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GENETICS SIMPLIFIED By Tom Drudik At the recent Corp I sale in Oklahoma, I visited with many sheep producers about DNA testing of their sheep for scrapie resistance genes and spider syndrome genes. To my amazement, I drew these conclusions; very few producers tested for these conditions, very few producers had
RRNN (scrapie-resistant and spider-free) animals available and very few people even understood simple genetic transmission of these traits. I thought it would be beneficial to give a simplified lesson on genetics
to demonstrate the trasmission for these two traits from the rams/ewes to their
lambs. Genetic Terms We can represent the genes for any trait in an organism by symbols such as letters. For example, we might let
N stand for a gene for the normal non-spider condition animal. If so, we would represent the genes of a pure normal lamb as
NN. A gene for the contrasting trait, spider condition, would be written as
S. A lamb with the spider condition would be SS. Such paired symbols show the
genotype of an animal. That is, they show the genes that are present in the animal's body cells. The effect caused in the animal by these genes is called its
phenotype. For example, a normal sheep has the genotype NN and the phenotype for
normal. The paired genes for a trait may be identical, as with
NN and SS. If so, we say that animal is homozygous for that trait. If the paired genes are not identical
(NS), the animal is heterozygous. The different forms of genes that have contrasting effects on the trait are called alleles. In the gene pair
NS, N is an allele of S, and S is an allele of
N. In the gene pair RQ, R is an allele of
Q, and Q is an allele of R. But N is not an allele of
R, and so on. A punnett square is a special chart, or grid system, named after its inventor, R. C. Punnett. You can use such squares to predict the results of crosses such as the scrapie resistance gene. For example, the Punnett square listed below will show the possible gene pairings that can result in different crosses. The alleles are R and Q - resistance and nonresistance respectively. For our example below the ram is genotypically RQ for scrapie resistance and the ewe is also RQ.
As you can see, genes present in gametes from the female parent are given across the top of the grid. Genes from gametes from the male are shown down the left side. To work the square, simply combine the genes from both parents in the proper section of the grid. Besides showing the possible gene pairings, a Punnett square will show how often, on the average, a given pairing will occur. For example, as shown above the probability ratio is one
RR to two RQ to one QQ . Remember that such a ratio represents an average. Thus, a small number of offspring may not have the exact ratio of gene pairings given by the square. But the larger the sample of crosses involved, the closer the results will be to the Punnett square probability ratio. You can see this effect for yourself by flipping two coins. In each flip, the coins may land two heads, two tails, or one head and one tail. The probability of one head and one tail is twice as great as that of either two heads or two tails. By chance, a quite different ratio may appear in a small number of flips. But the more times you flip the coins, the closer the results will be to the probability ratio. The
unfortunate reality of our example above is that the animals are phenotypically indistinguishable or they all
look the same. The only way to distinguish their genotype is to have the
resulting lambs DNA tested. When two pairs of contrasting traits are involved in a cross the same principles apply. However, this cross is more complex because there are more possible combinations of genes. Let us try a cross involving the two traits such as Spider Lamb Syndrome
(represented with N/S) and Scrapie Resistance (represented
with R/Q). For this example, let us assume we were to mate a ewe that was DNA tested as being
RQNS with a ram that also has the genotype of RQNS:
The above example gives us: 1
RRNN
The ratio of phenotypes of the resulting mating would be 13 normal appearing lambs out of 16 and three lambs would have the spider condition. The number of lambs that are normal appearing and carry the scrapie resistance gene (at least one
R) would be 9 out of 16, but only 3 out of 16 would not be a spider carrier and still have the scrapie resistance gene. Only one lamb out of 16 would be homozygous for the scrapie resistance gene and free of the spider gene. Without special selection emphasis put on these two economically important conditions, I can see why so few
RRNN rams were available at the sale or privately from any breeders.
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