Genetics - Complete and Incomplete Dominance, Polygenetics, Pleiotropy, Environmental Effects PPT

12.7 Do Mendel’s Rules Apply to All Traits?

• Inheritance Patterns That Depart from Mendel’s Rules

• Incomplete Dominance

• Multiple Alleles

• Codominance

• Polygenic Inheritance

• Pleiotropy

• Environmental Influence

Departure from Mendel’s Rules

• Assumptions drawn from Mendel’s Rules- All genes are governed by alleles found at a single locus on a pair of homologous chromosomes.

  • There are two alleles (gene forms) for each characteristic or gene type.

  • One allele is dominant over the other, which is recessive.

Non-Mendelian Inheritance

• A number of traits in humans show non-Mendelian inheritance patterns.

Incomplete Dominance & Codominance

• The world isn’t simply black or white- Mendel’s Dominance

  • Incomplete Dominance

  • Codominance

Incomplete Dominance

• Dominance of one allele over another breaks down in incompletely dominant characteristics.

• When the heterozygous phenotype is in between the two homozygous phenotypes, the pattern of inheritance is called incomplete dominance.

Incomplete Dominance: In Between

• Some alleles are not simply dominant or recessive- X - rr or red snapdragon flowers
  - crossed with ww or white snapdragon flowers
  - produce ****

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• In incomplete dominance:-

  • Both traits use their own lower case letters

  • The hybrid shows a blending of both traits

Example of Incomplete Dominance

• rr

• ww

• rw

• rw

• rw

• rw

F2 Generation of Incomplete Dominance

Incomplete Dominance: Hair Texture

• ___ genotype = Straight hair phenotype

• ___ genotype = Curly hair phenotype

• ___ genotype = Wavy hair phenotype

Incomplete Dominance: If They Mated

• X- Genotype =

  • Phenotype =Straight hair

  • Genotype =

  • Phenotype= Curly Hair

  • Genotype =

  • Phenotype =

Multiple Alleles

• A species may have more than two alleles for a given characteristic- Each individual still carries two alleles for this characteristic

Multiple Alleles

• Examples of multiple allelism- Thousands of alleles for eye color in fruit flies, producing white, yellow, orange, pink, brown, or red eyes

  • Human blood group genes producing blood types A, B, AB, and O- Three alleles in this system: A, B, and O

Table 12-1 Human Blood Group Characteristics

• Blood Type

• A- Genotype- AA or Ao

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  • Red Blood Cells- A glycoprotein

  • Has Plasma Antibodies to:- B glycoprotein

  • Can Receive Blood from:- A or O

  • Can Donate Blood to:- A or AB (no blood with B glycoprotein)

  • Frequency in U.S.- 40%

• B- BB or Bo- B glycoprotein
  - A glycoprotein
  - B or O
  - B or AB (no blood with A glycoprotein)
  - 10%

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• AB- AB- Both A and B glycoproteins
  - Neither A nor B glycoprotein
  - AB, A, B, O (universal recipient)
  - AB
  - 4%

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• O- 00- Neither A nor B glycoprotein (no blood with A or B glycoprotein)
  - Both A and B glycoproteins
  - O, AB, A, B (universal donor)
  - 0
  - 46%

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Codominance

• Some alleles are always expressed even in combination with other alleles

• Heterozygotes display phenotypes of **** ******

Codominance

• Both are equally important, so you see both

• Use !- Heterozygous genotype=

  • phenotype=

  • Homozygous genotype = Phenotype = Coat Color White

  • X

  • Homozygous genotype =

  • Phenotype =Coat Color of Gray

Codominance

• Example: Human blood group alleles- Alleles A and B are codominant

  • Type AB blood is seen where individual has the genotype AB

Codominance& Blood Types

• Anitbodies fight the blood type- ex. An$-A attacks A blood

• Antigens recognize and accept- ex. A antigen accepts A blood

Polygenic Inheritance

• Some characteristics show a range of continuous phenotypes instead of discrete, defined phenotypes- Examples include:

Polygenic Inheritance

• Phenotypes produced by polygenic inheritance are governed by * __*

• Human skin color is controlled by at least ___ genes, each with pairs of incompletely dominant alleles

Pleiotropy

• Some alleles of a characteristic may create ** – Mendel’s rules specify only one phenotype possible for any allele

Pleiotropy

• Example: The SRY gene in male humans- SRY gene stimulates development of gonads into testes, which in turn stimulate development of the prostate, seminal vesicles, penis, and scrotum

Environmental Influence

• The environment can modify how genes are expressed

Environmental Influence

• Example: Himalayan rabbit- Himalayan rabbits have the genotype for black fur all over the body

  • Black pigment is only produced in colder areas of the body: the nose, ears, and paws

Environmental Influence

• Both heredity and environment play major roles in the development of some characteristics- Identical twin studies in humans reveal different IQ scores between twins