TV4 dihybrid crosses

Dihybrid crosses
TV4
Ch21
Inheritance
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Slide 1: Slide
BiologieMiddelbare schoolvwoLeerjaar 4

This lesson contains 33 slides, with interactive quizzes and text slides.

time-iconLesson duration is: 60 min

Items in this lesson

Dihybrid crosses
TV4
Ch21
Inheritance

Slide 1 - Slide

Study the pedigree

How does this disease (filled symbols) inherit?
A
dominant
B
recessive
C
can't be determined

Slide 2 - Quiz

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

Give an argument why.

Slide 3 - Open question

Same pedigree again.

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

Can we determine that with 100% confidence from this pedigree?
A
yes, it is autosomal
B
yes, it is sex-linked
C
we can't say for sure, but are allowed to assume sex-linked

Slide 4 - Quiz

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)

Slide 5 - Slide

A tortoiseshell is crossed with a black male.

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

Slide 6 - Open question

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 

Slide 7 - Slide

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

Slide 8 - Slide

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?

Slide 9 - Open question

Linked genes

Both genes are on the SAME chromosome
Unlinked genes

Both genes are on DIFFERENT chromosomes

Slide 10 - Slide

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

Slide 11 - Slide

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 

Slide 12 - Slide

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

Slide 13 - Open question

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?

Slide 14 - Open question

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:

Slide 15 - Open question

What is the phenotype ratio in this F2?

Slide 16 - Open question

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 

Slide 17 - Slide

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 

Slide 18 - Slide

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 

Slide 19 - Slide

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?

Slide 20 - Open question

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?

Slide 21 - Open question

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?

Slide 22 - Open question

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?

Slide 23 - Open question

Step 4: Multiply these chances with each other

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

Slide 24 - Open question

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

Slide 25 - Slide

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. 



Slide 26 - Slide

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

It results in the following offspring:
What are the genotypes of the parents?

Slide 27 - Slide

Let's start with coat colour

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

Slide 28 - Open question

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

Slide 29 - Open question

Now for the ears

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

Slide 30 - Open question

What are the genotypes and phenotypes of the parents?

Slide 31 - Open question

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

Slide 32 - Slide

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 

Slide 33 - Slide