PRINCIPLES OF INHERITANCE
Genetics: Is a branch of biology concerned with the study of inheritance, as well as the variation of characters from parents to offspring.
Heredity: The process of genetic inheritance of characteristics from one generation to another;
Inheritance: The process by which characters are passed on from parent to progeny; it is the basis of heredity.
Variation: Is the degree by which progeny differ from their parents.
Gregor Johann Mendel : Father of Genetics
Hermann Joseph Muller : Father of Cytogenetics
Thomas Hunt Morgan : Father of experimental Genetics
Archibald Garrod : Father of Human genetics [Discovered Alkaptonuria]
Wilhelm Johannsen : Coined the term ‘GENE’
MENDEL’S LAWS OF INHERITANCE
Gregor Johann Mendel is considered as ‘father of genetics’
Gregor Mendel conducted hybridization experiments on garden peas for seven years (1856-1863) and proposed the laws of inheritance in living organisms.
Mendel conducted artificial pollination/cross pollination experiments using several true-breeding pea lines. A true-breeding line is one that, having undergone continuous self-pollination, shows the stable trait inheritance and expression for several generations.
Why Mendel selected garden pea (Pisum stivum) plant for his hybridization experiments?
• They can be grown easily in open ground or even in pots.
• They have a short period of growth cycle.
• They produce self-pollinating flowers, large number of seeds and fertile hybrids on cross-pollination.
• They show contrasting heritable characters.
The crossing between two plants, with respect to a single contrasting character is called monohybrid cross.
Mendel conducted hybridization experiment by crossing true-breed tall pea plant (TT) with true-breed dwarf pea plant (tt).
In F1 (First filial) generation only one of the parental traits (tall) was seen and trait of other plant (dwarf) was not seen.
Mendel self-pollinated the tall F1 plants and he found that in the F2 generation some of the offspring were ‘dwarf’; the character that was not seen in the F1 generation was now expressed.
The proportion of plants that were dwarf were 1/4th of the F2 plants while 3/4th of the F2 plants were tall.
F2 plants were identical to their parental type and did not show any blending.
Alleles: Gene which code for a pair of contrasting traits are known as alleles.
Homozygous: Similar pair of alleles present for a character (TT, RR).
Heterozygous: Dissimilar pair of alleles present for pair of character (Tt, Rr).
Phenotype: The observable or external morphological characteristics of an organism.
Genotype: Genetic constitution of an organism.
Dominant factor: The factor or allele of a character which express itself in the presence of alternative allele.
Recessive factor: The factor or allele which fails express in the presence of its alternative allele.
The cross made between unknown plant (or F1 hybrids) with the recessive parent is called test cross. It is useful to find the genotype an unknown plant.
In a monohybrid cross between violet colour flower (W) and white colour flower (w), the F1 hybrid was violet colour flower.
If all the F1 progeny are violet colour, then the dominant flower is homozygous (WW) and if the progenies are in 1 : 1 ratio, then the dominant flower is heterozygous.
Based on his observations on monohybrid crosses Mendel proposed two general rules to consolidate his understanding of inheritance in monohybrid crosses.
Today these rules are called the Principles or Laws of Inheritance:
The First Law or Law of Dominance and the Second Law or Law of Segregation.
Law of dominance: It states that “In a dissimilar pair of factors one member of the pair dominates (dominant) the other one (recessive).
Law of Segregation: This law states “A pair of alleles segregate from each other during gamete formation, such that a gamete receives only one of the two factors”.
Law of Independent assortment: The law states that “When two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters”.
It’s a phenomenon in which the F1 hybrid exhibits intermediate characters of the parental gene.
It is seen in flower colours of Mirabilis jalapa (4 O'clock plant and Antirrhinus majus (Snapdragon).
In a cross between true breeding red flowered (RR) and true breeding white flowered plants (rr), the F1 (Rr) was pink.
When the F1 was self-pollinated the F2 resulted in the following ratio 1 Red (RR) : 2 Pink (Rr) : 1 White (rr).
Here the phenotypic ratio deviates from the Mendel's monohybrid cross.
