  • He did his experiments because he wondered why
    pea plants had different characteristics.
  • Tall and short plants
  • Green and yellow seeds
  • Round (smooth) and wrinkled seeds
  • Each different form of a characteristic is called
    a trait.

5
Mendels Experiments
  • Fertilization is the process where an egg cell
    and a sperm cell join together.
  • Pollination is the process of the pollen reaching
    the pistil of a flower.
  • Pea plants are usually self-pollinating, meaning
    the pollen of a flower lands on the pistil of the
    same flower.
  • Mendel developed a method of cross-pollination.
  • He removed pollen from the flower of one plant
    and then brushed the pollen onto a flower on a
    second plant.

6
Crossing Pea Plants
  • Mendel decided to cross plants with opposite
    traits, for example tall and short plants.
  • He began his experiments with purebred plants.
  • Purebred organisms are the offspring of many
    generations that have the same trait.
  • Example Purebred short plants always come from
    short parent plants.
  • Purebred individuals are also called true
    breeding individuals.

7
The F1 Offspring
  • In Mendels experiments, the purebred parent
    plants are called the parental generation or P
    generation.
  • Example Mendel crossed a purebred tall plant
    with a purebred short plant.
  • The offspring of the P generation are called the
    first filial (Latin for daughter or son), or F1
    generation.
  • Example In Mendels F1 generation, all of the
    plants were tall.
  • Even though one of the parents was short, that
    trait seemed to disappear in the F1 generation.

8
The F2 Generation
  • Mendel let the fully grown F1 plants to
    self-pollinate.
  • The second filial, or F2 generation were a mix of
    tall and short plants.
  • The short trait reappeared even though none of
    the parents were short.
  • After counting the F2 plants, Mendel noted that ¾
    of the plants were tall and ¼ of the plants were
    short.

9
Experiments with Other Traits
  • Mendel did hundreds of crosses looking at other
    traits.
  • In all of his crosses, only one form of the trait
    appeared in the F1 generation, but that trait
    reappeared in the F2 generation in about ¼ of the
    plants.

10
Dominant and Recessive Alleles
  • Because of his experiments, Mendel concluded that
    individual factors must control the inheritance
    of traits.
  • He also reasoned that the factors that control
    each trait exists in pairs, one factor from each
    parent.
  • Based on the results of his experiments, Mendel
    concluded that one factor in each pair can mask,
    or hide, the other factor.
  • Example The tallness factor masked the shortness
    factor.

11
Genes and Alleles
  • Today, scientists call the factors that control a
    trait a gene.
  • The two different forms of a gene are called
    alleles.
  • Each pea plant inherits one allele from each
    parent.
  • A pea plant could inherit 2 tall alleles, 2 short
    alleles, or 1 of each.

12
Genes and Alleles
  • An organisms traits are controlled by the
    alleles it inherits from its parents.
  • Some alleles are dominant.
  • Dominant alleles are those whose trait always
    shows up in the organism when that allele is
    present.
  • Other alleles are recessive.
  • Recessive alleles are those whose traits are
    hidden whenever the dominant allele is present.
  • Recessive traits only show up if the organism
    does not have the dominant allele. In other
    words, the organism has two recessive alleles.

13
Genes and Alleles
  • In Mendels crosses, the allele for tall plants
    is dominant over the allele for short plants.
  • Only plants that inherit two short alleles will
    be short. Plants that receive one or two dominant
    alleles will be tall.

14
Alleles in Mendels Crosses
  • In Mendels experiments, the purebred tall plants
    had 2 alleles for being tall, while the purebred
    short plants had 2 alleles for being short.
  • All of the plants from the F1 generation received
    one tall allele and one short allele.
  • Organisms that has two different alleles for a
    trait is called hybrid.
  • All of the hybrid plants were tall because they
    received 1 tall and 1 short allele, but the tall
    is dominant over the short.

15
Alleles in Mendels Crosses
  • When the F1 plants self-pollinated, some of the
    F2 plants received two dominant alleles for
    tallness.
  • These plants were tall.
  • Other F2 plants received one dominant and one
    recessive allele.
  • These plants were tall.
  • The rest of the F2 plants received two alleles
    for shortness.
  • These plants were short.

