Inheritance, Genes, and Chromosomes

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Inheritance, Genes, and Chromosomes
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Chapter 8 Inheritance, Genes, and Chromosomes

• Key Concepts
• 8.1 Genes Are Particulate and Are Inherited
  According to Mendels Laws
• 8.2 Alleles and Genes Interact to Produce
  Phenotypes
• 8.3 Genes Are Carried on Chromosomes
• 8.4 Prokaryotes Can Exchange Genetic Material

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Chapter 8 Opening Question
How is hemophilia inherited through the mother,
and why is it more frequent in males?
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Concept 8.1 Genes Are Particulate and Are
Inherited According to Mendels Laws

• Early experiments with genetics yielded two
  theories
• Blending inheritancegametes contained
  determinants (genes) that blended when gametes
  fused during fertilization
• Particulate inheritanceeach determinant was
  physically distinct and remained intact during
  fertilization

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Concept 8.1 Genes Are Particulate and Are
Inherited According to Mendels Laws

• Mendel used the scientific method and studied
  garden peas.
• Their flowers have both male and female sex
  organs, pistils, and stamens, to produce gametes.
• Male organs can be removed to allow fertilization
  by another flower.

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In-Text Art, Ch. 8, p. 145
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Concept 8.1 Genes Are Particulate and Are
Inherited According to Mendels Laws

• Characterobservable physical feature (e.g.,
  flower color, seed shape)
• Traitform of a character (e.g., purple flowers
  or white flowers, wrinkled seeds)
• Mendel worked with true-breeding varietieswhen
  plants of the same variety were crossed, all
  offspring plants produced the same seeds and
  flowers.

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Concept 8.1 Genes Are Particulate and Are
Inherited According to Mendels Laws

• Mendels crosses
• Pollen from one parent was transferred to the
  stigma of the other parent.
• Parental generation P
• Resulting offspring first filial generation or
  F1
• If F1 plants self-pollinate, they produce second
  filial generation or F2.

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Concept 8.1 Genes Are Particulate and Are
Inherited According to Mendels Laws

• In Mendels first experiment, he crossed plants
  differing in just one character (P).
• This produced monohybrids in the F1 generation.
• The monohybrids were then allowed to
  self-pollinate to form the F2 generationa
  monohybrid cross.
• Mendel repeated this for seven characters.

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Figure 8.1 Mendels Monohybrid Experiments (Part
1)
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Figure 8.1 Mendels Monohybrid Experiments (Part
2)
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Concept 8.1 Genes Are Particulate and Are
Inherited According to Mendels Laws

• One trait of each pair disappeared in the F1
  generation and reappeared in F2 these traits are
  recessive.
• The trait that appears in the F1 is the dominant
  trait.
• The ratio of dominant traits to recessive traits
  in the F2 was about 31.

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Concept 8.1 Genes Are Particulate and Are
Inherited According to Mendels Laws

• Mendels observations rejected the blending
  theory of inheritance and supported the
  particulate theory.
• He proposed that the determinants occur in pairs
  and are segregated in the gametes.
• Each plant has two genes for each character, one
  from each parent.
• Diploidtwo copies of a gene
• Haploidone copy of a gene

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Concept 8.1 Genes Are Particulate and Are
Inherited According to Mendels Laws

• Alleles are different forms of a gene, such as
  smooth or wrinkled seeds.
• True-breeding individuals have two copies of the
  same allelethey are homozygous for the allele
  (e.g., ss).
• Heterozygous individuals have two different
  alleles (e.g., Ss).

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Concept 8.1 Genes Are Particulate and Are
Inherited According to Mendels Laws

• Phenotypephysical appearance of an organism
  (e.g., spherical seeds)
• Genotypethe genetic makeup (e.g., Ss)
• Spherical seeds can be the result of two
  different genotypesSS or Ss.

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Concept 8.1 Genes Are Particulate and Are
Inherited According to Mendels Laws

• Mendels first law
• The law of segregation states that the two copies
  of a gene separate when an individual makes
  gametes.
• Each gamete receives only one copy.

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Figure 8.2 Mendels Explanation of Inheritance
(Part 1)
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Concept 8.1 Genes Are Particulate and Are
Inherited According to Mendels Laws

• When the F1 self-pollinates, there are three ways
  to get the dominant trait (e.g., spherical), but
  only one way to get the recessive
  (wrinkled)resulting in the 31 ratio.
• Allele combinations can be predicted using a
  Punnett square.

