Principles of inheritance

 n The process by which characters are transferred from one generation to the next generation is called inheritance/heredity.

n The differences in traits of individuals of a progeny, from each other and from their parents are called variations.

n The branch of science which deals with inheritance and variation is called genetics.

1. Mendel’s Experiment

n Gregor Johann Mendel (1822–1884) is known as ‘Father of Genetics’.

n Mendel performed his experiments with garden pea plant (Pisum sativum).

n He conducted artificial pollination/cross-pollination experiments using several true-breeding varieties having contrasting traits as shown in Fig. 5.1.

n He observed one trait at a time.

n He hybridised plants with alternate forms of a single trait (Monohybrid cross). The seeds thus produced were grown to develop into plants of first filial generation (F1).

n Mendel then self-pollinated the F1 plants to generate plants of second filial generation (F2).

n Later, Mendel also crossed pea plants that differed in two characters (Dihybrid cross).

2. Mendel’s Experimental Plant

n Mendel selected garden pea as his experimental material because of the following reasons:

(i) It is an annual plant with a short life-cycle. So, several generations can be studied within a short period.

(ii) It has perfect bisexual flowers containing both male and female parts.

(iii) The flowers are predominantly self-pollinating. It is easy to get pure line for several generations.

(iv) It is easy to cross-pollinate them because pollens from one plant can be introduced to the stigma of another plant by removing the anthers.

(v) Pea plant produces a large number of seeds in one generation.

(vi) Pea plants could easily be raised, maintained and handled.

(vii) A number of easily detectable contrasting characters/traits were available.

n Phenotype: Visible expression of genetic constitution e.g., Tall/dwarf.

n Genotype: Genetic constitution of individual e.g., TT, Tt, tt.

3. Mendel’s Observations

n Monohybrid Cross: Cross involving study of inheritance of one character, e.g., height of plant. n Dihybrid Cross: Cross between plants differing in two traits/cross involving study of inheritance of 2 genes or characters. n Homozygous: The individual carrying similar alleles for a trait e.g., TT or tt. n Heterozygous: Individual carrying different alleles for a trait e.g., Tt.

n F1 progenies always resembled one of the parents and trait of other parent was not seen.

n F2 stage expressed both the parental traits in the proportion 3 : 1.

n The contrasting traits did not show any blending at either F1 or F2 stage.

n In dihybrid cross, he got identical results as in monohybrid cross.

n He found that the phenotypes in F2 generation appeared in the ratio 9 : 3 : 3 : 1 in dihybrid cross.

4. Mendel’s Laws of Inheritance

n Based on his hybridisation experiments, Mendel proposed the laws of inheritance.

n His theory was rediscovered by Hugo de Vries of Holland, Carl Correns of Germany and Eric von Tschermak of Austria in 1901.

n Hugo de Vries, Correns & Tschermark are thus referred to as rediscoverers of Mendelism.

(i) Law of dominance

n This law states that when two alternative forms of a trait or character (genes or alleles) are present in an organism, only one factor expresses itself in F1 progeny and is called dominant while the other that remains masked is called recessive.

n Characters are controlled by discrete units called factors. Factors occur in pairs.

(ii) Law of segregation

n This law states that the factors or alleles of a pair segregate from each other during gamete formation, such that a gamete receives only one of the two factors. They do not show any blending.

n Homozygous parent produces all gametes that are similar, heterozygous one produces 2 types of gametes each having one allele in equal proportion.

(iii) Law of independent assortment

n According to this law the two factors of each character assort or separate out independent of the factors of other characters at the time of gamete formation and get randomly rearranged in the offsprings producing both parental and new combinations of characters.

n When 2 pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters.

5. Incomplete Dominance

n It is a phenomenon in which the F1 hybrid exhibits characters intermediate of the parental genes.

n Here, the phenotypic ratio deviates from the Mendel’s monohybrid ratio.

n It is seen in flower colours of Mirabilis jalapa (4 o’ clock plant) and Antirrhinum majus (snapdragon), where red colour is due to genetic constitution RR, white colour is due to genetic constitution rr and pink colour is due to genetic constitution Rr.

6. Co-dominance

n 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.

