Gene interaction for a single phenotype

            We are all very familiar with the Mendelian studies, which show the independent gene assortment for each trait analysed in the peas. However, genes frequently do not act independently in their phenotypic expression. Gene interaction is the term used to call the interaction between expressions of genes at different loci. The effects of genes at one locus depend on the expression of genes at other loci (Pierce, 2008). Thus, the phenotype is not predictable from the perspective of a single locus effects alone, as the products of genes at different loci blend to produce the phenotype.

Figure 1: Gene interaction in fruit colour of Capsicum annum (Pierce, 2008) 

                The fruit colour in the pepper Capsicum annuum is a simple example of gene interaction between two loci producing a single trait. The Y locus and the C locus interact to form the colours. When two heterozygous cross with each other (YyCc x YyCc) the four possible colours of pepper fruit are present in the offspring in a proportion of 9:3:3:1, which are red, peach, orange and cream (Pierce, 2008). You can see on the picture the genotype for each colour.

                

Figure 2: Some coat colours in mice
 (Griffiths, 2000)

         
More complex gene interaction is observed in the genetics of coat colour of mammals. In dogs, many genes participate, and several loci interact with each other. The expression of a particular gene is modified by the effects of other genes. There are four most recognized loci producing many of the noticeable differences in colour and pattern among breed of dogs. However, other poorly known loci may modify the effects of these loci (Pierce, 2008).  In mice, five main genes interact to determine the coat colours (Griffiths et. al. 2000).

                In humans a set of genes are involved in the production of melanin, which provides colour for skin, hair and eyes. Mutations in any of these genes disturb the production of melanin, which reduces pigmentation in the skin, hair, and eyes and causes albinism. Mutations in six genes have been related to be responsible for different types of albinism (Oetting & Kin, 1999).

Figure 3: Albinism in humans

REFERENCE LIST

Griffiths AJF, Miller JH, Suzuki DT, Lewontin RC, Gelbart WM (2000). Gene interaction in coat color of mammals. In An Introduction to Genetic Analysis. 7th edition. New York: W. H. Freeman.

Oetting, WS and King, RA (1999), Molecular basis of albinism: Mutations and polymorphisms of pigmentation genes associated with albinism. Hum. Mutat., 13: 99–115

Pierce BA (2008). Genetic Maternal Effect. In: Genetics: A Conceptual Approach. (third edition) W. H. Freeman and Company, New York. pp 105-114

Figure 3: http://foter.com/search/instant/?q=albinism  

Genomic Imprinting: A different way of gene expression

             Well, I have discussed about sex influences on heredity on the last posts. I could not forget this amazing topic in my literature reviews here. Genomic imprinting is a phenomenon whereby the expression of the genotype is different when inherited from the male or female parent. Some genes have their expression significantly differentiated according to their parental origin (Pierce, 2008). The mechanisms of control of this expression are still not totally clear, but the methylation of DNA is undoubtedly fundamental to the process, being a way of epigenetic phenomenon.

Fig. 1: Genomic imprinting in fetal growth of mice. (Pierce, 2008)

               The gene Igf2 of mice and humans express under genomic imprinting process, encoding the protein insulin-like growth factor II, which determines the size of the fetus and placenta. Both alleles of the gene are inherited from the father and the mother, but only the paternal one is expressed, while maternal copy is completely silent. Demonstrations of deleted paternal copy of Igf2 in mice show small placenta and low-birth-weight in the offspring as result. Like in the placenta in mammals, plants have also exhibited differential expression of maternal and paternal genes in the endosperm, which provides nutrients for the growth of the embryo (Pierce, 2008).

            

Fig. 2: Genomic imprinting in deletion of region in chromosome
15 causing Prader-Willi and Angelman Syndrome.

Genomic imprinting has also received importance in human syndromes. It has been observed this event in the Prader-Willi and Angelman syndromes. Many individuals with the Prader-Willi syndrome do not have a small region on the long arm of chromosome 15. This deletion is always inherited from the father. When the same deletion is maternal, the expression results in a totally different set of symptoms, originating another syndrome: the Angelman syndrome (Pierce, 2008). Children with Prader-Willi syndrome present small hands and feet, mental retardation, poor sexual development and short stature. Symptoms of Angelman syndrome include frequent uncontrolled muscle movement, a large mouth and unusual seizures. A normal development requires the presence of both paternal and maternal regions of the chromosome 15(Pierce, 2008).

REFERENCE:

Pierce B. A. (2008). Genetic Maternal Effect. In: Genetics: A Conceptual Approach. (third edition) W. H. Freeman and Company, New York. p 119-120.

Figure 2: http://atlasgeneticsoncology.org/Deep/GenomImprintID20032.html (accessed 10/05/2014)

Genetic Maternal Effect on Phenotype

                As I have informed about the inheritance of maternal mitochondria, I remembered another sexual influence on phenotype, the genetic maternal effect on some treats. However, it is important to clarify that cytoplasmic inheritance is different from genetic maternal effect. In mithocondrial inheritance, the genes for the characteristic are inherited from only the mother. In genetic maternal effect, the genes are inherited from both parents, but the offspring’s treat is determined not by its own genotype but by its mother’s genotype. The maternal effect usually arises when substances in the egg’s cytoplasm, which are encoded by the mother’s nuclear genes, influence the offspring’s phenotype in its early development (Pierce, 2008). According to William et al., these substances are gene products of regulation, which active or repress the expression of the zygote genome.

Genetic maternal effect in shell coiling of Limnaea peregra (Pierce, 2008)

                The best didactic example of maternal effect on phenotype is seen on the species Limnaea pelegra. The shell of its snails usually coils to the right (dextral coiling) but some snails are left-coiling shell (sinistral coiling). The allele for dextral (s+) is dominant over the allele for sinistral (s). The direction of the coiling is influenced by the way in which the cytoplasm divides soon after the fertilization. This event is determined by a substance produced by the mother and passed to the offspring through the egg’s cytoplasm (Pierce, 2008). As the progeny’s phenotype is determined by the mother’s genotype, not her phenotype, the phenotype of the mother is not necessarily the same as the offspring.

                As you can see on the picture, dextral males s+s+ crossed with sinistral females ss produce a heterozygous offspring (s+s) that have all sinistral shell, as its mother’s genotype encodes sinistral coiling. As result of the F1 self-fertilized, the genotype of F2 is 1s+s+:2s+s:1ss, all with dextral coiling (as F1 genotype produces dextral phenotype). Males can influence phenotypes here just by contributing to the genotype of their daughters, affecting the phenotypes of the daughters’ offspring (Pierce, 2008).

REFERENCES:

Klug W. S.; Cummings M. R.; Spencer C.; Palladino M. A.(2009). Concepts of Genetics. (Ninth edition) Benjamin Cummings, San Francisco. p 489.

Pierce B. A. (2008). Genetic Maternal Effect. In: Genetics: A Conceptual Approach. (third edition) W. H. Freeman and Company, New York. p 119-120.