Mendel and Genetics

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Chapter 14

• Mendel and Genetics

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• Genetics
• Heredity
• Chromosome
• Gene
• Locus
• Alleles
• Example human blood group
• Genome

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• Modern genetics had its beginning in an abbey
  garden
• An Austrian monk, Gregor Mendel, documented the
  mechanisms of inheritance using garden peas
• He developed his theory of inheritance several
  decades before the behavior of chromosomes was
  observed

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• Heritable variations may be observed among
  individuals in a population
• What genetic principles account for the
  transmission of such traits from parents to
  offspring?
• One possible explanation of heredity is a
  "blending" hypothesis
• genetic material of two parents mix
• Everyday observations and breeding experiments
  contradict hypothesis
• fails to explain other phenomena of inheritance,
  such as traits skipping a generation

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• An alternative is a "particulate" hypothesis of
  inheritance the gene idea
• parents pass on discrete heritable
  units--genes--that retain their separate
  identities in offspring
• An organisms collection of genes can be sorted
  and passed along, generation after generation, in
  undiluted form
• The Mendelian model of genetics applies to the
  inheritance of human variations

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• Mendel brought an experimental and quantitative
  approach to genetics
• probably chose to work with peas because they are
  available in many varieties
• could control which individuals mated
• A character is a heritable feature that varies
  among individuals
• flower color
• A trait is a variation of a character
• purple or white flowers

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• Mendel chose to track only those characters that
  varied in an "either-or" manner
• plants had either purple flowers or white flowers
  
• Experimented with varieties that were
  true-breeding
• when the plants self-pollinate, all their
  offspring are of the same variety
• a plant with purple flowers is true-breeding if
  the all plants it gives rise to also have purple
  flowers

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• Mendel would cross-pollinate two contrasting,
  true-breeding pea varieties
• purple-flowered plants and white-flowered plants
• Hybridization crossing two true-breeding
  varieties
• true-breeding parents are the P or parental
  generation
• Hybrid offspring are the F1 generation
• F1 hybrids self-pollinate to produce an F2
  generation
• Analysis of F2 plants revealed the two
  fundamental principles of heredity
• law of segregation and the law of independent
  assortment

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• Law of segregation
• the two alleles for a characteristic are packaged
  into separate gametes 
• Law of independent assortment
• each pair of alleles segregates into gametes
  independently 
• Mendelian inheritance reflects rules of
  probability 

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• If the blending model of inheritance were
  correct, the F1 hybrids from a cross between
  purple-flowered and white-flowered pea plants
  would have pale purple flowers
• intermediate between the two varieties
• The F1 offspring all had flowers just as purple
  as the purple-flowered parents

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• F1 plants self-pollinated and the white-flower
  trait reappeared in the F2 generation
• Data fit a ratio of about three purple to one
  white
• In Mendels terminology, the purple flower is a
  dominant trait and the white flower is a
  recessive trait
• Mendel observed the same pattern of inheritance
  in six other characters, each represented by two
  different varieties
• Seed shape, seed color, flower location, pod
  shape, pod color and stem length

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• Alternative versions of genes account for
  variations in inherited characters
• The gene for flower color exists in two versions,
  one for purple flowers and the other for white
• Concept can be related to chromosomes and DNA
• Each gene resides at a specific locus on a
  chromosome
• Gene may vary in its sequence of nucleotides
  which varies its information content
• The purple-flower allele and the white-flower
  allele are two DNA variations possible at the
  flower-color locus on one of a pea plants
  chromosomes

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• For each character, an organism inherits two
  alleles
• one from each parent
• A diploid organism has homologous pairs of
  chromosomes
• one chromosome of each pair inherited from each
  parent
• A genetic locus is actually represented twice in
  a diploid cell
• These homologous loci may have identical alleles
  Or the two alleles may differ

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• If the two alleles differ, then one is dominant
• fully expressed in the organisms appearance
• The other is recessive
• has no effect on the organisms appearance
• Mendels F1 plants had purple flowers because the
  allele for that variation is dominant and the
  allele for white flowers is recessive
• The two alleles for each character segregate
  (separate) during gamete production
• corresponds to the distribution of homologous
  chromosomes to different gametes in meiosis

