Genes are the blueprint for all living things – plants and animals. When organisms of the same species mate and reproduce, they transfer information from one generation of species to the next. This is why offspring resemble one or both of their parents. A gene is a sequence of DNA, which is a unit of heredity located at a fixed location on a chromosome. Every characteristic of an organism is encoded by genes. All genes have variants called alleles, and the variant responsible for the presence of a specific characteristic such as eye color, hair color, height of an individual, size of nose, high or low level of resistance to disease , low or high susceptibility. Diseases such as diabetes, high blood pressure, obesity, presence or absence of genetic diseases
The parents of the child carry alleles of the same gene. All organisms have pairs of chromosomes. During fertilization of eggs and sperm, they each carry a set of chromosomes from the mother and father. For example, humans have 46 chromosomes or 23 pairs of chromosomes. Chromosome pairs split, and only half of the groups make their way into the sperm and egg. This is to keep the number of chromosomes the same after fertilization. The number of chromosomes is unique to each species and cannot be changed. After fertilization, the number of chromosomes recovers.
The expression of a particular trait depends on whether the gene representing that trait is dominant or recessive.
The presence of a dominant gene will determine whether a particular trait (phenotype) should be passed on. The dominant allele of this gene is indicated by capital letters. Recessive alleles of this gene are indicated in lowercase letters. When dominant and recessive alleles are present in the same individual, it is a dominant feature of expression. If an individual has a dominant allele (or two recessive alleles) for the same gene, it is said to be homozygous dominant or homozygous recessive. If he has one dominant and one recessive allele of the gene, he is said to be heterozygous.
Let us understand this with an example. Let’s denote the curly allele by C and the straight allele by c. If a person has a combination of Cc (heterozygous) at the gene that determines the hair characteristics on his chromosome, he will have curly hair because the dominant allele expresses itself and the recessive allele remains dormant. If he has a cc combination, his hair will be straight because the recessive allele will express itself without the dominant allele.
Let’s take another example about the height of an individual. We often say that a child’s height depends on the height of his parents. Let’s see – let’s say tall tall is denoted by H (dominant allele) and short tall is denoted by h (recessive allele).
So if the child has the first or fourth allele combination, he will be high and will be considered similar to the taller parents. In this case, a highly dominant allele is expressed over a recessive allele to produce a high trait (phenotype). But if the child has the second or third combination, he will be short and will be considered similar to the shorter parent. In this case, the recessive allele is expressed as the absence of the dominant allele. This is called complete dominance
Another case called codominance can be seen in blood types. Human blood group genes have equally dominant A and B antigen alleles. In a person’s existence, the characteristics that will be given to them and his blood type will be AB.
Partial dominance is seen when both the dominant and recessive alleles of a gene are partially expressed to produce a third variant. For example, when red flowers are cross-bred with white flowers, it produces pink offspring.
Genetic combinations determine the overall makeup of offspring. Genotype determines phenotype.
Dominant traits are passed on, and recessive traits remain dormant.