Published on Sep 05, 2023
Genetics is the study of genes, heredity, and variation in living organisms. It is generally considered a field of biology, but it intersects frequently with many of the life sciences and is strongly linked with the study of information systems.
The father of genetics is Gregor Mendel, a scientist and Augustinian friar. Mendel studied 'trait inheritance,' patterns in the way traits were handed down from parents to offspring. He observed that organisms (pea plants) inherit traits by way of discrete "units of inheritance". This term, still used today, is a somewhat ambiguous definition of what is referred to as a gene.
Mendelian inheritance is inheritance of biological features that follows the laws proposed by Gregor Johann Mendel in 1865 and 1866 and re-discovered in 1900. It was initially very controversial. When Mendel's theories were integrated with the chromosome theory of inheritance by Thomas Hunt Morgan in 1915, they became the core of classical genetics.
Mendel's law of segregation describes what happens to the alleles that make up a gene during formation of gametes. For example, suppose that a pea plant contains a gene for flower colour in which both alleles code for red. One way to represent that condition is to write RR, which indicates that both alleles (R and R) code for the colour red. Another gene might have a different combination of alleles, as in Rr. In this case, the symbol R stands for red colour and the r for "not red" or, in this case, white. Mendel's law of segregation says that the alleles that make up a gene separate from each other, or segregate, during the formation of gametes. That fact can be represented by simple equations, such as:
RR → R + R or Rr → R + r
Mendel's second law is called the law of independent assortment. That law refers to the fact that any plant contains many different kinds of genes. One gene determines flower colour, a second gene determines length of stem, and a third gene determines shape of pea pods, and so on. Mendel discovered that the way in which alleles from different genes separate and then recombine is unconnected to other genes. That is, suppose that a plant contains genes for colour (RR) and for shape of pod (TT). Then Mendel's second law says that the two genes will segregate independently, as:
RR → R + R and TT → T + T
Mendel's third law deals with the matter of dominance. Suppose that a gene contains an allele for red colour (R) and an allele for white colour (r). What will be the colour of the flowers produced on this plant? Mendel's answer was that in every pair of alleles, one is more likely to be expressed than the other. In other words, one allele is dominant and the other allele is recessive. In the example of an Rr gene, the flowers produced will be red because the allele R is dominant over the allele r.
Mendelian Inheritance Patterns And Genetic Disorders
Within a population, there may be a number of alleles for a given gene. Individuals that have two copies of the same allele are referred to as homozygous for that allele; individuals that have copies of different alleles are known as heterozygous for that allele. The inheritance patterns observed will depend on whether the allele is found on an autosomal chromosome or a sex chromosome, and on whether the allele is dominant or recessive.
If the phenotype associated with a given version of a gene is observed when an individual has only one copy, the allele is said to be autosomal dominant. The phenotype will be observed whether the individual has one copy of the allele (is heterozygous) or has two copies of the allele (is homozygous).
If the phenotype associated with a given version of a gene is observed only when an individual has two copies, the allele is said to be autosomal recessive. The phenotype will be observed only when the individual is homozygous for the allele concerned. An individual with only one copy of the allele will not show the phenotype, but will be able to pass the allele on to subsequent generations. As a result, an individual heterozygous for an autosomal recessive allele is known as a carrier.
Sex-linked or X-linked inheritance
In many organisms, the determination of sex involves a pair of chromosomes that differ in length and genetic content - for example, the XY system used in human beings and other mammals.
The X chromosome carries hundreds of genes, and many of these are not connected with the determination of sex. The smaller Y chromosome contains a number of genes responsible for the initiation and maintenance of maleness, but it lacks copies of most of the genes that are found on the X chromosome. As a result, the genes located on the X chromosome display a characteristic pattern of inheritance referred to as sex-linkage or X-linkage.
Females (XX) have two copies of each gene on the X chromosome, so they can be heterozygous or homozygous for a given allele. However, males (XY) will express all the alleles present on the single X chromosome that they receive from their mother, and concepts such as 'dominant' or 'recessive' are irrelevant.
A number of medical conditions in humans are associated with genes on the X chromosome, including haemophilia, muscular dystrophy and some forms of colour blindness
Pedigree Chart And Its Analysis
A pedigree chart is a diagram that shows the occurrence and appearance or phenotypes of a particular gene or organism and its ancestors from one generation to the next, most commonly humans, show dogs,and race horses.
Pedigree analysis is also useful when studying any population when progeny data from several generations is limited. Pedigree analysis is also useful when studying species with a long generation time.
A series of symbols are used to represent different aspects of a pedigree. To the right are the principal symbols used when drawing a pedigree.
Once phenotypic data is collected from several generations and the pedigree is drawn, careful analysis will allow you to determine whether the trait is dominant or recessive. Here are some rules to follow.
For those traits exhibiting dominant gene action:
• affected individuals have at least one affected parent
• the phenotype generally appears every generation
• two unaffected parents only have unaffected offspring
The following is the pedigree of a trait controlled by dominant gene action.
And for those traits exhibiting recessive gene action:
• unaffected parents can have affected offspring
• affected progeny are both male and female
To the right is the pedigree of a trait controlled by recessive gene action.
