Samacheer Class 12 Bio Botany - Classical Genetics

• True breeding lines (pure breeding strains) means it has undergone continuous self-pollination having specific phenotype trait inheritance from parent to offspring.
• Mating within pure breeding lines produces offsprings having, specific parental traits that are the same in inheritance and expression for many generations.
• Parents are homozygous for every trait.

• back cross is a cross off Fi offsprings with either one of the parental genotypes.
• The recessive back cross helps to identify the heterozygosity of the hybrid.
• It involves the cross between the fi offspring with either of the parents dominant.

“Genetics” is the branch of biological science which deals with the mechanism of transmission of characters from parents to offsprings. The term Genetics was introduced by W. Bateson in 1906.

• Alleles are alternative form of gene and they are responsible for differences in the phenotypic expression of a given trait. A gene for which atleast two alleles exist is to be polymorphic, so a particular gene may exist in three or more allelic forms known as multiple alleles
• eg) ABO of human blood is controlled by three alleles

Mendel was successful because:
* He applied mathematics and statistical methods to biology and laws of probability to his breeding experiments.
* He followed scientific methods and kept accurate and detailed records that include quantitative data of the outcome of his crosses.
* His experiments were carefully planned and he used large samples.
* The pairs of contrasting characters which were controlled by a factor (genes) were present on separate chromosomes.
* The parents selected by Mendel were pure breed lines and the purity was tested by self crossing the progeny for many generations.

Law of dominance
In cross of parents that are pure for contrasting traits only one form of the trait will appear in the next generation. They have hybrid or dominant trait in the phenotype.
eg) Monohybrid cross
Regarding F 1 generation the recessive allele is not lost, and it remain hidden or masked. But it reappears in the next generation.

Incomplete Dominance:
* In incomplete dominance, neither of the allele is not completely dominant to another allele rather combine and produce new trait
* New phenotype is formed due to character blending (not alleles)
* Example: Pink flowers of Mirabilis Jalapa
Co-dominance:
* In co-dominance, both the alleles in a heterozygote are dominant and the traits are equally expressed (joint expression)
* No formation of new phenotype rather both dominant traits are expressed, conjointly
* Example: Red and white flowers of camellia

• DNA is a universal genetic material.
• Genes located in nuclear chromosomes follow Mendelian inheritance.
• Certain traits are governed by the chloroplast (or) mitochondrial genes which is known as extranuclear inheritance.
• It is a kind of Non – Mendelian inheritance.
• The cytoplasmic organelles chloroplast and mitochondrion act as inheritance vectors so-called cytoplasmic inheritance.
• It is based on self – replicating extrachromosomal unit called plasminogen in the cytoplasmic Organelles, Chloroplast, and mitochondria.

Epistasis can be defined as a gene interaction whereby one gene interferes with the phenotypic expression of another non-allelic gene. The gene or locus which suppresses or masks the action of a gene at another locus is called the epistatic gene. The gene or locus where expressions are suppressed by an epistatic gene is called gene hypostatic.
Dominant epistasis A dominant epistasis suppresses the expression of a non-allelic gene, (dominant (or) recessive)
The F 2 phenotypic ratio is 12:3:1

* Polygenic inheritance, also known as quantitative inheritance, refers to a single inherited phenotypic trait that is combined bv two or more different: genes.
(or)
* Several genes combine to affect a single trait. A group of genes that together determine (or) contribute a characteristic of an organism is called polygenic Inheritance
(or)
* Polyinheritance occur when one characteristic is controlled bv two or more genes.
Eg. Human skin colour & eye colour and weight.
* H.Nilsson -Ehle (1909), a Swedish geneticist discovered a polygenic inheritance in wheat (kernel colour). Kernel colour is controlled by two genes each with two alleles, one with red kernel colour was dominant to white. He crossed the pure breeding wheat varieties dark red and a white.
* Dark red genotypes R 1 R 1 R 2 R 2 crosed unit r 1 r 1 r 2 r 2. In the F 1 generation medium red were obtained with genotype R 2 r 1 R, r 2. So the intensity of the red colour is determined by the number of R genes in the F 2 generation
* Four R genes: A dark red kernel colour is obtained.
* Three R genes: Medium – dark red kernel colour is obtained.
* Two R genes: Medium-red kernel colour is obtained.
* One R gene: Light red kernel colour is obtained.
* Absence of R gene:Results in White kernel colour is obtained.
The data produces a bell shaped curve which demonstrate continuous variation in wheat kernel from dark red to white in F 2 when the number F 1 were self crossed five different phenotypic classes appeared in F 2 in into ratio of 1:4:6:4:1
The phenotype ratio is Dark red:1 Medium dark red:4 Medium red: 6
light red: 4 white: 1
Hence the total ratio is 63 red: 1 white in F2 generation
1:6:15:20:15:6:1 in generation
He found that In F 2 generation plants have Kernel’s with range of colour variation. This is due to the fact that the genes are segregating and recombination takes place.

