What is a Gene?
Basic Genetic Concepts
Easy Review of Genetic Concepts
1. What is a gene?
A gene is a sequence of DNA nucleotides that codifies the production of a protein.
2. Is a gene a triplet of consecutive DNA nucleotides?
A gene is not a triplet of DNA nucleotides with their respective nitrogen-containing bases, like AAG or CGT. The nucleotide triplets may be pieces of genes but not genes.
A gene is a portion of a DNA molecule that codifies a specific protein. Thus it is formed by several DNA nucleotide triplets.
3. How is the concept of chromosome related to the concept of the gene?
A chromosome is a DNA molecule. A chromosome may contain several different genes and also DNA portions that are not genes.
4. What is meant by “gene locus”?
Gene locus (locus means place) is the location of a gene in a chromosome, i.e., the position of the gene in a DNA molecule.
5. What are alleles of a gene?
Diploid individuals have paired chromosomes. For example in humans there are 23 pairs of chromosomes totaling 46 chromosomes. Each pair comprehends homologous chromosomes, one chromosome from the father and another from the mother, both of them containing information related to the production of the same proteins (with the exception of the sex chromosomes, which are partially heterologous). So in the diploid individual it is said that each gene has two alleles, one in each chromosome of the homologous pair.
6. Are the alleles of a gene necessarily originated one from the father and the other from the mother? Are there exceptions?
It is natural that alleles have come one from the father and the other from the mother but it is not obligatory. In a “clone” generated by nucleus transplantation technology, for example, the alleles come from a single individual. In polysomies (as in trisomy 21) each gene of the affected chromosome has three alleles, in trisomies, or four, in tetrasomies.
7. What is a phenotype?
A phenotype is every observable characteristic of a living being conditioned by its genes. Some phenotypes may be altered by nongenetic factors (for example, artificial hair coloring). Specific phenotypes are also called phenotypical traits.
8. What is a genotype? What is the difference between genotype and phenotype?
Genotype is the genes, DNA nucleotide sequences contained in the chromosomes of an individual, that condition the phenotype. Phenotypes then are a biological manifestation of genotypes.
For example, the altered hemoglobin chain of sickle cell disease and the manifestation of the disease itself are the phenotype. The altered DNA nucleotide sequence in the gene that codifies the production of that abnormal hemoglobin chain is the genotype.
9. Does the environment exert an influence on the phenotype?
A phenotype may be altered (compared to the original situation conditioned by its genotype) by nongenetic means. Examples: some hormones may cease to be secreted due to diseases but the genes that determine their secretion remain intact; a person can go to a hairdresser and change the color of his/her hair; plastic surgery can be performed to alter facial features of an individual; colored contact lenses may be worn; a plant can grow beyond its genetically conditioned size by application of phytohormones.
Revealing cases of environmental influence on phenotypes are observed in monozygotic twins that have grown in different places. Generally these twins present very distinct phenotypical features due to the environmental and cultural differences of the places where they lived and to their different individual experiences in life.
(Biologically programmed phenotypical changes, like nonpathological changes of the skin color caused by sunlight exposure, tanning, or the variation of the color of some flowers according to the pH of the soil cannot be considered independent from the genotype. Actually these changes are planned by the genotype as natural adaptations to environmental changes.)
10. Are environmental phenotypical changes transmitted to the offspring?
Changes caused on phenotypes by the environment are not transmitted to the offspring (unless their primary cause is genotypical change in germ cells or in gametic cells). If a person changes the color of the hair or undergoes aesthetic plastic surgery the resulting features are not transmitted to his/her offspring.
11. What are the situations in which the environment can alter the genotype of an individual? What is the condition for this type of change to be transmitted to the offspring?
The environment can only alter genotypes when its action causes alterations in the genetic material (mutations) of the individual, i.e., deletion, addition or substitution of entire chromosomes or of nucleotides that form the DNA molecules.
Mutations are only transmitted to the offspring when affecting the germ cells that produce gametes or the gametes themselves.
12. What are some examples of phenotypical characteristics that present two or more varieties and of phenotypical features that do not vary? In relation to the genes correspondent to those characteristics that vary among individuals what can be expected about their alleles?
Color of the eyes, color of the hair, color of the skin, height, blood type are examples of phenotypical features that present two or more varieties. Other examples are the color of flowers and seeds in some plants, the sex of the individual in dioecius species, etc. Examples of phenotypical characteristics that do not present variation among individuals of the same species are: in general the number of limbs, the anatomical position of the organs, the general constitution of tissues and cells, etc.
Phenotype possibility of presenting natural variations (in beings of the same species) are necessarily determined by two or more different alleles of the correspondent gene. These different alleles combine and form different genotypes that condition the different phenotypes (variations).
13. Considering a pair of homologous chromosomes containing a gene having two different alleles how many different genotypes can the individual present?
If a gene of a diploid species has different alleles, for example, A and A’, the possible genotypes are: A’A’, AA, and AA’. So any of these three different genotypes may be the genotype of an individual.
14. For an individual having a genotype formed of two different alleles that condition different varieties of the same phenotypical trait, upon what will the phenotypical feature actually manifested depend?
If an individual presents a gene having different alleles (common situation), for example, A and A’, three types of genotypes may be formed: AA, A’A’ and AA’. The question refers to an individual bearing a genotype made of two different alleles, so it is the AA’ genotype (the heterozygous individual).
This AA’ individual may manifest the phenotype conditioned by the allele A or the phenotype conditioned by the allele A’ or still a mixed phenotype of those two forms. If the allele A is dominant over the allele A’ the form conditioned by A will manifest. If A’ is the dominant allele, the form determined by A’ will manifest. This phenomenon is known as dominance and occurs because the recessive (nondominant) allele manifests only when present in double in the genotype (in homozygosity), while the dominant allele manifests even when in heterozygosity. If none of the alleles dominate a mixture of the two varieties conditioned by both alleles appears or instead a third form may come out.
15. What is the difference between dominant allele and recessive allele?
Dominant allele is the allele that determines phenotypical features that manifest in homozygous or heterozygous genotypes.
In Genetics the dominant allele is represented in uppercase, e.g., “A”, and its recessive allele is written in lowercase, “a”.
In molecular terms generally the recessive allele has a nucleotide sequence previously identical to the corresponding sequence in the dominant allele but that during evolution was inactivated by mutation. This fact explains the expression of the dominant phenotype in heterozygosity (since one functional allele is still present).
16. Whenever a pair of alleles has different alleles is there dominance between them?
Not in all cases of a gene having two different alleles is the dominance complete. There are genes in which heterozygosity occurs with incomplete dominance (manifestation of an intermediate phenotype in relation to the homozygous, like in the color of roses, between white and red) and other genes that present codominance (expression of a third different feature, as in the MN blood group system).
17. What is the difference between homozygosity and heterozygosity?
Homozygosity occurs when an individual has two identical alleles of a gene, for example, AA or aa. Heterozygosity occurs when an individual has two different alleles of the same gene, in the example, Aa.
18. Why can it be said that a recessive allele can remain hidden in the phenotype of an individual and revealed only when manifested in homozygosity in the offspring?
A recessive allele can remain hidden because it does not manifest in heterozygous individual, i.e., it may be present in the genotype but not expressed in the phenotype. When this allele is transmitted to the offspring and forms homozygous genotype with another recessive allele from other chromosomal lineage the phenotypical characteristics that come out reveal its existence.
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