16
Symbols for Alleles
  • Letters are used to represent alleles.
  • Dominant alleles are represented by capital
    letters.
  • The tall allele would be T.
  • Recessive alleles are represented by lowercase
    letters.
  • The short allele would be t.
  • The alleles an organism receives for a trait are
    represented by a combination of letters.
  • The combination of alleles possible for pea
    plants are TT, Tt, and tt.

17
Homozygous and Heterozygous
  • An organism is said to be homozygous for a trait
    if both alleles are identical.
  • Example TT and tt are homozygous allele
    combinations.
  • TT is homozygous dominant.
  • tt is homozygous recessive.
  • An organisms is said to be heterozygous for a
    trait if the organism has both a dominant and
    recessive allele.
  • Example Tt is a heterozygous allele combination.
  • All hybrids are heterozygous individuals.

18
Why Mendel was Important?
  • Before Mendel, scientists thought that the traits
    of an individual were simply a blend of the
    parents traits.
  • Example If a tall plant and a short plant
    reproduced, they would make medium sized plants.
  • Because of Mendels experiments, traits are
    determined by individual, separate alleles
    inherited from each parent.
  • Mendels discovery was not recognized during his
    lifetime.
  • His work was rediscovered in 1900.
  • Mendel is known as the Father of Genetics.

19
Probability and Genetics
  • Mendel carefully counted all of the offspring
    from every cross he carried out.
  • When he crossed two tall hybrid plants, ¾ of the
    F2 generation were tall and ¼ were short.
  • Each time he repeated the cross, he obtained
    similar results.
  • He realized that probability applied to his work.

20
Probability and Genetics
  • Mendel could say that the probability of
    producing a tall plant in the F2 generation was 3
    in 4.
  • The probability of producing a short plant in the
    F2 generation was 1 in 4.
  • Mendel was the first scientist to recognize that
    the principles of probability can be used to
    predict the results of genetic crosses.

21
Punnett Squares
  • A Punnett Square is a chart that shows all the
    combinations of alleles that can result from a
    genetic cross.
  • Geneticists use these to show all the possible
    outcomes of a genetic cross, and to determine the
    probability of a particular outcome.

22
How to Make a Punnett Square
  • Draw a square and divide it into 4 smaller
    squares.

23
How to Make a Punnett Square
  • Place the alleles from one parent along the top
    of the Punnett square.
  • Make sure that only one letter is above each box.
  • Place the alleles from the other parent along the
    left side of the square.
  • Make sure that only one letter is beside each box.

T T
t
t
24
How to Make a Punnett Square
  • Copy the alleles from the top into each box under
    them.

T T
T
T
t
T
T
t
25
How to Make a Punnett Square
  • Now place each letter on the left of the box into
    the boxes to the right of them.
  • When you are finished, you should have two
    letters in each box.
  • You always should write the dominant allele on
    the left-hand side.
  • Tt instead of tT.

T T
Tt
Tt
t
Tt
Tt
t
26
How to Make a Punnett Square
  • The boxes in the Punnett square represent all the
    possible combinations of alleles that the
    offspring can inherit.
  • In this Punnett square, we see the results of
    crossing a purebred tall plant with a purebred
    short plant.
  • All of the offspring are hybrid tall plants.
  • From this cross, 4 in 4, or 100 will be tall.

T T
Tt
Tt
t
Tt
Tt
t
27
Using a Punnett Square
T t
  • In a genetic cross, the allele that each parent
    will pass on to its offspring is based on
    probability.
  • In the Punnett square to the right, there is a 3
    in 4 chance, or 75 chance that the offspring
    would inherit the tall trait.
  • The Punnett square represents the chances each
    time a pair reproduces.
  • This does not mean that if the pair to the right
    had 4 offspring, 3 would be tall and 1 would be
    short.
  • It says that each time they reproduce there is a
    75 chance for tall plants and 25 chance for
    short.

