Inquiry into Life Twelfth Editio

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					              23.1 Mendel’s Laws
• Gregor Mendel
  – Augustinian Monk
  – Around 1857, began
    breeding garden peas to
    study inheritance.
  – Performed crosses between
    true breeding lines
    of garden peas that differed
    in a single trait.
• Pea plants have several advantages for genetics:
   – Pea plants are available in many varieties with distinct
     heritable features (characters) with different variants
     (traits).

 Character                        Traits


Seed shape
                       Round                Wrinkled


 Seed color
                       Yellow                Green



 Pod shape
                       Inflated            Constructed


 Pod color
                       Green                  Yellow
• Mendel could control which plants mated with
  which.
  – Each pea plant has male (stamens) and female
    (carpal) sexual organs.
  – In nature, pea plants typically self-fertilize, fertilizing
    ova with their own sperm.
  – However, Mendel could also move pollen from one
    plant to another to cross-pollinate plants.
          Self-pollination
                                                 SELF-POLLINATION


Stigma (receives pollen)




                                                         Anthers
                                                         (produce pollen
                                                         grains, which
                                                         contain male
                                                         gametes)




               Ovules (produce female gametes)
Cross-Pollination
              1. Remove anthers
              from one plant.




              2. Collect pollen
              from a different plant.




              3. Transfer pollen
              to a stigma of the
              individual whose
              anthers have been
              removed.
                Mendel’s Experiment
• In a typical breeding experiment, Mendel would cross-
  pollinate (hybridize) two contrasting, true-breeding pea
  varieties.
    – The true-breeding parents are the P generation and their hybrid
      offspring are the F1 generation.


 Hypotheses:
 1. All offspring will be a blend of
    the two colors (lavender)
 2. All offspring will be some of
    each color
 3. All offspring will be one color
    or the other
• Mendel then allowed the F1 hybrids to self-pollinate to
  produce an F2 generation.
• When Mendel allowed the
  F1 plants to self-fertilize,
  the F2 generation included
  both purple-flowered and
  white-flowered plants.
    – The white trait, absent in the
      F1, reappeared in the F2.



• Based on a large sample size, Mendel recorded 705
purple-flowered F2 plants and 224 white-flowered F2 plants
from the original cross (a ratio of three purple to one white flowering
plant in the F2 offspring).
          Mendel’s Conclusions
• Mendel reasoned that the heritable factor for
  white flowers was present in the F1 plants, but it
  did not affect flower color.
   – Purple flower is a dominant trait and white flower
     is a recessive trait.
• Mendel’s quantitative analysis of F2 plants
  revealed the two fundamental principles of
  heredity:
   – law of segregation
   – law of independent assortment.
              Law of Segregation
• Four related ideas:
  1. Different factors or alternative versions of
  genes (alleles) account for variations in inherited
  characters.
  – Different alleles vary somewhat in the sequence of
    nucleotides at the specific locus (location) on paired
    chromosomes.
     • The purple-flower
       allele and white-flower
       allele are two DNA
       variations at the
       flower-color locus.
2. For each character, an organism inherits two
alleles, one on each homologous chromosome
from each parent.
– Each diploid organism has a pair of homologous
  chromosomes and therefore two copies of each locus.
– A diploid organism inherits one set of chromosomes
  from each parent.
– These homologous loci may be identical, as in the
  true-breeding plants of the P generation.
– Alternatively, the two alleles may differ
   • In the flower-color example, the F1 plants inherited a purple-
     flower allele from one parent and a white-flower allele from the
     other.
3. If two alleles differ, then one, the dominant
allele, is fully expressed in the the organism’s
appearance.
– The other, the recessive allele, has no noticeable
  effect on the organism’s appearance.
– Mendel’s F1 plants had purple flowers because the
  purple-flower allele is dominant and the white-flower
  allele is recessive.
  4. The two alleles for each character segregate
  (separate) during gamete production when the
  homologous chromosomes are separated and
  distributed to different gametes in meiosis.
  – If an organism has identical alleles for a particular
    character, then that allele exists as a single copy in all
    gametes.
  – If different alleles are present, then 50% of the
    gametes will receive one allele and 50% will receive
    the other.
• The separation of alleles into separate gametes
  is summarized as Mendel’s law of segregation.
   Law of Segregation - Summary

