Recessive phenotype

TOie frequency of cells with a recessive phenotype is associated with chromosomal euploidy. 

You may be familiar with Richard Dawkins s books The Selfish Gene and The Blind Watchmaker, and may perhaps have learned most of what you know about evolution, particularly the evolution of behavior, from these excellent sources. His second book. The Extended Phenotype, is less well known, and that is a pity, because in many ways it is his finest achievement. Nonetheless, even the most lucid books contain some obscure passages, and you could weU have been puzzled by a couple of pages of The Extended Phenotype that deal will the theory of modifier genes, which R. A. Fisher proposed in 1930 to explain why the phenotypes of some genes are dominant whereas others are recessive, phenotype being a technical term for the specific set of observable characteristics that indicate the presence of a particular variant of a gene. 

The specific-locus method consists essentially of mating treated and untreated wild-type mice, either male or female, to a strain homozygous for a number of recessive genes which have readily visible phenotypic effects. The offspring produced from the cross will be wild type unless a mutation has occurred at one of the loci represented by the recessive genes, when the recessive phenotype or perhaps a new or intermediate phenotype will be seen. The system thus provides a simple means of estimating the mutation frequencies to recessive visibles, including those with lethal effects. Dominant visibles would, of course, also be detected. 

S. Ohno, Aneuploidy as a possible means employed by malignant cells to express recessive phenotypes, in Chromosomes and Cancer (J. German, ed.), pp. 77-94, John Wiley and Sons, New York (1974). 

The genes responsible for production of the ABO substances are present on the long arm of chromosome 9. There are three alleles, two of which are codominant (A and B) and the third (O) recessive these ultimately determine the four phenotypic products the A, B, AB, and O substances. 

The three Squat (sqt) mutants have a characteristic dominant-dumpy/ recessive-roller phenotype, and of the two genes characterized sqt-1 and sqt-3), both encode cuticle collagens (Kramer, 1997). 

Birouk, N., Azzedine, H., Dubourg, O. et al. Phenotypical features of a Moroccan family with autosomal recessive Charcot-Marie-Tooth disease associated with the S194X mutation in the GDAP1 gene. Arch. Neurol. 60 598-604,2003. 

If only one copy of an allele is required for its phenotypic expression, the allele is dominant (i.e., it is observable in the heterozygous state). If two copies of the allele are required for its expression (i.e., the disease phenotype is observable only in the homozygous state), it is recessive. The expression of the recessive alleie is thus hidden in the heterozygote. The terms dominant and recessive provide a convenient classification of genetic diseases, as seen below. If two different alleles are both phenotypicaUy expressed in a heterozygous genotype, the alleles are said to be codominant. 

A 30-year-old man is phenotypically normal, but two of his siblings died from infantile Tay-Sachs disease, an autosomal recessive condition that is lethal by the age of five. What is the risk that this man is a heterozygous carrier of the disease-causing mutation ... 

The phenotypic complexity noted for other GCPR diseases is true also for PTHRl mutations. Opposite clinical manifestations have been reported to result from distinct recessive mutations in the gene. These rare variants present as Eiken syndrome, a distinct entity from JMC and Blomstrand s chondrodysplasia and from enchondromatosis. The skeletal features are opposite those in Blomstrand s chondrodysplasia. The Eiken syndrome variant, resulting in a truncation at position 485, may result in a paradoxical phenotype caused by the consequences of disrupting the carboxyl tail of the receptor (60). 

The W1 locus encodes flavonoid 3 5 -hydroxylase F3 5 H) [21]. F3 5 H diverts metabolic flux into the blue delphinidin branch of anthocyanin biosynthesis (Fig. 4.1). In the absence of F3 H activity (f), Wi and recessive wl give imperfect black and buff seed colors, respectively [10]. However, in black seeds, F3 H (T) phenotypically masks Wl. In contrast to its role in seeds, Wl has a prominent role in flower colors, as delphinidin-based anthocyanins are the major pigments in purple soybean flowers [22, 23]. Interestingly, F3 5 H is expressed at very low levels in flowers and seeds [21]. This suggests that, out of the two branch-point genes (i.e., F3 H and F3 5 H), it is the strong expression of F3 H in seed coats and weak expression in the flowers that determines preferential accumulation of cyanidin-based and delphinidin-based anthocyanins in these respective tissues [21]. 

T r) seeds [10]. O gives brown, whereas recessive o gives a red-brown phenotype. Preliminary analysis reveals the anthocyanin content of red-brown seed coats to differ from brown by the presence of 3-0-galactosides of cyanidin and delphini-din (Fig. 4.2). An enzyme with minor 3-0-galactosyltransferase activity has been identified [26], but it remains to be determined whether genetic differences in this gene account for red-brown versus brown seed coat color. 

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