1. Glossary of Terms used in Genetics
1.1 What is the Locus?
The position or location of a gene on a chromosome. For example, the locus of the gene for Huntington’s disease is chromosome 4.
1.2 What is a Gene?
It is a fundamental physical and functional unit of heredity located in a particular locus on the chromosome. It is a specific length or segment of DNA which contains ordered nucleotide sequence that codes for the linear sequence of amino acids of one polypeptide. It determines a characteristic or trait of an individual.
1.3 What is an allele?
It determines contrasting characteristics of the same trait. It is also known as an alternative form of a gene. Alleles of a particular gene occupy the same position on homologous chromosomes. A diploid (2n) individual possesses only 2 alleles at each gene locus at any one time. These 2 alleles may be the same or different. The expression of alleles depends on the interaction between alleles. They can be completely dominant, partially dominant or co-dominant.
1.4 What is a Genotype?
Genotype refers to the genetic makeup or the composition of alleles of an organism. In diploid organisms, almost all animals and many plants, there are 2 sets of chromosomes in each cell. Each parent contributes one pair of homologous chromosomes.
Homologous chromosomes carry the same genes in the same position / locus on the chromosome. Each cell of an organism therefore contains 2 copies of each gene. If that gene has 2 alleles, then there are 3 possible genotypes in the population.
Organisms in which the 2 alleles of a gene are the same are said to be homozygous. If the 2 alleles are different, then the organism is heterozygous.
1.5 What is a Phenotype?
It is the physical appearance of an individual which could be any measurable / observable characteristic or distinctive trait of an organism. It is the result of gene products i.e. the polypeptide or eventually the protein formed. Besides the gene, environmental factors may also modify the expression of the phenotype of an individual but they cannot be inherited.
1.6 What is Dominance?
Alleles interact with one another and may affect or inhibit the expression of another alleles to different degrees when present within the same individual. Dominance of alleles is best seen in heterozygotes when 2 different alleles are present.
Complete dominance is when one allele produces its effect in heterozygotes and the other is completely masked.
Partial or incomplete dominance is when both alleles in he heterozygotes are expressed and the combined effects produce a phenotype intermediate to the 2 homozygotes.
Codominance is when both alleles in the heterozygotes exert their effects distinctly also to produce an intermediate to the 2 homozygotes.
In genetics, the word dominant does not imply that a phenotype is either normal or more common than a recessive phenotype. Dominance means that an allele is expressed in a heterozygote as well as a homozygote for that allele. By contrast, he corresponding recessive allele is expressed only in a homozygote.
2. Mendel’s 1st Law: Principle of Segregation
Alternative versions of genes account for variations in inherited characters. In a nutshell, this is the concept of alleles. Alleles are different versions of genes that impart the same characteristic. Each human has a gene that controls height, but there are variations among these genes in accordance with the specific height the gene codes for.
For each character (each gene), an organism inherits 2 alleles, one form each parent. This alludes to the fact that when somatic cells are produced from 2 gametes, one allele comes from the mother, one comes from the father. These alleles may be the same of different.
The 3rd law, in relation to the 2nd, declares that if 2 alleles differ, then one, the dominant allele, is fully expressed in the organism’s appearance; the other, the recessive allele, has no noticeable effect on the organism’s appearance.
The 2 alleles for each character segregate during gamete production. This refers to meiosis when the chromosome count is changed form diploid to haploid number. The alleles are sorted into separate gametes, ensuring variation.
Mendel’s principle of segregation explains the inheritance of single characteristic.
3. Lethal Genes
Modification of the3:1 ration occurs when the bearer of certain genotypes are killed by drastic phenotypic effects. Most lethal genes are recessive but dominant lethal genes are also known. Most lethal genes prevent the embryo of the bearer to develop or permit it to develop but eventually aborts it.
4. Inheritance of hypercholestrolaemia
A recessive allele is responsible for hypercolestrolaemia, dangerously high levels of cholesterol in the blood. Normal individuals are HH
Heterozygotes Hh have blood cholesterol levels about twice normal. They are usually prone to atherosclerosis, the blockage of arteries by a build up of cholesterol in the arterial walls. They may have heart attacks from blocked heart arteries.