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Figure 8.2 Mendels Explanation of Inheritance
(Part 2)
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Concept 8.1 Genes Are Particulate and Are
Inherited According to Mendels Laws

• A gene is a short sequence on a longer DNA
  molecule.
• DNA molecules make up the chromosomes.
• Different alleles of a gene segregate when
  chromosomes separate during meiosis I.

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Figure 8.3 Meiosis Accounts for the Segregation
of Alleles (Part 1)
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Figure 8.3 Meiosis Accounts for the Segregation
of Alleles (Part 2)
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Figure 8.3 Meiosis Accounts for the Segregation
of Alleles (Part 3)
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Concept 8.1 Genes Are Particulate and Are
Inherited According to Mendels Laws

• Mendel tested his hypothesis by doing test
  crosses
• He did this to determine whether an individual is
  homozygous or heterozygous for a trait by
  crossing it with a homozygous recessive
  individual.
• Mendel crossed the F1 with known homozygotes
  (e.g., wrinkled or ss).

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Figure 8.4 Homozygous or Heterozygous? (Part 1)
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Figure 8.4 Homozygous or Heterozygous?
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Concept 8.1 Genes Are Particulate and Are
Inherited According to Mendels Laws

• Mendels next experiment involved
• Crossing peas that differed in two
  charactersseed shape and seed color
• True-breeding parents
• SSYYspherical yellow seeds
• ssyywrinkled green seeds

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Concept 8.1 Genes Are Particulate and Are
Inherited According to Mendels Laws

• F1 generation is SsYyall spherical yellow.
• Crossing the F1 generation (all identical double
  heterozygotes) is a dihybrid cross.
• Mendel asked whether, in the gametes produced by
  SsYy, the traits would be linked, or segregate
  independently.

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Concept 8.1 Genes Are Particulate and Are
Inherited According to Mendels Laws

• Two possibilities included
• Alleles could maintain associations seen in
  parental generationthey could be linked
• If linked, gametes would be SY or sy F2 would
  have three times more spherical yellow than
  wrinkled green.
• If independent, gametes could be SY, sy, Sy, or
  sY. F2 would have nine different genotypes
  phenotypes would be in 9331 ratio.

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Concept 8.1 Genes Are Particulate and Are
Inherited According to Mendels Laws

• Or
• The segregation of S from s could be independent
  of Y from ythe two genes could be unlinked
• If independent, gametes could be SY, sy, Sy, or
  sY in equal numbers.
• The F2 generation would have nine different
  genotypes and four phenotypes in a 9331
  ratio.
• This prediction was supported.

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Figure 8.5 Independent Assortment (Part 1)
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Figure 8.5 Independent Assortment (Part 2)
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Concept 8.1 Genes Are Particulate and Are
Inherited According to Mendels Laws

• Mendels second law
• The law of independent assortment states that
  alleles of different genes assort independently
  during gamete formation.
• This law doesnt always apply to genes on the
  same chromosome, but chromosomes do segregate
  independently.

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Figure 8.6 Meiosis Accounts for Independent
Assortment of Alleles
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Concept 8.1 Genes Are Particulate and Are
Inherited According to Mendels Laws

• One of Mendels contributions to genetics was the
  use of mathematical analysesthe rules of
  statistics and probability.
• His analyses revealed patterns that allowed him
  to formulate his hypotheses.
• Probability calculations and Punnett squares give
  the same results.

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Concept 8.1 Genes Are Particulate and Are
Inherited According to Mendels Laws

• Probability
• If an event is certain to happen, probability 1
• If an event cannot possibly happen, probability
  0
• All other events have a probability between 0 and
  1

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Concept 8.1 Genes Are Particulate and Are
Inherited According to Mendels Laws

• Two coin tosses are independent events, each will
  come up heads ½ the time.
• The probability that both will come up heads is
• ½ x ½ ¼
• To get the joint probability, multiply the
  individual probabilities (multiplication rule).

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Figure 8.7 Using Probability Calculations in
Genetics
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Concept 8.1 Genes Are Particulate and Are
Inherited According to Mendels Laws

• Probability in a monohybrid cross
• After self-pollination of an F1 Ss, the
  probability that the F2 offspring will have the
  genotype SS is ½ x ½ ¼ the same for ss
  offspring.
• There are two ways to get a heterozygote Ss the
  probability is the sum of the individual
  probabilities (addition rule)
• ¼ ¼ ½

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Concept 8.1 Genes Are Particulate and Are
Inherited According to Mendels Laws

• Human pedigrees can show Mendels laws.
• Humans have few offspring pedigrees do not show
  the clear proportions that the pea plants showed.
• Geneticists use pedigrees to determine whether a
  rare allele is dominant or recessive.

