NCERT Solutions for Class 12 Science Biology Chapter 5 - Principles Of Inheritance And Variation

Answer:

The genus located on the same chromosomes form one linkage group. It is defined as all the genes located on a single chromosome moves as a unit at the time of all division. Genes do not tend to separate until the crossing over occurs and if the distance between the two genus is very less, crossing over does not takes place.

Hence, the correct answer is option c.

Answer:

Aneuploidy is caused due to the failure of segregation of chromatids during cell division results in gain or loss of chromosomes.

For example:-
Down's syndrome us caused due to the gain in one extra copy of chromosomes 21.
Turner syndrome is caused due to the loss of an X-chromosome in human females.

Hence, the correct answer is option a.

Answer:

Linkage is the a term used to describe physical association of genes on a chromosome. While recombination is a term used to describe the generation of non-parental gene combinations. The distance between the two genes has direct relationship with the recombination percentage as with the increase in distance between the genes, the recombination percentage increases.

Hence, the correct answer is option a.

Answer:

In a sex-linked recessive disease, the genetic disorder is passed on from unaffected carrier female to the male progeny. For example, haemophilia, in this genetic disorder, an heterozygous carrier female for haemophilia can transmit the disease to their sons.

Hence, the correct answer is option d.

Answer:

The triples GUG codes for the aminoacid valine. Sickle cell anaemia is a genetic defect caused due to the substitution of glutamic acid (Glu) by Valine (Val) at the sixth position of the beta-globin chain of the haemoglobin molecule. The substitution of amino acid in the globin protein results due to the single base substitution at the sixth codon of the beta globin gene from GAG TO GUG.

Hence, the correct answer is option d.

Answer:

When the alleles I^A and I^B are present together, the both express their own types of sugars because of co-dominance. This is expressed as blood group AB.

Hence, the correct answer is option b.

Answer:

In birds, two different types of gametes in terms of sex chromosomes are produced by females and is called female heterogamety. The two different sex chromosomes of a female bird is designated as ZW. While the male birds have a pair of ZZ chromosomes.

Hence, the correct answer is option d.

Answer:

When a cross is made between two heterozygous tall plants(Tt), the offsprings produced are tall and dwarf. This can be best explained by a genetic cross:-

Figure

Tall: Dwarf

Hence, the correct answer is option b.

Answer:

Mendel Law of Independent Assortment states that "when two pairs of traits are combined in a hybrid, the segregation of one pair of characters is independent of the other pair of characters". The ratio of 9 : 3 : 3 : 1 in a dihybrid cross denotes that the alleles of two genes are segregating independently.

Hence, the correct answer is option d.

Answer:

Linkage is a term used to describe the physical association of genes on a chromosome. The linked genes occurs on the same chromosome and they are transmitted together. In this process, there will be a tendency to maintain the parental gene combinations except in the case of occasional crossovers. So, Linkage is a phenomenon that will not result in variations among siblings.

Hence, the correct answer is option c.

Answer:

Mendel’s Law of Independent Assortment states that 'when two pairs of traits are combined in a hybrid, the segregation of one pair of characters us independent of the other pair of characters'.

Hence, the correct answer is option .

Answer:

Pleiotropic gene are single gene that can exhibit multiple phenotyphic expression and this phenomenon is called pleiotropy. For example : phenylketonuria in humans. This disease is caused due to mutation in a gene that codes for phenylalanine hydroxylase enzyme (single gene mutation). The effect represents itself through phenotypic expressions which is characterised by mental retardation and reduction in hair and skin pigmentation.

Hence, the correct answer is option c.

Answer:

XQ- XX type of sex determination mechanism is seen in insects. Males produce two different types of gametes, (a) either with or without chromosomes or (b) some gametes with X-chromosomes and some with Y-chromosomes. In this type of sex determination, mechanism, males have only 18 chromosomes with autosomes while females have a pair of X-chromosomes with autosomes.
Males have 17 chromosomes while females have 18 chromosomes.

Hence, the correct answer is option a.

Answer:

Pedigree analysis is the inheritance of a particular trait is represented in a family tree over generations. This acts as a strong tool and is utilised to trace the inheritance of a specific trait, abnormality or diseases. So, this indicates that Mendelian principles are applicable to human genetics and the character studied in the pedigree analysis is equivalent to mendelian traits.

Hence, the correct answer is option b.

