13. Factor XI (FXI) is one of more than a dozen proteins involved in the early b

Question

13. Factor XI (FXI) is one of more than a dozen proteins involved in the early blood coagulation cascade. Bovines with FXI deficiency have a bleeding disorder and may have lower calving and survival rates and increased susceptibility to infectious diseases. It was discovered by Patel et al. (2007) that there is a 77-bp insertion in the FXI gene of animals with the disease (insertion the addition of 1 or more nucleotides in a DNA sequence so a 77-bp insertion means an additional 77 bp when the animals have the disease.). The gel image below shows the results of a PCR on genomic DNA extracted from white blood cells of four Holstein- Friesian bulls (lanes 1-6; lane 7 is a DNA ladder). 400 bp 300 bp 200 bp 247 bp Determine the genotype and phenotype of each bull. You can use 'F for the normal factor and f for the mutated (disease) factor, when writing down the genotype for the animal. Bull 1: Bull 2: Bull 3: Bull 4: Bull 5: Bull 6: Which bulls would you exclude from your breeding programs? Why? Prac: Gene-assisted selection for a2 milk

Explanation / Answer

The answer to the first part of the question

Before solving the question, let us understand the banding pattern of each sample run on an agarose gel

1. Sample 1 has shown only one band of 247 bp. We cannot see any band representing an insertion. So, this sample infers a healthy bull and the genes are homozygous dominant.

2. Sample 2 has shown two bands of 247 and 324 bp. In the problem, it is mentioned that the bleeding disorder is due to the insertion of 77 bp sequence into the FXI gene. When we add 247 and 77 we get 347 bp. So, this sample has one copy of a normal gene and another copy of the inserted gene. This represents a heterozygous condition.

3. Sample 3 has shown only one band of 347 bp and this is due to the presence of two copies of the inserted sequence in FXI gene. This represents a homozygous recessive condition and the bull is suffering from bleeding disorder.

4. The banding pattern of Sample 4 and 5 are similar to sample 2. Hence, they represent a heterozygous condition.

5. Sample 6 is similar to sample 1. Hence it represents a normal bull with homozygous dominant genes.

Now we shall find out the genotypes of each bull

Bull 1 - Normal FXI gene - The genotype is FF

Bull 2 - Heterozygous condition - The genotype is Ff

Bull 3 - Homozygous recessive (bull with bleeding disorder) - ff

Bull 4 - Heterozygous condition - The genotype is Ff

Bull 5 - Heterozygous condition - The genotype is Ff

Bull 6 - Homozygous dominant - The genotype is FF

The answer to the second part of the question

We must exclude Bull 2, Bull 3, Bull 4 and Bull 5

Reason:

Bull 2: This bull has one copy of a normal gene (dominant allele) and another copy of a diseased gene (recessive allele). During breeding, if the cow has a recessive trait then the cross between these two has 25% likelihood of diseased offsprings. This can be determined by doing the following cross:

Ff x Ff

FF, Ff, Ff, ff (1 diseased / 4 offsprings) = 0.25 or 25%

Bull 3: This bull is a suffering from bleeding disease and it is homozygous recessive. A cross between homozygous recessive and homozygous dominant results in the transfer of disease traits to all offsprings. This can be determined as follows:

FF x ff

Ff, Ff, Ff, Ff

So, this bull should not be used for breeding programs.

Bull 4 and 5 are similar to sample 2 and hence they should not be included for the breeding programs.

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