Home / 9.2: Mendelian Genetics & Related Concepts

9.2: Mendelian Genetics & Related Concepts

Heredity

Heredity is the passing the character (traits) from the parents to the offspring. Both the father & mother contribute to the traits in the offspring.

Few Functional Terms

  • Fertilization – The uniting of male(sperm) & female(egg) gametes (Sex cells).
  • Gene – Located on the chromosome, they control how an organism develops,  A particular segment of DNA which determines the heredity of a particular trait
  • Trait – The characters that are passed on from parents to the offspring
  • Locus – Is the spot which each gene has on the chromosome
  • Alleles – Different forms of same gene – each organism has two alleles each trait (exceptions)
  • Homozygous – If both the alleles for a trait are similar (TT)
  • Heterozygous – If both the alleles for a trait are different (Tt)
  • Phenotype – The way an organism looks  – Tall or short
  • Genotype – The genetic make up or genetic combination or constitution of an individual – TT, Rr, Aa etc.

Meiosis & Inheritance – Meiosis is the basis for heredity.

  • Chromosomes carry genes & each locus on a chromosome has alternative versions of the gene – allele
  • We inherit one allele from each parent
  • An allele can be dominant or recessive.

Gregor Mendel explained how heredity take place ⇔ Genetics is the study of heredity.

Mendelian Genetics & Population Genetics 

Mendelian Genetics– the Branch of genetics which deals with the study of pattern of transmission of genes from parents to progeny in a single family cross pedigree.

  • Here the unit of study is family / individual.
  • The main aim is to determine the mode/pattern of inheritance of phenotypes in humans.
  • Methods used – Family study, pedigree analysis, Twin/co-twin method etc

Population Genetics – It is the branch of genetics which studies of genetic structure of Mendelian Population.

  • Gene & Gene frequency & the forces that governs their change over generation.
  • Helpful in construction & testing of mathematical models
  • Useful to evolutionary biologists , studies Evolution (evolution – change of allele frequency from 1 generation to another)
Mendelian GeneticsPopulation Genetics
Deals with study of pattern of transmission of genes from parents to the progeny in a single family / cross/ pedigreeIt deals with study of pattern of transmission of genes from parental to progeny by all the members of Mendelian population (one 1 gen to another)
Unit of study family or Individualthe unit of study is population
Result of specific mating are followed from parents to progeny generation & so on in terms of genotypic & phenotypic ratio Involves study of all the possible mating among members of a population in terms of gene & genotypic frequencies
Primarily centres on genotypic & phenotypic ratios usually don’t require complicated mathematical formulauses mathematical models to describe the changes in the genetic composition of a population from gen to another.

Mendelism & its Application

Introduction – Mendelism refers to the principle of inheritance formulated by Gregor JOhann Mendel (July 22,1822). He conducted two types experiments on garden pea plant, based on which he gave his three laws which are k/n as Mendel’s Laws of Inheritance. He considered as pioneer of modern genetics. His approach  was simple, logical, scientific mathematical & analytical.

  • His generalizations on segregation & independent assortment were not widely accepted
  • Mendel however, concentrated on a particular character at a time & studied the pattern of inheritance of only one or two character or characters.
  • He died in  1884 & not until 1900s that his work was rediscovered by De Vires & Carl Correns.

Mendel’s Considerations for choice of suitable Material

  • Variation – Their traits are easy to isolate & have wide variation
  • Reproduction – Sexually reproducing
  • Controlled Mating
  • Short Life Cycle
  • Large Number of offspring
  • Convenience in handling

Why was Pea Plant Selected for Mendel’s Experiments

  • The pea plant can be easily grown and maintained.
  • They are naturally self-pollinating but can also be cross-pollinated.
  • It is an annual plant, therefore, many generations can be studied within a short period of time.
  • It has several contrasting characters. – in case of pea he identified 7
    • Seed
      • Form – grey & Round ; White & wrinkled
      • Cotyledons – yellow & green
    • Flower colour – white & Violet
    • Pod
      • Form – Full & Constricted
      • Colour – yellow & Green
    • Stem
      • Place – Axial pods flowers along & Terminal pods, flowers top
      • Size – Long (6-7ft) & Short (3/4 -1 ft)

Mendel’s Procedures

  • Pea Plant have both male & female sexual reproductive organs & as result they can self pollinate themselves or cross pollinate other plants
  • Mendel was able to selectively cross pollinate purebred plants with particulr traits & observe the outcomes over many generations
  • These are the bases for his conclusions about nature of genetic inheritance

Experiments & Law of Inheritance

Monohybrid Cross

In this experiment, Mendel took two pea plants of opposite traits (such as one short & one tall) and crossed them. 

