This figure shows a family pedigree chart of the inheritance of hemophilia disease : Answer the following questions: Q. A) is the allele for hemophilia dominant or recessive? Q. B) state with reasons the genotype of individuals 1,2 and 4. Q. C) what are the possible genotype of 3 and 5?
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Human Pedigree Analysis
In humans, controlled crosses cannot be made, so geneticists must resort to scrutinizing family records in the hope that informative matings have been made that can be used to deduce dominance and distinguish autosomal from X-linked inheritance. The investigator traces the history of some variant phenotype back through the history of the family and draws up a family tree, or pedigree, using the standard symbols given in Figure 4-17. The clues in the pedigree have to be interpreted differently depending on whether one of the contrasting phenotypes is a rare disorder or whether both phenotypes of a pair are common morphs of a polymorphism. The genetic disorders of human beings can be dominant or recessive phenotypes and can be either autosomal or X-linked. The four categories are discussed in the following sections.
Figure 4-17. Symbols used in human pedigree analysis.
Figure 4-17
Symbols used in human pedigree analysis.(After W. F. Bodmer and L. L. Cavalli-Sforza,Genetics, Evolution, and Man.Copyright © 1976 by W. H. Freeman and Company.)
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Autosomal Recessive Disorders
The unusual phenotype of a recessive disorder is determined by homozygosity for a recessive allele, and the unaffected phenotype is determined by the corresponding dominant allele. In Chapter 3 we saw that phenylketonuria (PKU) is a recessive phenotype. PKU is determined by an allele that we can call p, and the normal condition by P. Therefore, sufferers of this disease are of genotype p/p, and unaffected people are either P/P or P/p. What patterns in a pedigree would reveal such an inheritance? Two key points are that generally the disease appears in the progeny of unaffected parents and that the affected progeny include both males and females equally. When we know that both male and female phenotypic proportions are equal, we can assume that we are dealing with autosomal inheritance, not X-linked inheritance. The following typical pedigree illustrates the key point that affected children are born to unaffected parents:
Image ch4e12.jpg
From this pattern we can immediately deduce autosomal inheritance, with the recessive allele responsible for the exceptional phenotype (indicated by shading). Furthermore, we can deduce that the parents must both be heterozygotes, P/p. (Both must have a p allele because each contributed one to each affected child, and both must have a P allele because the people are phenotypically normal.) We can identify the genotypes of the children (in the order shown) as P/–, p/p, p/p, and P/–. Hence, the pedigree can be rewritten
Image ch4e13.jpg
Notice another interesting feature of pedigree analysis: even though Mendelian rules are at work, Mendelian ratios are rarely observed in single families because the sample sizes are too small. In the above example, we see a 1:1 phenotypic ratio in the progeny of what is clearly a monohybrid cross, in which we might expect a 3:1 ratio. If the couple were to have, say, 20 children, the ratio would undoubtedly be something like 15 unaffected children and 5 with PKU (the expected monohybrid 3:1 ratio), but in a sample of four any ratio is possible and all ratios are commonly found.
In the case of a rare recessive allele, in the population most of these alleles will be found in heterozygotes, not in homozygotes. The reason is a matter of probability: to conceive a recessive homozygote, both