Hypercolestrolaemia is even more serious in homozygous recessive individuals hh. They have 5 times the normal amount of blood cholesterol and may have heart attacks as early as age 2.
The dominant allele, which normal individuals carry in duplicate HH specifies a cell protein called LDL receptor. LDLs are cholesterol-containing particles in the blood. The LDL receptor picks up the LDL particles from the blood and promotes their uptake by cells that break them down. This process helps prevent the accumulation of cholesterol in arteries. Without the receptors, lethal LDL levels build up in the blood. Heterozygotes only have half the normal number of LDL receptors and homozygotes have none.
5. Inheritance of Human Blood Group
Blood group is controlled by an autosomal gene. Gene locus is represented by the symbol I. There are 3 alleles represented by IA, IB and IO. Alleles A ad B are codominant. Allele O is recessive. The letters refer to 2 glycoproteins designated A and B, that function as antigens on the plasma membrane of red blood cells. A person’s red blood cells may be coated with either antigen A or B, with both or with neither. Antigen A and B differ in the type of terminal sugar on the protein. When allele O is present, the antigen lacks the terminal sugar.
The presence of a single dominant allele results in the blood producing a protein, agglutinin, which acts as an antibody for the other antigen. Antibodies are substances that can be produced by the immune system in large quantities as a response to specific external antigens. An individual with IA will have antigen A and produces anti-B antibody against antigen B.
6. Mendel’s 2nd Law: Principle of Independent Assortment
The most important of this principle is that the emergence of one trait will not affect the emergence of another. While his experiments mixing one trait always results in a 3:1 ratio between the dominant and recessive phenotypes, his experiments with 2 traits showed a 9:3:3:1 ratio.
During gamete formation, the distribution of each allele from a pair of homologous chromosomes is entirely independent of the distribution of alleles of other pairs. It is the random alignment or assortment of homologous chromosomes on the equatorial spindle during metaphase I of meiosis and their subsequent separation during metaphase I and anaphase I that leads to the variety of allele recombination in the gamete cells.
It is possible to predict the number of allele combinations in either the male or female using the general formula 2n where n=haploid number of chromosomes. In the case of humans, where n=23, the possible number of different combinations is 8 388 608.
7. Test cross
A testcross is a mating between an individual of unknown genotype and a homozygous recessive individual.
8. Sex Determination
There are a variety of systems that determine sex of an organism. The XY system, XO system, ZW system and the chromosome number system.
9. Sex linked Genes
Besides bearing genes that specify sex, the so-called sex chromosomes also contain genes for other characteristics. Genes carried on the sex chromosomes are said to be sex-linked. Most known sex-linked genes reside on the X chromosome. Therefore the term “Sex-linked” is used with “X-linked”. The inheritance of sex linked genes is different from that for autosome-linked genes. In the heterogametic sex, there is a portion of the X chromosome for which there is n homologous region of the Y chromosome.
Characteristics determined by genes carried on the non-homologous portion of the X chromosome therefore appear in males even if they are recessive. This special form of linkage explains the inheritance of sex-linked traits.
10. Pedigree Analysis
Mendelian principles apply to traits in humans, but because human families are relatively small, determining modes of inheritance can be difficult. A pedigree chart is a diagram of a family tree over as many generations as possible showing the descendants from particular ancestors, their relationships, and the presence or absence of the trait in all members.
Males are represented as squares and females as circles. Shaded is the indication of the incidence of the particular phenotype under investigation. Each generation is set out along one horizontal level of the page. Succeeding generations are shown on following levels. Horizontal lines running directly between a circle and a square link 2 parents. Vertical lines run down from them to their children. The children of one family are linked by a horizontal line which runs above them. The birth sequence in the family running from left to right.
A dominant trait tends to occur in members of every generation. A recessive trait is seen infrequently often skip one or more generations.
Both sons and daughters of affected parents are equally likely to inherit the disease of the trait is autosomal. Sex linked traits tend to affect the sons since they are heterogametic. Sons tend to inherit the disease from affected mother while daughters from affected fathers and carrier mothers. An affected father can never pass sex-linked disease to their sons.