  • He found the first generation offspring’s were tall and called it F1 progeny.
  • Then he crossed F1 progeny and obtained both tall and short plants in the ratio 3:1.

Out of this experiment he gave following laws

  • Law of dominance also K/n as First Law of Inheritance
    • In crossing pure (homozygous) organism for contrasting character of a pair, only one character of the pair appears in the first filial generation which is dominant & the other allele which is hidden or masked is known as recessive. Eg. t allele is recessive
    • Mechanism of Dominance – In this law, each character is controlled by distinct unit called Factors, which occur in pairs. If the pairs are heterozygous, one will always dominate the other.
  • Law of segregation / Law of purity of Gametes – 2nd Law of Interitance
    • During the formation of gamete, each gene separates from each other so that each gamete carries only one allele for each gene
    • Mechanism of Segregation – This law explains that the pair of alleles segregate from each other during meiosis cell division (gamete formation) so that only one allele will be present in each gamete.Thus, the law of segregation is based on the fact that each gamete contains only one allele.
    • This law is based on four basic concepts:
      • A gene exists in more than one form of an allele.
      • When gametes are produced by meiosis, the allelic pairs separate, leaving each gamete with a single allele.
      • Every organism inherits two alleles for each trait.
      • The two alleles of a pair are different, i.e., one is dominant & one is recessive.
  • Law of one gene one character (Single factor Inheritance) – Unit factor
  • Law of equivalence

Dihybrid Cross

In the second experiment, Mendel studied two characters together, each having two contrasting aspects. Such a cross is called a dihybrid cross. Mendel crossed pure tall pea plants with red flowers and pure dwarf plants with white flowers.

  • In a dihybrid cross, the number of F2 plants is 16, phenotypic ratio 9 : 3 : 3 : 1 and the genotypic ratio is 1: 2: 2: 4: 1: 2: 1: 2: 1.

from it Mendel proposed the 3rd principle of inheritance which

  • Law of Independent Assortment – It states that when more than one pair of contrasting characters (alleles) are present in a hybrid, any one member of an allelic pair can assort or combine independently with any member of the other allelic pair.
    • For ex in a dihybrid self cross (TtRr), at the time of gamete formation, T can combine with R to form TR gamete. T can also be combined with r to form Tr gamete. Similarly, it can combine with R and also with r, independent of the other partner, to form tR and tr gametes, respectively.
    • only applicable to factors or genes, which are present on different chromosomes.

Mendelian Inheritance in Man

  1. Monohybrid Cross – ABO Blood Grouping
  2. Dihybrid Cross – Rollers & Tasters
  3. Other Example – Anonychia, chin fissure, mid digital hair, Darwin’s tubercle, free v/s attached ear lobes, black V/S blonde hairs, straight V/S bended little finger etc

Application of Mendel’s Law

  • Determination of blood groups → ABO & RH
  • Determination of sex of child
  • Pedigree Analysis – Medelian laws allow pedigree analysis with respect to those character that are caused by a single allele pair only.
  • Genetic Counselling by Pedigree Analysis – Progression of hereditary diseases can be predicted by pedigree analysis which can suggest whether one or both the parents are carrier. → counsellor’s advice → reduce the incidents of herediary diseases.
  • Medico-Legal Applications – paternity dispute by using a rare allele; like different body groups like Kidd, ABO, MN, Duffy
    • However this method can’t say who is the father but can say with certainty who is not father
  • Medical Application – These helped in many different kinds of histocompatibility & inborn errors of metabolism. Gives idea about aetiology of disease ex. Genetic disorders & their inheritance
  • Production of Hybrid Varieties of plant & animals

Limitation

  • Reproduction & transmission is too complex in human
  • Period of development is long to study & size of family is small
  • Ethical issues

Biological significance of Mendel’s Law

  • Science of eugenics : For betterment of human race ; is outcome of Mendelism
  • In obtaining disease resistance varieties of plants
  • Varieties of breed obtained by cross breeding
    • Breeds of Animal/ plant with better production & productivity
    • Disease resistant
  • Explain the inheritance pattern of qualitative traits
  • Understand the function & behaviour of genes (in the name of factors)
  • Helpful to increase production & productivity

Conclusion Thus Laws of Mendel can ensure food & nutritional security & promote social order & improve human health.

