Biology : Ch. 8 : Mendel and Heredity

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Biology Ch. 8 Mendel and Heredity

• In this chapter we will focus on the work of
  Gregor Mendel, which has led to our modern
  understanding of genetics.
• Genetics is defined as the study of heredity.
• Heredity is the passing on of characteristics
  from parents to offspring.

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Mendels Laws of Heredity

• Gregor Mendel (1843) was an Austrian Monk who
  studied the reproduction of garden pea plants.
• The pea plant reproduces sexually, meaning that
  two distinct sex cells (gametes) meet to form an
  offspring.
• In the pea (as in other plants) the pollen is the
  male gamete, and the ovule is the female gamete.

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Mendels Laws of Heredity

• The pollen from the male part of the pea plant
  (anther) falls on the ovule from the female part
  of the plant (pistil), and fertilization occurs.
• Pollination refers to the pollen meeting the
  ovule of plants.
• Pollination can be self pollination or cross
  pollination.

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Mendels Laws of Heredity

• To study the offspring of the garden pea plants
  Mendel had to carefully transfer pollen from one
  plant to another plant. This is called making a
  cross.
• By careful research Mendel was able to control
  which plants were fertilized by which plant, and
  was able to make some conclusions about the
  passing down of traits or characteristics from
  parents to offspring.

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Mendels Crosses

• Mendel gave the following symbols to describe the
  study of heredity or genetics in crosses
• P parents
• F1 first filial generation
• F2 second filial generation

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Mendels Crosses

• Mendel first cross involved studying one trait.
  (height). This is called a monohybrid cross
  since only one trait is studied from the parents
  to the offspring.
• He crossed a tall pea plant with a short pea
  plant, and found all the(F1) offspring were tall.
  
• He then crossed the F1with each other and found 1
  in 4 of their offspring was short.

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Mendels Monohybrid Cross Pea Plant Height
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Mendels Conclusions

• After studying genetics for 30 years Mendel
  concluded that each organism has two factors for
  each of its traits. These different gene forms
  are called alleles.
• Mendel also concluded that alleles can be
  dominant or recessive. The dominant allele seems
  to cover up the recessive allele for a given
  trait.

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End of Section 1
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Representation of Alleles

• Alleles, which are a form of a gene are
  represented by capital or lower case letters, to
  show the dominant or recessive allele of each
  trait.
• For pea plant height T dominant allele for
  tall plants, t recessive allele for short
  plants.
• A pea plant with Tt tall
• A pea plant with TT tall
• A pea plant with tt short

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Mendels Law of Segregation

• Mendel concluded that the two alleles for for
  each trait must separate when gametes are formed.
  A parent passes on at random one of its alleles
  for each trait.

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Trait Terminology

• Many traits which are passed on are controlled by
  two alleles. The following are the names for the
  types of alleles for each trait
• TT homozygous dominant
• Tt heterozygous
• tt homozygous recessive
• The way a trait is expressed or how an organism
  appear is the phenotype (TT tall, Tt tall)
• The actual alleles for a trait is the genotype.
  (TT,Tt,tt)

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Mendels Dihybrid Crosses

• A dihybrid cross involves studying two traits at
  the same time.
• When Mendel studied the color of the pea seed and
  the shape of the seed (wrinkled, or round), he
  found that different traits are inherited
  independently of each other. This is called the
  law of independent assortment.
• Today this law is true only if the alleles for a
  trait are located on a different chromosome.
  Some traits are not independently assorted to the
  offspring. Some traits are more packaged due
  to meiosis.

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Punnett Squares

• Punnett Squares are used to predict the
  offspring of a cross.
• A punnett square uses the male gamete on the top,
  the female gamete on the side, and possibilities
  of the offspring in the boxes.
• A punnett square will give you a predicted
  phenotypic and genotypical ratios of offspring.

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Punnett Squares
RRYy

• Dihybrid Crosses A dihybrid cross involves the
  study of two traits passed on to offspring.
• A dihybrid cross requires a 4 x 4 box to
  calculate.
• Possible alleles for the male and female are
  noted on the outside.