• Each individual has alleles for each trait
• The alleles segregate (separate) during the
  formation of gametes
• Each gamete contains only one allele from each
  pair of alleles
• Fertilization gives each new individual two
  alleles for each trait
Homologous Chromosomes
       Inheritance of a Single Trait

• Phenotype: physical appearance of the individual with
  regard to a trait
• Genotype: alleles responsible for a given trait
   – Two alleles for a trait
   – A capital letter symbolizes a dominant allele (W)
   – A lower-case letter symbolizes a recessive allele (w)
   – Dominant refers to the allele that will mask the expression
     of the alternate (recessive) allele
Example: Widow’s Peak
   Single Trait Gamete Formation

• During meiosis, homologous chromosomes
  separate so there is only 1 member of each pair
  in a gamete
  – There is one allele for each trait, such as hairline, in
    each gamete
  – Example: if one parent’s genotype is Ww, then some
    gametes from this individual will contain a W and
    others a w
                    One-Trait Cross




A homozygous man with a widow’s peak X a woman with a straight hairline
      Punnett Square
Two individuals who are both Ww
 One-Trait Crosses and Probability
• The chance of 2 or more independent events occurring
  together is the product of their chance of occurring
  separately

• In the cross Ww X Ww, what is the chance of obtaining
  either a W or a w from a parent?
   – Chance of W = ½ and the chance of w = ½
        • Therefore the probability of having these genotypes is as
          follows
   –   Chance of WW= ½ X ½ = ¼
   –   Chance of Ww = ½ X ½ = ¼
   –   Chance of wW= ½ X ½ = ¼
   –   Chance of ww = ½ X ½ = ¼
          One-Trait Test Cross

• Breeders of plants and animals may do a test
  cross to determine the likely genotype of an
  individual with the dominant phenotype
  – Cross with a recessive individual - the recessive has
    a known genotype (ww)
  – If there are any offspring produced with the recessive
    phenotype, then the dominant parent must be
    heterozygous
       Inheritance of Two Traits

The Law of Independent Assortment:

     • Each pair of factors assorts independently (without
       regard to how the others separate)


     • All possible combinations of factors can occur in
       the gametes
The Inheritance of Two Traits
Two-Trait Crosses (Dihybrid Cross)
Two-Trait Crosses (Dihybrid Cross)

• WwSs (X) WwSs

  – Phenotypic Ratio:
    • 9 widow’s peak, short fingers
    • 3 widow’s peak, long fingers
    • 3 straight hairline, short fingers
    • 1 straight hairline, long fingers
Two-Trait Crosses and Probability
 – Probability Laws
   •   Probability of widow’s peak = ¾
   •   Probability of short fingers= ¾
   •   Probability of straight hairline= ¼
   •   Probability of long fingers= ¼

 – Using the Product Rule
   • Probability of widow’s peak and short fingers =
         ¾ X ¾ = 9/16
   • Probability of widow’s peak and long fingers =
         ¾ X ¼ = 3/16
   • Probability of straight hairline and short fingers =
         ¼ X ¾ = 3/16
   • Probability of straight hairline and long fingers =
         ¼ X ¼ = 1/16
           Pedigree Analysis
• Information about the presence or
  absence of a particular phenotypic trait is
  collected from as many individuals in a
  family as possible and across as many
  generations as possible.
• The distribution of these characters is then
  mapped on the family tree.
• Example: If an individual in the third generation lacks a
  widow’s peak, but both her parents have widow’s peaks, then
  her parents must be heterozygous for that gene.
• If some siblings in the second generation lack a widow’ peak
  and one of the grandparents (first generation) also lacks one,
  then the other grandparent must be heterozygous and we can
  determine the genotype of almost all other individuals.
Beyond Simple Inheritance Patterns

