Trihybrid Cross (three genes)   Answer to Activity #1 Trihybrid Cross (three genes) ABD ABd AbD Abd aBD aBd abD abd AABBDD AABBDd AABbDD AABbDd AaBBDD AaBBDd AaBbDD AaBbDd AABBdd AABbdd AaBBdd AaBbdd AAbbDD AAbbDd AabbDD AabbDd AAbbdd Aabbdd aaBBDD aaBBDd aaBbDD aaBbDd aaBBdd aaBbdd aabbDD aabbDd aabbdd Phenotypic Ratio   27 9 3 1

Non- Mendelian Inheritance   Many other patterns of inheritance not based on Mendel's laws alone were discovered in plant and animals.   Gene interactions are of two types intragenic and intergenic.  

Intragenic interactions, also known as inter-allelic interactions occur between the two alleles of the same gene. Examples are incomplete dominance and co-dominance.   Intergenic interactions, also known as non-allelic interactions occur between alleles of different genes located on the same or different chromosomes. examples are complementary genes, supplementary genes, epistasis, pleiotropy and lethal genes.

Incomplete Dominance    interaction where both the alleles of a given trait is expressed as a blend (mixture) as against to a normal Mendelian pattern. an intermediate character is expressed. (This situation occurs due to the fact that the dominant gene can not completely suppress the expression of recessive gene. ) With the result, the heterozygous offspring will be phenotypically and genotypically different from either of the homozygous parent

        fig. 1. Flower Colour in Four O Clock Plant   This example very clearly indicates the phenomenon of incomplete dominance   That the genes responsible for red and white flowers do not actually mix, since both the pure characters reappear in the F2 generation   That there is no specific gene responsible for producing pink flowers  

Multiple Alleles a condition where more than two genes occupy the same locus, on the same pair of homologous chromosomes, in different organisms. Each of these genes expresses a totally different character. The inheritance of A B O blood groups in man is an example of multiple alleles. The fruit-fly, Drosphila melanogaster has 15 alleles for eye colour. In rabbits, there are 4 alleles for colour. In all these cases, at any given time, only two of the alleles can occupy the same locus on a pair of homologous chromosomes.

Polygenic traits (Polygenes) They are the result of the interaction of several genes. For instance, phenotypes like high blood pressure (hypertension) are not the result of a single "blood pressure" gene with many alleles (a 120/80allele, a 100/70 allele, a 170/95 allele, etc.) The phenotype is an interaction between a person's weight (one or more obesity genes),cholesterol level (one or more genes controlling metabolism), kidney function (salt transporter genes), smoking (a tendency to addiction), and probably lots of factors too. Each of the contributing genes can also have multiple alleles.

Codominance Co-dominance represents a situation where two allelic genes when present together in an individual, express their traits independently instead of showing a typical dominant recessive relationship.   A classical example of codominance is the expression of blood group AB. The genotype for this blood group is IA Ib. Each of the two genes produces the respective antigen and neither of them checks the expression of the other.

Heterodominance (inbreeding, outbreeding and hybrid vigour) When the heterozygotes have a more extreme phenotype than either of the corresponding homozygotes (homozygous parents), then it is usually referred to as overdominance, superdominance or heterodominance (Serra, 1959). For example, there is heterodominance, when the heterozygote Aa between a pair of factors which control size is bigger than the homozygotes  AA or aa. This type of allelic relation which implies interaction between the alleles, or of these with other factors of the genotype, may be found in qualitative characters and especially those such as size, production, vigour, etc., which are of importance in the breeding of animals and plants

Epistasis (Epi = above/over gene, static = standing) It is a pattern of inheritance where a pair of genes situated at one locus, prevent the expression of a pair of genes situated at another locus. Such genes are called inhibiting genes or epistatic genes (Epi = above/over gene, static = standing). Epistasis reduces the number of phenotypes in the F2 generation of a dihybrid cross.   A classical example of epistasis is seen in the white fowls. WHITE FOWL: COLOURED FOWL  13 : 3   gene C produces colour in the feathers. But the inhibiting gene I prevents the appearance of colour. As a result the leg horn fowl is white CCII. The plymouth rock fowl is white since it has recessive genes ccii

