ESSAY: Provide a Minimum, One Full Page Essay (Use Back Sheets If Needed)

ANSC 3335 - GENETICS OF LIVESTOCK IMPROVEMENT

FINAL EXAMINATION (100 points)

Sample Essays

ESSAY: Provide a minimum, one full page essay (use back sheets if needed).

1) Describe an actual breeding example of your choosing where the aim is to change gene and genotypic frequencies affecting a simply inherited trait in a population. What is the ultimate outcome and how was this achieved? Also, what is a breeding objective?

2) What does breeding value (BV) really mean? Why is there so much emphasis made on BV? What is the outcome of BV selection in a population? What does gene combination value (GCV) really mean? What is the emphasis here? How can GCV be exploited through inbreeding and crossbreeding?

3) What does heritability (h2) really mean? What are the implications for lowly versus highly heritable traits? Heritability is commonly erroneously defined as the “ability of parents to transmit traits to their offspring”. Explain why this is erroneous.

4) What are dam and sire breeds and traits? How do they factor into a terminal crossbreeding system? A rotational crossbreeding system? The development of a new composite breed? Be specific.

5) Imagine a trait influenced by two genes at a single locus. In other words, the trait is simply inherited. Figure that all breeding animals in a closed herd are heterozygous initially. First, describe a breeding program that you would design to eliminate the dominant gene from a herd (assume there is complete dominance). Next, do the opposite by describing a breeding program that you would design to eliminate the recessive gene in the herd.

6) From Chapter 4 on Population Genetics, describe the classic figure below by using a numerical value for q. Next, how do different values for q (e.g., 0, .5, and 1) relate to genotypic frequencies for all three genotypes? How does selection against q (or Q) affect genotypic frequencies over generation time? Feel free to relate to a specific trait example.

7) How does breeding value relate to heritability? Why is GCV not transmissible from parent to offspring? Why is only BV transmissible? How does homozygosity affect BV and GCV (i.e., as an effect of inbreeding)? Also, why are commercial producers more interested in G (genotypic value) in terms of trait performance?

8) Compare the relative advantages and disadvantages of terminally-crossbred, rotationally-crossbred, and composite animals. To organize your essay, I suggest you prepare a table with these three breed-types in columns and advantages and disadvantages being discussed in rows. Be specific.