| Talk about the case of polyallelism. | When three or more alleles of the same gene are present in a population, we say that the locus has multiple alleles. Any diploid organism has, at most, two homologous gene loci that may be occupied by different alleles of the same gene.
The simplest case of multiple alleles occurs when three alternative alleles of one gene exist. This situation is illustrated in the inheritance of the ABO blood groups in humans
A, B, AB, and O are all types of present antigens that there present is tested using antibodies of them, O has no antigens.
A crossmatch (compatibility test between a donor and a patient) is essential before blood transfusion. Group O is the universal donor; group AB is the universal recipient. |
| Talk about the Rhesus group. | The Rh factor is also inherited from the parents but independently of the ABO blood type alleles.
There are two different alleles for the Rh factor known as: Rh+ (or R or Rh or D) dominant and Rh- (or r or rh or d) recessive. Recessive rr individuals produce no Rh antigens and are Rh negative, while RR and Rr individuals have Rh antigens on the surface of their red blood cells and are classified as Rh positive. The function of Rh antigens remains unknown, but we suggest that they may be involved in maintaining the structural and functional properties of the erythrocyte membrane. |
| Talk about polyallelism in the coat color of rabbits. | In rabbits, a series of four alleles controls coat color in the following way: C is dominant to all other alleles and causes full dark gray color. The Chinchilla phenotype is due to the cch allele, which is dominant to all alleles other than C. The ch allele, dominant only to c a (albino), results in the Himalayan coat color. The order of dominance is C>cch>ch>ca. |
| Talk about the white eye locus of Drosophila. | the allele w+ (red eye) is dominant over all the other alleles: w+ > w= we = wa = wsat… In such cases, the heterozygotes commonly have phenotypes intermediate between those of their parents
It is X sex linked |
| Give the number of genotypes, homozygous and heterozygous in case of polyallelism. | Genotypes: N(N+1)/2
Homozygous: N
Heterozygous: N(N-1)/2 |
| Talk about lethal alleles. | Many gene products are essential to an organism’s survival. Mutations resulting in the synthesis
of a gene product that is nonfunctional can often be tolerated in the heterozygous state; that is, one
wild-type allele may be sufficient to produce enough of the essential product to allow survival.
However, such a mutation behaves as a recessive lethal allele, and homozygous recessive
individuals will not survive. The time of death will depend on when the product is essential |
| Give the phenotypic and genotypic ration of lethal allele. | the phenotypic ratio 3:1
will be deviated to 100% [A], and the genotypic
ratio 1:2:1 will be deviated to 1:2:0.
¼ of
the progeny of heterozygous flies (Aa) are
homozygotes (aa) for the lethal allele |
| Talk about abnoraml case of lethal allele. | In some cases, the allele responsible for a lethal effect when homozygous may also result in a
distinctive mutant phenotype when present heterozygously. It is behaving as a recessive lethal
allele but is dominant with respect to the phenotype. |
| Talk about multiple effects of expression of a single gene. | the converse situation, where expression of a single gene has multiple phenotypic effects, is also quite common. This phenomenon, which often becomes apparent when phenotypes are examined carefully, is referred to as pleiotropy.
In contrast, one gene governing one character is called monotropic
Examples albinism and sickle cell anemia |
| Talk about Marfan syndrome. | Marfan syndrome, a human malady resulting from an autosomal dominant mutation in the gene encoding the connective tissue protein fibrillin. Because this protein is widespread in many tissues in the body, one would expect multiple effects of such a defect. In fact, fibrillin is important to the structural integrity of the lens of the eye, to the lining of vessels such as the aorta, and to bones, among other tissues. As a result, the phenotype associated with Marfan syndrome includes lens dislocation, increased risk of aortic aneurysm, and lengthened long bones in limbs. |
| Talk about the combinations of two gene pairs with two modes of inheritance | Like the combination of albinsim which is a monohybrid dominance case and codominance of blood group
The first will give 2 phenotypes and 3 genotypes, the latter will give three genotypes and three phenotypes |
| Talk about the case of polymery | Soon after Mendel’s work was rediscovered, experimentation revealed that in many cases a given phenotype is affected by more than one gene. Instead of single genes controlling the development of individual parts of a plant or animal body, it soon became clear that phenotypic characters such as eye color, hair color, or fruit shape can be influenced by many different genes and their products. Polymeric genes can either interact in a simple manner (simple interaction), in an epistatic manner (epistasis) or in an additive manner (polygenic additive traits). |
| Talk about simple interaction between genes. | The term gene interaction is often used to express the idea that many genes may interact to affect a single phenotype.
When these F1 walnuts are mated in cross 4, all four comb shapes are produced in a ratio that approximates 9:3:3:1. This observation immediately suggests a cross involving two gene pairs, because the resulting data display the same ratio as in Mendel’s dihybrid crosses. Since only one character is involved (comb shape), we propose that the two gene pairs “interact” to produce each phenotype |
| What is epistasis? | Some of the best examples of gene interaction are those showing the phenomenon of epistasis, where the expression of one gene pair masks or modifies the effect of another gene pair. Sometimes the genes involved influence the same general phenotypic characteristic in an antagonistic manner, which leads to masking. In other cases, however, the genes involved exert their influence on one another in a complementary, or cooperative, fashion.
For example, the homozygous presence of a recessive allele may prevent or override the expression
of other alleles at a second locus (or several other loci). In this case, the alleles at the first locus
are said to be epistatic to those at the second locus, and the alleles at the second locus are hypostatic
to those at the first locus. |
| Talk about Homozygous recessive epistasis | Bombay phenotype discussed earlier is an example of the homozygous recessive condition at one locus masking the expression of a second locus. There we established that the homozygous presence of the mutant form of the FUT1 gene masks the expression of the IA and IB alleles. Only individuals containing at least one wild-type FUT1 allele can form the A or B antigen. As a result, individuals whose genotypes include the IA or IB allele and who have no wild-type FUT1 allele are of the type O phenotype, regardless of their potential to make either antigen. An example of the outcome of matings between individuals heterozygous at both loci is illustrated. If many such individuals have children, the phenotypic ratio of 3 A: 6 AB: 3 B: 4 O is expected in their offspring. |