Mendelian Inheritance of Man

Mendelian inheritance is a characteristic for most of the traits

  • Which are monogenic or single factor traits
  • when env. don’t influence the phenotypic expression of the genes.

ABO Blood Grouping : Monohybrid Cross –

  • It was identified by Landsteiner in 1901.
  • ABO blood type in human is determined by three alleles  IA, IB and IO.
    • IA, IB are codominnat alleles.
    • Both IA, IB are dominant to the allele in IO
  • Any of these two can occupy a locus on chromosome 9 of humans making genotye for blood groups.
    • Phenotype – Genotypes R/l
      • O           – ii
      • A            – IAIA or IA i
      • B            – IB IB or IB i
      • AB         – IA IB
  • Depending upon whether person is homozygous or hererozygous for blood groups A & B, the type of blood groups inherited by progeny with change.
    • Thus when I^A, I^B are together, the blood group AB develops. Such alleles are referred to as co-dominant.
  • It is also the example of Multiple Allelsism.
    • Possible Alleles from Male – IA, IB , I
    • Possible alleles from female- IA, IB , I
  • Thus inheritance of blood group is strictly Mendelian in fashion. Both parents contribute one form of allele to their progeny and when progeny reproduces,its two alleles are segregaed & alleles are capable of expression when placed together.

Rollers & Tasters : Dihybrid Cross

  • In it individuals are crossed for the inheritance of one pair of contrasting characters (+nce or -ve of of Huntington’s Disease, +nce or -ve of cystic fibrosis)
  • No experiment can be designed with human beings as subjects like one carried out with laboratory animals.
  • There are certain  characters which can be spottd out in th parents & its inheritance traced in their children.
  • Two such conditions are
    • Ability to taste phenylthiocarbamide – PTC (tasters vs Non tasters)
      • the allele for tasting is dominant T while the allele for nontasting is recessive t.
    • Ability to roll the tongue longitudinally (rollers vs non rolllers)
      • it is inherited as dominant trait
      • dominant homozygotes & heterozygotes have ability to roll their tonge longitudinally
  • it both of the couple are taster & roller, the state of their genotype can be ascertained by pedigree analysis.
  • if one of their parents were nontaster & non roller, it is certain they are heterozygote,
  • Inheritance of their character by their children would be found in the ratio of 9:3:3;1 as  predicted by Mendel.
    • Taster roller : 9
    • Taster – Non roller : 3
    • Non Taster – roller : 3
    • Non Taster Non roller : 1

Other Examples

  • Anonychia
    • This transmitted as dominant trait
    • Some or all of the nails of the fingers & toes are absent or rudimentary
  • Chin Fissure
    • Autosomal Dominant Allele
    • Chine fissure is a longitudinal fissure in the middle of the chin.
    • The character finds complete pentrance in males whereas 50 % penetrance in females
    • The trait is both sex & age influenced
  • Mid Digital Hair
    • Autosomal dominant allele
    • Presence of hair on middle of the fingers
  • Darwin’s Tubercle
    • Autosomal dominant allele
    • It is thickening of cartilage near the upper rim of the ear
  • Achondroplasis
    • Autosomal dominant trait & gene mapped on chromosome 4P 16.3 (Francis Roussaeau, 1994)
      • Homozygous – lethal in neonatal period
      • Hetroygous – 25% of the offspring affected
    • abnoramal condition characterised by short limbed dwarfism. Minimal proliferation of their of thir growth cartilage of long bones where as other cartilage are normal. Individual are fertile
  • Free Vs Attached ear lobe – free ear lobe is dominant
  • Black vs Blond Hair – Black hair is dominant over blond hair
  • Staight Vs Bended Little Finger – Straight finger is dominant over bended finger
  • Widow’s Peak
    • Autosomal – present in both sexes
    • mid forehead hair line.
  • Freckling
    • Folds or Wrinkles near the skin of mouth is dominant over noraml skin
  • Dimple – Autosomal dominant
  • Ability to taste phenythiocarbamide (dominant)
  • ACHOO Syndrome (dominant) (Photo Sneeze Reflex)
  • Albinism (Recessive)
  • Cleft Chin (Dominant)
  • Hitchhiker’s Thumb (Recessive)
  • Wet (Dominant) or dry (recessive) earwax
  • Huntingtons Disease (Neurodegenerative )dominant

