Chapter 10 Sexual Reproduction and Genetics

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Chapter 10Sexual Reproduction and Genetics

• 10.1 Meiosis

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Chromosomes and Chromosome Number

• Human body cells have 46 chromosomes
• DNA on the chromosomes are arranged in sections
  that code for a trait these sections are genes
• Humans have approximately 23,000 genes

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Chromosomes and Chromosome Number

• Of the 46 human chromosomes each parent
  contributes 23 chromosomes
• These chromosomes are paired

• Homologous chromosomesone of two paired
  chromosomes, one from each parent

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Homologous Chromosomes

• Same centromere position
• Same length
• Homologous chromosomes contain the same genes
  although they may contain different versions of
  the gene

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Chromosomes and Chromosome Number

• Haploid cells contain only one of the homologous
  pair of chromosomes (half the number of
  chromosomes)
• Diploid cell contain the chromosomes in pairs
  (ditwo)

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Chromosomes and Chromosome Number

• Gametes (sex cells) contain the haploid number of
  chromosomes (n)
• Body cells contain the diploid number of
  chromosomes (2n)
• Human gametes (sperm and egg) have 23 chromosomes
  and body cells have 46 chromosomes

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Meiosis

• Gametes are formed during the process of meiosis
• Meiosis reduces diploid cells to haploid cells
• Fertilization restores the diploid number

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Meiosis

• Meiosis involves two consecutive sets of cell
  divisions
• Meiosis only occurs in the reproductive
  structures of organisms who reproduce sexually
• Most animals
• Most plants
• Most fungi
• Most protists

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Meiosis I and Meiosis II

• Meiosis I is the reduction division cells start
  out diploid and end up haploid
• In Meiosis II sister chromatids are separated
  (much like mitosis)

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Meiosis I

• Interphase

• Chromosomes replicate.

• Chromatin condenses.

Interphase
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Meiosis I

• Prophase I

• Pairing of homologous chromosomes occurs.

• Each chromosome consists of two chromatids.

Prophase I

• The nuclear envelope breaks down.

• Spindles form.

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Meiosis I

• Prophase I

• Crossing over produces exchange of genetic
  information.

• Crossing overchromosomal segments are exchanged
  between a pair of homologous chromosomes.

Tetrads are groups of four sister chromatids
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Meiosis I

• Metaphase I

• Chromosomes centromere attach to spindle fibers.

Metaphase I

• Homologous chromosomes line up at the equator in
  tetrads

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Meiosis I

• Anaphase I

Anaphase I
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Meiosis I

• Telophase I

• The spindles break down.

Telophase I

• Chromosomes uncoil and form two nuclei.

• The cell divides.

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Meiosis II

• Prophase II

Prophase II
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Meiosis II

• Metaphase II

Metaphase II
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Meiosis II

• Anaphase II

Anaphase II
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Meiosis II

• Telophase II

Telophase II
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Meiosis II

• Cytokinesis results in four haploid cells, each
  with n number of chromosomes.

Cytokinesis
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Meiosis

• Meiosis consists of two sets of divisions

• Produces four haploid daughter cells that are not
  identical

• Results in genetic variation

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Meiosis

• Depending on how the chromosomes line up at the
  equator, four gametes with four different
  combinations of chromosomes can result.

• Genetic variation also is produced during
  crossing over and during fertilization, when
  gametes randomly combine.

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Meiosis and Variation

• Number of possible genetic variations in the
  gametes equals
• 2n where n is the haploid number
• In humans number of possible genetic combinations
  in gametes is 223
• Add the genetic combinations that exist when
  crossing over exists (at 3 per meiosis) and get
  (223)3

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Meiosis and Variation, cont

• The possibility that (223)3 variations exists for
  each gamete
• When fertilization occurs this number must be
  doubled 2 x (223)3
• You are unique no one else exists or ever has
  existed that is just like you (unless you have an
  identical twin).

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Advantages of Asexual Reproduction

• The organism inherits all of its chromosomes from
  a single parent.