Variation is the difference between individual with in a species. This can be caused by inherited or environmental factors. It can be continuous and discontinuous. Height, and weight of the human being are best examples of continuous variation. Human blood group, gender identity and eye colour are best example of discontinuous variation
Continuous Variation
Discontinuous Variation
Variation are fluctuate or mean
mean or average is absent
Direction is predictable
unpredictable
already exists in the population
variation occur previously
It is due to the chance of segregation of chromosomes during gamete formation & crossing over & chance pairing during fertilisation
Produced by changes in genome or genes
They can increase adaptability of the race
evolutionary based
It is also called fluctuation
It is also called fluctuation
graphically produce bell shaped curve
No curve is produced
Very common
appears occasionally
do not disturb the genetic system
They disturb the genetic system

• There are several patterns responsible for the inheritance of traits, gene causes one trait. But in some cases one gene is responsible for multiple traits. Sometimes two or more gene are required to produce one trait.
• It is otherwise called pleiotropy. It means, where one gene will code and control the phenotype or expression of several different and unrelated traits.
• Eg. Phenylketenuria disease.
• A gene that produces multiple or effect is called a Pleitropic gene. Multiple effects of a single gene is know as pleiotropy. A Pleitropic gene is a single gene that controls more that one trait.
• Eg. Human genetic disorder are often pleitropic ie, unusual tall height, thin finger and toes, dislocation of the lens of the eye, heart in the aorta (heart function)
• Eg: Pisum sativum plant with purple brown seeds and dark spot on the axis of the leaves were crossed with a variety of a peas having white flowers light coloured seed and no spot on the axils of the leaves, the three traits for peas colour, seed colour and a leaf axil spot all were inherited together as a single exist. This is due to the pattern of inheritances controlled by a single gene with dominant and recessive alleles,
• eg.Sickle cell anemia
• eg.Marfan syndrome
• A human genetic disorder called marfan syndrome is caused by a mutation in one gene, yet it affects many aspects of growth and development inducing height, vision and heart function. This is an example of pleiotropy or one gene affecting multiple characteristics.
• Gene also interact in pattern such a partial dominance or co-dominance, the trait is expressed a mix between two gene, Those are possibilities for one gene. Most trait are influenced by many genes. There are many different way for these gene to influence how trait is expressed.

* It is found in 4 O’clock plant (Mirabilis jalapa)
* There are dark green leaved plants and pale green leaved plants.
* When the pollen of dark green leaved plant (male) is transferred to the stigma of pale green leaved plant (female) the pollen of pale green leaved plant is transferred to the stigma of dark green leaved plant, the F 1 generation of both the crosses is identical as per mendelian inheritance.
* In the reciprocal cross the F 1 plant differs from each other.
* The F 1 plant reveals the character of the plant.
* The inheritance is due to the chloroplast gene found in the ovum of the female plant which contributes the cytoplasm during fertilization.
* The male gamete contribute only the nucleus.
12th Bio Botany Guide Classical Genetics Additional Important Questions and Answers
I. Match the following

(a) Genetics – (ii) W. Batson
(b) Mendel – (iii) Father of Geetics
(c) lethal gene – (i) E. Baeur
(d) H. Nillsson Ehle – (iv) Kernel colour

(a) Polygenic inherence – (iv) wheat kernel colour
(b) 4 O’clock pea plant – (iii) Mirabilis jalapa
(c) Garden pea plant – (i) Pisum sativm
(d) H. NillssanEhle – (ii) genetic material
II. Choose the correct statement

c) Pisum sativum
44.
Mendel’s experiments were rediscovered by
a. Hugo de vries & carl correns
b. E. Baur
c. H. Nilsson
d. T.H.Morgan
a) Hugo de vries & carl correns

