Biology 5 Marks Questions

TH Morgan's contribution in genetics are:

• Morgan contributed to the understanding of sex-linked traits.
• He carried out several experiments on fruit flies and his dihybrid crosses revealed that genes for some traits did not segregate independently and the F2 ratio was a lot different from 9 : 3 : 3 : 1.
• He inferred that when two genes were situated on the same chromosome, the proportions of parental gene combinations were significantly higher than the non-parental type.
• He established the principle of linkage crossing over, sex-linked inheritance and discovered the relation.
• He established the technique of chromosome mapping.
• He observed and worked on mutation.

Humans have 47wQbNPTDJp9hMYdvogK2hAUiHsGeiybwaWe36bwtRQ3UTpYV7YuZ8FV5j9nauFCWwcjM6dTzpL5s2N79Rp5unwdMvc8ZKU and the remaining 1 pair is the sex chromosome that determines the sex of the child. In humans sex chromosome is of two types X and Y. The genetic composition of females is 44+XX whereas males have a genetic composition of 47wQbNPTDJp9hMYdvogK2hAUiHsGeiybwaWe36bwtRQ3UTpYV7YuZ8FV5j9nauFCWwcjM6dTzpL5s2N79Rp5unwdMvc8ZKUchromosomes from one parent and the other half from another the diploid state is restored. Now, the gametes formed by the female is all 22+X where as males produce two types of gametes, 22+X and 22+Y. During fertilization if the female gamete with 22+X chromosome is fertilized by the sperm bearing 22+X chromosome the resulting zygote has the genetic composition as 44+XX thus giving rise to a female child. But, if the female gamete with 22+X chromosome is fertilized by the sperm bearing 22+Y chromosome the resulting zygote has the genetic composition as 47wQbNPTDJp9hMYdvogK2hAUiHsGeiybwaWe36bwtRQ3UTpYV7YuZ8FV5j9nauFCWwcjM6dTzpL5s2N79Rp5unwdMvc8ZKUsperm that fertilizes the ovum and not the other way round.

1.  

1. If the plants is homozygous for the dominant trait.

2. If the plants is heterozygous for the dominant trait.

Conclusion: If all progeny show axial flowers ( dominant) the plant is homozygous (AA) , If 50% of Progeny show Axial flower (Dominant) and 50% Terminal flower ( Recessive) the plant is heterozygous.

2. Law of Segregation, allelic pair segregate (separates) during gamete formation. (do not loose their identity)

 

1. Pleiotropy: A single gene that has an effect on the expression of two or more phonotypic traits is said to have a pleiotropic effect on the traits. For example, testosterone controls the development of what are referred to as secondary sexual characteristics (e.g., a hon’s mane), but testosterone also relates behavioral traits like aggraession/Thus, a gene that controls the level of testosterone would have a pleiotropic iffect on the expression of many secondary sexual traitjs which are morphological, as well as behavioral. The concept of pleiotropy is intimately related to the concept of trade-off (Steams- 1976). Pleiotropy describes the proximate genetic gene controls the expression of two or more traits and those traits are related to a fitness trade-off, then we have identified the proximate source of the trade-off.

Polygenic: If two or more genes are responsible for a single trait, the phenotypic.

2. If the gene for eye color is on the X chromosome, the red eyed male in the second cross will pass his red eyed X to only his daughters, who in turn received only a recessive white- carrying X from their mother. Thus all females had trend eyes like their father. Since the male fruit fly passes only the Y to his sons, their eye color is determined entirely by the single X chromosome they receive from their mother (in this case white). Thus all the males in the second cross were white eyed.

These experiments introduced the concept of sex-linkage, the occurrence of genes on that part of the X that lack a corresponding location on the Y. Sex- linked recessives (such as white eyes in fruit flies, hemophilia, baldness, and colorblindness in humans) occur more commonly in males, since there is no chance of them being heterozygous. Such condition is termedhemizygous.

Law of Independent Assortment- It states that when two pairs of traits are combgined
in a hybrid, seggregation of one pair of character is independent of the other pair of characters.

1. When two pairs of traits are combined in a hybrid, segregation of one pair of character is independent of the other pair of characters.
2. Both parental type and recombinant types are observed to show that genes for the colour and genes for the shape of seeds segregate independently during gamete formation.

