I will be using this article to correct some misconception about this topic.
Gene is a biological molecule responsible for transmissions of hereditary characters from parents to offspring. It is usually represented by letters, either capital or small letters.
Gene is a segment of a molecule known as deoxyribonucleic acid (DNA).
It is the arrangements of the nitrogenous bases, (adenine, guanine thymine and cytosine), in DNA molecule that determine the various structures of the genes, sudden change in the arrangement of these molecule produce what is known as mutation.
In other words, mutation is a sudden change in the sequence of base pair in chromosomal molecule, examples are
Genotype, is the genetic constituent or makeup of an organism and it could be expressed with respect to one character alone for example Tt or tt.
The different genotypes are AA, AS, AC and SS. They are referred to as the hemoglobin constituent on the red blood cells AS and SS are abnormal whereas, AC is rear.
Genetic Compatibility Chat
AA + AA = AA, AA, AA, AA (Excellent)
AA + AS = AA, AS, AA, AS (GOOD)
AA + SS = AS, AS, AS, AS (Fair)
AA + AC = AA, AA, AA AC (Good)
AS + AS = AA, AS, AS, SS (Very bad)
AS + AC = AA, AC, AS, SS (Bad)
SS + SS = SS, SS, SS, SS (Very Bad)
AC + SS = AS, AS, SS, SS (Very Bad)
AC + AC = AA, AC, AC, SS (Very bad, advice needed)
The most compatible genotype for marriage is AA + AA; this prevents us from having children that will have compatibility issue.
AA + AS, will produce offspring that shares both genotype which is good but most times, all the offspring will be AS, this is not good as it will lead to compatibility issue.
AS + AS, should abstain from marriage, there is the possibility of having a child with SS (sick cell disease).
SS + SS should also never think of marriage, there is a 100% chance of having a offspring with sickle cell disease.
The only solution to genetic incompatibility is Bone Marrow Transplant (BMT), this is also the only promising permanent cure to SS, SC and CC; However, this procedure has some set back, it is very expensive, and cannot be done in Africa, it is also associated with some risk.
Terms used in Genetics
Chromosome: Chromosomes are ribbon like structures found in the nucleus of each cell; it is the organelle that houses the gene and varies in number from one organism to the other. For example man has 46.
Sunflower – 34
Maize – 20
Mosquito – 6
In man, everybody cell (somatic cell), has 46 chromosomes, which occur in 23 pairs, 22 out of the 23 pairs are called autosomes, while the remaining one pair is the sex chromosomes, they determine the sex of an individual, both chromosomes are not identical, one is a rode shaped X, while the other is a hook shaped Y.
In male, the sex chromosome are XY, but in female they are identical, XX.
Alleles: alleles are pairs of gene responsible for a pair of contrasting character, e.g. the gene for tallness (T) and that of shortness (t) are alleles.
Homologous Chromosome: A pair of identical chromosome are called homologous chromosome.
Phenotype: This is the outward appearance of an organism.
Diploid number: This is the number of chromosomes in the body cell of an organism, in man, the diploid number is 46 and it is represented by 2n.
Haploid number: The number of chromosomes in the gamete of an organism is known as haploid number; it is the half the diploid number and is represented as n. The haploid chromosome in man is 23.
Homozygous gene: Two identical genes responsible for one character e.g TT or tt.
Heterozygous gene: Two dissimilar genes responsible for a one character.
Dominant gene: This is a gene that expressed its character in the presence of another whose character is suppressed, and is usually represented by capital letters, e.g. Tt, T is dominant over t.
Recessive gene: This is a gene that is suppressed in the presence of another, a recessive gene is only expressed when it occurs in homozygous form, it is usually represented by small letters.
Carrier: This happens when the recessive gene controlling the recessive character is present in the genotype of an individual; the individual is called a carrier.
Filial generation: The offspring of a parent make up filial generation. Thus we have first, second, and third filial generation.
Hybrid: This refers to the offspring of a cross between two genetically unlike parents. The phenomenon of artificial crossing is called hybridization. We have monohybrid cross when one contrasting character is considered in a cross. When two contrasting characters are considered it is called dihybrid cross. If more than two contrasting characters are crossed it is called polyhybrid cross.
Co-dominant genes: These are genes that exercise the same degree of dominance over each other; that is, none dominates over the other completely. Co-dominance is also known as incomplete dominance. The characters of co-dominance genes are expressed equally. For example, in the four o`clock flower when the read variety is crossed with the white variety, the two characters will blend to produce pink characters in the filial generation. The genes for the two characters (red and white flower) in the plant are co-dominant.
The simplest way to determine the chance of a child inherited a particular trait from their parentis by the use of Punnett square. It is the graphical representation of the phenotype and genotype (Gene combination) of the possible offspring.
Punnet square can be used to illustrate why offspring from the same parents have different traits, for example, why some have blue eyes while others have brown eyes, it can also give the odd of a parent transferring genetically inherited diseases such as dwarfism, cystic fibrosis, Huntington disease to offspring.
Punnet square is still used by genetic counselors today.