d) All of the above
IX. One Mark Question
1. The genetic constitution of the individual is called
Genotype
2. The observable characteristics of an organism are called
Phenotype
3. Who is father of genetics?
Gregor Johann Mendel
4. Name the Mendel’s published work.
Experiments on plant Hybrids.
5. Name the publication of Mendel research work
1899
6. What is the year of published work Mendel’s Research paper?
The proceedings of the Brunn Society & Natural History.
7. What is an allele?
It is another word for a Gene.
8. Individuals show a range of traits with small difference between them.
Continuous variation
9. When an individual show two or a few traits with large differences between them. This type of variation is called.
discontinuous variation
10. Human height is the good example of ………….. variation.
Continuous variation
11. Human skin colour is the good example of …………….. variation.
Continuous variation
12. Mention any two examples of continuous variation.
a. Human height
b. Human skin colour
13. Mention any two examples of discontinuous variation.
Style length of Primula & Height of the garden pea.
14. A trait that makes the expression of another trait when both version of the gene are present in the individual called
Dominant.
15. What is F 1 ?
It is the first filial generation in a cross; the offspring of the parental generation.
16. The letter ‘P’ denoted in genetics is
The parental generation in a cross
17. A variation in an inherited characteristics is
Trait
18. One pair genes can completely makes the expression of another pair of genes known as
Epistasis
19. Who discovered incomplete dominance?
Correns. (Germany)
20. Crosses between F1 offsprings with either of the two parents (hybrids) are known as
Back cross
21. Diploid organisms that have two different allele at a specific gene locus are said to be
Heterozygous
22. TT referred as…………….
Homogenous dominant variety.
23. ‘tt’ referred as ……………
Homozygous recessive character.
24. ‘Tt’ denotes for …………….
Heterogeneous hybrid variety.
25. The superiority of hybrid over either of its parents in one or more traits known as
Hybrid vigour or Heterosis
26. The site or position of a particular gene on a chromosome is
locus
27. An allele which has the potential to cause the death of an organism is called ……………….
Lethal genes
28. A single gene affects multiple traits are called ……………..
Pleiotropy
29. A single gene affects multiple traits and alter the phenotype of the organism is
Pleiotropy
30. Several genes combine to affect a single trait of an organism.
This kind of inheritance is ……………
Polygenic inheritance.
31. Who demonstrated first experiment on polygenic inheritance.
Swedish Geneticist H. Nilsson – Ehle (1909)
32. Which plant to use to identify the polygenic inheritance?
Wheat – Kernel colour (dark red & white variety)
33. List any two intragenic or allele interaction.
* Incomplete Dominance
* Co-dominance
34. List any two intergenic or non-allele interaction
* Dominant Epistasis
* Recessive Epistasis
35. Corren has used plant for studied incomplete dominance.
Mirabilis jalapa (4′ O clock plant)
36. Mention the botanical name of 4′ O clock plant.
Mirabilis jalapa.
37. Duplicate genes with cumulative effect of non-alleleic interaction is derived in
Fruit shape in Summer squash.
38. What is the FI phenotypic ratio of inhibitor genes in the intergenic interaction?
13:3
39. When the heterozygote exhibits a mixture of phenotypic character of both homozygous called as
Co-Dominance.
40. Name the two gene interaction.
* Intralocus interaction (allelic interaction)
* Interlocus interaction (non-allelic interaction)
41. A chart shows which genes are co-dominant. This is known as
A pedigree charts.
42. Each character is controlled by distinct units called factor, which occur in pairs. If the pairs are heterozygous, one wiil always dominant other. This is known as
First law of inheritance or Law of Dominance.
43. The second law of inheritance otherwise called as
Law of Segregation.
44. Give the name of the scientists who re-discovered Mendelism
* Hugo Devries
* Carl Correns
* Erich Von Tschermak.
45. is the prerequisite for Hybridization technique.
Emasculation.
46. Transmission of genes that occur outside the nucleus is called………………
Cytoplasmic Inheritance or Extra Nuclear
47. Cytoplasmic inheritance are found in
Mitochondria & Chloroplast
48. The interaction between separate gene in which one makes the effect of another
Epistasis
49. The acquisition of traits or conditions controlled by self replicating substances within the cytoplasm. This is a type of
Cytoplasmic Inheritance.
50. The hybrid progeny in the first generation is called as
F 1
51. The innate tendency of offspring to resemble their parents is called
Heredity
52. The tendency of offspring to differ from parents is called
Variation
53. Multiple allelic inheritances is otherwise called as
Co – dominance
54. What is the use of pedigree analysis in genetics?
It helps in genetic counselling.
55. Who proposed the genetic theory of inheritance?
T.H.Morgan
56. Give one good example for Atavism in plants.
Reemergence of sexual reproduction in Hieracium pilosella.
57. In pea plant, yellow seeds are dominant to green. If a heterozygous yellow seeded plant is crossed with a green seeded plant. What ratio of yellow and green seeded plants would you expect in FI generation?
50:50 (or) 1:1
58. Some genes have allele that prevents survival when homozygous or heterozygous. What is the kind of allele?
Lethal alleles
59. Recessive alleles of two different genes may give the same phenotype; This kind of genes also called
Complementary gene.
60. A gene is a functional unit of DNA which codes for a
Polypetide chain
61. Allele are the alternative form of the
gene
62. discovered incomplete dominance.
Correns
63. Human blood group is an example of variation.
Discontinuous
X. Two marks

Monohybrid inheritance looks at the inheritance of a single trait (a characteristics such as eye color, round or wrinkled seed type) coded by a single gene locus on a chromosome

Mendel’s first law is ‘The law of segregation’. Segregation means separation. The two alleles are separated from each other during meiosis, so each gamete produced is haploids that is contain one allele of each gene.

