The Science of Genetics

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The Science of Genetics

• Original Power Point Created by
• Andy Harrison

Modified by the GA Agriculture Education
Curriculum Office July 2002
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Introduction

• Genetics began as people noticed the similarity
  of offspring to parents
• Animals or plants were selected based on certain
  desirable traits
• Later became known as selective breeding

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Gregor Mendel

• Austrian monk 1822 1884
• Noticed the texture of seeds differed
• For example, some were smooth and round, others
  were not smooth and not round
• Also noticed the difference in colors of flowers,
  length of stems, etc.

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Mendels Experiments

• Used purebred plants
• Chose plants with traits that would be carried to
  next generation each time seeds produced.
• Crossed purebred plants with other purebred
  plants.
• Removed petals
• Removed pollen from stamens and placed on stigma
  of pistil of different type of plant

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Mendels Experiment

• Some produced red flowers, and some only white
• He then crossed all red with all white
• Yielded some plants with all red flowers
• Reasoned that some factor must cause the red to
  be dominant over white

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Mendels Experiment

• Law of Dominance overriding or dominant factors
  make certain recessive traits disappear.
• Allele matched pair of genes that control a
  trait
• Law of Segregation alleles responsible for
  trait separate and then combine with other parent
  at fertilization each parent provides one of
  two genes for the trait

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Inheritance

• Mendel let some plants self-pollinate
• Offspring called an F1 generation
• Some all red and some all white
• Let F1 reproduce
• Plants from all red produced some white flowers
  
• Called the F2 generation

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Inheritance

• Mendel concluded that traits or factors
  associated with red or white flowers separated
  before pollen merged with egg
• Law of segregation - alleles responsible for
  traits from each parent are separated and then
  combined with factors from other parents at
  fertilization.

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Inheritance

• Law of Independent Assortment factors or genes
  for certain characteristics are passed on to the
  next generation separate from the factors or
  genes that transmit other traits.

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Punnett Square

• Developed by mathematician R.C. Punnett
• Illustrates the possible combinations of a
  particular trait

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Punnett Square

• 75 Red
• 25 White
• 31 Red to White

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Genetics

• The study of how traits are passed on to future
  generations
• Genotype the genetic composition of an
  individual
• Phenotype how the allele expresses itself

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Genetics

• Homozygous Genes that contain 2 dominant
  alleles (RR) or 2 recessive alleles (rr)
• Heterozygous Genes containing 1 dominant allele
  and 1 recessive allele (Rr)

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Gene Transfer

• Traits are located on chromosomes
• Chromosomes are made of strands of DNA
• Contains proteins and amino acids
• Each segment of the chromosome that controls a
  trait is a GENE
• Some traits are controlled by one gene, others
  under multi-genetic control
• In Mendel's law of dominance, one allele is
  expressed and one is hidden

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Meiosis

• Similar to mitosis
• Has an additional step
• Chromosomes are divided so each sperm or egg
  contains a chromosome
• Results in one egg or 4 sperm

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Gene Transfer

• At fertilization the egg and sperm unite
• Newly formed cell contain two chromosomes
• There is a pair of alleles for each trait
• Haploid
• For example, sex cells have only one chromosome
• Diploid
• Chromosomes are in pairs
• Some cells have more than two pair
• Also known as polyploid

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Sex Determination

• Each parent contributes one chromosome which
  pairs with a chromosome from the other parent
• Sex Chromosomes
• X comes from the female
• Y comes from the male

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Sex Determination

• Female has XX

• Male has XY

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Gene Transfer

• DNA shaped in spiral a coil
• Also known as a double-helix
• If flattened out it would resemble a ladder
• The two sides of the ladder are connected by
  nitrogen containing bases
• Adenine - A
• Thymine - T
• Cytosine - C
• Guanine - G
• Adenine can only pair with Thymine
• Cytosine can only pair with Guanine

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Gene Transfer

• During mitosis and meiosis, the chromosome
  separates
• Prior to cell division, DNA copies itself by
  replication
• Messages in DNA are transferred by RNA (a
  messenger substance)
• This message determines how the molecules of
  new DNA are to be arranged

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Plant Breeding

• Systematic process of matching genetic factors
  from parent plants to produce offspring that are
  superior to parents
• Early man used a primitive form of plant breeding

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Plant Breeding

• All plants came from wild plants that were
  domesticated
• The domesticated species are called cultivars

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What Plant Breeders Look For

• Increased yield
• Disease resistance
• More efficient water use
• Temperature tolerance
• Ease of harvest
• Uniform maturity
• Quality of fruit or usable portion

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Plant Breeding

• Breeders have increased adaptation of plants by a
  systematic system of plant breeding
• Hybrid crossing two purebred lines to result in
  a superior plant
• Corn most common hybrid
• Many vegetables and flowers now hybrids
• Heterosis the result of the crossing of
  unrelated parents
• Results in increased performance

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Hybrid Corn

• First developed in 1909
• Information on use taught to growers by Extension
  Service and Vocational Ag Classes in schools
• Breeder develops parent lines by inbreeding
  selections from populations
• Then crosses parent lines to create hybrids

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Hybrid Corn

• Crosses are made by collecting pollen from one
  plant and depositing on silk of recipient plant.

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Animal Breeding

• Less animals than in 1950, but production has
  increased
• Most slaughter animals are crossbred due to
  hybrid vigor (heterosis benefit)
• Selected for weight gain, size, conformation,
  litter size, etc.

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Animal Breeding

• Some new breeds have been developed by
  crossbreeding
• Santa Gertrudis - cross of two different
  species Bos taurus X Bos indicus
• Most cattle in America or Europe Bos taurus
• Cattle from India - Bos indicus (Brahman)

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Santa Gertrudis

• Developed on the King Ranch in Texas
• All Santa Gertrudis are descendants of the bull
  Monkey

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Animal Breeding

• Texas cattlemen wanted to combine gentle nature
  and carcass quality of the Shorthorn breed with
  vigor, heat resistance, and insect resistance of
  the Brahman cattle
• Other breeds developed by crossing Bos indicus
  Brangus, Simbrah, Braford, and Charbray

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Animal Breeding

• Computers have aided in sorting data
• Results in expected progeny differences (EPDs)
• Use data of offspring to make selections of which
  animals to breed to others or to keep
• Animals selected with leaner carcasses and better
  quality muscle result in increased flavor