Both phenotypic and genotypic ratio will be the same 1:2:1.
The Alleles which are able to express themselves independently, even when present together are called co-dominant alleles and this biological phenomenon is called co-dominance.
In monohybrid and dihybrid cross, we observed the F1 progeny resembled either of the two parents (dominance) are was intermediate (incomplete dominance). But in the case of codominance the F1 generation resembles both parents.
A good example is different types of red blood cells that determine ABO blood grouping in human beings.
ABO blood groups are controlled by the gene I. The plasma membrane of the red blood cells has sugar polymers that protrude from its surface and the kind of sugar is controlled by the gene.
The gene (I) has three alleles IA, IB and i. The alleles IA and IB produce a slightly different form of the sugar while allele i does not produce any sugar.
Because humans are diploid organisms, each person possesses any two of the three I gene alleles. IA and IB are completely dominant over i.
In other words when IA and i are present only IA expresses (because i does not produce any sugar), and when IB and i are present IB expresses. But when IA and IB are present together they both express their own types of sugars: this is because of co-dominance. Hence red blood cells have both A and B types of sugars.
DIHYBRID CROSS (INHERITANCE OF TWO GENES)
The cross between two parents differed in two pairs of contrasting traits; is called dihybrid cross.
Mendel also worked with and crossed pea plants that differed in two characters, as is seen in the cross between a pea plant that has seeds with yellow colour and round shape and one that had seeds of green colour and wrinkled shape. The genotype of the parents can then be written as RRYY and rryy.
The gametes RY and ry unite on fertilisation to produce the F1 hybrid RrYy.
Mendel found that the seeds resulting from the crossing of the parents, had yellow coloured and round shaped seeds.
When Mendel self hybridised the F1 plants he found that 3/4th of F2 plants had yellow seeds and 1/4th had green.
The yellow and green colour segregated in a 3:1 ratio. Round and wrinkled seed shape also segregated in a 3:1 ratio; just like in a monohybrid cross.
The phenotypes round, yellow; wrinkled, yellow; round, green and wrinkled, green appeared in the ratio 9:3:3:1
This derivation can be written as follows: (3 Round : 9 Round, Yellow : 3 Wrinkled, Yellow: 3 Round, Green : 1 Wrinkled, Green.
CHROMOSOMAL THEORY OF INHERITANCE
Mendel published his work on inheritance of characters in 1865 but for several reasons, it remained unrecognised till 1900.
Firstly, communication was not easy (as it is now) in those days and his work could not be widely publicised.
Secondly, his concept of genes (or factors, in Mendel’s words) as stable and discrete units that controlled the expression of traits and, of the pair of alleles which did not ‘blend’ with each other, was not accepted by his contemporaries as an explanation for the apparently continuous variation seen in nature.
Thirdly, Mendel’s approach of using mathematics to explain biological phenomena was totally new and unacceptable to many of the biologists of his time.
Finally, though Mendel’s work suggested that factors (genes) were discrete units, he could not provide any physical proof for the existence of factors or say what they were made of.
In 1900, three Scientists (de Vries, Correns and von Tschermak) independently rediscovered Mendel’s results on the inheritance of characters.
Walter Sutton and Theodore Boveri noted that the behaviour of chromosomes was parallel to the behaviour of genes and used chromosome movement and proposed chromosomal theory of inheritance (in 1902).
According to Chromosomal Theory of inheritance;
All hereditary characters must be carried through sperm and egg cells.
The hereditary factors are carried in the nucleus in the form of chromosomes and genes.
Like the mendelian alleles chromosomes are also found in pairs.
The pairing and separation of a pair of chromosomes would lead to the segregation of a pair of factors they carried.
The sperm and egg having haploid set of chromosomes fuse to re-establish the diploid state.
LINKAGE AND RECOMBINATION
Later Thomas Hunt Morgan, experimentally verify the chromosomal theory of inheritance. Morgan worked with tiny fruit flies, Drosophila melanogaster.
Morgan carried out several dihybrid crosses in Drosophila to study genes that were sex linked. The crosses were similar to the dihybrid crosses carried out by Mendel in peas.
