GENETICS

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GENETICS

• Non-Mendelian Genetics

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Linked Genes

• Genes for 2 traits are located on the same
  chromosome and travel together into the same
  gamete

P
L
p
l
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Crossing Over

• New combinations of alleles in the offspring that
  occur when neighboring segments of tetrads break
  off where they meet and exchange genetic material

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Sex-Linked Genes

• X-linked Genes genes found only on the X
  chromosome
• Y-linked Genes genes found only on the Y
  chromosome

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X-Linked Genes

• Many hereditary diseases are X-linked, meaning
  the gene is found on the X chromosome
• Ex red-green color blindness
• hemophilia

x
x


x
XX
XX
Y
XY
XY
XX normal female
XX carrier female
XY normal male
XY affected male
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Test for Red-Green Color Blindness
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Sex-Limited Traits

• Expressed only in the presence of sex hormones
  and are only observed in one sex or the other
• Controlled by genes not found on the sex
  chromosomes
• Although both males and females have these genes,
  they are only expressed by one sex since the
  appropriate sex hormone must be present
• EX color in birds
• beard growth in human males

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Sex-Influenced Traits

• Expressed in both sexes, but they are expressed
  differently
• EX Baldness
• In the presence of the male sex hormone, the
  allele is dominant
• In the presence of the female sex hormone, the
  allele is recessive

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Intermediate Inheritance

• The heterozygous offspring have traits that are
  not exactly like the trait of either purebred
  parent
• There are 3 types of intermediate inheritance
• Incomplete Dominance
• Codominance
• Polygenic Inheritance

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Incomplete Dominance

• In the heterozygous offspring, the dominant
  allele does not completely mask the effect of the
  recessive gene
• The offspring shows a phenotype that is in
  between the phenotype of the two homozygous
  parents

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Example of Incomplete Dominance
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Codominance

• Both alleles in the heterozygote express
  themselves fully
• Ex Blood Types
• The IA IB genes are both dominant, so if an
  individual has one of each of these genes, their
  blood type is AB
• The iO gene is recessive
• EX Feather Color
• White (WW) and Black (BB) are both dominant,
  so a cross between a white chicken and a black
  rooster will produce a speckled offspring (BW)

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Inheritance by Multiple Alleles

• When there are more than two alleles in a
  population, these genes are said to have multiple
  alleles
• EX blood typethere are 3 alleles in the
  population IA, IB iO
• Possible combinations

Possible Genotypes IA IA IA iO IB IB IB iO IA IB iO iO
Blood Types Type A Type A Type B Type B Type AB Type O
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Polygenic Inheritance

• Some traits are controlled by 2 or more genes
• EX eye color

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Pleiotrophy

• When a single gene affects more than one trait
• EX sickle cell anemia
• Changes the cell shape
• Causes anemia
• Causes brain damage
• Causes spleen damage
• Causes heart damage

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Body Temperature and Gene Expression

• In some organisms, body temperature can affect
  the expression of genes
• EX Himalayan rabbits coat color
• The colder body parts (i.e. feet, ears, nose) are
  black
• The warmer body parts are white