Case studies

  • S,P Sinha & R.K Singh (1983-85) observed genetic variations in the santals & three Pahariya sub-tribes (the saurias,the Mals & Kumarbhags) distributed in the various districts of Jharkhand (then in Bihar). The study included 7 different anthropogenetic traits.
  • Bhatia etal (1976)studies I endogamous groups of Sarwals in W. India
  • Mukherjee, Malhotra & Kate (1979) study included Red- green colour blindness in some population of Delhi, MH & W. Bengal

Exceptions – However, there are a no. of exceptions to the rules of Mendelian inheritance in human genetics

  • Polygenic Traits – these traits are influenced by more than one pair of genes. These are also k/n as continuous taits or quantitative traits
    • Each of the allleles will have an additive effect (+ or -)
    • Ex. Human Stature – the combined size of all the parts from head to foot determine the height of an individual & the parts individual sizes are in turn influenced by many genes & also environment (Growth hormone, Nutrition, Emotions etc.)
    • Human disease like autism, hypertension, glaucoma
    • This law go against the law of one gene one  character
  • Incomplete Dominance – There is an apparent blending in the phenotype in heerozygous individualls ex. Red×White=Pink
    • In humans, the male voice pitch is an intermediate expression i.e Intermediate baritones are heterozygous Aa
    • This goes against the law of dominance
    • The child killer disease Tay Sachs (fluid pressure on brain & subsequent degeneration) is characterized by incomplete dominance where the hertrozygous individuals are gentically programmed to produces only 40-60% of the enzyme hexosaminidase A
  • Codominance – Both alleles express themselves in heterozygous condition and the phenotype is not an intermediate
    • This goes against the law of dominance
    • Sickle Cell Trait
      • The gene for Hemoglobin Hb has two alleles – HbA (normal) and HbS (mutated gene) and three phenotypes
      • HbAHbA – Normal Phenotype and all RBCs and hemoglobin are normal
      • HbAHbS – Sickle Cell Trait – 50% of hemoglobin in every RBC is abnormal and RBCs are slightly distorted / suffer only under hypoxia / resistant to Malaria
      • HbSHbS – Sickle Cell Anemia – 100% abnormality in hemoglobin and all RBCs min sickle shape – fatal
  • Multiple – Allele Series – trait can be controlled by more than just a pair of alleles.
    • Ex. ABO system – there are three alleles A, B and O but the individual is inheriting only two of them!
    • Some traits are believed to be controlled by far more than two alleles – Ex. HLA system (Human Leukocyte Antigen)
      • HLA – identifying and rejecting foreign tissue by regulating antibody production
      • Known to have 30,000,000 different genotypes
      • Play a very important role in rejection of organ transplant
  • Modifying & Regulator Genes –
    • Modifying Genes alter how certain other genes are expressed in a phenotype –
      • Ex. Cataract Gene is dominant but produces varying degree of vision impairment depending on the presence of specific allele on companion modifying gene (also environment like alcoholism, diabetes etc)
    • Regulator Genes initiate or block expression of phenotype.
      • Ex. Time of production of specific proteins or new structural parts during aging – Homeotic Genes
  • Incomplete Penetrance – Incomplete penetrance is a situation where the effect of a particular gene does not normally occur unless there are certain environmental factors present Ex.
    • One may inherit the gene for Type 2 Diabetes , but can avoid it unless they are obese, stressful and insomniac
    • Multiple Sclerosis ( a progressive neurodegenerative disorder leading to paralysis) – Triggered due to a specific virus called Epstein Barr virus
  • Pleiotropy – Pleio – More; Tropy – Convert : A single gene may be responsible for a variety of traits
    • Albinism – The gene for this trait not only results in typical melanin deficiency of skin, hair and eyes, but also cause defects in vision
    • Sickle Cell Trait is another example – can also cause hearth, lung, kidney and vision problems, strokes, osteoporosis etc
      • This law go against the law of one gene one  character
  • Epistatis– Epi – Over; Statis – to sit – “To Sit on Top” ; Interaction between genotypes at two different genetic loci
    • Exception to Mendel’s Independent Assortment –9:3:3:1 integrity is lost
    • Ex. If gene at one locus determines if the color should be red or white, another gene at a different locus determines if the pigment should be produced or not!
    • Dermatoglyphics in man are believed to be epistatic
  • Sex Related Genetic Effects – Three types
    • Sex limited Genes – Inherited by both men & women but are normally expressed in the  phenotype of one of them. Ex. Heavy Male beard (Gene for facial Hair)
    • Sex Controlled Genes – Expressed in both sexes differently  –  Ex, Gout – Joint Swellings – Men 8 times more likely to have severe symptoms
    • Genome Imprinting – Genes express differently depending on the sex of the parent from whom they are inherited
  • Sex Linked Inheritance – Besides Sex information, there are 80,000 genes in X (responsible for X linked traits) & 90 in the Y chromosome – responsible for Y linked traits .
    • Most of the X linked genes are recessive – Colorblindness, Hemophilia, Baldness
    • Men more likely to show them b/c they are hemizygous – when only on copy of the gene is present
    • Women who are homozygous also show the traits
  • Suppressor Gene
    • When a gene product can suppress the expression of dominant alleles then gene inheriting the expression is called suppressor gene.
    • It goes against the law of dominance for e.g. Proto-oncogene
  • Gene imprinting
    • with the change in the source of alleles, the expression in offspring’s also changes.
    • This phenomenon goes against the principle of law of equivalence.
    • Huntington chorea syndrome – dominant disorder ; when the allele comes from mother it express differently which can be compared with allele coming from father
  • Linkage – law of independent assortment was not universally applicable. In some cases there is marked tendency for parental combinations to remain linked & produce new combinations.
    • It goes against the law of independent assortment
  • Environmental Influences
    • Environmental factors alter phenotypes
    • Abnormal growth & conditions affecting growth
      • Nutrition
      • Oxygen deprivation during pregnancy, childbirth – autism ?
      • Habit – smoking can casue some mutations
      • Radition exposures etc.