For example, ABO blood grouping in humans.

l ABO blood groups are controlled by gene I. Gene I has three alleles IA, IB and IO/i.

l IA and IB produce RBC surface antigens which are sugar polymer A and B, respectively, whereas i does not produce any antigen.

l IA and IB are dominant over i hence IA and IB are dominant alleles and i is recessive allele as in IAi and IBi.

l When IA and IB are present together, both express equally and produce both the surface antigens A and B, hence show co-dominance.

l Since humans are diploid, each person possesses any two of the three ‘I’ gene alleles, resulting into six different genotypic combinations and four phenotypic expressions.

Table showing the genetic basis of blood groups in human population

 

Allele fromParent 1	Allele fromParent 2	Genotype of offspring	Blood groups of offspring	Antigen present	Antibody
IA	IA	IAIA	A	A	Anti–B
IA	IB	IAIB	AB	A & B	No antibody
IA	i	IAi	A	A	Anti–B
IB	IB	IBIB	B	B	Anti–A
IB	i	IBi	B	B	Anti–A
i	i	i i	O	No antigen	Both Anti–A Anti–B

 

7. Test Cross

n It is a method devised by Mendel to determine the genotype of an organism.

n In this cross, the organism with dominant phenotype (but unknown genotype) is crossed with the recessive individual.

n In a monohybrid cross between violet colour flower (W) and white colour flower (w), the F1 hybrid was violet colour flower. The test crosses are:

n If all the F1 progeny are violet colour, then the plant is homozygous dominant, i.e., WW and if the progenies are in 1 : 1 ratio, then the plant is heterozygous, i.e., Ww.

8. Pleiotropy

n It is the phenomenon in which a single gene exhibits multiple phenotypic expressions.

n The pleiotropic gene affects the metabolic pathways, resulting in different phenotypes.

n For example, phenylketonuria is caused by mutation in the gene, coding for the enzyme phenylalanine hydroxylase. The affected individuals show mental retardation as well as reduction in hair and skin pigmentation.

n In Drosophila, gene for wing size influences nature of balancers, colour of eye, dorsal bristles, fertility and longivity.

9. Polygenic Inheritance

n It is a type of inheritance, in which a trait is controlled by three or more genes. Such traits are called polygenic traits.

n The phenotype reflects contribution of each allele and is also influenced by the environment.

n For example, human skin colour. Suppose 3 genes A, B and C control skin colour with A, B, C being the dominant alleles and a, b, c being the recessive alleles. Then,

n The F2 generation will have varied skin tones, with each type of allele in the genotype determining the darkness or lightness of the skin.

10. Chromosomal Theory of Inheritance

n The chromosomal theory of inheritance was proposed independently by Walter Sutton and Theodore Boveri in 1902. They stated that behaviour of chromosomes was parallel to behaviour of genes and used chromosome movement to explain Mendel’s laws. According to this theory,

(i) The hereditary factors are carried in the nucleus.

(ii) Like the Mendelian alleles, chromosomes are also found in pairs.

(iii) The sperm and egg having haploid sets of chromosomes fuse to re-establish the diploid state.

(iv) The two alleles of a gene pair are located on homologous sites on homologous chromosomes in a linear order. As there are two chromosomes of each kind in somatic (diploid) cell there must be two genes of each kind, one in each of the two homologous chromosomes.

(v) Homologous chromosomes synapse during meiosis and get separated to pass into different cells. This forms the basis for segregation and independent assortment. A gamete receives only one chromosome of each type and thus has only one gene for a trait. The paired condition is restored by fusion of gametes.

A Comparison between the Behaviour of Genes and Chromosomes

 

S.No.	Genes	Chromosomes
(i)	Occur in pairs	Occur in pairs
(ii)	Segregate at the time of gamete formation such that only one of each pair is transmitted to a gamete	Segregate at gamete formation and only one of each pair is transmitted to a gamete
(iii)	Independent pairs segregate independently of each other	One pair segregates independently of another pair

 

11. Linkage and Recombination

n T. H. Morgan carried out several dihybrid crosses in Drosophila to study the genes that are sex-linked. He observed that when the two genes in a dihybrid cross are located on the same chromosome, the proportion of parental gene combinations in the progeny was much higher than the non-parental or recombination of genes.

n Reason for selecting Drosophila melanogaster (fruit fly):

(i) They could be grown on simple synthetic medium in the laboratory.