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Concept 8.1 Genes Are Particulate and Are
Inherited According to Mendels Laws

• Pattern of inheritance for a rare dominant
  allele
• Every person with the abnormal phenotype has an
  affected parent.
• Either all (if homozygous parent) or half (if
  heterozygous parent) of offspring in an affected
  family are affected.

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Figure 8.8 Pedigree Analysis and Inheritance
(Part 1)
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Concept 8.1 Genes Are Particulate and Are
Inherited According to Mendels Laws

• Pattern of inheritance for a rare recessive
  allele
• Affected people often have two unaffected
  parents.
• In an affected family, one-fourth of children of
  unaffected parents are affected.

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Figure 8.8 Pedigree Analysis and Inheritance
(Part 2)
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Concept 8.2 Alleles and Genes Interact to
Produce Phenotypes

• Different alleles arise through mutation rare,
  stable, inherited changes in the genetic
  material.
• The wild type is the allele present in most of
  the population. Other alleles are mutant alleles.
• A gene with a wild-type allele that is present
  less than 99 percent of the time is called
  polymorphic.

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Concept 8.2 Alleles and Genes Interact to
Produce Phenotypes

• A given gene may have more than two alleles.
• Multiple alleles increase the number of possible
  phenotypes and may show a hierarchy of dominance
  in heterozygotes.
• One example is the coat color in rabbits.

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Figure 8.9 Multiple Alleles for Coat Color in
Rabbits
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Concept 8.2 Alleles and Genes Interact to
Produce Phenotypes

• Some alleles are neither dominant nor recessive.
• A heterozygote has an intermediate phenotype in
  incomplete dominance.
• Red white snapdragons pink in F1
• Red and white colors reappear in F2 as well as
  pink.

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Figure 8.10 Incomplete Dominance Follows
Mendels Laws (Part 1)
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Figure 8.10 Incomplete Dominance Follows
Mendels Laws (Part 2)
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Concept 8.2 Alleles and Genes Interact to
Produce Phenotypes

• Codominancetwo alleles of a gene produce
  phenotypes that are both present in the
  heterozygote.
• Example
• ABO blood group system has three alleles of the
  gene IA, IB, and IO.

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Figure 8.11 ABO Blood Reactions Are Important in
Transfusions
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Concept 8.2 Alleles and Genes Interact to
Produce Phenotypes

• Epistasisphenotypic expression of one gene is
  influenced by another gene
• Example
• Coat color in Labrador retrievers
• Allele B (black) dominant to b (brown)
• Allele E (pigment deposition) is dominant to e
  (no pigment depositionyellow)

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Figure 8.12 Genes Interact Epistatically
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Concept 8.2 Alleles and Genes Interact to
Produce Phenotypes

• Hybrid vigor, or heterosis, is a cross between
  two different true-breeding homozygotes.
• It can result in offspring with stronger, larger
  phenotypes.
• Most complex phenotypes are determined by
  multiple genes.
• Quantitative traits conferred by multiple genes
  are measured, rather than assessed qualitatively.

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In-Text Art, Ch. 8, p. 154
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Concept 8.2 Alleles and Genes Interact to
Produce Phenotypes

• Genotype and environment interact to determine
  the phenotype of an organism.
• Two parameters describe the effects
• Penetrance is the proportion of individuals with
  a certain genotype that show the phenotype.
• Expressivity is the degree to which genotype is
  expressed in an individual.

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Concept 8.3 Genes Are Carried on Chromosomes

• Genes are sequences of DNA that reside at a
  particular site on a chromosomea locus (plural
  loci).
• The genetic linkage of genes on a single
  chromosome can alter their patterns of
  inheritance.

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Concept 8.3 Genes Are Carried on Chromosomes

• Genetic linkage was discovered by Thomas Hunt
  Morgan and students at Columbia University using
  the fruit fly Drosophila melanogaster.
• Much genetic research has been done with
  Drosophila, which is considered a model organism
  because of its size, ease of breeding, and short
  generation time.

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Concept 8.3 Genes Are Carried on Chromosomes

• Some crosses performed with Drosophila did not
  yield expected ratios according to the law of
  independent assortment.
• Instead, some genes for body color and wing shape
  were inherited together.
• Morgan theorized that the two loci were linked on
  the same chromosome and could not assort
  independently.

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Figure 8.13 Some Alleles Do Not Assort
Independently (Part 1)
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Figure 8.13 Some Alleles Do Not Assort
Independently (Part 2)
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Concept 8.3 Genes Are Carried on Chromosomes

• Some offspring showed recombinant phenotypes,
  different from their parents.
• Genes may recombine during prophase I of meiosis
  by crossing over.
• Homologous chromosomes exchange corresponding
  segments.
• The exchange involves two chromatids of four in
  the tetradboth chromatids become recombinant
  (each ends up with genes from both parents).