It is a term which describes how genes interact to affect a phenotype whereby an allele at one locus prevents an allele at another locus from manifesting its effect.
(or)
One gene is effectively interfering with or masking the effects of another gene.

A sort of cross multiplication matrix used in the prediction of the outcome of a genetic cross, in which male and female gametes and their frequencies are arranged along the edges.

Four R genes are produced dark red kernel color. Three R genes are produced medium dark red kernel colour. Two R genes are produced medium red kernel colour. One R gene is produced medium red kernel colour and absence of R genes in results in white kernel colour.

• It was called Pea Gene A which encodes a protein that functions as a transcription factor which is responsible for the production of anthocyanin pigment.
• So the flowers are purple. Pea plants with white flowers do not have anthocyanin, even though they have the gene that encodes the enzyme involved in anthocyanin synthesis.

It states that a dominant allele expresses itself in a monohybrid cross and suppresses the expression of recessive allele. However this recessive allele for a character is not lost and remain that hidden or masked in the progenies of F:l generation and reappear in the next generation.

Alleles are alternative form of a gene. A gene for which at least two alleles exist is said to be polymorphic. Instances in which a particular gene may exist in three or more allelic forms are known as multiple allele conditions.

The set of theories prepared by Gregor Mendel, which attempt to explain the inheritance pattern of genetic characteristics based on simple breeding experiments involving single gene on chromosome pairs.

• Green plants with chlorophyll (CC)
• Yellowish green plants with carotenoids are referred to as pale green, golden or a urea plants (Cc)
• White plants without any chlorophyll, (cc)
• The genotype of the homozygous green plants is CC. The genotype of the homozy¬gous white plant is cc.

It refers to genetic situation in which one allele does not completely dominate another allele, and therefore results in a new phenotype.
(or)
It is a form of intermediate inheritance in which one allele for a specific trait is not completely expressed over its paired allele. This results in third phenotype in which the expressed physical traits is a combination of the phenotypes of both alleles.

It deals with heredity in groups of individuals for trait which is determined by a few genes.
(or)
Population genetics is the study of genetic variation with in population, and the examination and moddling of changes in the frequencies of gene and allele in populations over space and time.

It is the field of that biology that studies the structure and function of genes at a molecular level
(or)
Study of structure and function of genes at molecular level
(or)
A branch of genetics that deals with structure and function of genes at molecular leve

A permanent, heritable change in the nucleotide sequence in a genes or a chromosome, the process in which such a change occurs in a gene or in chromosome.

A gene where individual effect on a phenotype is too small to be observed but which can act together with others to produce observable variation.
(or)
Characters are determined by two or more gene pairs, and they have additive or cumulative effect. Such genes are called polygenes or multiple factors or cumulative gene. Eg. Human skin colour.

> (2 Marks) Fertilisation in a plant or animal by the fusion of male and female gametes produced by the same individual
(or)
> (3 Marks) Fertilisation that occurs when male and female gamete produced by the organism unite self fertilisation occur in many protozoans and invertebrate animal. It result from self pollination in plants. Seeds fertilization allows an isolated individual organism to reproduce but restricts the genetic diversity of a community.

The fertilisation of an organism by the fusion of an egg from one individual with a sperm or male gamete from a different individual’s is opposite to the self.
(or)
Cross fertilisation is a term used in the field of biological reproduction describing the fertilisation of an occurs from one individual with spermatozoa of another. It is also called allogamy.
(or)
The fusion of male and female gamete (sex cells) from different individual of the same species.
It is mostly occur in dieocious plant and in animal species which they are separate male and female individual.

True breeding means that the parents with also pass down a specific phenotypic trait to their offspring. True breeding organism will have a pure genotype (genetic expression of a trait) and they will produce a certain phenotype. True breed is sometime also called pure breed.

• Anthocyanin are naturally occurring pigment of red, purple and blue.
• Anthocyanin pigments are more stable at low PH (Acidic condition) which gives a red pigment. Measurable higher the PH value of anthocyanin will provide of colour fading of the colour blue or purple.

The word progeny is the progeny of the Latin verb “progignere” meaning “to beget”. In biology, offspring are the young born of living organism, produced either by a single organism or in the case of sexual reproduction, true organism. Collective offspring may be known as a brood or progeny. It is also called as offspring of animals or plants or the children and other descendants.