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• If an organism has identical alleles for a
  particular character-then that allele exists in a
  single copy in all gametes
• If different alleles are present, as in the F1
  hybrids, then 50 of the gametes receive the
  dominant allele, while 50 receive the recessive
  allele
• Mendels law of segregation
• Derived from experiments in which he followed
  only a single character
• The F1 hybrids produced are called monohybrids

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• What would happen if he followed two characters
  at the same time?
• Seeds may be either yellow or green. They also
  may be either round (smooth) or wrinkled
• He crossed two true-breeding pea varieties
  differing in both of these characters--a parental
  cross between a plant with yellow-round seeds
  (YYRR ) and a plant with green-wrinkled seeds
  (yyrr )
• The F1 plants were dihybrids, heterozygous for
  both characters (YyRr)

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• Were these two characters, seed color and seed
  shape, transmitted from parents to offspring as a
  package?
• Will the Y and R alleles always stay together,
  generation after generation?
• Or are seed color and seed shape inherited
  independently of each other?

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• Mendel experiments showed independent
  segregation of each pair of alleles during gamete
  formation
• Law of independent assortment

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• Punnett square is used for predicting the results
  of a cross between individuals of known genotype
• capital letter is used to symbolize a dominant
  allele and a lowercase letter for a recessive
  allele
• (P1 cross) PP purple flowers X pp white
  flowers
• p p
• P Pp Pp
• P Pp Pp
• F1 generation are all Pp hybrids with purple
  flowers

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• Crossing the F1 hybrids yields different results
• F1 cross - Pp X Pp
• P p
• P PP Pp
• p Pp pp
• F2 generation displays 2 Pp hybrids with purple
  flowers, 1 PP with purple flowers and the
  reemergence of a pp individual with white flowers

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• Homozygous organisms have a pair of identical
  alleles for a character
• PP is dominant homozygous
• pp is homozygous, but for the recessive allele
• If we cross dominant homozygotes with recessive
  homozygotes, every offspring will have two
  different alleles (Pp)
• Organisms having two different alleles for a
  trait are said to be heterozygous
• A Pp plant of the F1 generation will produce both
  purple-flowered and white-flowered offspring when
  it self-pollinates
• Phenotype and genotype are not always the same

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• Not all traits exhibit the typical
  dominance/recessive inheritance pattern
• Incomplete dominance
• The heterozygote will appear as an intermediate
  of the two homozygotes
• Snapdragon flowers - RR red, rr white, Rr pink
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• 2. Codominance full expression of both alleles
• MN blood group in humans
• MM type M, NN type N, MN type MN
• MN phenotype is not an intermediate between the M
  and N phenotypes
• both the M and N phenotypes are individually
  expressed by the presence of the two types of
  molecules on red blood cells

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• 3. Multiple Alleles more than two alternate
  forms
• ABO blood group in humans, type A, type B, or
  type O
• The O allele is recessive to the A and B allele,
  but the A and B allele act codominantly.
• These groups are based on blood-cell molecules
  different from those used for the MN
  classification discussed earlier
• A persons blood cells may have one substance or
  the other (type A or B), both (type AB), or
  neither (type O)
• The four blood groups result from various
  combinations of three different alleles of one
  gene

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• Most genes have multiple phenotypic effects
• Pleiotropy a single gene affects several
  phenotypic characters
• certain hereditary diseases in humans, like
  sickle-cell disease, cause multiple symptoms
• Epistasis a gene at one locus alters the
  phenotypic expression of a gene at a second
  locus.
• Mouse hair color 1 gene determines color and a
  2nd determines if hair will accept color

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• Polygenic inheritance additive effect of two or
  more genes on a single phenotypic character
• the converse of pleiotropy, where a single gene
  affects several phenotypic characters
• Skin pigmentation is controlled by 3 genes

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• Environmental conditions can influence phenotypic
  expression
• A single genotype can have a range of phenotypic
  expression
• Hydrangea flowers of the same genetic variety
  range in color from blue-violet to pink,
  depending on the acidity of the soil