• Incomplete Dominance
  – Occurs when the heterozygote shows a distinct
    intermediate phenotype not seen in the two
    homozygotes
  – Offspring of a cross between heterozygotes will show
    three phenotypes: each parent and the heterozygote.
  – The phenotypic and genotypic ratios are identical,
    1:2:1.
• A clear example of incomplete dominance
  is seen in flower color of snapdragons.
  – A cross between a
    white-flowered plant
    and a red-flowered
    plant will produce all
    pink F1 offspring.
  – Self-pollination of the
    F1 offspring produces
    25% white, 25% red,
    and 50% pink
    offspring.
Incomplete Dominance
            Codominance
– Occurs when alleles are equally expressed in
  a heterozygote
– Example: the M, N, and MN blood groups of
  humans are due to the presence of two
  specific molecules on the surface of red blood
  cells.
– People of group M (genotype MM) have one
  type of molecule on their red blood cells,
  people of group N (genotype NN) have the
  other type, and people of group MN (genotype
  MN) have both molecules present.
        Multiple Allele Inheritance
• A trait is controlled by multiple alleles, the gene
  exists in several allelic forms.
   – Each person has only two of the possible alleles.
   – ABO Blood Types
      • IA = A antigens on red blood cells
      • IB = B antigens on red blood cells
      • i = has neither A nor B antigens on red blood cells
      • Both IA and IB are dominant over i, IA and IB are
        codominant
       ABO Blood Types
Phenotype                 Genotype
A                         IAIA or IAi
B                         IBIB or IBi
AB                        IAIB
O                         ii

   Both IA and IB are dominant over i, IA and IB are
   codominant

   The Rh factor is inherited separately from ABO
   blood types.
Inheritance of Blood Types
Sex-Linked Inheritance in Humans
• 22 pairs of autosomes, 1 pair of sex
  chromosomes
  – X and Y
     • In females, the sex chromosomes are XX
     • In males, the sex chromosomes are XY
     • Note that in males the sex chromosomes are not
       homologous
  – Traits controlled by genes in the sex
    chromosomes are called sex-linked traits
  – X chromosome has many genes, the Y
    chromosome does not
             Sex-Linked Alleles
• Red-green colorblindness is X-linked
  – The X chromosome has genes for normal color vision
     • XB = normal vision
     • Xb – colorblindness

  Genotypes          Phenotypes
    XBXB             female with normal color vision
    XBXb             carrier female with normal color vision
    XbXb             colorblind female
    XBY              male with normal color vision
    XbY              colorblind male
Cross involving an X-linked Allele
          Polygenic Inheritance

• Occurs when a trait is governed by two or more
  sets of alleles.
• Each dominant allele codes for a product
• The effects of the dominant alleles are additive.
• The result is continuous variation.
• Examples of traits include size or height, shape,
  weight, and skin color.
Polygenic Inheritance – Skin Color
        Environmental Influences
• Environmental factors can influence the expression
  of genetic traits.

   Example:

      Siamese cats
      and Himalayan
      rabbits are
      darker in color
      where body
      heat is lost to
      the
      environment.
     Inheritance of Linked Genes

• All the alleles on one chromosome form a
  linkage group.

• Recall that during meiosis crossing over
  sometimes occurs

• If crossing over occurs between two alleles of
  interest, then four types of gametes are formed
  instead of two
Linkage Groups
• The occurrence of crossing-over can help
  determine the sequence of genes on a
  chromosome
• Crossing-over occurs more often between
  distant genes than genes that are close together
• In the example below, it is expected that
  recombinant gametes would include G and z
  more often than R and s.

				
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posted:4/17/2010
language:English
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