Thus all the laws of Mendel have exception except the law of segregation which is universal or absolute law.

Difficulties In the Primates Model

It is difficult to analyse human & primates pedigree on the basis of Mendelian inheritance.

  • The determination of genetic basis of human & the primates traits is fraught with difficulties owing to
    • incomplete penetrance.
    • Variable age of onset
    • Sex influence
  • Environmental Effects
    • No clearcut association b/w genotype & phenotype.
    • Phenotype in majority cases, depend on evn. also
    • example
      • Diabetes mellitus & hypertension are genetic diseases but time of onset & severity largely depend on evn. factor such as composition of the diet, stress etc.
      • Sometimes a genetic character si mimicked by environmental character (phenocopy)
  • Double Mutant Loci
    • A genetic trait is modifiable not only by environment but by composition of total genotype which may include several modifier gens for the characters.
    • sometimes a 2nd gene locus is involved in the expression of phenotype e.g. in case of thalassaemia
      • α globin locus on chromosome 16
      • β globin locus on chromosome 11
  • Uncertain Dominance – Sickle cell anaemia illustrates
    • On microscpic investigation, the trait displays a dominant inheritance pattern as rbc in both Heterozygotes & mutant homozygotes are seen to be sikle. i.e 3/4 of the offspring mating will display the sickning phenomenon.
    • on Clinical viewpoint, the inheritance is autosomal recessive b/c only only homozygous Hb^s gene develop serious symptoms of anaemia.
      • Pedigree analysis indicate that, on avg. 1/4 offspring of carrier parents are affected.
    • At the molecular level, where the hb protein produced can be assessed directly. the Mendelian pattern is co-dominant

Monogenic Inheritance or Single Factor Inheritance

Introduction – refers to inheritance patterns resulting from a single gene/factor i.e character determined by alleles, occupying a single locus is known as single factor inheritance. It is based on Mendel’s Law of Segregation. Currently more than 4000 human traits are known to be inherited according to simple Mendelian principles. Ex include several blood group system such as ABO

Features of Monogenic Inheritance – Mendelian Traits

  • alleles follow Mendelian principles / laws i.e Dominance, recessives, codominance – when two different alleles occur in heterozygous condition
  • Environment has no role to play ; different in phenotype results from alternative genotype of a single gene.
  • Mendelian traits are said to be discrete or discontinuous b/c their phenotypic expression do not overlap; rather than fall not clearly defined categories I.e these don’t show continuous variation i.e it shows / occur in discrete categories.
  • The inheritance of single factor of uni-factorial inheritance can be analysed by the pedigree method. It is simplest inheritance pattern.

