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Page 1: NOTES – Complex Inheritance Patterns

NOTES – Complex NOTES – Complex Inheritance PatternsInheritance Patterns

Most human traits have more than 2 or 3 Most human traits have more than 2 or 3 forms. Types of inheritance that are more forms. Types of inheritance that are more complex than simple dominance, complex than simple dominance, incomplete dominance or codominance incomplete dominance or codominance control these traits.control these traits.

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Multiple Allelic InheritanceMultiple Allelic Inheritance (Multiple (Multiple

Alleles)Alleles) Traits are controlled by 1 gene with Traits are controlled by 1 gene with more more

than 2than 2 alleles alleles

Ex. The ABO Blood Type System in HumansEx. The ABO Blood Type System in Humans

The gene that controls ABO blood type has 3 The gene that controls ABO blood type has 3 alleles:alleles:

AA = Type A blood = Type A blood BB = Type B blood = Type B blood oo = Type O Blood = Type O Blood

Alleles Alleles AA and and BB are are codominantcodominant and allele and allele oo is is recessiverecessive

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There are 6 possible genotypes and 4 There are 6 possible genotypes and 4 possible phenotypes:possible phenotypes:

A A or A oA A or A o == Type A BloodType A Blood

B B or B oB B or B o = = Type B BloodType B Blood

ABAB == Type AB BloodType AB Blood

o o o o = = Type O BloodType O Blood

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Blood CellsBlood Cells

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There are four basicThere are four basic Blood Blood typestypes: :

Type AType A with A antigens on with A antigens on the red cells and anti B the red cells and anti B antibodies in the plasma. antibodies in the plasma.

Type BType B with B antigens on with B antigens on the red cells and anti A the red cells and anti A antibodies in the plasma. antibodies in the plasma.

Type ABType AB with both A and B with both A and B antigens on the red cells antigens on the red cells and no blood type and no blood type antibodies in the plasma. antibodies in the plasma.

Type O Type O with no antigens on with no antigens on the red cells and both anti A the red cells and both anti A and anti B antibodies in the and anti B antibodies in the plasma plasma

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Polygenic InheritancePolygenic Inheritance

Traits are controlled by Traits are controlled by more than 1more than 1 genegene

There are many possible phenotypesThere are many possible phenotypes Ex. Skin color is controlled by at least Ex. Skin color is controlled by at least

3 genes 3 genes A = dark skinA = dark skin a = light skina = light skin B = dark skinB = dark skin b = light skinb = light skin C = dark skinC = dark skin c = light skinc = light skin

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Differences in skin color are largely Differences in skin color are largely due to differences in the amount of due to differences in the amount of melanin, a dark pigment produced by melanin, a dark pigment produced by skin cells.skin cells.

Each “dark skin” allele (A, B, C) codes Each “dark skin” allele (A, B, C) codes for high melanin productionfor high melanin production

Each “light skin” allele (a, b, c) codes Each “light skin” allele (a, b, c) codes for low melanin productionfor low melanin production

Each dark skin allele (A, B, C) in the Each dark skin allele (A, B, C) in the genotype adds a small but equal genotype adds a small but equal amount of pigment to the skinamount of pigment to the skin

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Some possible genotypes and Some possible genotypes and phenotypes:phenotypes:

aabbccaabbcc == very light skin very light skin AABBCCAABBCC == very dark skinvery dark skin AaBbCcAaBbCc = = medium skin medium skin

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What would a Punnett Square What would a Punnett Square for skin color look like?for skin color look like?

Cross two medium skin tone parentsCross two medium skin tone parents

AaBbCcAaBbCc X X AaBbCcAaBbCc

Each parent could produce 8 different Each parent could produce 8 different possible combination to pass on: possible combination to pass on: ABC, ABc, AbC, Abc, aBC, aBc, abC, or ABC, ABc, AbC, Abc, aBC, aBc, abC, or abcabc

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Blood Type Is PolygenicBlood Type Is Polygenic

Another level of specificity is added to Another level of specificity is added to blood type by examining the presence or blood type by examining the presence or absence of the Rh proteinabsence of the Rh protein Each blood type is either positive "+" (has the Each blood type is either positive "+" (has the

Rh protein) or negative "-" (no Rh protein) Rh protein) or negative "-" (no Rh protein) For example, a person whose blood type is "A For example, a person whose blood type is "A

positive" (A +), has both type A and Rh positive" (A +), has both type A and Rh proteins on the surface of their red blood cells proteins on the surface of their red blood cells

The Rh protein gene has two alleles:The Rh protein gene has two alleles: RR = protein present (+) = protein present (+) rr = protein absent = protein absent

(-) (-)

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Polygenic Blood TypePolygenic Blood Type

What is the phenotype of a person who What is the phenotype of a person who has the blood type genotype ABRr?has the blood type genotype ABRr? Type AB +Type AB +

What is the phenotype of a person who What is the phenotype of a person who has the blood type genotype oorr?has the blood type genotype oorr? Type O – Type O –

What would be the genotype of a What would be the genotype of a person who is A +person who is A + AARRAARR or or AoRRAoRR or or AARrAARr or or AoRrAoRr

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