1. It could be homozygous dominant, heterozygous dominant.
2. By test cross.

Thalassemia- Anaemia.
Haemophilia- Non stop bleeding.
Haemophilia- Sex linked recessive disorder, is generally passed on from (carrier) mother to some of her sons/from affected father to daughter. (carrier)


Thalassemia- Autosome linked recessive blood disease , inheritance is like Mendelian inheritance pattern.

According to Morgan and his group if genes were very tightly linked they showed very low recombination.
(shown in cross A)
If genes were loosely linked they showed very high recombination.
(shown in cross B)
The group used the frequency of recombination between gene pairs on the same chromosome as a measure of distance between genes and ‘mapped’ their position on the chromosome.

1. Cause- sex-linked recessive disorder.

Symptoms- failure to discriminate between red and green colour.

2. Since males have only one X chromosome gene for colour blindness, if present in any one parent will always be expressed, whereas in female it will be expressed only if it is present on both the X chromosomes, when both parents are carrying gene for colour blindness.

Law of Independent Assortment: when two pair of traits are combined in a hybrid, inheritance of one pair of characters is independent of the other pair of characters/when two pairs of contrasting characters or genes or traits are inherited together in a dihybrid cross (in a pea plant ) the inheritance of one pair of character is independent of inheritance of the other character in the progeny
Explanation: Mendel took homozygous pea plant producing yellow and round seeds and crossed them with homozygous pea plant producing green and wrinkled seeds/shown in a flow chart of a dihybrid cross given.

Phenotypes	–	Yellow round	:	Yellow wrinkled	:	Green round	:	Green wrinkled
Phenotype ratio	–	9	:	3	:	3	:	1

(Formation of new phenotypes along with parental phenotypes is possible because inheritance of two pairs of contrasting traits or genes in the progeny is independent of each other)

1. Thalassemia is a group of autosomal recessive blood disorders characterised by sever anaemia as a result of production of faulty haemoglobin chains. Mutations in the genes coding for the alpha, beta or delta chains constituting haemoglobin lead to the synthesis of improperly folded haemoglobin that is incapable of transporting oxygen efficiently.
2. Thalassemia is an autosomal recessive disease, which means the mutation is carried on one of the autosomes, so the carrier can be any one of two parents. It has an equal probability of coming from the mother or the father, so to just blame the mother for the child’s abnormality is unjustified.
3. The values counselling can propagate in the families are-

1. Give healthy diet to the child.
2. Accepting their child with all his/her positives and negatives.
3. Neither of the parents is responsible for giving birth to a sick baby.
4. The defect is caused by a random change in the genes of child.
5. Encouraging the child to follow his/her treatment regularly and lead a happy and normal Life.
6. Support the child emotionally by taking about fear, anxiety and depression or stress the child feels.

A single gene controls the size of the starch grains and the seed shape.(Deviation fromMendelian Law of Dominance): The trait of size of starch grain shows incomplete dominance. Hence in heterozygous condition the starch grain are of intermediate size. The trait of seed shape follows Law of Dominance and the hybrid will show only dominant trait.

1. Drosophila melanogaster:

Morgan carried out several dihybrid crosses in Drosophila to study gens that were sex-lined. Morgan and his group knew that the genes were located on the X chromosome and saw quickly that when the two genes in a dihybrid cross were situated on the same chromosome, the proportion of parental gene combinations were much higher than the non-parental type. Morgan attributed this due to the physical association or linkage of the two genes and coined the term linkage to describe this physical association of genes on a chromosome and the term recombination to describe the generation of non-parental gene combination. Morgan and his group also found that even when genes were grouped on the same chromosome, some genes were very tightly linked (showed very low recombination) while others were loosely linked.

2. Morgans student Alfred Sturtevant used the frequency of recombination between gene pairs on the same chromosome as a measure of the distance between genes and ‘mapped’ their position on the chromosome. Today genetic maps are extensively used as a starting point in the sequencing of whole genomes.

If parents are heterozygous then child can have O blood group but if parents are homozygous then child cannot have O blood group.

Down’s syndrome is a chromosomal disorder. A person suffering from Down’s syndrome shows following symptoms:

• Short stature.
• Small round head.
• Furrowed tongue and partially open mouth.
• Palm is broad with characteristic palm crease.
• Physical, psychomotor and mental development is retarded in the person.