Example – ABO System, is governed by 3 alleles A, B, O found at ABO locus on 9th chromosome.  It is occupied by any of the 3 alleles (A, B or O)  → code for production of antigens

  • Antigen A only → A blood group
  • Antigen B only → B blood group
  • A & B Antigen → AB Blood group
  • No Antigen → O blood group

The different patterns of such inheritance are –

  • Autosomal Inheritance – is a pattern of inheritance in which the transmission of the traits depends on the presence or absence of certain alleles on the autosomes. They pattern may be
    • Autosomal dominant Trait Inheritance e.g polydactyl
    • Autosomal Recessive Trait Inheritance e.g Cystic fibrosis
  • Sex Linked Inheritance – is a pattern of inheritance in which the transmission of traits depends upon the presence or absence of certain alleles on the sex chromosomes.
    • X-Linked Dominant Trait Inheritance e.g Rickets
    • X-linked Recessive Trait Inheritance e.g Haemophilia 
    • Y-Linked Inheritance (Holandric)  e.g Hairy ears

Polygenic or Multifactorial or Quantitative Inheritance  

Introduction – There are several quantitative traits like height & skin colour that display continuous variations & can’t be simply Mendelian inheritance. Distinct classes be identified for such traits making it difficult to be analysed by the conventional genetic methods. So QG deals with such complex traits. 

Quantitative Genetics – Is also referred to as the genetics of complex traits, is the study of such characters and is based on a model in which many genes influence the trait and in which non-genetic factors may also be important.The framework can also be used for the analysis of traits such as litter size that take a few discrete values, and of binary characters such as survival to adulthood that have a polygenic basis.

  • The quantitative genetics approach has diverse applications:  it is fundamental to an understanding of variation and covariation among relatives in natural and managed populations, of the dynamics of evolutionary change, and of methods for animal and plant improvement and alleviation of complex disease.
  • it is based on measurements of individuals within a population of  organisms. 
  • It recognize two imp. factors
    • Most quantitative traits involve contribution of many different genes. A single gene may exert primary influence but usuallu there are many equally influential genes
    • Quantitative traits are often influenced by environmental factors. A quantitative trait will show phenotypic variation over & above that caused by genotypic differences.

Complex or Multifactorial Traits – Traits in which a range of phenotypes can be produced by gene interacts & gene-environment interaction are known as Complex or Multifactorial.  these traits are influenced by more than one pair of genes (Polygenic) . These are also k/n as continuous traits or quantitative traits

Character of Polygenic Inheritance

  • Characteristic that are determined by an interaction of genes on serval chromosomes or at server also places on one chromosome is called polygenic inheritance
  • In such cases the characters, instead of being sharply marked off, show continuous variation between two extremes.
  • Each of the allleles will have an additive effect (+ or -)
  • Each contributing allele in a series produces an equal effect
  • There is no dominance involved
  • Epistasis (i.e suppression of one gene by other)  doesn’t exit
  • When there are so many genes involved in the producation of traits the latter often take the form of distribution curves.
  • one such trait is stature in man , where b/w a few extremely short & tall individuals there are many in b/w these extremes.
    • Ex. Human Stature – the combined size of all the parts from head to foot determine the height of an individual & the parts individual sizes are in turn influenced by many genes & also environment (Growth hormone, Nutrition, Emotions etc.)
  • Polygenic charactersare normally distributed in population.Those with extreme traits are in lower frequency where as those with mean value are in highest frequency.
  • Human disease like autism, hypertension, glaucoma

Multiple Factor Hypothesis

As it was difficult to reconcile Medelian Genetics with quantitative traits. To a/c for such traits R.A Fisher proposed the Multiple Factor Hypothesis. It postulates that there are traits whose inheritance is determined by multiple factors both environmental & genetics. This si known as Polygenic / quantitative inheritance.

