Definitions of Genetic Terms

The study of genetics is involved with a lot of terminology. The discipline has its own language or vocabulary. Hence, before we sink deep into the study of genetics in subsequent parts of our lesson, we must get to know basic definitions in order to facilitate the understanding process. It should be equally noted that not only genetics is this way. Every discipline comes with its own vocabulary as far as science is concerned. The words commonly used in genetics by geneticists include but may not be limited to

1. Genetics: This is the study of heredity and variation.
2. Heredity: This is the concept of the passing on of characteristics via genes from parent to offspring.
3. Variation: These are the changes observed among organisms of a given population.
4. Genotype: This is the genetic make-up of an organism. Of course the genotype cannot be seen been can be studied and identified.
5. Phenotype: This is the physical expression of the genes or genotype of an organism. The way you look is in actuality, the ways your genes commanded it. The number of hair strands on your head, your posture, the way you look, your scent and everything about you has been coded in your genes. Genes are observed as triplet codes of base pairs.
6. Gene: This is the basic unit of inheritance of a given organism. A gene is a triplet code comprising three base pairs in a DNA strand. A gene is found at a specific point on the chromosome of an organism, called the locus. They occur in pairs.
7. Genome: This is the total genetic make-up of an organism. That is, a combination of all the genes in an organism.
8. Allele: An allele is the different forms of the same gene occupying the same locus on a chromosome. One of a group of genes that occur alternatively at a given locus. In principle, genes are in pairs, an allele is one of the pair. If a gene is represented as ‘Aa’, then the alleles for this gene are ‘A’ and ‘a’
9. Homozygous: This is a condition whereby both alleles that make up a gene are similar or identical e.g. “AA” or “aa”.
10. Heterozygous: This is a condition whereby both alleles that make up a gene are not identical e.g. Aa (in a case of dominant recessive alleles) or AB (in a case of codominant alleles).
11. Dominant allele: These are alleles whose presence masks the presence of another allele for the same character. They are always represented by a capital letter. If we assume that long pods in a pea plant are dominant over short pods, the allele for long pods could be represented by ‘A’ while the allele for short pods could be represented by ‘a’. If a pea plant then possesses a gene of ‘Aa’, it will have long pods because long pods is dominant.
12. Recessive allele: These are alleles that expresses themselves only in the homozygous state. Remember homozygous means both alleles are the same, if they express themselves only in this state, then, base on the example just above in “Dominant Alleles”, the pea plant can only have short pods when it has a genotype of ‘aa’.
13. Sex-linked: This is a situation where the locus for which a gene is found is located on the sex chromosomes. That is, either the X or Y chromosome. Nonetheless, it has been noticed that the X chromosomes carry a lot more of such genes than the Y chromosome. This is due to the fact that the X chromosome is much bigger than the Y chromosome and hence has a lot more space. More details will be said under sex-linkage in part four (non-mendelian genetics)
14. Lethal genes: These are genes that cause death when found in an individual. Generally, this happens when the alleles are in a homozygous condition. It is important to note that lethal genes must not kill the individual immediately the mutation occurs (e.g achondroplasia, Epiloia), it can equally kill the individual a while after birth (e.g sickle cell anaemia). It is important to note that it must not be recessive but could also be as a result of dominant alleles.
15. Epistasis:  The gene interaction in which, the presence of a dominant allele in one locus masks the other gene in another locus.
16. Epistatic genes: These are genes that carryout apistasis. With more detail, we could say epistatic genes are genes that block the expression of other genes in their dominant state.
17. Test cross: This is a cross between an individual with an unknown genotype with a homozygous recessive individual. This will permit the scientist to identify the genetic situation of the individual with unknown genotype.
18. Hybrid: An organism originating from breeding 2 organisms with contrasting phenotype.
19. Monohybrid cross: This is a cross involving only one characteristic e.g fur length, fur colour, eye colour, etc.
20. Dihybrid cross: This is a single cross involving two characteristics, e.g a cross involving both fur length and fur colour.
21. Mutation: This is a change in the genetic make-up of an organism. It could be natural or artificial. (More details in part six)
22. Chromosome: This is a threadlike feature found in the nucleus of an organism, containing a part of the overall DNA of the organism.
23. Chromatid: This is one half of a chromosome. Chromosomes appear like two threads tied to each other at a particular point. Each thread in this case is a chromatid.
24. Chromatin: This is a tread-like feature made up of DNA or RNA and protein, which condenses to for chromosomes in the nucleus of an organism.
25. Centromere: This is the site where two sister chromatids attach themselves to form a chromosome. It is only when they attach that they are referred to as sister chromatids. (Non-sister chromatids are simply chromatids of different chromosomes, not like they are offspring of a particular parent)
26. Chiasmata: This is the point where 2 non-sister chromatids attach and crossing over occurs.
27. Dominance: This is a situation where a gene is expressed in both the homozygous and heterozygous state. For instance, if long pod pea plants are crossed with short pod pea plants and all the offspring have long pods, then, long pods is dominant over short pods. The phenomenon is therefore referred to as dominance.
28. Codominance: This is a situation where there are two or more observations for a particular trait on a single species. This is the reason for which there exist organisms like cats with more than one fur colour.
29. Absence of Dominance: This is a situation whereby an organism expresses neither of two characteristics in it heterozygous state. A good example here is the snap dragon flower. under such a situation, a completely new observation is gotten. for the snap dragon flower as example, the black and white colours exhibit absence of dominance. hence, in the heterozygous state, a pink colour is observed.

It is important to note that these terms are in no way the limit to terms in genetics but are especially useful for students of this level of education. That is according to analysis from the Cameroon GCE syllabus and past questions. Nonetheless, the definitiona will be regularly updated.

Thanks.

Notes Written by Buo Genesis Kelly

Last edited on the 26/07/2023

Link to Previous parts:

A level Genetics Part 1 of 7

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