A cross between homozygous parent that differ in three gene pairs is called to trihybrid cross. A self fertilising trihybird plants forms 8 different gemeter and 64 different zygote. So these combination of three pair crosses operating together.
Phenotypic ratio -27:9:9:9:3:3:3:1 F 2
Mendel laws of segregation and independent assortment are also applicable to three pairs of contrasting traits ie. Trihybrid cross

The cross between the F 1 offspring with either of the two parents. The parent may be dominant or recessive
(or)
When F 1 individuals are crossed with one of the true parenst from which they were derived, then such cross is called back cross
Explanation
* When TT is crossed with tt we get Tt as F 1 generation
* TT x tt = Tt
* when Tt (F 1 ) is crossed with either TT or tt (parent) it is called a back cross.

Interactions take place between the alleles o the same gene.
alleles at the same locus is called intragenic or intralocus gene interactions.
* Incomplete dominance
* co dominance
* multiple alleles
* pleiotropic genes.

The chloroplast arid mitochondrial genes show special pattern of inheritance known as Extra chromosomal inheritance.
Chromosomal inheritance:
The other aspects are
* They have vegetative segregation involving cytoplasmic plasmagenes..
* It has uniparental inheritance (only from female parent)
* Both have reduced rate of recombinations.

* It is a genetic cross which involves individuals differing in two characters.
* Dihvbrid inheritance is the inheritance of two separate genes each with two alleles.
XI. Three marks

Mendelian experiments prove that a single gene controls one character. But in the post mendelion findings, various exception have been noticed, in which different types of interaction are possible between the genes. FTence the expression of a single character by the interaction of more one pair of genes is called genic interaction or interaction of genes. According to this hypothesis some character are produced by the interaction of two or more pairs factor (gene).

It is the major human blood group system. The ABO type of a person depends on they presence of absence of two gene, A and B. These gene determine the configuration of the red blood cell surface. A person who has two A gene or an A and O gene has bloodcells of type A. There are four main group of blood A,B,AB and O. The phenotype ratio is given below.
Blood group inheritance phenotype only

An individually ABO type results from the inheritance of 193 alleles is A,B,0 from each parent. The possible out comes are given below
Both A and B alleles are dominant over O. As a results individual who have an AO gene type will- have an A phenotype. People who are type O have OO genotype. In other words, they inherited a recessive ‘O’ allele from both parents. The A and B alleles are co-dominant. Therefore, if an A is inherited from one parent and a B from the other the phenotype will be AB.

It is closely related to incomplete dominance is co-dominance is which both alleles are simultaneously expressed in the heterozygote. In both co-dominonce and incomplete dominance both alleles for a trait are dominate in co-dominance a hetrozygous individual express both simultaneously with out any blending. People who are to type O have OO genotype. In other words they inherited a recessive O allele from both parents. The A and B alleles are co-dominant. Therefc )re is an A is inherited from one parent and a B from other the phenotype will be AB

sickle cell anemia is a disease, in which the haemoglobin protein is produced incorrectly and the red bloodcells have a sickle shape. A person that is homozygous recessive for the sickle cells traits wills have red blood cells that all have the incorrect haemoglobin.

Morphologically, physiologically and genetically similar chromosome present is a diploid cell are called homologous or homologous chromosomes. In each pair of homologous chromosomes, one chromosome maternal and the other is paternal.

homozygous:
* Organism having identical alleles for a character are homozygous.
* It is pure or true breeding
* They form only one type of gametes
* (eg) Tall (TT) dwarf (tt)
heterozygous:
* Organism having dismillar alleles for a character are heterozygous.
* It is hybrid
* They form more then one type of gametes.
* es (Tt)

Dominant character
Recessive character
1. The character that are expressed in F1 generation are dominant
The characters that are not expressed in F1 generation are recessive
2. It is expressed in presence of dominant as well as recessive allele.
Eg. Tt, TT = tall
It is expressed only when both the recessive allele of a gene are present Eg. tt – dwarf
3. In pea plants tallness and red flowers are dominant character.
In pea plant dwarf and white flowers are recessive characters.
4. Dominant character can expression in both homozygous as well as hetrozygous condition
Recessive character can be expressed only in homozygous condition

Phenotype
* It is the physical appearance of and organism
* It can be directly seen
* phenotype can be determined from genotype, (eg) Tt =Tall
Genotype
* It is the genetic constitution of an organism
* It is determined by inheritance pattern
* Genotype can not be determined from phenotype (eg) Tall can either Tt (or) TT

Monohybrid cross:
* The cross between to pure parents differing in a single pair of contrasting character is called Monohybrid cross
* Phenotypic ratio is 3:1
* Genotypic ratio is 1:2:1
* The law of segregation is explained by this method
Dihybr id cross:
* The cross between two pure parents differing in two pairs of contrasting character is called dihybrid cross
* phenotype ratio is 9:3:3:1
* genotype ratio is 1:2:2:4:1:2:1:2:1
* The law a independent is explained by this cross.