(ii) They complete their life cycle in two weeks.

(iii) A single mating could produce a large number of progeny.

(iv) There was clear differentiation of the sexes, i.e., male and female flies are easily distinguishable.

(v) It has many types of hereditary variations that can be seen with low power microscopes.

n Morgan and his group found that when genes are grouped on the same chromosome, some genes are tightly linked or associated and show little recombination.

n F2 generation ratio deviated from 9:3:3:1.

n When the genes are loosely linked they show higher percentage of recombination.

n Morgan hybridised yellow bodied and white eyed females with brown bodied and red eyed males (wild type) (cross-A) and inter-crossed their F1 progeny.

n Alfred Sturtevant determined that genes of Drosophila are arranged in a linear order. He measured the distance between genes and prepared chromosome maps with the position of genes on the chromosomes based on percentage of recombinants. These are also called genetic maps.

Linkage = Physical association of genes on a chromosome. Recombination = Generation of non-parental gene combinations, arising from crossing over between non-homologous chromosomes.

 

12. Sex Determination Mechanism

n Finalisation of sex at the time of zygote formation is called sex determination.

n Two types of chromosomes are present in individuals – sex chromosomes (which determine the sex of individuals) and autosomes.

(i) XX–XY type

n Seen in many insects and mammals including humans, Drosophila melanogaster.

n Males have X and Y chromosomes along with autosomes [A] and females have a pair of X chromosomes.

(ii) XX–XO type

n Seen in grasshopper.

n Males have only one X chromosome besides autosomes and females have a pair of X chromosomes.

(iii) ZZ–ZW type

n Seen in birds, fowl and fishes.

n Females have one Z and one W chromosome whereas males have a pair of Z chromosomes.

(iv) ZO–ZZ type

n Seen in cockroaches.

n Females have only one Z chromosome besides autosomes and males have a pair of Zchromosomes.

13. Sex Determination in Humans

n Humans show XY type of sex determining mechanism.

n Out of 23 pair of chromosomes, 22 are autosomes (same in both males and females).

n Females have a pair of X-chromosomes.

n Males have an X and a Y chromosome.

n During spermatogenesis males produce two types of gametes with equal probability – sperm carrying either X or Y chromosome.

n During oogenesis, females produce only one types of gamete – having X chromosome.

n An ovum fertilised by the sperm carrying X-chromosome develops into a female (XX) and an ovum fertilised by the sperm carrying Y-chromosome develops into a male (XY).

n Hence, it is evident that genetic constitution of sperm determines the sex of the child. In every pregnancy, there is always 50% probability of either male or female child. So it is not correct to blame women for producing female child.

14. Sex Determination in Honeybee

(Haplodiploidy Sex Determination System)

n Honeybee show haplodiploid sex-determination system.

n Offsprings formed from union of a sperm and an egg develops as a female (queen or worker), which are diploid, having 32 chromosomes.

n Unfertilised eggs developed by parthenogenesis form male (drone), which are haploid having 16 chromosomes.

n Males produce sperms by mitosis, so they, neither have fathers nor sons but have grandfathers and grandsons.

15. Pedigree Analysis

n The study of inheritance of genetic traits in several generations of a human family in the form of a family tree diagram is called pedigree analysis.

n Advantages:

(i) It helps in genetic counselling to avoid disorders in future generations.

(ii) It shows the origin of a trait and flow of a trait in a family.

(iii) It is important to know the possibility of expressive recessive allele that can cause genetic disorders like colour blindness, haemophilia, etc.

(iv) Control crosses cannot be made in humans, so pedigree analysis helps us to study inheritance pattern of a trait.

(v) It helps us to understand whether the trait is dominant or recessive autosomal or sex-linked.

(vi) It predicts the harmful effects of marriage between close relatives.

(vii) It is extensively used in medical research.

16. Mendelian Disorders

n Mendelian disorders are caused due to alteration or mutation in single gene.

n These follow Mendel’s principles of inheritance.

(i) Haemophilia

(i) It is a sex-linked recessive disorder.

(ii) Patient continues to bleed even with a minor cut because of a defect in blood coagulation.

(iii) The gene for haemophilia is located on X chromosome.

(iv) More males suffer from haemophilia than females because in males single gene for the defect is able to express as males have only one X chromosome.

