Question

In 2016, The American Sheep Industry Association released data showing that the nation's 88,000 sheep producers generated a total economic impact of $5.8 billion. Some of the issues that impact the profitability of sheep include their coat color, how quickly they gain weight, and whether they are resistant to infection caused by parasitic worms called nematodes. In the UK last year, approximately 100 million dollars were spent on reducing and curing worm infections in sheep. A team of researchers led my Riggio and Matika are working on understanding the genetics of sheep in order to increase profitability. While sheep have hundreds of thousands of genes on their total of 54 chromosomes (n=27), the researchers are focusing on 3 genes. The NR gene is correlated with nematode resistance and is located on chromosome 14. Only individuals with two recessive copies show resistance to nematode infection. The GTH gene is correlated with growth rate and overall bodyweight. The researchers would like to have sheep grow quickly so they are ready for market sooner. Individuals with GG or Gg grow the fastest and reach market weight sooner than individuals with gg. Finally, there are two genes involved in coat color. They are both on chromosome 3, meaning they are linked. The first determines if the individual is Black (BB or Bb) or brown (bb). The second determine if there are white spots on the coat. The spotting gene shows no spotting with the NN genotype, spotting only on the head with the SN genotype, and spotting covering a majority of the body with the SS genotype.

Q1. Fill in the table below to help you keep track of the genes.  

Chromosome #           Possible Alleles             Possible Genotypes                 Resulting Phenotyp

3                                       B or b                            BB or Bb or bb Black, black, brown

Coat Color S or N   

13

Growth Rate                  G or g

14

Nematode Resistance      R or r

Q2. In order to confirm that the NR gene controls resistance to infection from nematodes, the researchers want to perform a study to test their idea that individuals with the genotype “rr” have a lower risk of getting infected by nematodes than individuals with RR or Rr genotypes.

a. What is the null hypothesis for the study?

b. What is the alternative hypothesis for the study?

The methods used in the study are explained below:

The genotype of 300 baby sheep were determined prior to the study and approximately the same number of genotype RR, Rr and rr were used. They were raised in the same pastures and fed similar food. Fecal samples were collected from the rectum of the baby sheep at approximately 16, 20 and 24 weeks of age. Fecal egg counts of nematodes were measured on four replicates of each fecal sample and the average of the four replicates at each time point was used for analyses.

Q3. Draw a figure that would show support for the alternative hypothesis. Make sure to clearly label your axes, values (tick marks), and include an appropriate figure legend below the figure.

Q4. One of the sheep in the study is Dolly. She is brown with spotting on her head. She is heterozygous for the growth rate gene and shows resistance for nematode infection.

a. What is the genotype of Dolly?

b. Draw one of Dolly’s cells during metaphase of mitosis. Show chromosomes 3, 13, and 14. Label all the alleles. Label sister chromatids once in your model. Label homologous pairs once in your model.

c. Describe what the products of mitosis would be for Dolly.

d. Describe one reason why Dolly would need to perform mitosis.

Answer 1

Q1

Chromosome#	Trait	Alleles	Genotypes	Phenotypes
3	Coat color Spotting	B or b N or S	BB, Bb, bb NN, NS, SS	Black, black, Brown No spots, spots in head, spots in body
13	Growth rate	G or g	GG, Gg, gg	Fast growth, fast growth, slow growth
14	Nematode resistance	R and r	RR, Rr, rr	Vulnerable, vulnerable, resistant

Q2

a) Null: RR and Rr genotypes have the same resistance/vulenrability levels to nematodes as rr genotype

b) Alternative: rr genotype has more resistance to nematodes compared to RR and Rr genotypes

Q3

Fig. Nematode egg count for each genotype during different times in the lamb development since birth. The values used are the count means for each genotype population. Note the different behavior presented by the recessive homozygous genotype, we can see rr genotype presents a lower count in each stage.

Q4

a) bb NS Gg rr

b) Metaphase of mitosis is the stage in which the chromosomes align in the center, getting ready for segregation:

c) Well, the products of mitosis will be just 2 identical cells, with 54 normal chromosomes, with the exact same normal genotype of Dolly (bb NS Gg rr)

d) For replacing tissue like epithelial cells, in general to replace any cell in the body. Epithelial cells are the ones with one of the highest division rates, they constantly undergo mitosis to allow for dermal layers to be replenished and hair to grow from the follicles