Figure 9.8A

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Figure 9.8A
Dominant Traits
Recessive Traits
No freckles
Freckles
Widows peak
Straight hairline
Free earlobe
Attached earlobe
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Table 9.9
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Variations of Mendelian Genetics

• 5.5 and 5.6

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Dihybrid Cross

• Looks at two pairs of contrasting traits at the
  same time

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Dihybrid CrossExample

• Round or wrinkled and green or yellow
• Law of segregation still applies

4 different alleles, 9 possible genotypes, 4
possible phenotypes
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Independent Assortment

• The inheritance of an allele for one
  characteristic does not affect the inheritance of
  an allele for a different characteristic.
• Related to independent orientation during
  meiosis- homologous chromosomes randomly line up
  on the left and right side.
• Independent assortment still applies even if the
  alleles for the different characteristics are on
  the same chromosome because of crossing over
  which occurs during prophase I of meiosis

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Law of Segregationvs.Law of Independent
Assortment

• Law of segregation is talking about how haploid
  gametes are produced through the process of
  meiosis
• Law of independent assortment is talking about
  how the inheritance of one characteristic does
  not effect the inheritance of other
  characteristics

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Complete DominancePea Plant
Genotype Phenotype
Homozygous dominant (PP) Purple
Heterozygous (Pp) Purple
Homozygous recessive (pp) White
2 different alleles, 3 possible genotypes, 2
possible phenotypes
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Incomplete Dominance

• Neither allele is purely dominant or recessive
• The phenotype of the heterozygote is a result of
  both alleles being expressed
• Results in an intermediate phenotype

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Incomplete DominanceExample

• Many common garden flowers like the snapdragon
  have genes for flower color that display
  incomplete dominance.

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Incomplete DominanceSnapdragon
Genotype Phenotype
Homozygous for red (RR) Red
Heterozygous (RW) Pink
Homozygous for white (WW) White
2 different alleles, 3 possible genotypes, 3
possible phenotypes
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Incomplete DominanceHypercholesterolemia

• One example of incomplete dominance in humans is
  hypercholesterolemia, in which
• dangerously high levels of cholesterol occur in
  the blood and
• heterozygotes have intermediately high
  cholesterol levels.

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Figure 9.11B
GenotypesHhHeterozygous
hhHomozygousfor inability to makeLDL receptors
HHHomozygousfor ability to makeLDL receptors
Phenotypes
LDL
LDLreceptor
Cell
Normal
Mild disease
Severe disease
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Co-dominance

• Neither allele is purely dominant or recessive
• The phenotype of the heterozygote is a result of
  both alleles being expressed
• Does not result in a blend or intermediate
  characteristics
• Both alleles are expressed separately and equally

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Co-dominanceExample

• Many mammals have genes for hair color that
  display co-dominance

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Co-dominanceHorse Hair
Genotype Phenotype
Homozygous for red (RR) Red
Heterozygous (RW) Roan
Homozygous for white (WW) White
2 different alleles, 3 possible genotypes, 3
possible phenotypes
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Multiple Alleles

• In each example weve looked at so far, the trait
  is a result of the interaction of 2 different
  alleles.
• With many traits, however, there are more than 2
  possible alleles that you could inherit.

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Multiple AllelesExample

• There are 3 possible alleles for blood type and
  they display a combination of complete dominance
  and co-dominance.
• Allele for blood type A and B are both dominant
  over the allele for blood type O
• Allele for blood type A and B are co-dominant

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Multiple AllelesABO Blood Type

• There are three possible alleles IA, IB, and i.
• The combination of two of those alleles
  determines an individuals blood type.

Blood Types (Phenotypes) Possible Genotypes
Type A IAIA or IAi
Type B IBIB or IBi
Type AB IAIB
Type O ii
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Multiple AllelesABO Blood Type
3 different alleles, 6 possible genotypes, 4
possible phenotypes
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Polygenic Inheritance(Multiple Genes)

• Multiple genes at different loci and/or on
  different chromosomes effect a single
  characteristic
• Most human traits have some degree of polygenic
  inheritance
• Skin color, hair color, eye color, height, body
  build, intelligence

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Polygenic Inheritance(Multiple Genes)Example

• There is at least 3 genes that control skin color
• Combinations result in a blend from very light to
  very dark
• Behavior and the environment play an important
  role as well

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Polygenic Inheritance(Multiple Genes)Skin Color
6 different alleles, 27 possible genotypes, 7
possible phenotypes
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Sex Determination

• Humans have 46 chromosomes (23 pairs)
• 22 pairs of autosomes and one pair of sex
  chromosomes
• Sex chromosomes determine the sex and we
  represent them with an X and a Y.
• Female XX
• Males XY
• It is the mans gene that determines the sex of
  the child

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Sex-Linked Traits

• X chromosome is much larger than the Y chromosome
• Genes on the X chromosome are important for both
  males and females
• Genes on the Y chromosome are what make a male a
  male
• In females characteristics are determined by the
  interaction of the alleles on each X chromosome
• In males, since they only have one X chromosome
  and the Y chromosome is much smaller, some
  characteristics are determined by a single allele
  on the X chromosome.

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Sex-Linked TraitsExample

• Red-green colorblindness
• Reduced amount of a chemical in the retina of
  their eyes
• Red-green colorblindness is a recessive
  characteristic that is found on the X chromosome

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Sex-Linked TraitRed-Green Colorblindness
Male Male
Genotype Phenotype
XNY Normal
XnY Colorblind
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Sex-Linked TraitRed-Green Colorblindness
Female Female
Genotype Phenotype
XN XN Normal
XN Xn Normal
Xn Xn Colorblind
Carrier- a carrier for an X-linked trait is a
female individual who does not exhibit the
characteristic but does carry the gene for the
trait
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Sex-Linked Trait

• Colorblind father x Heterozygous mother
• Father with normal vision x Heterozygous mother
• Colorblind father x Homozygous mother with normal
  vision
• Father with normal vision x Colorblind mother

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Pedigree

• Chart that geneticists use to trace the presence
  or absence of a trait in a number of generations
• Square male
• Circle female
• Shaded has trait
• Not shaded does not have trait

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Hemophilia

• X-linked recessive trait
• Bleeders disease
• Hemophilia- genetic disorder that causes
  individuals to lack an enzyme in the blood that
  is essential for normal blood clotting
• Normally minor cuts, bruises, and scrapes can
  become very serious
• In the past, individuals with this disorder
  typically died very young
• Modern medication can replace the enzyme

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Pedigree
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