Rules of Inheritance

The rules of inheritance can be deduced from the rules of chromosomal behavior. Because chromosomes occur in pairs, so do individual genes. A person receives one member of each gene pair from the mother and the other from the father. The gene is the unit of inheritance. 

Alleles are classified by how they interact with each other. The red-hair allele is recessive —i.e., one such gene on each member of the chromosome pair is required for the trait to be manifest. The nonred-hair allele, in contrast, is dominant —i.e., a person with either one or two of these genes has nonred hair. It is customary to designate dominant alleles with capital letters and recessive alleles with lower-case letters. If r stands for the red-hair gene and R for its alternative, rr persons will have red hair, and Rr and RR will not. 

The genetic makeup of a person (or other organism) is the genotype. If two alleles are alike (RR or rr), the person is homozygous for the trait in question. If they are different (Rr), the person is heterozygous. The manifestation of the trait itself, in this case hair color, is the phenotype. Two persons, one with Rr and the other with RR, have different genotypes, but the same phenotype. 

Sometimes both members of a pair of alleles express their phenotypes. In such cases, the alleles are codominant. An example is found in blood groups a person with both A and B genes has both A and B antigens and has blood type AB. Dominance is often incomplete, the heterozygote being somewhere between the two homozygotes in its phenotype. 

One pair of chromosomes are the sex chromosomes. The two members of this pair, called the X and Y chromosomes, cure not identical in size and shape the Y is much smaller. 

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Biology becomes much more understandable in light of genetics (Ayala and Kiger 1984). This is true even more so in the case of the theory of evolution proposed by Darwin (1859). It seems the theory of evolution would have been placed on a solid foundation from the start if Darwin would have been aware of the Mendelian rules of inheritance. There is some indication that a copy of Mendel s publication was received by Darwin, which remained unopened during his lifetime. It is believed that this caused Darwin s failure to provide a firm basis on which selection works during the process of evolution. 

After cells L and R divide, we get four cells LT, LB, RT, and RB (where L is left, R is right, T is top, and B is bottom). All cells have the same genome (and, thus, update rules), but they also inherit the steady state vectors of their parents. The steady states of new cells RT and RB are not perturbed by any external influence, in accordance with the update rules of Table 10.1. However, for new cells LT and LB, the resetting of bit 1 in the neighborhood vector results in bit 2 of their state vectors switching ON. Since this bit is part of the 3-bit cell type bits, the type (and, thus, color) of both resulting cells change. This process is summarized below. 

Perhaps because of this focus on actual practice, chemists did not write much directly on the question of continuity and discontinuity. Among those who did, the most prominent in the eighteenth century was Pierre-Joseph Macquer, who attempted a system of chemistry based on a set of rules of affinity (see Chapter Eight). His third rule, which Duncan says he inherited from Stahl, reads Substances that unite together lose some of their separate properties and the compounds resulting from their union partake of the properties of the substances which serve as their principles. 

In the investigation of inherited diseases, it is often useful to sketch a patient s family tree to illustrate at a glance his or her family history. The simple rules and conventions for constructing these diagrams are shown in Figure 4. 

The rules of segregation of alleles originally defined by Gregor Mendel explained mnch of the phenomena associated with inheritance and have been dogmatically applied in the field of genetics. However, there are situations in which the rules of Mendelian inheritance cannot explain observed phenomena. A variety of molecular mechanisms have been identified that explain certain phenomena that are not easily explained by traditional Mendelian patterns of inheritance. These non-Mendelian mechanisms differ on a molecnlar basis, but can be described as a group by the term nontraditional mechanisms of inheritance or nontraditional inheritance. Stated simply, nontraditional inheritance refers to the pattern of inheritance of a trait or phenotype that occurs predictably, recurrently, and in some cases familiaUy, but does not follow the rules of typical Mendelian antosomal or sex chromosome inheritance. 

Thymine can also be described as 5-methyluracil. Other methylated pyrimidines exist in some nucleic acids e.g., methylation of cytosine (Fig. 3-30) in both DNA and RNA has important biological effects with respect to protection of the genetic material and its expression. Methylation is considered to be of central importance in controlling phenotypical traits whose inheritance appears to violate the usual rules of Mendelian genetics, and hence are termed epigenetic traits. 

After birth of a child with severe Potter phenotype due to bilateral agenesis or dysplasia in up to 10% unilateral agenesis or dysplasia can be found in one parent indicating a genetic basis. Empirical recurrence risks favour mostly multifactorial inheritance with a recurrence risk of about 5% after birth of a child with renal agenesis/dysplasia. In single pedigrees, autosomal dominant or X-linked mode of inheritance seems to be likely with incomplete penetrance and variable expressivity. Specific syndromes have to be ruled out (Table 3.2). 

A genetic basis is well known, many reported families indicate autosomal dominant inheritance with variable expressivity and incomplete penetrance. In about 10% of parents and siblings similar observations can be made, indicating multifactorial inheritance for the majority of cases. According to the general rule of thumb, the recurrence risk doubles with each additionally affected close relative. Isolated duplication anomalies have to be distinguished from genetic syndromes (Table 3.5). 

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