Patterns of Inheritance

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Patterns of Inheritance

• Chapter 10

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Blending Hypothesis of Inheritance

• Trait
• A variation of a particular characteristic
• Blending hypothesis (1800s)
• Early explanation of how offspring inherit trait
  from both parents
• Example if a red flower plant crossed with a
  yellow flower, the offspring would be orange
• Later discarded

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Gregor Mendel

• Austrian monk
• Father of Genetics (study of heredity)
• Said parents pass on to their offspring separate
  and distinct genes
• Studied 7 characteristics in pea plants

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• True breeding plants
• A true plant will show the same physical
  appearance generation after generation after
  self-fertilization
• Cross fertilization
• The sperm from the pollen of one true flower
  fertilizes the eggs in the flower of a different
  plant

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Mendel's Experiments

• Cross-fertilized 2 true-breeding plants each with
  contrasting traits (i.e. white and purple
  flowers)
• What color of flowers do you think the offspring
  plants were?

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Principle of Segregation

• P generation
• Parental plants (purebred and true breeding)
• F1 generation ( F for filial son)
• Hybrid offspring
• Hybrids
• The offspring of 2 different true-breeding
  varieties
• F2 generation
• When F1 self-fertilize or fertilize each other

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Monohybrid Cross

• Monohybrid cross
• Cross fertilization in which only one physical
  characteristic is considered
• In Mendel's cross, all F1 were purple but ¼ of F2
  were white

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Gene Hypotheses 1

• There are alternative forms of genes which
  determine physical appearances
• Allele is the term
• Example Flower color can be white or purple

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Gene Hypotheses 2

• For each characteristic, an organism has 2
  alleles for genes controlling the physical
  appearances (one from each parent)
• If 2 alleles are the same homozygous
• If 2 alleles are different heterozygous

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Gene Hypotheses 3

• Dominant alleles determine the physical
  appearance in a heterozygous individual.
• Recessive allele is the other allele that does
  not affect the physical appearance
• Capital letter represents dominant allele P
• Lower case letter represents recessive allele p

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• Phenotype is the physical appearance
• purple
• purple
• white
• Genotype is the genetic makeup
• Possible genotype are PP, Pp, pp.

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Gene Hypotheses 4

• The two alleles for a character segregate
  (separate) during meiosis so that each gamete
  carries only one allele for each character, known
  as principle of segregation.

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Punnett Square

• The alignment of combination of gametes to form
  zygotes with pairs of alleles is random
• Like tossing a coin.
• Punnett Square is a diagram that shows all
  possible outcomes of a genetic cross.
• Used to predict probabilities of outcomes if you
  know the genotypes of the parents

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The Testcross

• Breeds an individual of unknown genotype, but
  dominant phenotype (purple) with a homozygous
  recessive individual (white)
• Appearance of F1 will reveal the genotype of the
  mystery parent
• If white flowers are produced, the unknown parent
  must be heterozygous (Pp) and have a recessive
  trait

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Intermediate Dominance

• Heterozygotes have a phenotype intermediate
  between the phenotypes of the two homozygote
• This is referred to as INCOMPLETE DOMINANCE
• Rules (example snapdragon flowers)
• Capital/lower case letters not used
• Instead, a C for color is paired with a
  superscript R for red and W for white
• CR CR is red and CW CW is white
• CR CW is pink

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There is a breed of chicken called Andalusians,
black and white parents produce F1 hybrid
offspring, called "blues," with grayish-blue
feathers. Because neither the black nor white
allele is dominant, capital and lowercase letters
are not used to represent them.
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Instead, a C for "color" is paired with a
superscript B for "black" or W for "white" to
represent the two alleles. A heterozygote
chicken has one of each allele, CBCW, and is
grayish-blue in color
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Although the F1 phenotypes are intermediate, this
inheritance pattern does not support the blending
hypothesis. This is because the parent
phenotypes can reappear in the F2 generation.
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Multiple alleles

• Heterozygote express the distinct traits of both
  alleles
• Example Human blood system
• A, B, AB, or o
• The letters are antigens found on the surface of
  red blood cells
• Red blood cells may be coated with one protein
  (A), the other (B), both (AB), or neither (O)
• There are six possible genotype combinations

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ABO blood type is a genetic example of multiple
alleles. There are three alleles in the gene pool
for ABO blood type. IA IB i
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IA codes for protein A IB codes for protein
B i codes for neither protein A nor protein B.
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Within this multiple allele pool the gene
interactions illustrate both simple dominance as
well as co-dominance.
Remember each individual has only two alleles for
each trait even if there are multiple alleles in
the gene pool. IAIA both
code for A type blood IAi
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Phenotype Genotype Protein on RBC (antigen) Antibodies in the blood plasma
Type A IA IA and IA i A b
Type B IB IB and IB i B a
Type AB IA IB A and B -------------
Type O ii ----------- a and b
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ABO Blood System

• Antibodies (proteins) also found in the blood
  serum that attacks foreign antigens
• Blood A has antibody Anti-B
• Blood B has antibody Anti-A
• Blood AB has no antibody
• Blood O has Antibody Anti A and B
• Blood O is the universal donor
• Blood AB can receive any blood type

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Rh Factor

• Rh positive (Rh ) has protein in blood
• Rh negative (Rh -) has no protein in blood
• Rh is dominant

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Blood Typing
Blood Type Anti-a Sera Anti-b Sera
A Clumping No clumping
B No clumping Clumping
AB Clumping Clumping
O No clumping No clumping
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Sex-linked genes

• The eggs contain a single X chromosome and sperm
  contain either an X or a Y
• Sex of the offspring depends on whether the sperm
  that fertilizes the egg has an X or a Y
• Any gene located on a sex chromosome (X) is
  called a sex-linked gene
• Most are found on the X (2,000) and few on the Y
  (24)

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Sex-linked traits

• Written as a XRXr for heterozygous.
• Y chromosome carries no allele and the phenotype
  is dependant upon the womans allele
• Therefore, males carry one allele for a
  sex-linked trait.

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Sex-linked disorders

• Red-green blindness
• Hemophilia (inability of blood to clot)