Analysis Of My Family Pedigree
CHARACTERISTIC 1: PRESENCE OF ATTACHED EARLOBE
The earlobe character of whether it remains attached to the head or remains free such that its end hangs down from point of attachment is a characteristic inherited from our ancestors (parents, grandparents etc). The presence of an attached ear lobe is due to a recessive autosomal allele pair or gene and the presence of free earlobe is due to a dominant gene.
In the adjoining pedigree (of my family) it is observed that in the first generation person 1(grandpa) and person 2(grandma) have free earlobes therefore genotypically both of them have a dominant allele for this characteristic. In the next generation it is seen that one of their sons (Person5: my 2nd uncle) has attached earlobes making him homozygous recessive (i.e.ee) and indicating that both the persons of first generation (i.e. Grandma and grandpa) were heterozygous (i.e. Ee).
Their other four sons and the daughter remain heterozygous (Ee) or homozygous dominant thus possessing free ear lobes. Person 1(of generation II) gets married to person 2 who phenotypically has free earlobes (thus genotipically Ee or EE).They have a daughter (person 1 of generation III: my cousin) who again has free ear lobes hence genotipically Ee or EE. Person 3 and 7 (of generation II: my father and 3rd uncle) get married to person 4 and 8 of (generation II: my mother and 3rd aunt) respectively who have attached earlobes hence genotipically ee. Persons 3and 4 (of generation II) have two sons (persons 2 and 3 of generation III: me and my brother) out of this person 2 (me) has attached earlobe making him genotipically ee (homozygous recessive), While his brother (person 3 of generation III) has free earlobes thus making him genotipically Ee (homozygous dominant). Similarly persons 5 and 6 (of generation II) have two sons (persons 5 and 6 of generation III: my cousins) .
Out of this person 5 has attached ear lobe hence making him homozygous recessive (i.e. ee) and his brother (person 6, generation III) is heterozygous dominant (Ee).person 5 (generation II: my second uncle) gets married to person 6(generation III: second aunt) who has free earlobes hence making her homozygous or heterozygous dominant. They have a son (person 4 generation III: my second cousin) who has free ear lobes and hence is heterozygous recessive (Ee). Person 9 (generation III: my aunt) gets married to person 10 (generation III) who is homozygous recessive (ee) as he has fused earlobe, they have two children person (7 and 8 of generation III: my 7th and 8th cousins) who happen to have free earlobes hence are homozygous dominant (EE) or heterozygous (Ee).
Characteristic 2: Tongue Roller
This is the particular ability to roll the tongue into ‘u’ shaped tube. This ability arises due to the presence of a dominant gene (may be homozygous or heterozygous). It is an inherited characteristic and follows the Mendelian laws of inheritance. Non rollers are homozygous recessive.
In the adjoining pedigree, we can observe person 1 (of generation I: my grandfather) is unable to roll his tongue hence he is homozygous recessive (rr).Person 2 of generation I (my grandmother) posses the ability to roll her tongue, hence she may be homozygous dominant or heterozygous. In the second generation it is observed that out of the six progenies, person 1 and 11 (my fist and 6th uncle) are unable to roll their tongue and the others posses this ability hence confirming two things:
Person 2 of first generation (grandma) is heterozygous dominant (Rr).
Persons 1 and 2 (of generation II: my uncles) are homozygous recessive (rr). Persons 3, 5, 7 and 9 of generation II are heterozygous (i.e. Rr).
In second generation person 1 gets married to person 2 who is homozygous dominant (RR, able to roll the tongue) and have a daughter person1 (of generation III: my cousin) who naturally is a tongue roller and hence heterozygous dominant (Rr). Person 3 (of generation II: my father) marries person 4 (of generation II: my mother) who is heterozygous dominant (Rr). They have 2 children (person 2 and 3 of generation III: me and my brother).
Out of this person 2 (of generation III: me) is a non roller and hence homozygous recessive (rr), while his brother (person 3, generation III) is a tongue roller and hence heterozygous dominant (Rr). Person 5 (of generation II: my uncle) gets married to person 6 (of generation II: my aunt) who is a non roller and hence homozygous recessive ( rr). They have a son who is also a non - roller and hence his homozygous recessive (rr). Person 7(of generation II) gets married to person 8 who is homozygous dominant (RR, a Tongue –roller).
They have 2 sons (persons 5 & 6 of generation III), both of them being tongue rollers may be homozygous dominant (RR) or heterozygous (Rr). Person 9 (of generation III: my aunt) gets married to person 10(of generation III) who is a non roller and hence homozygous recessive (rr). They have 2 children out of which, progeny one i.e., person 7 of generation III is a tongue roller and hence heterozygous dominant (Rr) while her brother (person 8, generation III) is non roller and hence homozygous recessive (rr).
Characteristic 3 : Interlaced Fingers.
On interlacing the fingers of our hands the way in which our thumbs are crossed is controlled by an inherited gene. It follows the Mendelian Laws of inheritance. If the left thumb covers the right it means that the person has a homozygous dominant or heterozygous allele pair. Whereas if the right thumb covers left it means the person has homozygous recessive gene.
