Biology Notes Chapter 9

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Biology Notes Chapter 9

• Introduction to Genetics

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Gregor Mendel

• Gregor Mendel, an Austrian is credited
  as being the father of
  genetics.
• Mendel worked a lot with the pea plant.
• He was able to control the
  manner in which pea plants
  reproduced.
• He was able to self pollinate his
  plants, by taking pollen grains from
  the plant and pollinating
  the egg of the same plant.
• Seeds produce by self pollination inherit all
  their characteristics from the single plant that
  bore them.
• These plants are referred to as purebreds,
  meaning that they are not the product of any
  cross pollination.

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Mendel Continued

• Mendel found that self pollination could be
  prevented if the male part of the plant was
  removed.
• When the male part was removed and pollen grains
  from another pea plant was used cross pollination
  has occurred.
• Cross pollination produces seeds that are the
  offspring of two different plants.
• With this technique, Mendel was able to cross
  plants with different characteristics.

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Mendel Continued

• Mendel studied the traits, which are
  characteristics that are easily observed, of
  peas.
• Mendels experimentation with pea plants led him
  to produce hybrids.
• A hybrid is an organism that is produced by
  crossing parents with differing characteristics.

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Mendel Continued

• From his experiments Mendel was able to form two
  conclusions.
• The first is that individual factors, which do
  not blend with one another, control each trait of
  a living thing.
• The individual factors that control each trait
  are called genes.
• The different forms of a gene are called alleles.
• An example of alleles would be a tall pea plant
  gene and a short pea plant gene (both control the
  height of a plant.)

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Mendel Continued

• The second of Mendels conclusions is often
  called the principle of dominance.
• Some alleles are dominant, whereas some are
  recessive.
• The effects of a dominant allele are seen even if
  it is present with a contrasting recessive
  allele.
• The effects of a recessive allele are not
  observed when the dominant allele is present.

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Segregation

• Mendel noticed that recessive traits disappeared
  when two purebreds were crossed.
• In his search to find out what happened to these
  recessive traits, Mendel allowed several of the
  hybrid plants to reproduce by self pollination.
• To keep the different groups of seeds and plants
  clear in his mind, Mendel gave them different
  names.
• The purebred parental plants are called the P
  generation.
• The first generation produced by cross
  pollination is called the first filial
  generation, or F1.
• Each successive generation is labeled in order.
  (F2, F3, F4 ..................)

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Punnett Squares

• Biologists represent a particular gene by using a
  symbol.
• The dominant allele is represented by a capital
  letter.
• The recessive allele is represented by the
  corresponding lower case letter.
• The Punnett square shows the types of
  reproductive cells, or gametes, that are produced
  by each of the parents.
• It also shows each possible gene combination for
  the next generation.

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Punnett Squares

• The phenotype is the characteristics that are
  shown after a cross. (These are the physical
  characteristics.)
• The genotype is the actual genetic make up of the
  organishas two identical alleles.
• The term heterozygous means that an organism has
  two different alleles for the same trait.

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Procedure For A Single Factor Cross

• Always use the first letter of the dominant
  trait.
• Always put the males alleles across the top of
  the Punnett square, and the female alleles along
  the left side.
• The dominant traits are always capitalized.

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Punnet square practice problem

• Cross a purebred male tall pea plant with a
  purebred short female pea plant.
• Tall is dominant in pea plants.
• Phenotype All 4 will be tall so it is expressed
  as 40
• Genotype All 4 will be Tt so it is expressed
  as 40
• Since all of them have both the dominant and
  recessive trait they are heterozygous tall.

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Practice Crosses

• Cross a Road Island red rooster with a leghorn
  white hen. (Both are purebreds) Red is dominant.
• Cross a heterozygous tall pea plant (male) with a
  homozygous short pea plant (female) Tall is
  dominant.
• Cross a heterozygous tall male pea plant with a
  heterozygous tall female pea plant. Tall is
  dominant.

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Independent Assortment

• Independent assortment means that genes are not
  connected to one another and that they may
  segregate independently.
• This means that just because a pea plant is tall
  that it does not necessarily mean that it will
  have axial flowers. It could have terminal
  flowers since the genes undergo independent
  assortment.

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Summary of Mendels Work

• Mendels work on the genetics of peas can be
  summarized with four basic statements.
• The factors that control heredity are individual
  units called genes.
• In organisms that reproduce sexually genes are
  inherited from each parent.
• In cases in which an organism possesses two
  forms of a gene for a single trait, some forms
  of the gene may be dominant and others may be
  recessive.
• The two forms of each gene are segregated during
  the formation of the reproductive cells
• The genes for different traits may assort
  independently of one another.

