HEREDITY

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HEREDITY

• 2-1 The Chromosome Theory

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INTRODUCTION

• The work of Gregor Mendel provided many early
  solutions to the riddle of genetics however, he
  did not have all of the answers.
• Mendel did not know where the hereditary factors,
  or genes, are located in the cell.
• The first clue came in 1882 when the German
  biologist Walther Flemming discovered chromosomes.

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INTRODUCTION

• Chromosomes are rod-shaped structures that are
  found in the nucleus of every cell in an
  organism.
• The next clue was provided by Walter Sutton, an
  American graduate student who in 1902 was doing
  research on grasshopper chromosomes.
• From his work, Sutton concluded that chromosomes
  carried Mendels hereditary factors, or genes,
  from one generation to the next.

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Grasshoppers have 24 chromosomes, arranged in 12
pairs.
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Each of a grasshopper's sex cells contains 12
chromosomes, or half the number in body cells.
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INTRODUCTION

• In other words, genes are located on chromosomes.
• Suttons idea came to be known as the chromosome
  theory of heredity.
• According to the chromosome theory, genes are
  carried from parents to their offspring on
  chromosomes.

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Chromosomes and Genes

• Today, scientists know that chromosomes play an
  essential role in heredity.
• Chromosomes control all the traits of an
  organism.
• The main function of genes on chromosomes is to
  control the production of substances called
  proteins.
• Proteins determine the size, shape, and other
  physical characteristics of an organism.

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Chromosomes and Genes

• The kind and number of proteins in an organism
  determine the traits of that organism.
• So by controlling the kind and number of proteins
  produced in an organism, chromosomes are able to
  determine the traits of that organism.

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Chromosomes and Genes

• Chromosomes are found in pairs within the nucleus
  of a cell.
• For any particular trait, the gene contributed by
  one parent is on one of the paired chromosomes.
• The other gene for that trait, contributed by the
  other parent, is on the second chromosome of the
  pair.
• Each genes major role is to control the
  production of a specific protein.

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Meiosis

• The process of meiosis produces the sex cells,
  the sperm or egg cells.
• As a result of meiosis, the number of chromosomes
  (and genes that they carry) in each sex cell is
  half the normal number of chromosomes found in
  the parent.
• When the sex cells combine to form the offspring,
  each sex cell contributes half the normal number
  of chromosomes.
• Thus, the offspring gets the normal number of
  chromosomes half from each parent.

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Sex Chromosomes

• In 1907, the American zoologist Thomas Hunt
  Morgan began his own studies in genetics.
• He experimented with insects called fruit flies.
• Morgan quickly discovered something strange about
  the fruit flies four pairs of chromosomes.
• In female fruit flies, the chromosomes of each
  pair were the same shape.
• In males, however, the chromosomes of one pair
  were not the same shape.

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Sex Chromosomes

• One chromosome of the pair was shaped like a rod,
  and the other chromosome was shaped like a hook.
• Morgan called the rod-shaped chromosome the X
  chromosome and the hook-shaped chromosome the Y
  chromosome.
• After performing a number of experiments and
  analyzing his results, Morgan discovered that the
  X and Y chromosomes determine the sex of an
  organism.

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Sex Chromosomes

• For this reason, the X and Y chromosomes are
  called the sex chromosomes.
• In general, an organism that has two X
  chromosomes (XX) is a FEMALE.
• An organism that has one X chromosome and one Y
  chromosome (XY) is a male.
• An exception female birds are (XY).

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