Double Trait Inheritance – Dihybrid Cross

Dihybrid Cross

Dihybrid Cross: a type of cross that involves two genes

Example: Mendel self-fertilized a pea plant that was heterozygous for purple colour and tall traits

Use a Punnett square to show the possible genotypes and phenotypes of the offspring?

Dihybrid Cross

Step 1: determine which alleles are dominant/recessive

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Step 2: determine the genotypes of the parents

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Dihybrid Cross

Step 3: Before making a Punnett Square it is useful to determine the possible gamates that can be made by the parents

* Keep in mind the law of independent assortment and use FOIL*

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Double Trait Inheritance

Step 4: Prepare your Punnett square to determine the F1 generation. You will need a grid that fits all possible gamates – in this case _______ x _______ grid

Step 5: Fill in the grid Stick to the ordering of traits from the parents (Colour,

Height) Put the alleles for the same trait back together Put dominant letter first

Step 6: State the final answer (remember to re-read the question)

Dihybrid Cross

Dihybrid Cross

Mendelian ratio for a dihybrid cross (2 traits, both heterozygous) is 9 : 3 : 3 : 1

The genotypic or phenotypic ratio you find is the percent chance or the probabilty (aka frequency) that an individual born from those parents will have those traits. It is not the exact results, or birth order

Test your understanding

In humans, curly hair is dominant over straight hair and the ability to curl your tongue is dominant over not being able to

A man who is homozygous for curly hair and heterozygous for the ability to roll his tongue has children with a woman with straight hair who cannot roll her tongue

What are the genotypic and phenotypic ratios of their possible offspring?

Example

Challenge

A male and female guinea pig are heterozygous for fur colour and fur texture. Both dark fur (D) and rough fur (R) are dominant traits

What are the parents phenotypes? How many different gametes are formed and what

are they? What is the chance of having offspring that are

homozygous for both traits? What is the chance of having offspring with dark,

rough fur? What is the chance of having offspring with both

recessive traits?