Pleiotropy It is a phenomenon where a given gene has multiple phenotypic effects. As a result, the gene not only influences the trait which it expresses, but also influences many other traits. Pleiotropic genes generally do not have the same influence on all the traits that they control. A pleiotropic gene may cause an evident expression of its specific trait, representing a major effect or a less evident expression of its other traits, representing a secondary effect. in the sweet pea plant, the genes that control the colour of the flower also control the colour of the seed coat and appearance of red spots in the axils of the leaves. In Drosophila, the gene that controls wing size also affects the nature of balancers, eye colour, fertility and life span of the insect. 

Lethality and Lethal Alleles The term Iethal is applied to those changes in the genome of an organism which produce effects severe enough to cause death. Lethality is a condition in which death of a certain genotype occurs prematurely. The fully dominant lethal allele kills the carrier individual both in its homozygous and heterozygous conditions. The recessive lethal allele kills the carrier individual only in homozygous condition. They maybe of two kinds (i) one which has no obvious phenotypic effect in heterozygotes and (ii) one which exhibits a distinctive phenotype when in heterozygous condition.

Heritability: Heritability has two definitions. The first is a statistical definition, and it defines heritability as the proportion of phenotypic variance attributable to genetic variance. The second definition is more common "sensical". It defines heritability as the extent to which genetic individual differences contribute to individual differences in observed behavior (or phenotypic individual differences) A heritable trait is most simply an offspring's trait that resembles the parents' corresponding trait.

Penetrance is a term used in genetics describing the proportion of individuals carrying a particular variation of a gene (an allele or genotype) that also express an associated trait (the phenotype). In medical genetics, the penetrance of a disease causing mutation is the proportion of individuals with the mutation who exhibit clinical symptoms. For example, if a mutation in the gene responsible for a particular autosomal dominant disorder has 95% penetrance, then 95% of those with the mutation develop the disease, while 5% do not.

Examples of Incomplete Penetrance I. Polydactyly in man is thought to be produced by a dominant gene P. The normal condition with five digits on each limb is produced by the recessive genotype (pp). Some heterozygous individuals (Pp) are not polydactylus and therefore has a penetrance of less than 70%. 2. In man, the tendency to develop diabetes mellitus (a condition in which there is an excess of sugar in the blood) is controlled by certain genes. However, not everyone carrying the gents for diabetes actually develops the condition, for the genes have incomplete penetrance.

Effects of environment on penetrance- The environmental factors and genetical background have some definite effect on the degree of penetrance of a gene. Accordingly, the percentage of penetrance of a given gene may be altered by changing the conditions of temperature, moisture, nutrition and so forth, under which the organism develops. For example, when various twins which carry genes for diabetes mellitus are studied, it is found that the disease appears only in those cases with one eating more carbohydrate foods (starch and sugars).

Variable expressivity Although some genetic disorders exhibit little variation, most have signs and symptoms that differ among affected individuals. Variable expressivity refers to the range of signs and symptoms that can occur in different people with the same genetic condition. For example, the features of Marfan syndrome vary widely— some people have only mild symptoms (such as being tall and thin with long, slender fingers), while others also experience life-threatening complications involving the heart and blood vessels. As with reduced penetrance, variable expressivity is probably caused by a combination of genetic, environmental, and lifestyle factors, most of which have not been identified. If a genetic condition has highly variable signs and symptoms, it may be challenging to diagnose.

LATE ONSET Some autosomal dominant diseases do not express themselves until later in life, well beyond the reproductive years. The individuals who will develop the disease have passed the mutant allele along to their offspring before they themselves know they are affected. In some cases even grandchildren are born before the affected grandparent shows the first signs of the disease. Huntington disease, sometimes called Huntington's Chorea because of the choreic movements expressed as the disease progresses, is a good example of a late onset disease. Age of onset varies from the teens to the late sixties, with a mean age of onset between ages 35 and 45. Nearly 100% of the individuals born with the defective allele will develop the disease by the time they are 70. The disease is progressive with death usually occurring between four and twenty-five years after the first symptoms develop. Emotional changes often are the first symptoms.