TT
Tt
T
tt
Tt
t
28
Phenotypes and Genotypes
  • A pheontype is an organisms physical appearance
    or visible traits.
  • Example tall, short, purple flowers, white
    flowers, wrinkled seeds, round seeds, black fur,
    white fur
  • A genotype is its genetic makeup or allele
    combinations. In other words, the combination of
    letters.
  • Example TT, Tt, tt, BB, Bb, bb, RR, Rr, rr, WW,
    Ww, ww

29
Codominance
  • For all the traits that Mendel studied, one
    allele was dominant while the other was
    recessive.
  • This does not happen 100 of the time.
  • In codominance, the alleles are not dominant nor
    recessive.
  • As a result, both alleles are expressed in the
    offspring.

30
Genetic Laws
  • The Law of Dominance states that when an
    organism has two different alleles for a trait,
    the allele that is expressed, overshadowing the
    expression of the other allele, is said to be
    dominant. The allele whose expression is
    overshadowed is said to be recessive.

31
Genetics Laws
  • The Law of Segregation states that the alleles
    for a trait separate when gametes (egg and sperm)
    are formed. These allele pairs are then randomly
    united at fertilization. Mendel arrived at this
    conclusion by performing monohybrid crosses.
    These cross-pollination experiments were with pea
    plants that differed in one trait, such as pod
    color.

32
Genetics Laws
  • The Law of Independent Assortment states that
    alleles for different traits are distributed to
    sex cells and offspring independently of one
    another.
  • This means that the inheritance of one trait has
    nothing to do with the inheritance of another.
  • Example Just because a pea plant inherits the
    tall trait does not mean that they must also
    inherit the trait for having wrinkled seeds.

33
Genetic Disorders and Recessive Genes
  • Many genetic disorders are caused by recessive
    genes.
  • If an offspring receives two recessive alleles
    from the parents, the child inherits the disease.
  • If a person is heterozygous, he/she will not show
    the symptoms.
  • These people are known as carriers.

34
Cystic Fibrosis
  • This is a genetic disorder in which the body
    produces abnormally thick mucus in the lungs and
    intestines.
  • The mucus fills the lungs and makes it hard to
    breathe.
  • It is caused by a recessive allele on one
    chromosome.
  • It is the result of a mutation in which three
    bases are removed from DNA.

35
Sickle Cell Disease
  • Sickle cell anemia results from a substitution
    mutation of the DNA in the sex cells. This has
    resulted in a recessive trait.
  • Sickle cell commonly affects people of African,
    Indian, and Mediterranean descent.
  • It causes the red blood cells to become
    sickle-shaped.
  • This prevents the blood from passing normally
    through the capillaries, resulting in oxygen not
    being passed on to the tissues.

36
Hemophilia
  • This is a genetic disorder in which a persons
    blood clots very slowly or not at all.
  • They do not produce one of the proteins needed
    for normal blood clotting.
  • Have a high risk of internal bleeding from small
    bumps and bruises.
  • Caused by a recessive allele on the X chromosome,
    making it a sex-linked disorder.
  • Occurs more often in males than females.

37
Heredity and Meiosis
  • Sometimes mistakes happen during meiosis, the
    production of egg and sperm cells.
  • This can result in individuals having more or
    fewer chromosomes than normal.
  • Individuals with Downs Syndrome have an extra
    copy of chromosome 21.
  • This results in a variety of physical and/or
    mental conditions.

38
Sex Chromosomes
  • The sex chromosomes carry genes that determine
    whether a person is male or female. They also
    carry genes that determine other traits.
  • The sex chromosomes are the only pair that do not
    always match.
  • In females, the chromosomes match. The female
    genotype is XX.
  • In males, the chromosomes do not match. The male
    genotype is XY.

39
Sex-Linked Genes
  • Genes on the X and Y chromosomes are called
    sex-linked genes because their alleles are passed
    from parent to child on a sex chromosome.
  • Traits controlled by sex-linked genes are called
    sex-linked traits.
  • One sex-linked trait is red-green colorblindness.
  • A person with this trait cannot distinguish
    between the colors red and green.

40
Pedigrees
  • A pedigree is a chart or family tree that
    tracks which members of a family have a
    particular trait.
  • Pedigrees include two or more generations.
  • Females are represented by circles, while males
    are represented by squares.
  • Those with a trait are shaded, while those that
    do not have a trait are left clear.
  • If the organism is a carrier of a trait, but does
    not show the trait, their symbol is only shaded
    halfway.

41
A Pedigree for Albinism(A condition where the
skin, hair, and eyes lack normal coloring)
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