This condition happens because of an extra copy of chromosome 21. As per various research reports, chances of having a child with Down’s syndrome increases with advancing maternal age because ova are present in females right from their birth. More is the age of the mother, cells will be more older. Hence, chances of chromosomal non-disjunction will be more because of various physico-chemical exposures during the mother’s life-time.

1. Thalassemia is a blood disorder passed down through families (inherited) in which the body makes an abnormal form or inadequate amount of hemoglobin. Hemoglobin is the protein in red blood cells that carries oxygen.
2. Mother cannot be blamed for the disease because it is an autosomal recessive blood disorder. The genes for the synthesis of globin chains are present on one of the autosomes. So, the carrier can be any one of the two parents. It has equal probability of coming from the mother or father. So, to blame the mother for the child's abnormality is unjustified.
3. Values that can be propogated in families are:

1. One should be aware of such autosomal diseases and should get themselves checked for the same before marriage.
2. Give a healthy diet plan to child.
3. Neither of the parents is responsible for giving birth to a sick baby.
4. Accepting their child with all his/ her positives and negatives.

Morgan worked with the tiny fruit files, Drosophila melanogaster, which were found very suitable for such studies, as:

1. They could be grown on simple synthetic medium (ripe banana) in the laboratory.
2. They complete their life cycle in about two weeks.
3. A single mating could produce a large number of progeny flies.
4. There was a clear differentiation of the sexes – the male and female flies are easily distinguisable.
5. It has many types of hereditary variations that can be seen with low power microscopes.

The plants must be crossed with a white flowered (homozygous recessive) plant, i.e. a test cross is conducted.
Garden Pea:

1. If the plant is homozygous dominant, all the plants in the progeny would bear red flowers (heterozygous) and if the plant is heterozygous, the progeny would consist of red-flowered plants and white-flowered plants in the ratio of 1 : 1.

1. Homozygous Red:

2. Heterozygous Red:

Red flowered : White flowered, 1 : 1.
Snapdragon:

1. In snapdragon, when a red-flowered plant is crossed to a white-flowered plant, the progeny would consist of pink-flowered plants.
2. So, red-flowered plants are homozygous dominant, while heterozygous condition produces pink-flowered plants.
3. The cross is as follows:

4. It is because of incomplete dominance, where neither of the two alleles is completely dominant over the other and the hybrid is intermediate between the two i.e. pink-flowered.

Inheritance pattern of flower colour in garden pea follows principle of dominance whereas inheritance in snapdragon shows incomplete dominance.

Phenotypic ratio–3 : 1 Genotypic ratio–1 : 2 : 1. Inheritance of flower colour in snapdragon:

Phenotypic ratio–1 : 2 : 1 Genotypic ratio–1 : 2 : 1.

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.

Chromosomal Theory of Inheritance:
The chromosomal theory of inheritance was proposed independently by Walter Sutton and Theodore Boveri in 1902. According to this theory,

1. Since the sperm and egg cells provide the only bridge from one generation to the other, all hereditary characters must be carried in them. ii. The hereditary factors are carried in the nucleus.
2. Like the Mendelian alleles, chromosomes are also found in pairs.
3. The sperm and egg having haploid sets of chromosomes fuse to re-establish the diploid state.
4. The genes are located on the 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.
5. 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.

Both are caused due to alteration or mutation, in a single gene and follow Mendelian pattern of inheritance.
Symptoms:
Thalassemia: Anaemia (caused due to defective/ abnormal Hb).

Haemophilia: Non–stop bleeding even in minor injury.
Pattern of inheritance:

Thalassemia: Autosomal recessive inheritance pattern inherited from eterozygous/ parent carrier.

Haemophilia: X–linked recessive inheritance inherited from a haemophilic father/carrier mother (females are rarely haemophilic).



Thalassemia is an autosome–linked recessive blood disease. Its inheritance is like Mendelian inheritance pattern.

Law of Segregation is universally accepted. It states that allelic pair segregate (separates) during gamete formation.

Results of a dihybrid cross where the two parents differed in two pairs of contrasting traits: seed colour and seed shape.

Law of Dominance: In a contrasting pair of factors, one member of the pair dominates (dominant) the other (recessive).
Law of Segregation: Factors or allele of pair separate from each other such that gamete receives only one of the two factors.