Test Cross
* The cross between F1 hybrid and its recessive parent is called test cross
* A test cross is always a back cross
* Test cross determines the genetic constitution of an organism
* Test cross produces both dominant and recessive character is equal proportion
Back Cross:
* The cross between F1 hybrid and any one of its parents (either dominant or recessive) is called back cross.
* A back cross is not always a test cross
* Back cross helps in improving and obtaining desirable character
* Back cross helps in improving and obtaining desirable character

Genetic disorders are nothing but malfunctioning of genes due to some changes in their arrangement brought by mutation. Often these disorders characterized by absence or inactive protein products.

Sudden heritable change in DNA or chromosome is called mutation. There are agents which cause mutation called Mutagens. Due to mutations many abnormalities will appear in new generations which may be useful or harmful.

* The phenomenon in which two alleles are both expressed in the heterzygous individual is known as codominance
Example:
* Red and white flowers of camellia, inheritance of sickle cell haemoglobin.
* ABO blood group system in human beings.
* In humanbeings, IA and IB alleles of I gene are codominant which follows mendels law of segregation.
* The co-dominance was demonstrated in plants with the help of electrophoresis or chromatography for protein or flavonoid substance.

In sex linked diseases the defected gene are present on the sex chromosomes attached to them whereas in sex influenced diseases defective gene are present on the other chromosome but affects the sex chromosomes.

Lethal allele are alleles that cause the death of the organism that carries them. They are unusually a result of mutation is gene that are essential to growth or development. Lethal allele may be recessive, dominant or conditional depending on the genes or genes involved.

The diseases sickle cell anemia is causes by a gene (Hbs) which is lethal in homozygous condition. But has a slight denotable effect is the heterozygous conditions, producing sickle cell trait. The homozygous for this gene (Hbs/HbS) generally die of fatal anemia. The hetrozygotes or carriers for Hbs. (ie) HbA/HbS) show signs of mild anemia as their RBC become sickle – shaped in oxygen deficiency. A marriage between two carriers, therefore results in carrier and normal offspring in the ratio 2:1

Plants that fail to produce functional pollengrains are said to be male-sterile. If the traits conditioning the sterility is not inherited according to mendelion rules, but is instead maternally transmitted, it is referred to as cytoplasmic male sterility(cms). So in this male-sterility is inherited maternally.
The gene for cytoplasmic male sterility is found in the mitochondrial DNA
(or)
When plants fails to produce functional pollengrain, they are called male sterile mole. Male sterility may be conditioned by either nuclear or Cytoplasmic genes. If the sterility trait is inherited is a non -Mandelian fashion, it is designated as cytoplasmic male sterility (CMS). Cytoplasmic gene are most often maternally transmitted in plants.

Atavism derives via French from Latin atavius, meaning “ancestor”. Avus in Latin means ‘grand father’; and its is believed that the ‘at’ is related to atta a word for “Daddy”. Atavism is a term rooted in evolutionary study referring to instances when an organism possesses trait closer to a more remote ancestor, rather than its own parents. It is modification of a biological structure whereby an ancestral traits re appears after having been lost through evolutionary changes is the previous generations.
(eg) Re-emergence of sexual reproduction in the flowering plant Hieracium pilosella is the best example for Atavism in plants

* To identify whether an organism exhibiting a dominant trait is homozygous or hesterozy- gous for a specific allele a scientist can perform a test cross.
* The organism in question can be crossed with an organism that is homozygous for the recessive trait – and the offspring’s of the test cross are examined.
XII. Five Marks

Pleiotropy is when one gene affect multiple characters eg. Marfan syndrome and polygene inheritance is when one traits is controlled by multiple genes (eg), skin colour (or) skin pigmentation

• In co-dominance neither allele is dominant over the other, so both will be expressed equally in the heterozygote.
• In incomplete dominance, there is an intermediate heterozygote. Such as pink flower when the parent phenotypes are red. and white.

Monohybrid
* It is one sided or both sided
* It is used to study the inheritance of single pair of alleles.
* It cannot distinguish between nuclear and Cytoplasmic (or) sex linked and autosomal traits
Reciprocal cross
* It is both sided cross in which female of one type is crossed with male of the second type and vice versa
* It may study inheritance of one, two or more traits
* It can distinguish between nuclear and cytoplasmic inheritance as well as sea linked l j and autosomal inheritance.

