A Brief HISTORY OF HEREDITY by sir17308

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									        A Brief HISTORY OF
             HEREDITY
• The idea that traits are passed down is an
  old one.
• Characteristics that seem to run in families
  were noticed ever since humans were able
  to notice them.
• The earliest recordings of this are from
  Hippocrates, who lived from 460 – 370
  BCE.
• He wrote many aspects on medicine
  (Hippocratic oath is named after him).
• He mentioned the inheritance of baldness,
  crossed eyes, epilepsy, and an eye
  disease that develops in middle age
  (glaucoma).
Pierre Maupertuis (1698-1759)
• recorded instances of unusual inheritance
  such as polydactyly—the presence of
  extra fingers, toes, and nipples.
• He also reported instances of albinism, a
  lack of pigment which causes the skin to
  be very fair and the hair white.
• Sometimes the eyes appear violet
  because the blood vessels in the irises are
  more easily seen, or light blue.
• Albino animals can be albino too.
• Because they are relatively rare, seeing
  one was often considered important.
Black bear with a partially albino cub
Mother with albino daughter.
Jean Baptiste Lamarck(1744-1829)
• suggested that acquired traits could be
  passed down.
• That is, if you father was muscular, you
  would be muscular.
• If your mother was educated, you would
  be smart. (There is more to what he
  suggested, but we will revisit Lamarck in a
  later unit).
• Enter Gregor Mendel, a humble monk
  born in 1822.
• By 1856 he had founded the scientific
  study of heredity, but never received the
  credit for it within his life-time.
Mendel’s Laws of Heredity
     (Ch 10 – P 253-279)
               A Brief Bio
• Mendel was born in 1822, attended
  university of Vienna where he studied
  chemistry, biology and physics.
• He left before graduating, probably for
  health reasons.
• Entered the Augustinian monastery in
  Brno, Austria, and with the support of the
  abbot, began his investigation of the
  inheritance of certain traits in pea plants
  (Pisum sativum).
• Modern genetics began in the 1860’s with
  Mendel’s discovery of the principles of
  genetics in breeding garden peas.
• Mendel chose the garden pea for the
  following reasons:
  – They were easy to grow because they
    reproduce and grow quickly
  – They were readily available in many
    distinguishable varieties with contrasting traits
  – He was able to exercise strict control over
    which plants were bred with another.
     Anatomy of a pea flower
               (p 254 & 642)