Answer:

It was observed by the Mendel's law that when two pairs of different characters are analyzed in a dihybrid cross. The behaviour of a specific character is independent of another character.

Hence, the correct answer is option b.

Answer:

Figure
 

Answer:

In a dihybrid cross, the number of phenotypes are 4 (round yellow, round green, wrinkled yellow, wrinkled green) while the number of genotypes are 9.

Hence, the correct answer is option d.

Answer:

Figure

It is evident from the above-mentioned cross that the genotypes of mother and father are I^AI^O and I^BI^O.

Hence, the correct answer is option c.

Answer:

A cross is made between the F1 progeny and homozygous recessive parent is called test cross. This cross is used you the determination of the genotype of an unknown dominant parents.

Answer:

Yes, the Mendel's law of inheritance would have been different it the characters were located on the same chromosome because they would not assort independently and linkage percentage depends on the distance between the two genes.

Answer:

Steps involved in cross-pollination are:-
(i) Selection of parents with desired characters.
(ii) Emasculation and bagging techniques are done to protect the stigma from contamination.
Cucurbits are bisexuals plants that have both male and female flowers on the same plant. Self-pollination occurs in such plants and emasculation is required to prevent contamination.

Answer:

(i) The organisms must have easily identifiable sets of contrasting characters.
(ii) They have short life cycle so that their study can be completed within a couple of years.
(iii) Hybridization is easily induced in that organism.

Answer:

The given Pedigree chart represents autosomal recessive trait. Both the parents are carriers (heterozygous condition), so one male and female offspring are suffering from genetic disorder while the other two are either carrier or normal.

Answer:

To obtain the F₁ generation, Mendel pollinated a pure-breeding tall plant (TT) with a pure-breeding dwarf plant (tt) and the progenies obtained were all tall.
Figure
The progeny obtained in F₁ generation are heterozygous and allow self-pollination to raise F₂ offspring. 50% genotype of the offspring would resemble one parent and the rest to the other parent. He also wants to understand the inheritance of the selected trait over several generations.

Answer:

Figure
The genes found in an organism express a specific trait by forming a certain products and this can be achieved by the process of transcription and translation (Central dogma).

Answer:

Alleles are the polymorphs and have different nucleotide sequences results in different phenotypic expression. They are the alternative forms of a same gene.
For example: Genes that codes for height have  2 allele:- one for dwarfness (tt) and one for tallness(TT)
Significance:
They are responsible for variation in the population, as a character.

Answer:

Genetic cross:-
Figure

Selfing:-
Figure

3 : 1
In this cross the upper case letters are used to denote dominant characteristics while the lower case letters are used to denote the recessive characteristics of the same alleles. It shows a monohybrid cross with 3 : 1 in the F₂ generation.
It indicates that the parents must be true-breed and parents are diploid as well as homologous chromosomes carry alliles with same type. They are represented by RR and rr.

Answer:

Yes, genes are not only responsible for the determination of phenotype. As, environment is also responsible for the expression of traits. They are active throughout the lives, switching their expressions on and off according to the change in environmental condition. Apart from internal factors such as hormones and metabolism affects the gene expression, external factors such as temperature, light, nutrition and so on also affects the gene expression and exhibit phenotypic changes.

Answer:

The genotypes of offspring produced are:-
 

Answer:

Human have 23 pairs of chromosomes and out of them, 22 pairs are autosomes. They are same in both male and female. Males have one X and Y chromosomes (sex chromosome) while females have two X chromosomes. When the ovum fertilises with the sperm carrying X-chromosome, the zygote develops into a female (XX). When the ovum fertises with the sperm carrying Y-chromosome, the zygote develops into a male (XY). It is evident that father is responsible for the sex of child and there is always 50 percent probability of either a male or female child.

Answer:

The shape of pea seeds (round and wrinkled) are controlled by two alleles (Rr). R is dominant allele for round seed shape while r is recessive allele for wrinkled seed shape. The phenotype for wrinkled seed shape is shown when genotype contains both the recessive alleles and is represented by rr.

Answer:

Most of the organisms are diploid in nature and the alleles are present only in pairs. So, apart from multiple allelism, an individual will have only two alleles because individual develops from a zygote. It is formed by the fusion of male and female gametes. So, the egg and sperm contain one gene for each trait and the zygote formed is diploid as they have two alleles for each trait.
For example:- ABO blood grouping in humans as they have I^A, and I^B i alleles but any given blood group has only two alleles, like AB blood group (I^AI^B) or O blood group (ii).