Down’s syndrome: The individuals have overall masculine development but they express feminine development like development of east, i.e., gynaecomastia. They are sterile.
Klinefelter’s syndrome: The females are sterile as ovaries are rudimentary. Other secondary sexual characters are also lacking.

1. Symptoms of Haemophilia:

• Since a protein necessary for blood clotting is not formed, the blood does not clot and there is non-stop bleeding in case of an injury in the affected individual.

2. Symptoms of sickle-cell anaemia:

• The RBCs become sickle-shaped.
• The oxygen transport to the tissues is impaired.

3. Differences:

S.No	Haemophilia	Sickle-cell anaemia
1.	It is due to a defective recessive allele present on the X-chromosome, i.e. it is a sex-linked disorder.	It is due to point mutation, i.e. a single base pair change leading to a change in an amino acid, i.e. it is an autosomal disorder.
2.	More males than females are affected.	Both males and females are affected equally.
3.	The female parent passes on the disorder to male progeny, but father never passes it on to the male progeny.	The female parent passes on the disorder to male or female progeny in equal frequency and father also passes on the disorder to male and female progeny.

Sex Determination in Humans:

• Humans show XY type of sex determining mechanism.
• Out of 23 pair of chromosomes, 22 are autosomes (same in both males and females).
• Females have a pair of X–chromosomes.
• Males have an X and a Y chromosome.
• During spermatogenesis males produce two types of gametes with equal probability – sperm carrying either X or Y chromosome.
• During oogenesis females produce only one types of gamete – having X chromosome.
• 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).

These are disorders or illness caused by one or more abnormlities in the autosomes or sex chromosomes of the person. Thus, referred to as autosomal disorders or sex-linked disorders, respectively.
The generic disorders are divided into:

Differentiate between Mendelian and Chormosomal disorders are:

S.No.	Mendelian disorders	Chormosomal disorders
(i)	The disorders are caused by allelic abnormalities.	The disorders are caused by chromosomal abnormalities
(ii)	Allelic abnormalities develop due to mutations.	Chromosomal abnormalities develop due to defective synapsis and disjunction.
(iii)	The defect can be predicted through pedigree analysis.	The defect can be known through amniocentesis.
(iv)	The disorder is transmitted to the progeny.	The disorder is rarely transmitted.
(v)	Examples: Haemophilia, Cystic fibrosis, Colour blindness, Sickle-cell anaemia, Thalassemia, Phenylketonuria, etc.	Examples: Down's syndrome, Turner's syndrome, Klinefelter's syndrome, etc.

Law of Segregation:

1. law of segregation states that the members of the allelic pair that remained together in the parent/ hybrid, segregate during gamete formation and enter different gametes.
2. As a result, gametes have only one allele for a trait and are pure for a character.

3. The phenotypic ratio is 3 tall plants : 1 dwarf plant.
4. The genotypic ratio is 1TT : 2Tt : 1tt
5. In this case, tallness is dominant and dwarfness is recessive.
6. The F, hybrid is tall (dominant character).
7. The recessive character, dwarfness, remains hidden in the F, but reappeares in the F, generation without any change.
8. This is because the factors Tandt remained together in the hybrid, but segregated during gamete formation and entered different gametes.

1. 
2. Mendel’s Law of Independent Assortment: This law states that the different factors or allelomorphic pair in gametes assort themselves and segregate independently of one another.

• The phenotypic ratio of violet flowers : white flowers is 3 : 1.

2.  

1. It is a monohybrid cross.
2.  

• Law of dominance states that dominance is a phenomenon, in which one allele of a gene expresses itself and suppresses the expression of the other allele of the same gene, when they are present together in a hybrid.
• Law of segregation states that the two factors of a trait that remained together in the hybrid segregate during gametogenesis and enter different gametes.

Law of Independent Assortment: This law states that the different factors or allelomorphic pair in gametes and zygotes assort themselves and segregate independently of one another.