RRYy
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Example Dihybrid Cross
Traits B black coat b red coat , T
tall , t short What is the predicted offspring
of a heterozygous black coat bull which is
heterozygous for tall and a heterozygous black
coat cow which is heterozygous for tall?
BbTt
BT
Bt
bT
bt
BT
Bt
BbTt
bT
bt
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Inheritance of Traits

• Traits are inherited on the autosomes or sex
  chromosomes.
• If a gene for a trait is located on the sex
  chromosome the trait is termed sex-linked
• Examples baldness, color blindness, hemophilia

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Pedigree

• A pedigree is a chart used to study inheritance
  of traits.
• A pedigree is a family history diagram.

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Pedigree

• A pedigree uses circles to represent females, and
  squares to represent males.
• The allele to be studied is represented by
  shading of the circles and squares.
• A pedigree consists of different generations of
  the individual

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Inheritance Patterns

• Mendelian inheritance dominant and recessive
  alleles. The dominant allele covers the
  recessive allele.
• Individuals who are heterozygous are termed
  carriers of the recessive allele.

Tongue rolling explained by mendelian
inheritance
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Inheritance Patterns

• Incomplete dominance
• The mixing of alleles to provide a trait.
  Heterozygous genotype is an intermediate between
  the two alleles.
• Red flower x White flower pink flower.

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Inheritance Patterns

• Codominance pattern of inheritance in which
  different alleles may dominate different areas of
  an organism.
• Ex calico cats, cattle, speckled chickens,
  baldness patterns, etc.

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Inheritance Patterns

• Polygenic complex traits which are a result of
  many genes.
• Ex. Eye color, height, weight, skin color, etc.

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Inheritance Patterns

• Multiple alleles inheritance pattern involving
  three or more alleles to determine a trait.
• Ex. Blood type

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Blood Types Multiple Alleles

• Human blood types are determined by proteins
  found on the Red Blood Cells.(Antigens)
• Possible alleles
• IA - protein A
• IB - protein B
• I no surface proteins

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Blood Types

• Human Blood Type is determined by one allele from
  the mother and one allele from the father. The
  alleles show codominance.

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Blood Types

• Blood antigens(proteins) are important to
  identify when donating and receiving blood.
  Different proteins will bond with other proteins
  and cause clotting or coagulation of the blood.
  

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Blood Types

• Blood Types
• O universal donor
• AB universal acceptor

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Rh Factor

• Another protein may be present on the RBCs. This
  protein is called the Rh factor.
• If a mother is Rh- and a developing baby is Rh
  the mother will build up anti Rh proteins
• The mothers second child, if Rh will have
  complications due to the mothers antibodies
  fighting the Rh proteins of the child

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Environmental Influence

• A phenotype of an individual is also linked to
  their environment.
• Example Arctic fox in the winter white coat,
  Arctic fox in summer darker coat.

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Genetic Disorders

• Sickle Cell Anemia
• Autosomal recessive trait
• Red Blood Cells lack proper shape to carry oxygen
  and carbon dioxide to and from cells

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Cystic Fibrosis

• 1 in 2000 children are born with CF.
• To be born with CF you need both recessive
  alleles.
• CF alleles cause thick mucus in the lungs and
  respiratory tract

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Hemophilia

• Hemophilia is a sex-linked trait. A hemophiliac
  lacks the ability for blood to clot.
• Most hemophiliacs are male. (one allele)
• A recessive sex-linked disorder.

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Huntingtons Disease

• Lethal disorder caused by a dominant allele.
• Results in a degeneration of the nervous system
• Generally is expressed at age 30 50

Dominant allele
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Tay Sachs Disease

• Autosomal Recessive disorder
• Lack of an enzyme which breaks down a lipid. The
  lipid builds up in tissue.
• Blindness, loss of movement, and mental
  deterioration.

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Treating Genetic Disorders

• Gene therapy involves trying to isolate and
  replace certain genes in cells.
• Viruses have been used to transfer genes into
  cells to alter the desired gene.
• Viruses however are naturally attacked by the
  immune system.