Answer:

There are several chemical and physical factors that can induce mutations and they are called mutagens. Mutagens induce mutation by altering the sequence of nitrogenous base pair of DNA during the process of DNA replication. Ethylmethanesulfonate (EMS) is a chemical mutagen that can induce mutation in the genome of fly.

Answer:

The plants produced in a F₁ generation, are when allowed to reproduce without a genetic cross, the alleles segregate. In this, one alleles goes to one parent while the another allele goes to one another parent and this process is similar to the transfer of halved the number of chromosome during meiosis. The process of segregation is a random process and there will be 50%, chance of a specific allele to become either male gamete our female gamete and because of this, the genotypes produced by F₂ generation is same as that with the phenotype produced in the same generation. Hence, the plants produced with pure genotypes and those produced with mixed genotypes will be the
same (50%).

Answer:

If the parents have blood group I^A I^O and I^B I^O (heterozygous condition) then their will be a possibility for a child born having O blood group. It can be best explained with the help of a given genetic cross.

Figure

It is evident from the above-mentioned cross that if the genotypes of the parents is I^A I^O and I^B I^O, the child born have O blood group.

Answer:

Down syndrome is a genetic disorder caused due to the presence of an additional copy of chromosome number 21 (trisomy of 21 chromosome) This is first described by Langdon Down in 1866.
Symptoms:
The affected individuals are short statured with small round head, furrowed tongue and partially open mouth. Their palm is broad with characteristics palm crease, physical, psychomotor and mental development is retarded.
With the increase in age of females above 40, the chances of having a child with Down's syndrome increases because ova or egg cells are present in females since birth and older eggs cells are more prone to chromosomal non-disjunction due to several physical and chemical factors.

Answer:

Walter Sutton and Theodore Boveri observed that the behaviour of chromosomes was parallel to the behaviour of genes. They have studied the behaviour of chromosomes during mitosis and meiosis. The chromosomes and genes occurs in pairs from this study they have concluded that the genes are located on chromosome.

Answer:

When a plant with red flowers and crossed with plant having yellow flowers, orange flowers are produced in the F₁ generation. This type of inheritance is called incomplete dominance.
In this type of inheritance, the phenotype produced during F₁ generation did not resemble either of the two parents and was in between the two.
This can be explained with the help of a genetic cross:-
Figure

Answer:

The characteristics of true-breeding lines are:-
(i) They must be self-pollinating for several generations.
(ii) They must be able to stable trait inheritance for several generations.
(iii) They must show stable expression of characters for several generations.

Answer:

Figure

Generation:-

Answer:

Colour blindness is a sex-linked inheritance. This disorder is caused due to the defect in either red or green cones of the eye results in failure to discriminate between red and green colour. It occurs commonly in males because the genes responsible for red-green colour blindness are located on X-chromosome and males only have one X-chromosome while female have two X-chromosome. A female will be colourblind only when her mother is carrier or colourblind and father is colourblind.

Answer:

Colour blindness is a sex-linked inheritance and genes responsible for the red-green colour blindness are located on X-chromosomes.
The mother is carrier for colour blindness and may passed the genes to her son. As sex-linked inheritance is not transferred from father to son. In this inheritance, the transmission of disease occurs from unaffected carrier female to male progeny.
This can be explained with the help of a given cross:-

Figure

Figeneration

Answer:

Drosophila has been used for genetic studies because
(i) They can be grown on simple synthetic medium in the laboratory.
(ii) They complete their life cycle into two weeks.
(iii) A single mating could produce a large number of progeny flies.
(iv) Male and female flies are easily distinguishable.

Answer:

The similarities between genes and chromosomes are as follows:-
(i) Both genes and chromosomes occurs in pair.
(ii) They Segregate during gamete formation as only one, of each pair is transmitted into the gametes.
(iii) The independent pairs segregate independently of each other in them.

Answer:

Recombination is a process that involves the generation of new combination of genes. It is different from the parental types.
Recombination occurs due to crossing over that takes place at the time of meiosis prior to gamete formation.

Applications of recombinations:-
(i) This process helps in the formation of new combinations of genes and traits.
(ii) It also increases the variability among population that is useful for the process of natural selection under changed environmental condition.
(iii) The frequency of crossing over depends upon the distance between the two genes and this phenomenon is used for the preparation of linkage chromosome maps.

