O if some siblings in the second generation lack a

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o If some siblings in the second generation lack a widow’s peak and one of the grandparents (first generation) also lacks one, then we know the other grandparent must be heterozygous, and we can determine the genotype of many other individuals. We can use the same family tree to trace the distribution of attached earlobes (f), a recessive characteristic. Individuals with a dominant allele (F) have free earlobes.
Some individuals may be ambiguous, especially if they have the dominant phenotype and could be heterozygous or homozygous dominant. A pedigree can help us understand the past and predict the future. We can use normal Mendelian rules, including multiplication and addition, to predict the probability of specific phenotypes. o For example, these rules could be used to predict the probability that a child with WwFf parents will have a widow’s peak and attached earlobes. The chance of having a widow’s peak is 3/4 (1/2 [WW] + 1/4 [Ww]). The chance of having attached earlobes is 1/4 [ff]. This combination has a probability of 3/4 × 1/4 = 3/16. Many human disorders follow Mendelian patterns of inheritance. Thousands of genetic disorders, including disabling or deadly hereditary diseases, are inherited as simple recessive traits. These conditions range from relatively mild (albinism) to life-threatening (cystic fibrosis). The recessive behavior of the alleles causing these conditions occurs because the allele codes for a malfunctioning protein or for no protein at all. o Heterozygotes have a normal phenotype because one normal allele produces enough of the required protein. A recessively inherited disorder shows up only in homozygous individuals who inherit a recessive allele from each parent. Individuals who lack the disorder are either homozygous dominant or heterozygotes. While heterozygotes may lack obvious phenotypic effects, they are carriers who may transmit a recessive allele to their offspring. Most people with recessive disorders are born to carriers with normal phenotypes. o Two carriers have a 1/4 chance of having a child with the disorder, 1/2 chance of having a child who is a carrier, and 1/4 chance of having a child without a defective allele. Genetic disorders are not evenly distributed among all groups of humans. This results from the different genetic histories of the world’s people during times when populations were more geographically and genetically isolated. Cystic fibrosis strikes one of every 2,500 whites of European descent. o One in 25 people of European descent is a carrier for this condition. o The normal allele for this gene codes for a membrane protein that transports Cl? between cells and extracellular fluid. o If these channels are defective or absent, there are abnormally high extracellular levels of chloride.
o This causes the mucus coats of certain cells to become thicker and stickier than normal.

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