• Garden peas reproduce sexually.
• This means they produce male and female
  sex cells called GAMETES.
• The male gamete forms in the pollen
  grain.
• The female gamete forms in the ovule.
• The male gamete unites with the female
  gamete in the process of
  FERTILIZATION.
 Flower
Anatomy
Anther and Stigma
• The transfer of pollen from the male part of
  the plant to the female part of the pea
  plant is called pollination. (Fig 24.12 p 647)
• Petals of pea flowers almost completely
  enclose the stamen (male) and carpel
  (female) so in nature, plants self-fertilize
  (self pollination).
• Mendel ensured self-pollination by
  covering the flower with a bag so that no
  pollen from another plant could reach the
  carpel.
 Flower –
Life Cycle
 The Genetics of Garden Peas
Mendel chose to follow 7 traits in pea plants (pg 256)
• Flower colour:      purple or white
• Seed colour:        yellow or green
• Seed shape:         round or wrinkled
• Pod shape:          inflated or constricted
• Pod colour:         green or yellow
• Stem length:        tall or short
• Flower position:    axial (side) or terminal (tips)
See Fig 10.3
 on p 256
• He picked these traits because they were
  distinct and easily followed.
• The traits in bold were later found to be
  significantly more common.
• Over the next 8 years, Mendel conducted
  experiments and maintained detailed
  records of his results.
• He designed simple experiments that
  allowed him to observe the inheritance of
  one trait at a time.
• He used his mathematics to formulate
  conclusions based on his results.
 How did Mendel Conduct his Experiments?
• He wanted to study what would happen when
  bred (crossed) different varieties of pea plants.
• Mendel always started with true-breeding
  varieties (pure breeding)—plants that showed
  the same trait over several generations. For
  example, if he worked with tall plants, he used
  plants from populations of plants that had been
  tall for many generations and had always
  produced tall offspring.
   – Recall that he covered the plants with a bag to
     prevent cross pollination
• To cross-fertilize (cross pollinate), Mendel
  would do the following: (see Fig. 10.1 on p 254)
  – He would cut off immature stamen to prevent
    self-fertilization. This plant was now a female
    plant.
  – He dusted the carpel with pollen from another
    plant.
  – The carpel would develop into a pod,
    containing the seeds he would later plant.
  – When these seeds grew into offspring plants,
    he observed them for certain traits.
• In this way, Mendel was certain of the
  parentage of the new plants.
• In this way, Mendel also developed some
  vocabulary. (See Fig. 10.2 p 255)
The Generations:
• Analyzing a single trait in this manor is
  called a MONOHYBRID CROSS
• The parental plants are called the ―P
  Generation‖
• The offspring from the cross are called the
  ―F1 Generation‖ (F for filial from the Latin
  word for son)
• When two F1 generation plants are
  allowed to pollinate, this new offspring set
  are called the F2 generation.
• What would happen if a purple flowered
  plant were crossed with a white flowered
  plant?
Explaining the Monohybrid Cross
• In pea plants, there are two different forms
  of the traits that Mendel studied.
• Later, these forms of the same trait were
  called ALLELES—there is an allele for
  purple on the genes of one plant, an allele
  for white on the genes of the other plants.
• In reproduction through meiosis, genes
  are passed onto offspring through each
  parent’s gametes.
• Each offspring receives one copy of each
  gene from each parent
  – I.E. an offspring receives an allele for a trait
    from each parent. (HOMOLOGOUS PAIR)
  – Problem Solving Lab 10.2 p 264
  – See Figure 10.10 p 265
• Mendel did the same type of experiment
  with all 7 traits and discovered the same
  patterns:
  – In the F1 generation, one trait was always
    hidden, or masked. That hidden trait would
    show up in the F2 generation in about ¼ of
    the plants.
  – He called the hidden traits in the F1
    generation RECESSIVE and the trait that
    exerted itself was called DOMINANT. In pea
    plants, purple flowers are dominant to white
    flowers.
• To show this, the capital letter for the
  allele trait represents the dominant allele.
• The lowercase letter represents the
  recessive allele:
P – purple allele p – white allele
  – A true breeding purple flower: PP
  – A true breeding white flower: pp
             Homozygous
• These represent the P generation in
  Mendel’s experiment.
• True breeding plants have both alleles the
  same and are called HOMOZYGOUS. We
  say ―homozygous dominant‖ (PP) or
  ―homozygous recessive‖ (pp)
             Heterozygous
• HETEROZYGOUS plants show the
  dominant trait but carry one dominant
  allele and one recessive allele (often
  called CARRIERS of the recessive trait).
• A heterozygous purple flower: Pp
• These represent the F1 generation of
  Mendel’s experiment.
• Mendel also used the term HYBRID for
  plants that were heterozygous for a trait.
                           See Fig. 10.5 p 258
Mendel also studied TWO traits at the same time. This type of cross is more
complex. It is called a DIHYBRID CROSS. More on that later.
        The CONCLUSION:
•   Mendel’s use of mathematics allowed
    him to formulate conclusions based on
    his results.
•   These conclusions are known as
    Mendel’s Laws or Principles:
1. The Rule of Unit Factors
Traits are controlled by unit factors
(genes) that exist in pairs in individuals.
One factor (gene) is passed on to the
offspring from each parent so that each
offspring has a pair of unit factors
(genes).
2. The Rule of Dominance:
  When two unlike factors for a trait are
present in an individual, one masks the
expression of another. That is, one
factor is dominant to the other, which is
recessive. (p 256)
3. The Law of Segregation
During gamete formation in meiosis,
factors separate (segregate) randomly
so that each gamete receives one form
of the trait or the other—each gamete
can have only one allele for a trait. The
passing down of alleles occurs
randomly
4. The Law of Independent
   Assortment
During gamete formation, segregating
pairs of factors (genes) assort
independently of each other.
  E.g. Colour factors are not linked to
height factors—these sort
independently of each other.
• In 1865 Mendel published his findings.
• The scientific community did not seem to grasp
  the significance of his findings, as a result, it was
  largely ignored.
• Mendel later become abbot (1868) and died in
  1884.
• In 1900, three scientists working independently
  rediscovered and confirmed Mendel’s laws of
  heredity—Hugo de Vries, Carl Correns and Erich
  von Tschermak-Seysenegg all gave credit to
  Mendel.
• Mendel never knew the world would come to
  embrace him as the father of modern genetics.
             Assignment
• Worksheet packet
  – Pages 9, 10, 18-22, 25 & 26

								
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