Because the allele is recessive parents may be

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Because the allele is recessive, parents may be carriers of the disease-causing CF allele, but show no signs of the disease. In other words, healthy parents may be het- erozygous for the CF gene. Genetic testing can determine whether a person is a carrier of the recessive CF allele. If a couple knows from genetic tests that the mother is het- erozygous for the CF gene and that the father does not carry the disease-causing allele, they can use a Punnett square to predict that the chances of having a child who is a carrier are equal to the chances of having a child who is not: 50% in each case.They can predict that if they had 10 children, about 5 of them would probably be CF carriers and 5 would be homozygous for the normal allele. But until their baby is tested for the CF allele, they cannot know for certain whether that child will be a carrier of cystic fibrosis. And even if they had 99 children, they could not predict whether or not child number 100 would carry the CF allele. Donating alleles during reproduction is like tossing a coin. Both are random events. The best possible prediction of the outcome of a random event is nothing more than the probability that each of the possible outcomes will occur.To better understand segregation, independent assortment of alleles and the different ratios of phenotypes and genotypes predicted from hybrid crosses, it is useful to know some facts about probability. Probability Every time a coin is tossed, there are two possible and equally likely out- comes: Heads or tails.When a single die is tossed, there are six possible and equally likely outcomes: The numbers one through six. In any random event, the probability, or likeli- hood,that any single outcome will occur is equal to the number of times it can or actually does occur divided by the total number of possibilities. Thus, the probability of getting Figure 3-12 Cystic fibrosis treatment. In 1992, an experimental treatment was introduced in which the patient inhales a mist containing enzymes that thin the mucus that accumulates in the lungs.

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3-1 How Are Traits Passed from Generation to Generation? 77 heads in a single toss of a coin is or 0.5. Heads is one of two possible outcomes. Like- wise, the probability of getting a specific number, say a four, on any given roll of the die is or 0.17.An event that always occurs has a probability of or 1.0.An impossible event has a probability of 0. We can apply probability theory to the distribution of alleles at conception. Imag- ine a parent who is heterozygous for some hypothetical gene we will call G . Because the genotype is Gg , each gamete (egg or sperm) produced by that parent will contain one of the two alleles, either G or g , but not both. If neither allele is favored when gametes are produced, as we predict, the basis of Mendel’s law of segregation, the probability, P , that any one gamete will contain the G allele and, hence be passed on, is or 0.5. Likewise, the probability that the g allele is passed on is or 0.5. Notice that the probability that either the G or the g
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