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Figure 8.14 Crossing Over Results in Genetic
Recombination (Part 1)
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Figure 8.14 Crossing Over Results in Genetic
Recombination (Part 2)
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Figure 8.14 Crossing Over Results in Genetic
Recombination (Part 3)
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Concept 8.3 Genes Are Carried on Chromosomes

• Recombinant offspring phenotypes (non-parental)
  appear in recombinant frequencies.
• To determine the recombinant frequencies, divide
  the number of recombinant offspring by the total
  number of offspring.
• Recombinant frequencies are greater for loci that
  are farther apart on the chromosome.

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Figure 8.15 Recombination Frequencies (Part 1)
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Figure 8.15 Recombination Frequencies (Part 2)
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Concept 8.3 Genes Are Carried on Chromosomes

• Recombinant frequencies can be used to make
  genetic maps showing the arrangement of genes
  along a chromosome.
• Recombinant frequencies are converted to map
  units corresponding to distances between genes.

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In-Text Art, Ch. 8, p. 157 (1)
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Concept 8.3 Genes Are Carried on Chromosomes

• The fruit fly genome has four pairs of
  chromosomesthree pairs are similar in size,
  called autosomes.
• The fourth pair are of different size, the sex
  chromosomes.
• Many genes on the X chromosome are not present on
  the Y chromosome.

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In-Text Art, Ch. 8, p. 157 (2)
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Concept 8.3 Genes Are Carried on Chromosomes

• Genes on sex chromosomes dont follow Mendelian
  patterns.
• The Y chromosome carries few genes the X
  chromosome carries many.
• Thus, males have only one copy of these
  geneshemizygous.

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Concept 8.3 Genes Are Carried on Chromosomes

• Sex-linked inheritanceinheritance of a gene that
  is carried on a sex chromosome
• One example is the eye color in Drosophila.

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Figure 8.16 A Gene for Eye Color Is Carried on
the Drosophila X Chromosome (Part 1)
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Figure 8.16 A Gene for Eye Color Is Carried on
the Drosophila X Chromosome (Part 2)
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Concept 8.3 Genes Are Carried on Chromosomes

• X-linked recessive phenotypes
• They appear much more often in males than
  females.
• A male with the mutation can only pass it on to
  daughters.
• Daughters who receive one X-linked mutation are
  heterozygous carriers.
• Mutant phenotype can skip a generation if it
  passes from a male to his daughter (normal) and
  then to her son.

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Figure 8.17 RedGreen Color Blindness Is Carried
on the Human X Chromosome
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Concept 8.3 Genes Are Carried on Chromosomes

• Besides the genes in the nucleus, mitochondria
  and plastids contain small numbers of genes.
• Mitochondria and plastids are inherited only from
  the mother.
• The inheritance of organelles and their genes is
  non-Mendelian and is called maternal or
  cytoplasmic inheritance.

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Figure 8.18 Cytoplasmic Inheritance
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Concept 8.4 Prokaryotes Can Exchange Genetic
Material

• Bacteria exchange genes by bacterial conjugation.
• Sex pilus is a projection that initiates contact
  between bacterial cells.
• Conjugation tube is a cytoplasmic bridge that
  forms between cells.
• The donor chromosome fragments and some material
  enters the recipient cell.

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Figure 8.19 Bacterial Conjugation and
Recombination (Part 1)
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Figure 8.19 Bacterial Conjugation and
Recombination (Part 2)
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Concept 8.4 Prokaryotes Can Exchange Genetic
Material

• Bacteria have plasmidssmall circular DNA
  moleculesbesides the main chromosome.
• Genes on the plasmids are in categories
• Metabolic tasks, breaking down hydrocarbons
• Involved in conjugation
• Antibiotic resistance

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Concept 8.4 Prokaryotes Can Exchange Genetic
Material

• Plasmids can move between the cells during
  conjugation.
• They can
• Replicate independently of the main chromosome
• Add their genes to the recipient cells genome

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Figure 8.20 Gene Transfer by Plasmids
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Answer to Opening Question

• In hemophilia, the mutant gene for factor VIII,
  the clotting factor, is carried on the X
  chromosome.
• The affected males inherited their single X
  chromosome from their mothersif the mutated form
  of the gene was present, they would develop the
  disease.
• Daughters would inherit a normal X chromosome as
  well and would not express the recessive trait,
  though could be carriers.

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Figure 8.21 Sex Linkage in Royal Families of
Europe