Answer:

Artificial selection is the identification and selection of desirable traits in plants and animals by humans to produce desirable traits.
Yes, the process of artificial selection, affects the process of national selection.
The process of natural selection involves the selection for or against the trait on the basis of their effect on the fitness of the organism.
Natural selection results in the evolution any change in the expression of the trait in the population.
Artificial selection is a faster process but cause threat on the diversity.

Answer:

Answer:

Sickle cell anaemia is an autosome linked recessive trait. This can be transmitted from parents to the offspring when both the parents are carriers for the gene (heterozygous condition). The disease is controlled by a single pair of a allele Hb^A and Hb⁵. Homozygous individuals such as Hb⁵ Hb⁵ show the diseased phenotype. While heterozygous individuals are unaffected but they are carrier of the disease. People having sickle cell anemia mutation is protected against malaria which may be the reason of persistance of sickle cell anemia gene in the population.

Answer:

A cross between a tall plant with red flowers (TTRR)  and a dwarf plant with white flowers (ttrr). All the plants produce in F₁ generation will be tall and produce red flowers (TtRr)
But when the F₁ plants are self-pollinated, the phenotype of F₂ plants can be represented by punnett square.

Answer:

a. Humans have 23 pairs of chromosomes. Out of them, 22 pairs are autosomes while 23rd chromosome is sex chromosome. Females have two X chromosome and they are homogametic while males have X and Y chromosome and they are heterogametic.
In case of birds, Males have a pair of Z-chromosomes while females have one Z and W chromosome. Female birds produce two different types of gametes i.e., female heterogamety.
b. In humans, the sex of the child is determined by what they inherit from their father. As males have X and Y sex chromosomes while females have two pair of X chromosomes. All children will inherit an X-chromosome from their mother. A child who inherits an X-chromosome from their father will be a girl and one who inherits an Y-chromosome from their father will be a boy.
In some reptiles, for example: Crocodile, temperature is required for the incubation of fertilized eggs helps in the determination of sex of the animals whether it will be male or female. When the crocodile eggs are incubated at higher temperature, led to the birth of male crocodiles.

Answer:

Fig.
Colourblindness is a sex-linked recessive disorder caused due to defect in either red or green cone of the eye. This results in the failure to discriminate between red and green colour. The genes responsible for this defect are located on the X-chromosome. So males have one one chromosome while females have two X chromosome.
In this case, female is carrier as her father is colourblind marries a normal man. So, there will be a probability of having one colourblind son, one carrier daughter, 2 normal offsprings (one son and one daughter.)

Answer:

Morgan carried out dihybrid cross in Drosophilla to study that genes were sex-linked. For this, he hybridised yellow-bodied, white-eyed female to brown-bodied, red-eyed males. He intercrossed their F₁ progeny. He observed that the two genes did not segregate independently to each other and the F₂ ratio deviated from the 9 : 3 : 3 : 1 ratio.
He knew that the genes were located on X-chromosome and noticed that when two genes in a dihybrid cross were situated on the same chromosome, the proportion of parental gene combinations were higher than the non-parental type. He attributed this because of the physical association or linkage of the two genes. He coined a term linkage in order to describe the physical associations of genes on a chromosome. While the term recombination is used to describe the formation of non-parental gene combinations.
Morgan and his colleagues also observed that even when the genes were grouped on the same chromosome, some were very tightly linked and show low recombination. Whereas other genes were loosely linked and show higher recombination.
Alfred Sturtevant was the student of Morgan used the frequency of recombination between gene pairs located on the same chromosome as a measure of the distance between genes and 'mapped' their position on the chromosome.

Answer:

Aneuoploidy: It is an chromosomal abnormality caused due to the failure of segregation of chromatids during cell division that results in gain or loss of a chromosomes.
Polyploidy: It is an chromosomal abnormality that occurs due to failure of cytokinesis after the telophase stage of cell division. It results in an increase in an increase in a whole set of chromosomes in an organism. This condition only occurs in plants.
a. Trisomy of 21st chromosome:
Down syndrome is an genetic disorder caused due to the presence of an additional copy of chromosome number 21 or trisomy of 21 chromosome.
b. XXY: Klinefelter's syndrome is a genetic disorder caused due to the presence of an additional copy of X-chromosome results into a karyotype of 47 + XXY. This occurs in males.
c. XO: Turner's syndrome is a genetic disorder caused due to the absence of one x chromosomes (45 + XO). This occurs in females.