23'1 Mendels Laws

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23.1 Mendels 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.

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• 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
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• 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.

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Self-pollination
SELF-POLLINATION
Stigma (receives pollen)
Anthers (produce pollen grains, which contain
male gametes)
Ovules (produce female gametes)
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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.
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Mendels 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
• All offspring will be a blend of the two colors
  (lavender)
• All offspring will be some of each color
• All offspring will be one color or the other

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• 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).

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Mendels 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.
• Mendels quantitative analysis of F2 plants
  revealed the two fundamental principles of
  heredity
• law of segregation
• law of independent assortment.

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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.

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• 2. For each character, an organism inherits
  twoalleles, 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.

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• 3. If two alleles differ, then one, the
  dominant allele, is fully expressed in the the
  organisms appearance.
• The other, the recessive allele, has no
  noticeable effect on the organisms appearance.
• Mendels F1 plants had purple flowers because the
  purple-flower allele is dominant and the
  white-flower allele is recessive.

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• 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 Mendels law of segregation.

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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

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Homologous Chromosomes
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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

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Example Widows Peak
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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 parents genotype is Ww, then
  some gametes from this individual will contain a
  W and others a w

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One-Trait Cross

• A homozygous man with a widows peak X a woman
  with a straight hairline

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Punnett Square

• Two individuals who are both Ww

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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

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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

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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

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The Inheritance of Two Traits
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Two-Trait Crosses (Dihybrid Cross)
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Two-Trait Crosses (Dihybrid Cross)

• WwSs (X) WwSs
• Phenotypic Ratio
• 9 widows peak, short fingers
• 3 widows peak, long fingers
• 3 straight hairline, short fingers
• 1 straight hairline, long fingers

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Two-Trait Crosses and Probability

• Probability Laws
• Probability of widows peak ¾
• Probability of short fingers ¾
• Probability of straight hairline ¼
• Probability of long fingers ¼
• Using the Product Rule
• Probability of widows peak and short fingers
• ¾ X ¾ 9/16
• Probability of widows 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

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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.

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• Example If an individual in the third generation
  lacks a widows peak, but both her parents have
  widows 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.

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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, 121.

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• 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.

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Incomplete Dominance
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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.

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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

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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.
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Inheritance of Blood Types
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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

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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

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Cross involving an X-linked Allele
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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.

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Polygenic Inheritance Skin Color
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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.
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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

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Linkage Groups
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• 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.