Multifactor Hypothesis ” Quantitative traits are not determined by single gene but many & their alleles each has small & approx. additive effects”

  • Thus, a phenotype of an individual depended on its genotype at all the relevant loci, with each allele adding or subtracting a small amount.
  • Fisher also proposed that many environmental factors influenced the trait by adding or subtracting effects in manner similar to that of all the genetic loci. This combination of multiple environmental & gentic factors determined the phenotype of an individual

Multiple Factor Hypothesis explain Three features of Quantitative Traits

  • Multiplicity of factors contributing to a trait which a/c for the variability among phenotypes.
  • Fisher’s hypothesis suggested an explanation that, if a trait is influenced by two types of alleles + or -, Mendelian segregation should lead to many individuals carrying a mixture of plus or minus. Thus, extreme values of trait are rare & intermediate are common.  
  • It explains why offspring of phenotypically different parents mostly expresses an intermediate phenotypes. For ex – Mating b/w homozygous tail & homozygous dwarf parent will have heterozygous medium tall progeny.
  • Conditions attributed to polygenic inheritance includes diabetes, cancer, obesity etc

Significance of Multifactor Hypothesis

  • It readily explains variablity among phenotypes of quantitative traits through large no. of genotypes & environmental factors
  • Explains why offspring of parents who are phenotypically distinct often lead to intermediate types
  • Explains why intermediate phenotypes are more common than extreme phenotypes which are relatively rere, because of the operation of multiple genotypes having plus or minus effect.

Phenotypic Threshold Value : Sometimes in the polygenic inheritance is introduced a condition of threshold. A character is not expressed until mutant genes at the polygenic sites don’t reach a crtical threshold level. ExHuman eye color is a polygenic trait that shos threshold effect.

  • Eye color varies from pale blue to very dark brown, depending on the concentration of melanin,
  • Two blue eyed parents can produce brown-eyed offsping. This is presumably b/c the child inherits from each parent the appropriate combination of alleles at the polygenic foci which enables the child to cross the phenotypic threshold.

Case study –

  • The hypothesis of Multi-factor inheritance was later proved to be correct by Nilsson Elle in an analysis in a graded series of seed pigmentation in wheat crosses. 
  • Polygenic inheritance of skin colour studied by C.B Davenport (1913)
    • The presence of melanin pigment in the skin determine the skin colour,
    • study concludes that amount of melanin producted is always proportional to the number of contribution genes, showing discontinuous variations.
    • Hence polygenic inheritance  served as a basis for recital classification.

Conclusion – Thus QG gives a solution to analysis of genetic variation & states that multiple factors are responsible for genetic variations which in turn lead to phenotypic variations. 

Lethal Gene Action

Introduction – One of the most imp. assumption in gentics is equal survival of all the gametes & zygotes that are produced as a result of segregation. This assumption is true in majority of cases.

  • But Some genes affect the survival of the gamete or the zygote (the individual) if present in specific genotypes.
  • their effect ranges from improving survival to no effect to even death.

Concept of Lethality ; A gene may sometimes interfere with the development & may cause the death of the individual.

Categorisation of Lethal Genes – Based on their effect on survival, genes can be categorized into

  • Depending Upon Lethality
    • Semi Lethal – Death after attainment of reproducible age
    • Sub Lethal  -Death in early infancy
    • Conditional lethal
  • Depending on Zygosity
    • Dominant Lethal – it kills possessor even in hetero zygosity
      • Sub lethal e.g. epiloia
      • Semi lethal e.g. Huntington’s disease
    • Recessive Lethal – death only in homozygous condition
      • Sub lethal e.g. Thalassemia, Sickle cell anemia
      • Semi Lethal eg. Heamophillia

Lethal Gene

Introduction – lethal Gene is a gene which casues death of all individuals carrying this gene in appropriate genotype before they reach adulthood

  • The appropriate genotype for an allele would depend on its dominance relationship with other alleles
  • The stage of development at which a lethal gene produces its lethal affect,  by interfering in the production of the gene product, varies considerably.
    • Some casue death very early in development
    • other allow survival of individuals to reproductive age
  • Time of death will depend on
    • When the product is essential for life
    • Whether the gene is in position to produce the gene product or not
    • If produced whether it is sufficient quantities or not

Based on the nature of Lethality they can be Classified into

  • Gametic Lethal –
    • the genes which makes genetic  inviablity of gametes or make them incapable of fertilisation
    • Gametic lethals lead to a drastic departure from the typical ratio expected in a segregating generation- this phenomenon is c/a ‘Segregation Distortion or Meiotic Drive‘
    • They are different from Zygotic lethals
  • Sub Lethal
    • The genes which kill the possessor before attaining reproductive age
  • Semi Lethal
    • The genes which kills the possessor after attainment of reproductive age ex – Huntington Disease (40 yrs)
    • casue the death of more than 90% of the individuals, only less than 10% of the individuals surivive