Monohybrid:
* Mono refer to single and hybrid means mixed breed
* It is used to study the inheritance of single pair of alleles.
* Genotype ratio is 1:2:1
* Phenotypic ratio is 3:1
* One pair of contrasting character are involved
Dihybrid
* Di refers to two or double and hybrid means breed.
* It is used to study the inheritance of two different alleles.
* Genotype is ratio is 1:2:1:2:4:2:1:2:1
* Phenotypic ratio is 9:3:3:1
* Two pair of contrasting character are involved.

A monohybird is a genetic cross which occurs between two individuals, focusing on the inheritance of one trait at one time. Monohybrid cross is also known as single trait cross. Two homozygous parent are selected for this cross.
Each homozygous parent in the P generation produces only one kind of gamete.
The heterozygous F] offspring produces two kinds of gamete
The heterozygous Fi offspring produces two kinds of gamete

A dihybrid cross is a genetic cross that occurs between two individuals, focusing on the inheritance of two independent traits at one time. It is also known as two trait cross.
Two parents considered for this cross have two independent traits (eg: pea colour and pea shapes of plants). Thus a dihybrid cross involves two pairs of genes. The following figure explains the process of dihybrid crossing.
F 1 phenotype: All round yellow cotyledon
Fi genotype: All RrYy
RrYy x RrYy (Fj generation selfied)
Ry Ry rY rY x Ry Ry ry ry (Haploid gametes)
How to do a Dihybrid Cross
* Analyze the data!
* Make a tally of all possible phenotypes.
In a dihybrid cross, traits are considered as not linked, and they have an equal probability of sharing up in offspring. Each pair of alleles segregates independently of the gametes. Offspring is predicted and assessed for two trait inheritance. The phenotypic ratio of the offspring generation is 9:3:3:1 in a dihybrid cross.

A trihybrid cross is between two individuals that are homozygous for three different traits. (Eg: Pea shape, colour and pea shape)
(or)
A cross between homozygous parents that differ in three gene pairs, (ie: producing trihybrid) is called trihybrid cross. A seed fertilizing trihybrid plant forms 8 different gametes and 64 different zygotes. So a combination of three single pair crosses operating together. The three contrasting characters of a trihybrid crosses are
* F 2 Phenotypic ratio – 27:9:9:9:3:3:3:1
* 27 – round, green, smooth pod
* 9 – round, green, constructed pod
* 9 – round, yellow, smooth pod
* 9 – wrinkled, green, smooth pod
* 3 – round, yellow, constructed pod
* 3 – wrinkled, green, constructed pod
* 3 – wrinkled, yellow, smooth pod
* 1- wrinkled, yellow, constructed pod

A trait controlled by one gene but multiple allele is blood type. There are four phenotypes A, B, AB, O. Type A and B are co-dominant and ‘O’ is recessive to A and B. None are dominant. Some traits are controlled by a single gene with two alleles. Mendelian heredity had only two alternative expression or alleles. However many genes can change in several different ways or changes. Those changes give rise to several alternative states which are called multiple alleles.
(or)
Blood type is an example of a common multiple allele trait. There are three different alleles for blood type A, B & O. A is dominant to O, B is also dominant to O. A and B are both co-dominant.

• Carl Correns’s (1905) experimented in 40′ clock plant, Mirabilis jalapn.
• When the pure breeding homozygous red (R 1 R 1 ) parent is crossed with homozygous white (R 1 R 1 )
• The phenotype of the F 1 hybrid is heterozygous pink (R 2 R 2 )
• The F1 heterozygous phenotype differs from both the parental homozygous phenotype.
• This cross did not exhibit the character of the dominant parent but has an intermediate colour pink.
• The phenotypic and genotypic ratios were found to be same as 1:2:1 (1 red: 2 pink: 1 white). Genotypic ratio is 1 R 1 R 1: 2 R 1 R 2: 1 R 2 R 2 in F 2 interbreed.
• In the F2 generation, R 1 and R 2 genes segregate and recombine to produce red, pink and white in the ratio of 1:2:1.
• R 1 allele codes for an enzyme responsible for the formation of red pigment and R 2 allele codes for defective enzyme. R 1 and R 2 genotypes produce only enough red pigments to make the flower pink.
• Mendel’s particulate inheritance takes place in this cross which is confirmed by the reappearance of original phenoty in F 2

Allele that cause an organism to die are called lethal alleles or lethal genes. Lethal genes are usually a result of mutations in genes that are essential to the growth or development. Lethal gene can cause death of an organism prenatally or anytime after birth. Lethal genes are first discovered by Lucien cuenot in the study of coat colour in mice.

