GENETICS AND HEREDITY

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GENETICS AND HEREDITY
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Heredity and Genetics

• Heredity is the passing of physical
  characteristics from parents to offspring.
• Genetics is the scientific study of heredity.

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Mendel

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

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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How to Make a Punnett Square

• Draw a square and divide it into 4 smaller
  squares.

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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
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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
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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
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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
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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
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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

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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