Question
Asked Aug 6, 2019
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Draw a dihybrid Punnett Square between 2 individuals heterozygous for both traits. Identify the genotypic ratio. Identify the phenotypic ratio when both conditions are: autosomal recessive, autosomal dominant, incomplete dominant, and polygenically linked.

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Expert Answer

Step 1

A cross that involves two pairs of contrasting characters is called dihybrid cross. Genotype refers to the genetic composition of the organism. Phenotype refers to the morphological appearance of the organism that is controlled by the genotype.

Step 2

Assume that A/a and B/b are two alleles responsible for two different phenotypic traits. As it is given that the parents are heterozygotes, their genotype is AaBb. A dihybrid cross between two heterozygotes parents is given below:

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Parent 2 Parent AaBb AaBb Gametes AB Ab aB ab Ab ab AB ав АВ ААВB АAВЬ| AаBB | AaBb Ab AABb AAbb AaBbAabb ав | АаBB | АаBb | aaBB | aaBЬ ab AaBbAabb aaBbaabb

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Step 3

From a genetic cross between two parents, the ration of different genotype in the offspring is called genotypic ratio. From the Punnett Square, the following genotypes can be observed:

  • AABB: 1
  • AaBB: 2
  • AABb: 2
  • aaBB: 1
  • AaBb: 4
  • AAbb: 1
  • aaBb: 2
  • Aabb: 2
  • aabb: 1

The genotypic ratio for above Punnett Square is 1:2:2:1:4:1:2:2:1

From the Punnett Square, the following phenotypic ratios can be observed:

Autosomal recessive: In order to develop a recessive trait presence of two copies of recessive alleles are necessary. The phenotypic ratio when both conditions are autosomal recessive (both a and b are recessive; “aabb”) is 1.

Autosomal dominant: In order to develop a dominant trait at least one copy of allele. The phenotypes when both conditions are autosomal dominant are AABB, AaBB, AABb, AaBb and their respective ratios are 1:2:2:4.

Incomplete dominance: It refers to the form of intermediate inheritance in which one allele cannot be expressed over its paired allele, so that the resulting trait is neither dominant nor recessive. Incomplete dominance occurs when phenotype will ...

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Gametes Ab ab AB ав ААBB AаBB AABb АаBb AB RED DARK DARK MEDIUM AABB AAbb AaBb Aabb Ab DARK MEDIUM MEDIUM LIGHT AaBb aаBb AaBB aaBB AB MEDIUM LIGHT DARK MEDIUM aaBb LIGHT Aabb aabb AaBb ab WHITE MEDIUM LIGHT

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