1. Mendelian disorders are mainly determined by the alteration or mutation in the single gene. These mutations are transmitted to the offspring. On the other hand, Chromosomal disorders are caused due to absence or excess of one or more chromosomes.
2. The two chromosomal aberrations that are most commonly seen are Down's syndrome (trisomy of 21st chromosome) and Turner's syndrome (absence of one of the X chromosome i.e. 45 + X0).
3. The characteristics of the two chromosomal disorders are as follows:

1. Characteristics of Down's syndrome are furrowed tongue, partially opened mouth, broad palm with characteristic palm creases, short-statured, small round head physical, psychomotor and mental development is retarded.
2. Characteristics of Turner's syndrome are sterile or rudimentary ovary, lack of secondary sexual characteristics. It occurs only in females.

The number of different types of gametes and their genotypes are:

1. Aa : 2 (A, a).
2. AABB : 1 (AB).
3. AaBb : 4 (AB, Ab, aB, ab).
4. EeCc : 4 (EC, Ec, eC, ec).
5. FFIIJJ : 2 (FIJ, FIj).

The alleles are present on homologous chromosomes.
The recessive allele does not code for its product or codes for a defective product.
The other allele remains normal and thus expresses itself.

1. Short stature/ small round head/ furrowed tongue/ partially open mouth/ mental development retarded.
2. Both.
3. Klinefelter’s syndrome.
4. Male.
5. 1. Sterile ovaries;
   2. Lack of secondary sexual characters.
6. Female.

1. The given tall pea plant with yellow seeds need to be crossed with a dwarf plant with green seeds:

1. The dominant traits are tallness and yellow colour of seeds. The recessive traits are dwarfness and green colour of seeds.
2. Cross between tall plant/ yellow seeds and dwarf plant/green seeds. Cross showing heterozygous nature for both traits.

In this cross, the F-generation shows four phenotypes in the ratio of 1 : 1 : 1 : 1. So, the given plant is heterozygous for both the traits.

2.  

A - Dominant trait.

B - Dominance.

C - Phenotype obtained in between of two parental traits.

S.No	Column I	S.No	Column II
A.	XO	3.	Male grasshopper
B.	XX	4.	Female Drosophila
C.	XY	1.	Human male
D.	ZZ	4.	Female bird

Human height or skin colour are examples of polygenic inheritance.
Height trait is controlled by at least three gene pairs.
Additive effect allele contributes to the phenotypic expression of the trait.
The dominant alleles more are, more pronounced is the phenotypic expression or more in other word.
The recessive alleles less pronounced is the phenotypic expression.

In pleiotropy a single gene can exhibit multiple phenotypic expressions. In phenylketonuria single mutated gene express multiple phenotypic expression like mental retardation and reduction in hair and skin pigmentation.

Thalassemia is categorised as Mendelian disorder because these are caused due to alteration or mutation in single gene and follow the Mendel’s principles of inheritance.
It occurs due to either mutation or deletion resulting in reduced rate of synthesis of one of globin chains of haemoglobin.

Both are caused due to mutation or alteration in a single gene, and follow Mendelian inheritance, therefore, they are called Mendelian disorders.
Symptoms of colour blindness: Unable to discriminate between red and green colours.
Symptoms of thalassemia: Formation of abnormal haemoglobin resulting in Anaemia.

S. No.	Male heterogamety	Female heterogamety
(i)	Males produce two types of gametes.	Females produce two types of gametes
(ii)	Example, male grasshopper produce gametes of two types–X and O.	Example, female birds produce gametes of two types–Z and W.

1. Law of Independent Assortment: This law states that when two pairs of traits are combined in hybrid, the factors of every character segregate independently of the factors of other pair of characters.
2.  

Embryo has (trisomy of sex chromosome) XXY karyotype or Klinefelter’s syndrome.
She was advised to undergo MTP since the child will have the following problems:

1. Male with feminine traits.
2. Gynaecomastia.
3. Underdeveloped testes.
4. Sterile.

From the above cross law of independent assortment can be derived which states that when two pairs of traits are combined in a hybrid, segregation of one pair of character is independent of the other pair of characters.

1. 
2. Both the phenotypic and genotypic ratio are same, i.e., 1: 1 : 1 : 1.
3. This cross is known as test cross.

Dominance: It is a phenomenon in which when two contrasting alleles are present together, only one expresses itself and is called dominant whereas the other which does not express itself is called recessive.
Co–dominance: It is a phenomenon in which when two contrasting alleles are present together, both of the alleles express themselves.