Morgan hybridized yellow-bodied, white-eyed females to brown-bodied, red-eyed males and intercrossed their F1 progeny.
He observed that the two genes did not segregate independently of each other and the F2 ratio deviated very significantly from the 9:3:3:1 ratio.
Linkage: The physical Association of two genes on a chromosome is termed as linkage.
Recombination: The generation of non-parental gene combinations is termed as recombination.
The mechanism of sex determination has always been a puzzle before the geneticists. The initial clue about the genetic/chromosomal mechanism of sex determination can be traced back to some of the experiments carried out in insects.
In fact, the cytological observations made in a number of insects led to the development of the concept of genetic/chromosomal basis of sex-determination. Henking (1891) could trace a specific nuclear structure all through spermatogenesis in a few insects, and it was also observed by him that 50 percent of the sperm received this structure after spermatogenesis, whereas the other 50 percent sperm did not receive it. Henking gave a name to this structure as the ‘X body’ but he could not explain its significance.
Finalization of sex at the time of zygote formation is called sex determination.
Two types of chromosomes are present in individuals – sex chromosomes (which determine the sex of individual) and autosomes.
I. XX-XY type
This type of sex determination seen in many insects and mammals including humans.
Males have X and Y chromosomes along with autosomes and females have a pair of X chromosomes.
Male heterogamety: In this case males produce two different types of gametes in term of the sex chromosomes.
II. XX-XO type
This type of sex determination seen in grasshopper.
Males have only one X chromosomes along with autosomes and females have a pair of X chromosomes.
III. ZZ-ZW type
This type of sex determination seen in birds, fowl and fishes.
Females have one Z and one W chromosome whereas males have a pair of Z chromosomes.
Female heterogamety: In this case females produce two different types of gametes in term of the sex chromosomes.
IV. ZO-ZZ type
This type of sex determination seen in cockroaches.
Females have one Z chromosome besides autosomes and males have a pair of Z chromosomes.
SEX DETERMINATION IN HUMANS
Humans show XY type of sex determination mechanism.
Out of 23 pair of chromosomes, 22 are autosomes (same in both males and females).
Females have a pair of X chromosomes.
Males have an X and a Y chromosome.
During spermatogenesis Males produce two types of gametes with equal probability - sperm carrying either X or Y chromosome.
During oogenesis females produce only one type of gamete having X chromosome.
An ovum fertilized by the sperm carrying X-chromosome develops into a female (XX) and an ovum fertilized by the sperm carrying Y-chromosome develops into a male(XY).
SEX DETERMINATION IN HONEYBEE
Honeybee show haploid sex-determination system.
Offsprings formed from union of a sperm and an egg develops a female (queen or workers), which are diploid, having 32 chromosomes.
Unfertilized eggs developed by parthenogenesis form male (drone), which are haploid having 16 chromosomes.
Male produce sperms by Mitosis, so they, neither have fathers nor sons but have grandfathers and grandsons.
Mutation is a phenomenon which results in alteration of DNA sequences and consequently results in changes in the genotype and the phenotype of an organism.
The sudden, stable, inheritable change in genetic material of an organism is termed as mutation.
The organism which undergoes mutation is called mutant.
The agent of mutation is called mutagen. E.g., UV radiations and some chemicals.
When mutation takes place due to change in a single base pair of DNA is called point mutation. E.g., Sickle-cell anaemia.
When mutation takes place due to deletion or insertion of a segment of DNA, alteration of chromosomes is called chromosomal aberration. Its common in cancer cells.
The study of Inheritance of genetic traits in several generations of human family in the form of a family tree diagram is called pedigree analysis.
It helps in genetic counselling to avoid disorders.
It shows the origin of trait and flow of trait in a family.
It is possible to know the expressive recessive allele that can cause genetic disorders.
It predicts the harmful effects of marriage between close relatives.
Genetic disorders may be grouped into two categories Mendelian disorders and chromosomal disorders.
Mendelian disorders are mainly caused due to the alteration or mutation in the single gene.
