Gene Interactions and Multifactorial Inheritance in Genetics, Summaries of Genetics

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Chapter 4

! !ExtensionsExtensions toto MendelianMendelian GGeneticsenetics ! Gene Interactions

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Gene Interactions –

Extensions to Mendelian Genetics

• Just as different alleles of 1 gene can interact in complex ways,
• 2 different genes can also act together to modify a phenotype:
  • 2 genes 1 phenotype (Additive Gene Action) Complementation (complementary gene action) Epistasis (recessive and dominant) Redundancy

4 You can tell this genotype is caused by more than one gene :

• because there are 4 phenotypes not 3 in F
• 1 gene F2 would have 3 phenotypes 1:2:1 ratio (Additive Gene Action)

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F 2 phenotypes

• Dominance Relationships :
  • Tan is dominant to green
  • Gray is dominant to green
  • Brown is dominant to gray, green and tan.
  • Tan and Gray are incompletely dominant, giving rise to brown.
• Genotypic classes :
  • Brown: A_B_
  • Tan: A_bb
  • Gray: aaB_
  • Green: aabb

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10 Recessive Epistasis

• Example 1 : Coat color of Labrador retriever
• Example 2 : ABO blood groups: Bombay

phenotype.

• Phenotypic ratios are 9 : 3 : 4 in F 2.

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P^ X

F 1

Coat-Color Inheritance in Labrador Retrievers golden black black

13 9 black: 3 brown: 4 golden (9 B-E-: 3 bbE-: 3 B-ee: 1 bbee) x F

BE Be bE be

BE

Be

bE

be

AACC AACc Aa c CC AACc AAcc Aa c Cc Aa c cc Aa c Cc a c a c CC a c a c Cc a c a c cc a c a c Cc Aa c cc Aa c Cc Aa c AACc CC BbEe BbEe BBEE (^) BBEe BbEE BbEe BBEe BbEE BbEe BbEe BBee (^) BbEe Bbee bbEe bbee bbEE Bbee (^) bbEe Dihybrid Cross:

14 Molecular Explanation Pigment production (B) and subsequent incorporation (E) into the hair shaft are controlled by two separate genes. To be black, both genes must function. Mutations in B (b) lead to brown pigment. Mutations in E (e) lead to no pigment in coat. gene B gene E 3 4 9

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Dominant Epistasis

• caused by the dominant allele of one gene,

masking the action of either allele of the

other gene.

• Ratio is 12 : 3 : 1 instaed f 9 : 3 : 3 : 1
• Example: Summer Squash

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AB Ab aB ab

AB

Ab

aB

ab

AACC AACc Aa c CC AACc AAcc Aa c Cc Aa c cc Aa c Cc a c a c CC a c a c Cc a c a c cc Aa c AACc CC AABB AABb AABb AAbb AaBB AaBb aaBB AaBb AaBB AaBb Aabb aaBb AaBb (^) Aabb aaBb (^) aabb A-B- X A-B- 15/16 A-B- red; 1/16 aabb white Whenever a dominant gene is present, the trait is expressed. One allele is sufficient to produce the pigment.

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Hints for figuring out gene interactions:

Look at the F 2 phenotypic ratios!!

• If one gene is involved in the trait, then the monohybrid phenotypic ratio is: 3:1 or 1:2:1 or 2:
• If two genes are involved in the trait, then the dihybrid phenotypic ratio is: 9:3:3:1 or some permutation (9:4:3 or 9:7 or 12:3:1) "The 1/16 class is always the double homozygous recessive. " Look for internal 3:1 ratios, which will indicate dominance/recessive relationships for alleles within a gene.