(v) The defective alleles produce non-functional proteins which later form a non-functional cascade of proteins involved in blood clotting.

(vi) Females suffer from this disease only in homozygous condition, i.e., XcXc.

(vii) Queen Victoria was a carrier of this disease and produced haemophilic offsprings.

(ii) Sickle-cell anaemia

(i) It is an autosome-linked recessive trait.

(ii) The disease is controlled by a single pair of allele HbA and HbS.

(iii) Only the homozygous individuals for HbS, i.e., HbSHbS show the diseased phenotype.

(iv) The heterozygous individuals are carriers (HbAHbS).

(v) Due to point mutation, glutamic acid (Glu) is replaced by valine (Val) at the sixth position of β-globin chain of haemoglobin molecule.

(vi) It occurs due to single base substitution at 6th codon of b-globin gene from GAG to GUG. Mutated hemoglobin molecule undergoes polymerisation under low oxygen tension causing the change in the shape of RBC from biconcave disc to elongated sickle like structure. As a result, the cells cannot pass through narrow capillaries. Blood capillaries are clogged and thus affect blood supply to different organs.

(iii) Phenylketonuria

(i) It is an inborn error of metabolism and is inherited as autosomal recessive trait.

(ii) The affected individual lacks an enzyme called phenylalanine hydroxylase that converts the amino acid phenylalanine into tyrosine in liver.

(iii) Phenylalanine is accumulated and gets converted into phenylpyruvic acid and other derivatives. This affects the brain, resulting in mental disorder.

(iv) Phenylalanine is also excreted through urine because of its poor absorption by kidney.

(iv) Thalassemia

(i) It is an autosome-linked recessive disease.

(ii) It occurs due to either mutation or deletion resulting in reduced rate of synthesis of one of globin chains of haemoglobin.

(iii) Anaemia is the characteristic of this disease.

(iv) Thalassemia is classified into two types:

l 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 of the four genes.

l β-thalassemia—Production of β-globin chain is affected. It occurs due to mutation of one or both HbB genes on chromosome 11.

(v) Colour blindness

(i) It is a sex-linked recessive disorder.

(ii) It results in defect in either red or/and green cone of eye, resulting in failure to discriminate between red and green colour.

(iii) The gene for colour blindness is present on X chromosome.

(iv) It is observed more in males (XcY) because of presence of only one X chromosome as compared to two chromosomes of females. It occurs in 8% males and 0.4% females.

17. Chromosomal Disorders

n Chromosomal disorders are caused due to excess, absence or abnormal arrangement of one or more chromosomes.

n Aneuploidy: Sometimes the chromatids fail to segregate during cell division, resulting in gain or loss of a chromosome. This is called aneuploidy. It is of two types:

(i) Trisomy: Additional copy of a chromosome in an individual, i.e., (2n+1).

(ii) Monosomy: Lack of copy of a chromosome in an individual, i.e., (2n – 1).

n Polyploidy: Failure of cytokinesis after telophase stage of cell division results in an increase in whole set of chromosomes in an organism. It is called polyploidy. It is often seen in plants.

(i) Down’s syndrome

Cause: Additional copy of chromosome number 21 or trisomy of chromosome 21.

Symptoms:

(i) Short statured with small round head.

(ii) Partially open mouth with protruding furrowed tongue.

(iii) Palm is broad with characteristic palm crease.

(iv) Physical, psychomotor and mental development retarded.

(ii) Klinefelter’s syndrome

Cause: Presence of an additional copy of X chromosome resulting in the karyotype 44+XXY. i.e., 47 chromosomes.

Symptoms:

(i) Sex of the individual is masculine but possess feminine characters.

(ii) Gynaecomastia, i.e., development of breasts.

(iii) Poor beard growth and often sterile.

(iv) Feminine pitched voice.

(v) They are sterile.

(vi) Tall stature.

(iii) Turner’s syndrome

Cause: Absence of one of the X chromosomes, resulting in the karyotype 44+XO i.e., have 45 chromosomes.

Symptoms:

(i) Sterile female with rudimentary ovaries.

(ii) Lack of other secondary sexual characters.

(iii) Underdeveloped feminine characters.

(iv) Poor development of breasts.

(v) Short stature, small uterus, puffy fingers.