TV4 dihybrid crosses

• Biologie
• Middelbare school
• vwo
• Leerjaar 4

Dihybrid crosses

TV4

Ch21

Inheritance

Study the pedigree

How does this disease (filled symbols) inherit?

1. dominant
2. recessive
3. can't be determined

This is the same pedigree as the previous question.
At first the disease looks to be sex-linked

Give an argument why.

Same pedigree again.

The question is if the disease is actually sex-linked.

Can we determine that with 100% confidence from this pedigree?

1. yes, it is autosomal
2. yes, it is sex-linked
3. we can't say for sure, but are allowed to assume sex-linked

The colour and pattern of the fur in cats in determined by at least ten different genes. Sex-linked are the alleles XD for orange and Xd for black.  

Usually one of the X-chromosomes is inactivated in each of the cells. Groups of cells can differ in which X-chromosome is active. Heterozygous cats can therefore have patches of orange and black fur, called tortoiseshell. The exact pattern depends on the stage during embryonic development in which the X-chromosome got inactivated. 

Tortoiseshell 

Calico: tortoiseshell with white patches (autosomal trait)

A tortoiseshell is crossed with a black male.

What is the chance of the female offspring being tortoiseshell? Show the Punnett square.

Learning objectives

• You can explain what a dihybrid cross is 
• You can use Punnett squares to solve dihybrid crosses 
• You can systematically work your way through dihybrid crosses 
• You can, depending on the goal, solve dihybrid crosses in two different ways 
• You can determine the genotype and phenotype of the parents when given the ratios in the offspring 

If we look at two genes at the same time in a cross, we call it a dihybrid cross. 

Fur colour in cattle is determined by 1 gene. The dominant allele codes for black fur and the recessive for red fur. 

The pattern is determined by another gene. The dominant allele codes for a solid colour and the recessive De vachttekening wordt bepaald door een ander gen. Het dominante allel zorgt voor een effen vachttekening. Het recessieve allel zorgt voor een bonte vachttekening

See the cross in the image

Colour: A = black; a = red
Pattern: B = solid; b = patches

What is the genotype of the red-patched bull?
What can be the genotype(s) of the solid black cow?

Linked genes

Both genes are on the SAME chromosome

Unlinked genes

Both genes are on DIFFERENT chromosomes

Fur colour and fur pattern in cattle are unlinked genes. 

We are making the following cross: 

AABB   x   aabb

List the gametes they will produce.

Principles:

- Gametes are formed through meiosis, the result is haploid cells 

- Gene A and gene B are unlinked

- There will be 1 allele of EACH gene in every gamete 

- All chromosome combinations need to be made

Let's take a look at an individual with AaBb as an example to help you understand:

Diploid cell before meiosis:

-heterozygote for both alleles

- genes are on different chromosomes: unlinked

Gametes after meiosis:

- haploid: 1 allele of every gene 

- One of each type of chromosome 

- Which of the two long and which of the two short chromosomes ends up in a daughter cell is random

- This means all chromosome/allele combinations are possible 

Write down all possible genotypes for the sperm cells of this bull (AaBb)

Write down all possible genotypes of the egg cells of this cow (AABB).
Do the same for the sperm cells of the bull (aabb)
What is the genotype of all offspring in the F1?

We take a cow and a bull from the F1 and cross them to create the F2.

Write down the Punnett square for this and upload below:

What is the phenotype ratio in this F2?

phenotype ratio: 9 : 3 : 3 : 1

When is a punnett square useful with a dihybrid cross? 

- if you want an overview 

- to see all possible outcomes 

- to determine a genotype/phenotype ratio 

When is it not that useful? 

- to find the chance of one specific genotype/phenotype 

Is there another way? For sure! 

If you want to calculate the chance of one specific genotype you can apply the following principle:

In tomato plants we find a gene for leaf length and a gene for leaf edge. Both genes are located on separate chromosomes. That means they are not linked. We perform the following crosses:

P.     long, smooth leaves   x   short, serrated leaves 

F1.   100% long, serrated leaves 

        F1 is crossed 

What part of the F2 has long, smooth leaves 

How to solve:

1. Determine the genotypes of the P generation 

2. Determine the genotypes of the F1 generation 

3. Determine the chance in the F2 for the two traits SEPARATE from each other 

4. Multiply these chances with each other 

Step 1: Determine the genotypes of the P generation

P. long, smooth leaves x short, serrated leaves

F1. 100% long, serrated leaves

What are the genotypes of the parents?

Step 2: Determine the genotypes of the F1 generation

P. long, smooth leaves x short, serrated leaves

F1. 100% long, serrated leaves

What are the genotypes in the F1?

Step 3: Determine the chance in the F2 for both traits separate.

First trait: leaf length
F1: Ll x Ll

What is the chance of long leaves in the F2?

Step 3: Determine the chance in the F2 for both traits separate.

First trait: leaf: leaf edge

F1: Ee x Ee

What is the chance of smooth leaves in the F2?

Step 4: Multiply these chances with each other

What is the chance of long and smooth leaves?
Show your calculation

To conclude....

Do you want to determine ratios or you need an overview of all possibilities?  --> Create a dihybrid punnett square 

Do you want to determine the chance of one specific phenotype/genotype? --> Calculate the chance per trait and multiply the chances with each other

Cross in reverse: determine genotypes of parents

The other way around is also possible. If you have the ratios of the offspring, you can usually determine the genotypes of the parents. 

In bunnies we have two coat colours: black (A) and brown (a)

There are two types of ears: hanging (b) and standing (B)

Both genes are on separate chromosome pairs. 

We have the following cross: ??? x ???

It results in the following offspring:

What are the genotypes of the parents?

Let's start with coat colour

What is the (approximately) the phenotype ratio of the offspring?

If the phenotype ratio is 1:1, what MUST the genotypes of the parents be? (use the letter A)

Now for the ears

What is the phenotype ratio of the offspring and what should the genotypes of the parents therefore be?

What are the genotypes and phenotypes of the parents?

And that's how you do that!!

The phenotype ratios in the offspring can be used to deduce what the genotypes of the parents must be

Learning objectives

• You can explain what a dihybrid cross is 
• You can use Punnett squares to solve dihybrid crosses 
• You can systematically work your way through dihybrid crosses 
• You can, depending on the goal, solve dihybrid crosses in two different ways 
• You can determine the genotype and phenotype of the parents when given the ratios in the offspring