S.No	Column I	S.No	Column II
A.	ABO blood group in humans	5.	Multiple allelism
B.	Flower colour in snapdragon	4.	Incomplete dominance
C.	Human skin colour	1.	Polygenic inheritance
D.	Phenylketonuria	2.	Mendelian genetic disorder

All daughters are normal visioned and 50% of sons are likely to be colour blind.

Law of Independent Assortment: When two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of character.

Two chromosomal aberration–associated disorders are Down’s syndrome and Klinefelter’s syndrome.

1. Klinefelter's syndrome It occurs due to trisomy of sex chromosome.
2. She was advised to undergo MTP since, the child will have the following problems.

1. Male with feminine traits, i.e. gynaecomastia.
2. Underdeveloped testes leading to sterility.

3. Yes, this test is actually meant to determine the genetic defects or metabolic disorders in foetus by doing a chromosomal analysis. In such extreme cases that could be incurable, a decision to abort the foetus could be taken. But, amniocentesis is misused now a days.

It is used to determine the sex of the foetus and in many cases it leads to female foeticide. It has became so serious that it disturbed the male-female ratio, that can have a negative impact on society. So, it should be banned.

Yes, the ratio will deviate if the two genes are interacting with each other. In this condition the genes do not independently assort with each other, so, ratio will be changed from 9 : 3 : 3 : 1.

1. Pleiotropy:

• In this, a single gene controls more than one phenotype.
• The F, resembles one of the parents, i.e., dominant parent.
• There is no gradation in the phenotype.

2. Dominance:

• It is the phenomenon in which one allele of a gene expresses itself and suppresses the expression of the other (recessive) allele of the same gene, when they are present together in a hybrid
• The hybrid resembles one of the parents.

1. Codominance:

• It is the phenomenon in which two alleles of a gene are equally dominant and express themselves in the presence of the other.
• The hybrid shows characters of both the parents.

2. Incomplete dominance:

• It is the phenomenon in which neither of the two alleles of a gene is completely dominant over the other.
• The hybrid is intermediate between the two parents.

Mendelian Inheritance	Polygenic inheritance	Pleiotropy
One gene controls one trait/ character/ phenotype.	Two or more genes influence the expression of one trait/ character/ phenotype.	One genes controls the expression of more than one trait/character/ phenotype.

S. No.	Mendelian disorder	Chromosomal disorder
(i)	This disorder is mainly due to alteration or mutation in the single gene.	This disorder is caused due to absence or excess or abnormal arrangement of one or more chromosomes.
(ii)	This follows Mendel’s principles of inheritance.	This does not follow Mendel’s principles ofinheritance.
(iii)	This may be recessive or dominant in nature.	This is always dominant in nature.
(iv)	For example, haemophilia, sickle–cell anaemia.	For example, Turner’s syndrome.

Two mendelian disorder are:

1. Sickle cell disease is a genetic disease which is caused due to molecular mutation of gene Hb on chromosome 11 which produces the beta chain of adult hemoglobin. The mutated gene Hbs produces sickle cell haemoglobin.The 6th amino acid in the beta chain of normal haemoglobin is glutamic acid, in sickle cell haemoglobin this amino acid is replaced by valine. The children that are homozygous produce rigid chains. When oxygen level of the blood drops below a certain level, RBC undergoes sickling. Such cells do not transport oxygen efficiently, they are removed by spleen causing severe anaemia. Sickle cell is an autosomal linked disease in which both male and female progenies can be equally affected.
2. Haemophilia is a sex-linked disorder. In haemophilia, the blood fails to clot when exposed to air and even a small skin injury results in continuous bleeding and can lead to death from the loss of blood. The recessive X linked gene for haemophilia shows characteristic criss-cross inheritance. It's a single gene in male results and disease haemophilia, where is a woman needs two such genes for the same.

Alfred Sturtevant used the frequency of recombination between gene pairs on the same chromosome as a measure of the distance between genes and mapped their position on the chromosome.

If the plant is homozygous for the dominant trait.

Conclusion: If all progeny show axial flowers (dominant) the plant is homozygous (AA), If 50% of progeny show axial flower (Dominant) and 50% terminal flower (Recessive) the plant is heterozygous.

