Brooker Chapter 4

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CHAPTER 2 EXTENSIONS OF MENDELIAN
INHERITANCE MISS NUR SHALENA SOFIAN
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INTRODUCTION

• Mendelian inheritance describes inheritance
  patterns that obey two laws
• Law of segregation
• Law of independent assortment
• Simple Mendelian inheritance involves
• A single gene with two different alleles
• Alleles display a simple dominant/recessive
  relationship

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INTRODUCTION (cont.)

• In this chapter we will examine traits that
  deviate from the simple dominant/recessive
  relationship
• The inheritance patterns of these traits still
  obey Mendelian laws
• However, they are more complex and interesting
  than Mendel had realized

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2.1 INHERITANCE PATTERN OF SINGLE GENES

• There are many ways in which two alleles of a
  single gene may govern the outcome of a trait
• Mendelian inheritance describes several patterns
  that involve single gene. What are they? (submit
  19/1/2011)
• These patterns are examined with two goals in
  mind
• 1. Understanding the relationship between the
  molecular expression of a gene and the trait
  itself
• 2. The outcome of crosses

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• Prevalent alleles in a population are termed
  wild-type alleles
• These typically encode proteins that
• Function normally
• Are made in the right amounts
• Alleles that have been altered by mutation are
  termed mutant alleles
• These tend to be rare in natural populations
• They are likely to cause a reduction in the
  amount or function of the encoded protein
• Such mutant alleles are often inherited in a
  recessive fashion

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• Consider, for example, the traits that Mendel
  studied

Wild-type (dominant) allele Mutant (recessive) allele
Purple flowers White flowers
Axial flowers Terminal flowers
Yellow seeds Green seeds
Round seeds Wrinkled seeds
Smooth pods Constricted pods
Green pods Yellow pods
Tall plants plants

• Another example is from Drosophila

Wild-type (dominant) allele Mutant (recessive) allele
Red eyes White eyes
Normal wings Miniature wings
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• Genetic diseases are caused by mutant alleles
• In many human genetic diseases , the recessive
  allele contains a mutation. What kind of genetic
  diseases that you know of containing recessive
  alleles? (Submit 19/1/2011)
• Preventing the allele from producing a fully
  functional protein
• In a simple dominant/recessive relationship, the
  recessive allele does not affect the phenotype of
  the heterozygote
• So how can the wild-type phenotype of the
  heterozygote be explained?

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FIGURE 1
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Lethal Alleles

• Essential genes are those that are absolutely
  required for survival
• The absence of their protein product leads to a
  lethal phenotype
• Nonessential genes are those not absolutely
  required for survival
• A lethal allele is one that has the potential to
  cause the death of an organism
• Resulting mutations in essential genes
• Inherited in a recessive manner

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• Many lethal alleles prevent cell division
• These will kill an organism at an early age
• Some lethal alleles exert their effect later in
  life
• Huntington disease
• Characterized by progressive degeneration of the
  nervous system, dementia and early death
• The age of onset of the disease is usually
  between 30 to 50
• Conditional lethal alleles may kill an organism
  only when certain environmental conditions
  prevail
• Temperature-sensitive (ts) lethals
• A developing Drosophila larva may be killed at 30
  C
• But it will survive if grown at 22 C

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• Semilethal alleles
• Kill some individuals in a population, not all of
  them
• Environmental factors and other genes may help
  prevent the detrimental effects of semilethal
  genes
• A lethal allele may produce ratios that seemingly
  deviate from Mendelian ratios
• An example is the creeper allele in chicken
• Creepers have shortened legs and must creep along
• Such birds also have shortened wings
• Creeper chicken are heterozygous

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• Creeper X Normal

Creeper X Creeper
Creeper is lethal in the homozygous state
Creeper is a dominant allele
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Incomplete Dominance

• In incomplete dominance the heterozygote exhibits
  a phenotype that is intermediate between the
  corresponding homozygotes
• Example
• Four oclock (Mirabilis jalapa) flower color
  plant
• Two alleles
• CR wild-type allele for red flower color
• CW allele for white flower color

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121 phenotypic ratio NOT the 31 ratio observed
in simple Mendelian inheritance
In this case, 50 of the CR protein is not
sufficient to produce the red phenotype
Figure 2
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Incomplete Dominance

• Whether a trait is dominant or incompletely
  dominant may depend on how closely the trait is
  examined
• Take, for example, the characteristic of pea
  shape
• Mendel visually concluded that
• RR and Rr genotypes produced round peas
• rr genotypes produced wrinkled peas
• However, a microscopic examination of round peas
  reveals that not all round peas are created
  equal

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FIGURE 3
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Multiple Alleles

• Many genes have multiple alleles
• Three or more different alleles

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• An interesting example is coat color in rabbits
• Four different alleles
• C (full coat color)
• cch (chinchilla pattern of coat color)
• Partial defect in pigmentation
• ch (himalayan pattern of coat color)
• Pigmentation in only certain parts of the body
• c (albino)
• Lack of pigmentation
• The dominance hierarchy is as follows
• C gt cch gt ch gt c
• Figure 4 illustrates the relationship between
  phenotype and genotype

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• Phenotype Genotype
• Agouti (wild type) cc, ccch, cch, cc
• Chinchilla (mutant) cchcch
• Himalayan(mutant) chch,chc
• Light grey cchch, cchc
• Albino (mutant) cc

FIGURE 4
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HOW CAN THIS BE???

• Caused by tyrosinase producing melanin
• Two types of melanin eumelanin (black pigment)
  and phaeomelanin (orange/yellow pigment)

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(No Transcript)
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• The himalayan pattern of coat color is an example
  of a temperature-sensitive conditional allele
• The enzyme encoded by this gene is functional
  only at low temperatures
• Therefore, dark fur will only occur in cooler
  areas of the body
• This is also the case in the Siamese pattern of
  coat color in cats

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• The ABO blood group provides another example of
  multiple alleles
• It is determined by the type of antigen present
  on the surface of red blood cells
• WHAT ARE ANTIGENS? (Submit 19/1/2011)
• As shown in Table 1, there are three different
  types of antigens found on red blood
• Antigen A, which is controlled by allele IA
• Antigen B, which is controlled by allele IB
• Antigen O, which is controlled by allele i

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• Allele i is recessive to both IA and IB
• Alleles IA and IB are codominant
• They are both expressed in a heterozygous
  individual

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• The carbohydrate tree on the surface of RBCs is
  composed of three sugars
• A fourth can be added by the enzyme glycosyl
  transferase
• The i allele encodes a defective enzyme
• The carbohydrate tree is unchanged
• IA encodes a form of the enzyme that can add the
  sugar N-acetylgalactosamine to the carbohydrate
  tree
• IB encodes a form of the enzyme that can add the
  sugar galactose to the carbohydrate tree
• Thus, the A and B antigens are different enough
  to be recognized by different antibodies

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• For safe blood transfusions to occur, the donors
  blood must be an appropriate match with the
  recipients blood
• For example, if a type O individual received
  blood from a type A, type B or type AB blood
• Antibodies in the recipient blood will react with
  antigens in the donated blood cells
• This causes the donated blood to agglutinate
• A life-threatening situation may result because
  of clogging of blood vessels