Lethal Gene can be classified into

  1. Incomplete Dominant Lethal
    • lethal in homozygous continues
    • In heterozygous state produce some abnormal phenotypes & cause death only if they are serious
    • Ex – Sickle cell anaemia (Gene-Hbs)
      • Sickle cell Disease (SCD) – lethal homozygous condition
      • Sickle cell trait (SCT) – heterozygous condition – mild anaemia & causes sickling only in oxygen deficiency.
  2. Dominants Lethal
    • Lethal in both homozygous & heterozygous conditions
    • I.e reduce viability in the heterozygous conditions as well
    • Ex – Huntington’s disease/chorea in Man – (semi – lethal dominant)
      • rare & fatal inherited disease of CNS → progressive degeneration of nervous system
      • expressed only in middle age usually after 40 yrs
      • Due to dominant autosomal H chromosome – expressed even when a single dominant allele is preset.
      • Casuses damage to brain cells, leading to gradual loss of co-ordination, decline in metal ability & change in personality & finaly death.
    • Ex – Epiloia Lethal Inheritance ( Sub Lethal- Dominant)
      • Expressed before individual reaches reproduction age So can’t be maintained in the population unlike recessive lethals in heterozygous state
      • These have to be produced in every generation through mutation
      • Causes abnormal skin growths, sever metal defects, multiple tumour in the heterozygous condition, so that they die befor reaching adulthood
  3. Recessive Lethals
    • Those genes in which Lethality is expressed only in homozygous conditions
    • The survival of heterozygotes is unaffected.
    • However many genes in heterozygotes show Dominant Phenotypic Effect & Recessive Lethal Effect.
    • Ex – Achondroplastic, Dwarfism, Xeroderma Pigmentosum,
    • Sub lethal Recessive  – Thalassaemia, Sickle cell anaemia –
    • HaemophiliaSemi Lethal Recessive
      • females seldom suffer from this disease b/c in homozygous condition the gene casues death of zygote or embryo,
      • males are sufferers
      • In it lack of clotting factor VIII causes a weal platelet plug to form → incomplete &/or delayed fibrin clot allows bleeding to continue.
  4. Conditional Lethals
    • those lethal genes which may allow normal development & survival, & may prove lethal  when environment is changed.
    • So, Those lethal genes,  which require a specific condition for their lethal action are termed conditional lethals
    • A gene lethal in one environmetntal condition may not be so in other environmental condition e.g. mutants of phenylketonuria are normal if reared on phenylalanine free diet.
    • Ex. Phenylketonuria, Diabetes, Xeroderam Pigmentosum (condition is light, nutrition
    • Ex – Erythroblastosis foetalis (Rh Haemolytic Disease) → Father is Rh Positive & mother is Rh negative ; Rh is not lethal in normal condition but RH -ve mother carrying RH positive baby it becomes lethal

Most of lethal genes are recessive lethal & they exist among us in carrier state. Hence to eliminate lethal genes from the population → identifying the carriers & preventing them from mating. 

Lethal Gene & Human Population

Many recessive lethal genes which are harmful for man are found to be circulationg in populations at high frequency & defy rejection by natural selection.

Two theories have been proposed to explain this dilemma

  • School of Dobjhansky –
    • maintains that recessive lethal genes in heterozygous condition provides heterosis & heterozygotes are superior to both homozygotes.
    • Ex-  Sickle cell anaemia & thalassaemia
    • It has been found that heterozygotes have resistance against malaria.
  • School of Muller
    • believes that there is artificial reduction of natural selection by use from medical sciences & sociocultural practices.
    • Thus a person suffereing from phenylketonuria can hope to survive & live a full life if reared on phenylanine free diet. Such practices ensure circulation of lethal genes in population.

Conclusion – Thus lethal genes are responsible for elimination of weaker traits in the population by way of natural selection.

Others Gene Classification based on Vitality

  • Sub Vital – kill less than 90% of the individuals who carry them
  • Vital – don’t affect the survival of the individuals carrying. neither enhace nor reduce the viablitity
    • It doesn’t imply that these genes are no necessary for the survival of the concerned org. It simply means that the survival of the organism is not affected by the fact that the concerned vital genes are present on homozygous or heterozygous state.
  • Super Vital – these mutant genes enhance the survival of those individuals that carry them in the appropriate genotype as compared to that of the wild type (naturally occurrin) allele.
    • Ex, Disease resistant crops – high productivity etc