In the adjoining pedigree person 1 of generation I (grandfather), shows homozygous recessive trait (ff) as his right thumb covers over his left. However person 2(of generation 1: my grandmother) shows a dominant gene as her left thumb over laps her right. She must be heterozygous as three of her six children show recessive trait, i.e., persons 1, 5, 7 (of generation II: my uncles) show homozygous recessive trait (ff).
Person 1(of generation II: my uncle) gets married to persons 2(of generation II: my aunt) whose right thumb overlaps the left hence she is homozygous recessive (ff).They have a daughter (person 1 of generation III: my cousin) who is also obviously homozygous recessive (ff). Person 3(of generation II: my father) who is heterozygous (Ff) marries person 4(of generation II: my mother) whose right thumb overlaps the left; hence she is homozygous recessive (ff). They have two children (person 2&3 of generation III: me and my brother), both of them are homozygous recessive (f f).
Person 5 (of generation II: my uncle) who is homozygous recessive marries person 6 (of generation II: my aunt) whose left thumb overlaps her right, making her homozygous dominant (FF).They have a son (person 4 of generation III) whose left thumb overlaps the right making him heterozygous(Ff) .
Person 7 (of generation II) who is homozygous recessive marries person 8(of generation III: my aunt) whose left thumb overlaps her right hence making her homozygous dominant (FF).This is evident from the fact that both of her children (person 5&6 of generation III: my cousins) appear to be heterozygous (Ff) as for both of them their left thumb overlaps their right.
Persons 9 (of generation II: my aunt) who appears to be homozygous dominant (FF) gets married to person 10 (of generation II: my uncle) who also appears to be homozygous dominant (FF).They have two children (persons 7&8: my cousins) who are also naturally homozygous dominant (FF).
Characteristic 4: Hitchhikers Thumb
Hitchhikers thumb is a kind of bent thumb that a person possesses. It is seen when a person gives a thumbs up! to some one. If the thumb is straight a dominant gene either homozygous or heterozygous (i.e. SS or Ss) is indicated. The bent thumb is the hitchers thumb and people with such thumb have homozygous recessive gene (ss).
In the adjoining pedigree we see that person 1 of generation I (my grandfather) possesses the hitchhikers thumb as he is homozygous recessive (ss). She marries person 2 (my grandmother) who as a straight thumb. She may hence be heterozygous/ homozygous dominant (Ss/ss). However it is evident that she is heterozygous as out of their 6 children persons 3, 5, 9,) possess the hitchhikers thumb and hence are homozygous recessive (ss)
Out of this person 9(of generation II: my aunt) marries person 10(of generation II: my uncle) who also possess hitchhikers making him homozygous recessive (ss). They have 2 children (person 7&8 of generation: III) who are also evidently homozygous recessive (ss) and hence possess hitchhikers thumb. Person 7 (of generation II) who is heterozygous (Ss) marries person 8 who is homozygous recessive (ss) possessing hitchhikers thumb. They have 2 children out of which their first child (person 5 of generation III: my cousin) is homozygous recessive (ss) as he has the hitchhikers thumb while his brother is heterozygous (Ss) as he has a straight thumb.
Person 1(of generation II: my uncle) who is heterozygous (Ss) gets married to person 2(of generation III) who also possesses a straight thumb and hence is heterozygous (Ss). They have a daughter (person 1 of generation III) who has a hitchhikers thumb and hence as a homozygous recessive genotype (ss). Person 3(of generation II: my father) possesses the hitchhikers thumb and hence is homozygous recessive (ss).He marries person 4 (of generation II: my mother) who has a straight thumb. It is clearly evident that person 4 of generation II is heterozygous as one her children (person 2 of generation III: me) is homozygous recessive (ss) while his brother (person 3, generation III) is heterozygous dominant and hence shows straight thumb.
Person 5(of generation II: my uncle) being homozygous recessive (ss) gets married to person 6(of generation II: my aunt) who is also homozygous recessive (ss), i.e. both of them possess the hitchhiker’s thumb. They have a son (person 4 of generation III: my cousin) who naturally possesses the hitchhiker’s thumb, so he has a homozygous recessive gene (ss) straight thumb and he is heterozygous dominant
Gregor Mendel the father of genetics postulated his three Laws of Inheritance, which eventually became the basis of modern day genetics.
These laws namely “Law of Dominance”, “Law of Segregation” & “Law of Independent Assortment” can be used to analyse types of genetic disorders and inheritance patterns.
Pedigree charts, made based on Mendel’s Laws can be used to analyze and study Inheritance patterns of different genes in a family hence helping in diagnosing and curing several genetic disorders.
On the whole genetics, an endless study has helped us, human beings to understand ourselves in way that is unimaginable. The vastness of this study is far greater than anything known to man and now as this study deepens into the vastness perhaps there is hope in the future for a society free from any kind of disease and sufferings.
Who knows, one day it may even provide us with answers to questions about our origin
• Class XI practical textbook .
• http://www.texascollaborative.org/spencer_Pedigree Analysis/ds_sub2.htm