1. Dihybrid cross:

• Since white flowers have appeared in the progeny, both the parents must be heterozygous for flower colour, i.e. Vv.
• Since all plants in the progeny have axial flowers, the parent with axial flowers, must be homozygous for the trait, i.e. AA.
• Genotypes of the parents are as follows:

Terminal, violet flowers : aaVv
Axial, violet flowers : AAVv

The phenotypic ratio:
3 Axial, violet flower: 1 axial, white flowers is justified

2. Law of independent assortment: It states that when two pairs of traits are combined in a hybrid, segregation of one pair of traits is independent of the other pair of trait.

Failure of segregation of chromatids during cell division cycle results in the gain or loss of a chromosome(s), called aneuploidy. – Difference between aneuploidy and polyploidy

1. Failure of segregation of chromatids during cell division cycle results in the gain or loss of a chromosome(s), called aneuploidy. Failure of cytokinesis after telophase stage of cell division results in an increase in a whole set of chromosomes in an organism and,this phenomenon is known as polyploidy.
2. Polyploidy occurs due to altering set of chromosome number such as 2n, 3n, 5n, whereas aneuploidy occurs due to altering particular chromosome or part of a chromosome such as 2n + 1 (trisomic) and 2n – 1 (monosomic).
3. Aneuploidy can be seen in human as genetic disorders; for example, Tuner syndrome, Klinefelter syndrome and Down syndrome, whereas polyploidy is common in plants.

Down’s Syndrome (Mongolism),

• The cause of this genetic disorder-is the presence of an additional copy of the chromosome number 21 (trisomy of 21) due to non-disjunction of chromosomes during sperm or ova formation.
• The affected individual is short statured with small round head, furrowed tongue and partially open mouth. Palm is broad with characteristic palm crease.
• Physical, psychomotor and mental development is retarded.

Klinefelter’s Syndrome,

• This genetic disorder is also caused due to the presence of an additional copy of X-chromosome resulting into a karyotype of 47, XXY.
• Such an individual has overall masculine development, however, the feminine development (development of breast, i.e., Gynaecomastia) is also expressed. Such individuals are sterile male.

Turner’s Syndrome,
Such a disorder is caused due to the absence of one of the X chromosomes, i.e., 45 with XO. Such females are sterile as ovaries are rudimentary besides other features including lack of other secondary sexual characters.

1. In humans, the 23rd pair of chromosome contains and X chromosome and a Y chromosome. Hence, males are called heterogametic. Females, on the other hand, have XX chromosomes in the 23rd pair. Hence, females are called homogametic. But in bird’s females have ZW chromosome and male have ZZ Chromosome. So, in some cases, males can be homogametic and females can be heterogametic.
2. In case of humans, sex is determined by X and Y-chromosomes. Out of the 23 pairs of chromosomes in human beings, the 23rd pair is called sex chromosome while the remaining 22 pairs are called autosomes. All males have X and Y-chromosomes in the 23rd pair while females have XX-chromosomes in the 23rd pair. Thus, a sperm can have either X or Y chromosome, while all the eggs will have X-chromosome. When a sperm with X chromosome fertilizes the ovum; the zygote will result in development of a girl child. If a sperm with Y chromosome fertilizes the ovum; the zygote will result in development of a male child.

Temperature dependent sex determination is found in many animals, e.g. in crocodiles. When eggs are incubated at higher temperature, it results in birth of male crocodiles.

Morgan and his group conducted various experiments in the field of genetics.
Sturtevant was a student of Morgan. Some of the contributions by them are as follows:
Morgan carried out several dihybrid crosses of Drosophila. He observed that the phenotypic ratio was not similar to the standard phenotypic ratio as observed by Mendel. Morgan and his team were aware that the genes were located on X chromosome. Thy inferred that when the genes were situated on the same chromosome, they did not segregate independently of each other.
When the genes are situated on the same chromosome, the chances of parental combination are much higher than non-parental combination. The physical association of genes on the same chromosome was termed as linkage; by Morgan. Morgan also coined the term recombination to describe generation of non-parental combination.
Sturtevant came out with the finding that relative distance between two genes on the same chromosome was an important factor in recombination or lack of recombination. If the genes were tightly linked, they did not show recombination. But if the genes were far apart then chances of recombination were higher. Today’s genetic mapping could be developed because of contributions made by Morgan and his team.