These disorders follow Mendel’s principle of inheritance.
E.g., Haemophila, Sickle-cell anaemia, colour blindness, phenyleketonurea, thalassemia, cystic fibrosis.
It is a sex-linked recessive disorder. Which shows its transmission from unaffected carrier female to some of the male progeny.
Patient continues to bleed even with a minor cut because of a defect in blood coagulation
The gene for haemophilia is located on X-chromosome.
More males suffer from haemophilia than females because in males single gene for the defect is able to express as male have only one X-chromosome.
The defective alleles produce non-functional proteins which later form a non-functional cascade of proteins involved in blood clotting.
Females suffer from this disease only in homozygous condition i.e., XhXh when father is haemophilic and mother is a carrier.
Queen Victoria was a career of this disease and produced hemophilic offsprings.
b. Sickle-cell anaemia
This is an autosome linked recessive trait.
The disease is controlled by a single pair of allele, HbA and HbS.
That can be transmitted from parents to the offspring when both the partners are carrier for the gene (or heterozygous).
Only the homozygous individuals for HbS (HbSHbS) show the diseased phenotype.
Heterozygous (HbAHbS) individuals appear apparently unaffected but they are carrier of the disease.
The defect is caused by the substitution of Glutamic acid (Glu) by Valine (Val) at the sixth position of the beta globin chain of the haemoglobin molecule.
The substitution of amino acid in the globin protein results due to the single base substitution at the sixth codon of the beta globin gene from GAG to GUG.
This leads into change in the shape of the RBC from biconcave disc to elongated sickle like structure.
It is an in born error of metabolism and is inherited as the autosomal recessive trait.
The affected individual lacks an enzyme called phenylalanine hydroxylase that converts the amino acid phenylalanine into tyrosine in liver.
Phenylalanine is accumulated and converted into phenylpyruvic acid and other derivatives. This affects the brain results in mental retardation. These are also excreted through urine because of its poor absorption by kidney.
It is an autosome-linked recessive disease.
It occurs due to either mutation or deletion resulting in reduced rate of synthesis of one of the globin chains of haemoglobin.
Anaemia is the characteristic of this disease.
Thalassemia is classified into two types:
a. α-thalassemia- Production of α-globin chain is affected. It is controlled by the closely linked genes HbA1 and HbA2 on chromosome 16. It occurs due to mutation or deletion of one or more gene of the four genes.
b. β-thalassemia- Production of β-globin chain is affected. It occurs due to mutation of one or both HbB gene on chromosome 11.
e. Colour blindness
It is a sex-linked recessive disorder.
It results in defect in either red or/and green cone of eye, resulting in failure to determinate between red and green colour.
The gene for colour blindness is present on X-chromosome.
It is observed more in males (XcY) because of presence of only one X-chromosome as compared to two chromosomes of female.
Chromosomal disorders are caused due to absence or excess or abnormal arrangement of one or more chromosomes.
Failure of segregation of chromatids during cell division cycle results in the gain or loss of a chromosome(s), called aneuploidy.
E.g., Down’s syndrome, Turner’s syndrome, Klinefelter’s syndrome.
a. Down’s syndrome
Down’s syndrome is caused by an extra copy of chromosome number 21 (trisomy of 21).
This disorder was first discovered by Langdon Down (1866).
Short statured with small round head.
Partially open mouth with protruding furrowed tongue.
Palm is broad with characteristic palm crease.
Physical, psychomotor and mental development is retarded.
b. Klinefelter’s syndrome
Klinefelter’s syndrome is caused by the presence of an additional copy of X-chromosomes, resulting in the karyotype of 44+XXY.
Sex of the individual is masculine but possess feminine characters.
Gynaecomastia i.e., development of breasts.
Poor beard growth and often sterile.
Feminine pictched voice
c. Turner’s syndrome
Turner’s syndrome is caused by the absence of one of the X-chromosomes. i.e., 44+XO
Sterile female with rudimentary ovaries.
Shield shaped thorax.
Poor development of breasts.
Short stature, small uterus, puffy fingers.