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Why you look the way you do (Part 2)

The manner in which one behavior is transferred from one person to another follows a certain pattern called Inheritance Patterns. Scientifically, Inheritance patterns are the predictable patterns that are seen in the transmission of genes from one generation to the other,

Sylvester Torsman
Sylvester Torsman

So what determines whether a person will be black or white, tall or short...?

‌‌Inheritance Patterns‌‌

The manner in which one behavior is transferred from one person to another follows a certain pattern called Inheritance Patterns. Scientifically, Inheritance patterns are the predictable patterns that are seen in the transmission of genes from one generation to the other, and the way they are expressed in the organism that possesses them. For an explanation on what a gene is, click on this article.

Before we delve deeper, let's discuss these generative terms in biology  for the sake of the topic at hand.‌‌

Phenotype: Phenotype is descibed as an organism's observable characteristics. In simple terms, it refers to what can be seen on the outside of a person such as height, hair texture, skin color, or ear shape.‌‌It is influenced by the genes inherited from the parents and also from environmental factors such as nutrition, temperature, height or altitude, etc.‌‌ For instance, malnutrition of infants could result in stunted growth, heart related problems and many other challenges which affects a person's outward appearance. Also, a couple may give birth to two identical children, which means, they possess similar genes. However, when one is sent abroad and the other stays in a rural area, their appearance seem to differ.‌‌

Genotype: This refers to the set of genes an organism inherits. Basically, the genetic makeup of the organism. Genes carry the information that determines your traits. They are hereditary, meaning they are passed on from parents to offspring.‌‌

Dominant and recessive gene: Dominant refers to the relationship between two versions of a gene. Individuals receive two versions of each gene, known as alleles, from each parent. As explained in my previous article, an allele is a variant form of a gene.‌‌Some genes have a variety of different forms, which are located at the same position, or genetic locus, on a chromosome. Humans are called diploid organisms because they have two alleles at each genetic locus, with one allele inherited from each parent. If the alleles of a gene are different, one allele will be expressed; that is the dominant gene. The effect of the other allele, called recessive, is concealed or overpowered.‌‌ Now to the main topic...‌‌

Understanding how genes are passed on to us‌‌

how the genotype looks_ eg of the male genotype 

Animals and most other multicellular organisms possess two sets of chromosomes in each cell, one set inherited from the mother, and the other from the father. Such an organism is said to be a diploid . In humans, the maternal and paternal sets each include 23 chromosomes, so humans have 46 chromosomes in each cell.‌‌ Analyses show that the maternal and paternal chromosome sets are virtually identical, and they can be matched up to form 23 pairs. One pair, however, may not be a pair at all. These are the sex chromosomes so called because they determine the sex or gender of the organism. In humans, the female carries two identical sex chromosomes, called X chromosomes, while the male carries two dissimilar chromosomes, an X and a Y. The other 22 pairs of chromosomes are referred to as autosomes.‌‌

Members of each autosome carry the same set of genes, resulting in a cell carrying two copies of (almost) every gene, one on the maternally derived chromosome, and one on the paternally derived chromosome, basically the female and male parent respectively. These two copies may be precisely identical, meaning the two genes could have precisely the same sequence of nucleotides, or their sequences may be slightly different. These differences in the sequence may result in no effect at all on the phenotype, or they may lead to different forms of the same trait, such as brown versus blue eye color, and dark skinned vs light skinned offspring. The two different forms of the gene are called alleles , and so we speak of the brown eye color allele or dark skinned allele.

‌‌Dominance Relations‌‌

When both alleles for a particular trait are identical, the organism is said to be homozygous for that trait and when the alleles are different, the organism is heterozygous . The two different alleles present raise the question of whether one or the other, or both, will determine the phenotype of the organism.

‌‌Kinds of Inheritance Relations‌‌

Autosomal Dominance Inheritance‌‌

Autosomal dominance inheritance is due to a dominant allele being carried on one of the autosomes. For the trait to be expressed on the phenotype, alleles need only be inherited from one parent, either the mother or the father. Mathematically speaking, any child has a 50 % of inheriting the allele and expressing the trait if just one parent has it. Many normal human traits are derived from autosomal dominance inheritance pattern. Example of such traits are dark hair, dimples, baldness, etc. Certain disorders also could show autosomal dominance such as Huntington disease, Marfan syndrome, etc. Because inheritance of a harmful dominant allele can be lethal, these alleles tend to be quite rare.

Autosomal Recessive Inheritance‌‌

This form of inheritance is due to recessive alleles carried on autosomes. A person possessing only one recessive allele is known as a carrier. An individual must inherit two recessive alleles, one from each parent, to be able to express the recessive trait. When two carrier parents have offspring, each offspring has a 25 % of inheriting two alleles and expressing the trait. Disorders mostly are obtained from autosomal recessive inheritance than for autosomal dominant conditions. This is because inheritance of one harmful recessive allele does not produce symptoms, and so the individual can reproduce and easily pass the allele unto the children. Thus, most harmful recessive alleles are not removed from a population's genes as rapidly as the dominant ones, using the 50 % and 25 % examples, so the likelihood of inheriting two recessive copies is consequently higher. Most humans harbor a small handful of known harmful alleles; it is only when they mate with another who has the same set that there is a chance of bearing children that express the disorder. Therefore it is highly advised that close relations don't engage in sexual intercourse because of the high probability of obtaining two homozygous recessive conditions.‌‌

Sex linked Inheritance‌‌

Sex linked is a trait in which a gene is located on a sex chromosome. In humans, the term generally refers to traits that are influenced by genes on the X chromosome. This is because the X chromosome is large and contains many more genes than the smaller Y chromosome. Therefore more males are susceptible to inheriting sex linked disorders particularly because they have a single copy of X chromosome that carries the mutation. In females, the effect of the mutation may be masked by the second healthy copy of the X chromosome.‌

The "a" recessive allele will not be expressed in her phenotype
The "a" recessive allele will be expressed in his phenotype


There are other inheritance patterns but these three patterns described above are mainly responsible for the human traits we see around. In some African societies, some traits expressed in some individuals are termed abominations and has led to many unnecessary sacrifices because of lack of understanding of genetic makeup and inheritance patterns. Let's embrace progress through right education. Remember, education is a key to success.