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Applying Mendels Principles

• Mendel applied the math concept of probability to
  biology.
• Probability is the likelihood that a particular
  event will occur.
• Probability the number of times an event occurs
• the number of opportunities for the
  event to occur. (number of trials)

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Probability Continued

• The rules of probability apply to genetics as
  well as they do to flipping a coin.
• Expected genetic ratios may not show up when only
  a few pea plants (or other organisms) are
  considered.
• The larger the number of organisms examined, the
  closer the numbers will be to the expected
  values.
• Genetic ratios do not indicate what the outcome
  of a single event will be.
• Because previous events do not affect future
  outcomes, it cant be assumed that a particular
  event will occur just because it is over due.
• Genetic ratios are useful because they make it
  possible to predict the most likely outcome for a
  large number of events.

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Meiosis

• Meiosis is a process of reduction division in
  which the number of chromosomes per cell is cut
  in half and the homologous chromosomes that exist
  in a diploid cell are separated.
• Each chromosome in one sex of a species of an
  organism has a corresponding chromosome in the
  member of the opposite sex of that species.
• These corresponding chromosomes are said to be
  homologous, and are called homologs.

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Meiosis Continued

• A cell that contains both sets of homologous
  chromosomes (one set from each parent) is said to
  be diploid. (The diploid number is referred to as
  2N.)
• A diploid cell contains two complete sets of
  chromosomes and two complete sets of genes.
• This agrees with Mendels idea that all of an
  organisms cells (except gametes) contain two
  alleles for a given trait.

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Meiosis Continued

• Cells that contain a single set of chromosomes
  are said to be haploid.
• These are represented by the symbol N.
• In order for gametes to be produced, there must
  be a process that divides the diploid number of
  chromosomes in half.
• This is done during meiosis.

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The Phases of Meiosis

• Haploid gametes are produced from diploid cells
  during meiosis.
• Meiosis is a process of reduction division in
  which the number of chromosomes per cell is cut
  in half and the homologous chromosomes that exist
  in a diploid cell are separated.
• In most organisms, meiosis takes place in two
  stages, know as the first and second meiotic
  divisions.

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Meiosis I

• During meiosis I, cells in the reproductive
  organs undergo DNA replication.
• In a cell with a diploid number of 8 (2N 8)
  during prophase I, each chromosome seeks out its
  corresponding homologous chromosome to form a
  special structure called a tetrad.
• A tetrad contains 4 chromosomes in a cell where
  2N 8

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Meiosis I Continued

• Tetrads line up, rather than individual
  chromosomes lining up in the center of the cell
  during metaphase I of meiosis.
• Crossing over may occur when chromosomes pair up
  and form tetrads, in prophase I, they may
  exchange portions of their chromatids.
• Crossing over is the exchange or swapping of
  portions of their genetic information producing
  new combinations of genes.

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Meiosis I Continued

• The homologous chromosomes then separate and two
  new cells are formed.
• Even though each cell still has 8 chromatids as
  it would in mitosis, something is different.
• The cells no longer have two complete sets of
  chromosomes.
• Instead the maternal and paternal chromosomes
  have been shuffled like a deck of cards.
• The 2 cells produced by meiosis I have sets of
  chromosomes that are different than each other
  and the cell that began the division.

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Meiosis II

• In meiosis II the two cells produced by meiosis I
  divide, but there is no DNA replication.
• Each of the cells chromosomes contains 2
  chromatids.
• During metaphase II, 4 chromosomes line up in the
  center of each cell.
• In anaphase II the paired chromatids split.
• Each of the 4 daughter cells produced in meiosis
  II receives 4 chromatids.
• The 4 daughter cells contain the haploid number
  (N) with 4 chromosomes each.
• The amount of genetic material has been reduced
• In addition, the combinations of chromosomes in
  each gamete have been made at random.

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Meiosis and Genetics

• As cells undergo meiosis they follow the path
  that Mendel predicted that structures that carry
  genes would do.
• Meiosis I results in segregation and independent
  assortment.
• During Anaphase I the homologous chromosomes
  separate and are segregated to different cells.

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Gamete Formation

• In male animals, the haploid gametes produced by
  meiosis are called sperm.
• In higher evolved plants, pollen grains contain
  haploid sperm cells.
• In female animals, generally only one of the
  cells produced by meiosis is used for
  reproduction.
• The female gamete is called an egg in animals and
  an ovule in plants.

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Female gametes continued

• In female animals, the cell division at the end
  of meiosis I and meiosis II are uneven, so that
  the egg or ovule receives most of the cytoplasm.
• The other three cells produced in the female
  during meiosis are known as polar bodies and
  usually do not participate in reproduction .
• These cells will usually be reabsorbed by the
  body.