Epistasis is a type of polygenic interaction where one gene controls the phenotype of another gene for a trait. Both genes have an influence on the physical appearance of the traits, but the one that shows epistasis masks the effect of the other. Eg: albinism.
Dominant epistatsis: It happens when the dominant allele of one gene masks the expression of all allele of another gene.
Recessive Epistasis:
Recessive epistasis is when the recessive allele of one gene in a homozygous slate masks the phenotypic expression of the dominant allele of another gene.
(eg) Mice,
In Mice, body Colour is determined by a gene A. A is hypostatic to an allele C of another gene, which mean that C marks the expression of A. C is the presence of a gives cinnamon mice, While C in the prsence of A gives agouti mice.
Recessive Epistasis:
*
In dominant epistasis, the majority of the individuals are affected. There is a 12:3:1 ration.
Genes that show recessive epistasis can only mask a phenotype if two alleles are present The ratio is 9:3:4

If both gene loci have homozygous recessive alleles and both of them produce identical phenotype the F2 ratio 9:3:3:1 would be 9:7. The genotype aaBB, aaBb, AAbb, Aabb and aabb produce same phenotype. Both dominant alleles when are present together only than they can complement each other. This is known as complementary gene.
Complementary Genes (9:7)
Ex: In Lathyrusodoratus,Bateson and punnet crossed two varieties(CCpp x ccPP),each with white flowers.
* Eg: complete dominance at both gene pairs, but either recessive homozygote is epistatic to the effect of the other gene.
* In sweet pea flower colour.
* Gene pair A = purple dominant over white
* Gene pair B = colour dominant over white
* Interaction = Homozygous recessive of either gene A or B produce white

* Certain phenotype traits despond on the dominant alleles of two gene loci. When dominant is present it will share its phenotype. The ratio will be 9:6;1 Eg: Fruit shape in summer squash.
* Complete dominance at both gene pair, interaction between, both dominance to give new phenotype.
* Gene pair ‘A’ sphere shape dominant over long.
* Gene pair ‘B’ sphere shape dominant over long.
* So interaction at ‘AB’ when present together form disc shaped fruit.
* Finally disc shaped fruit 9/16 Sphere shaped fruit 6/16
Duplicate genes with cumulative effect (9:6:1):

* If a dominant allele of both gene low produces the same phenotype without cumulative effect i.e., independently the ratio will be 15:1
* Eg: seed capsule of shephered’s purse complete dominance at both gene pair, but either gene when dominant, epistatic to the other.
* Gene pair ‘A’=Triangular shape dominant over ovoid
Gene pair ‘B’=Triangular shape dominant over ovoid (double recessive)
Duplicate dominant genes (15:1):
15/16 = Triangular
1/16= Ovoid (top shaped)
AABB x aabb
Triangular ovoid
AaBb x AaBb
Triangular

Sometimes the dominant alleles of one gene locus (A) in homozygous and heterozygous (AA, Aa) condition and homozygous recessive alleles bb of another locus (B) produces the same phenotype. The F2 ratio will become 13:3. The genotype AABB, AaBB, AAbb, Aabb and aabb produce one type of phenotype and genotype aaBb, aaBB, will produce another type of phenotype.
* Eg: Feather colour of Fowl
* Complete dominance at both gene pair, but are gene when dominant epistatic to the other and the second gene when homozygous recessive epistatic to the first.
* Gene ‘A’ colour inhibition is dominant to colour appearance.
* Gene ‘B’ colour in dominant to white.
Interaction:
* Dominant colour inhibitors prevents colour even when colour is present, colour gene, when homozygous recessive prevents colour when dominant inhibitor is present.
Dominant and recessive interaction (13:3):
13/16 = white
3/16 = coloured

Male sterility found in pearl maize (sorgum Vulgare) is the best example for mitochondria cytoplasmic inheritance. so it is called cytoplasmic male sterility.
In this, male sterility is inherited maternally.
The gene for cytoplasmic male sterility is found in the mitochondrial DNA.
In this plant there are two types, one with normal cytoplasm (N) which is male fertile and the other one with aberrant cytoplasm (s) which is male sterile.
These types also exhibit reciprocal differences as found in Mirabilis jalapa
Recently it has been discovered that cytoplasmic genetic male sterility is common in many plant species.
This sterility maintained by the influence of both nuclear and cytoplasmic genes.
There are commonly two types of cytoplasm N (Normal) and S (Sterile)
The genes for these are found in mitochondrian.
There are also restores of fertility (Rf) genes. Even though these genes are nuclear genes, they are distinct from genetic male sterility genes of other plants.
Because the Rf genes do not have any expression of their own, unless the sterile cytoplasm is present.
Rf genes are required to restore fertility in S cytoplasm which is responsible for sterility.
So the combination of N cytoplasm with rfrf and s cytoplasm with RfRf products plants with fertile pollens, while S cytoplasm with rfrf produces only male sterile plants.