Term
IntroductionWhat is a trait? Example? Difference between continuous and discontinuous? What is a phenotype? |
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Definition
- Our interests in animals begin with the traits they express. A trait is an observable or measurable characteristic of an animal. An example would be the amount of milk produced by a cow in a 305-day lactation, or the weight of a steer at 12 months of age. Those would be continuous traits, characters that can range across a continuum of values. Another trait, though discrete or discontinuous, might be the presence of disease, such as a dog affected or unaffected by epilepsy. The trait is just more simply observed as presence or absence.
- Once we have a trait we wish to study (or improve through breeding) we move to measuring this character in a group of animals – and that measurement is called a phenotype. A phenotype is something you can SEE, and record, and it is with phenotypes that we begin our trip into genetics. That trip starts with a model – an algebraic expression of how we believe the world around us operates.
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Term
Phenotype = __ + __ Genotype is? Environment is? Genotype is 50% ____ and 50% ____ for an offspring |
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Definition
Phenotype = Genotype + Environment, or
P = G + E
The genotype is the genetic makeup of the individual. We can’t see it directly (though the hope of biotechnology and molecular biology is to better understand this component) but it is there, and the result of the combined action of all the genes an animal carries that influence the trait of interest. The environment term is, put simply, everything that isn’t genetic – the influences on phenotype that are not passed from parent to offspring. Because as you might guess, our simplest assumptions about genotype are that:
Goffspring = ½ Gsire + ½ Gdam where
the G is meant to represent the genotype and the subscript outlines the genotype of which animal. In words, the genotype of the offspring is the combination of the genotype from the sire (father) and the dam (mother). This model, as you will learn later is greatly simplified, but for now gets the point across, that the genotype differs from the environment in that it can be passed across the barriers of time – the real hallmark of genetics. |
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Term
Variation in P = Variation in G + Variation in E Heritability is? Algebraically expressed as? Upper bound? Lower Bound? |
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Definition
That is – phenotypes vary because individuals have different genes and are reared in different environments, this can’t be surprising. Geneticists go further, and are particularly interested in the percentage of all the variation we see that is due to variation in genotypes. The term is called heritability (with symbol h2) and can be expressed algebraically as:
h2 = (Variation in G)/ (Variation in P) |
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Term
Basic Genetics Basic unit of inheritance is? where are they located? what's a locus? what is the individual gene on a locus called? notation for it? Genotype or phenotype? Homozygous means? heterozygous means? Sperm and eggs are? how are they formed? |
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Definition
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- The basic unit of inheritance is called a gene.
- Genes are located on chromosomes.
- Locus, the Latin word for location, denotes the site of a particular gene.
- At each locus there is a pair of genes, one on a chromosome inherited from the father, and the other on a chromosome inherited from the mother. The individual genes at a locus are called alleles.
- The notation for alleles is often a letter, like A for one form of the gene and a for a second form.
- The combination of alleles at a locus is called the genotype at that locus and can take the form of AA, Aa or aa, in our little two allele example mentioned above. Note, a locus can have more than 2 alleles (in fact, most do).
- Genotypes of the form AA or aa are called homozygous (the same allele on both maternal and paternal chromosomes). The genotype Aa is called heterozygous, a different allele on each chromosome.
- Sperm and egg (also called gametes) are formed in a process called meiosis, a process where cells divide but the pairs of alleles (actually chromosomes) are separated into single forms in each gamete. This is called segregation, the separation of paired genes during meiosis. When complete, the gametes contain only ONE copy of a gene, so when sperm and egg are united to form a zygote (a new organism), all the genes are back in pairs.
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Definition
| Dominant, Recessive and Additive |
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Now, if you mate the Angus by Hereford crossbred to one another you would create the following table – also called a Punnett square.
| | Bull’s Gametes | | | P | p | Cow’s Gametes | P | | | p | | |
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Definition
Now, if you mate the Angus by Hereford crossbred to one another you would create the following table – also called a Punnett square.
| | Bull’s Gametes | | | P | p | Cow’s Gametes | P | PP (polled) | Pp (polled) | p | Pp (polled | pp (horned) |
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Term
Change in P = Change in G + Change in E
Change in G comes from artificial selection and natural selection
What is natural selection? Artificial selection? |
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Definition
| “Change in G”. And the principal way of changing G will come through artificial selection. Selection is that process that determines which individuals become parents and which do not, how many offspring they produce and how long they remain in the breeding population. Of course there is natural selection, the process that drives evolution. But for domesticated animals, humans control mating, and this brings us the term artificial selection. |
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Term
| Change in Genotypes = ____ x _____ x _____ |
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Definition
Intensity – The more intensely you select the more you will change genotypes. In other words, if you pick the best 5 of 100 animals you will make more change than if you pick the top 50 of 100 animals.
· Variation – The more genetic variation in a population, the more you will change genotypes through selection. Not every one in a population has the same genotype – so the more variable the genes, the more “raw material” there is to select. In other words, traits with a larger heritability will respond faster to selection.
· Accuracy – Remember, we only see phenotypes, not genotypes. When we select individuals it is done on what we can see. So when we select the parents of the next generation, some may be “good” because of the environment, not genes. So how accurately we can identify the “good” genes is an issue in changing a population. |
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Term
| Inbreeding and Crossbreeding |
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Definition
- Inbreeding – the mating of relatives. Mating relatives has the effect of increasing the percentage of loci in an individual that are homozygous (we say that inbreeding increases homozygosity). The impact on phenotypes is increasing uniformity (or if you wish, decreasing variability). The increase in homozygosity, however, also means we increase the probability that two recessive “bad” alleles match up causing health and fitness problems. This phenomenon is called inbreeding depression, and is part of the common thought that inbreeding is “bad”.
- Crossbreeding – the mating of individuals from different breeds (or from distant populations). Within a breed there is a tendency to increase the amount of inbreeding, so animals become more homozygous. By crossbreeding, we increase the percentage of loci that are heterozygous (or, we increase the heterozygosity of the population). This often has the effect of increasing the health and fitness of individuals in a phenomenon called hybrid vigor (technically we call is heterosis).
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