• The new individual is genetically identical to
  its parent.
• Usually occurs more rapidly than sexual
  reproduction

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Advantages of Sexual Reproduction

• Beneficial genes multiply faster over time.
• The organisms inherits genes from two parents and
  is not genetically identical to either parent.
• Ensures genetic variation

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Chapter 10Sexual Reproduction and Genetics

• 10.2 Mendelian Genetics

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Gregor Mendel

• Lived in Europe in what is now Czech Republic
  near the Austrian border.
• Father of Genetics
• Monk, entered monastery 1843

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Gregor Mendel

• Failed teachers exam
• When to U of Vienna
• Studied with physicist Doppler- science through
  experiment, applied math to science
• Studied with botanist Unger- interest in causes
  of variation in plants
• Passed teachers exam and taught at monasterys
  school also responsible for schools garden
• Published 1866, mathematics and plant breeding

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Mendel Studied Peas

• Available in many varieties
• Self pollinating (can manipulate pollination)
• either-or inherited traits
• Had true breeder for parental generation (P)
  due to flower structure

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Mendel Studied Peas

• The petals enclose the stamen (with pollen) so
  that cross pollination does not occur
• Cross pollination is easily accomplished by
  peeling back the petals and moving pollen with a
  paint brush

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Inheritance of Traits
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Inheritance of Traits

• The offspring of this P cross are called the
  first filial (F1) generation.

• The second filial (F2) generation is the
  offspring from the F1 cross.

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Pea Traits Studied by Mendel
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Inheritance of Traits

• Mendel studied thousand of pea plants for the
  seven traits.
• He concluded that
• Genes are in pairs
• Different versions of genes (alleles) account for
  variation in inherited characteristics
• Alleles can be dominant or recessive

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Dominant and Recessive

• Alleles can be dominant or recessive.
• An allele is dominant if it appears in the F1
  generation when true breeder parents are crossed.
• An allele is recessive if it is masked in the F1
  generation.

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Symbols

• To help make genetics easier symbols are used
• Capital letters are used for dominant alleles
• Lower case letters are used for recessive alleles
• The letter to use is based on the dominant trait
• Example purple is dominant to white, P would be
  the dominant allele and p the recessive allele

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Homozygous and Heterozygous

• Dominant traits can be homozygous or heterozygous
• Homo same the alleles would be the same, PP
• Heterodifferent the alleles would be different,
  Pp
• For the recessive trait to be expressed both
  alleles would be recessive, pp

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Genotype and Phenotype

• Genotype is the organism's gene pairs PP, Pp or
  pp
• Phenotype is the outward physical appearance or
  expression of the genotype purple or white

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Genotype and Phenotype

• If the phenotype displays the recessive trait
  (white) then you know the genotype pp

• If the phenotype displays the dominant trait
  (purple) then the genotype could be homozygous
  dominant (PP) or heterozygous (Pp)

Genotype is PP or Pp
Genotype is pp
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Punnett Squares

• Mathematical device for predicting the results of
  genetic cross
• Male gametes are written across the top
• Female gametes are written along the side
• Genetic possibilities of the offspring are in the
  boxes
• Expect 31 phenotypic ratio

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Monohybrid Cross

• Mono one
• One trait is studied at a time
• This one is seed color
• Monohybrid crosses provided evidence for the Law
  of Segregation

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Mendels Law of Segregation
Two alleles for each trait separate during meiosis
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Dihybrid Cross

• Di two
• Two traits are studied at a time
• This one is seed color (yellow or green) and seed
  shape (wrinkled or round)

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Dihybrid Cross

• Four types of alleles from the male gametes and
  four types of alleles from the female gametes can
  be produced.
• The resulting phenotypic ratio is 9331 which
  gave evidence for the Law of Independent
  Assortment

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Mendels Law of Independent Assortment

• Random distribution of alleles occurs during
  gamete formation

• Genes on separate chromosomes sort independently
  during meiosis.

• Each allele combination is equally likely to
  occur.

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Mendels Law of Independent Assortment
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Probability

• Genetic crosses predict what to expect in the
  phenotypes and genotypes of the offspring.
• Observed results are what you actually see with
  the organisms.
• The larger the number of offspring the closer the
  expected and observed results usually are.

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Chapter 10Sexual Reproduction and Genetics

• 10.3 Gene Linkage and Polyploidy

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Gene Linkage

• The linkage of genes on a chromosome results in
  an exception to Mendels law of independent
  assortment because linked genes usually do not
  segregate independently.

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Polyploidy

• Polyploidy is the occurrence of one or more extra
  sets of all chromosomes in an organism.
• Approximately 30 of flowers are polyploidy
• Strawberries are octoploidy (8n)

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Polyploidy

• Horticultural important plants are forced to
  polyploidy to increase the size and flavor of
  flowers and fruits and overall vigor of the
  plants.
• Polyploidy is uncommon in animals.