2nd PUC Biology Important Questions Chapter 5 Principles of Inheritance and Variation

Karnataka 2nd PUC Biology Important Questions Chapter 5 Principles of Inheritance and Variation

Question 1.
Define test cross?
Answer:
Test cross is a cross between F; hybrid and its recessive parent.

Question 2.
What are alleles? .
Answer:
Genes which code for a pair of contrasting traits are termed as alleles.

Question 3.
Mention the phenotypic ratio of Monohybrid cross?
Answer:
3:1

Question 4.
Mention the phenotypic ratio of dihybrid cross?
Answer:
9:3:3:1

Question 5.
State the law of dominance?
Answer:
It states that in a dissimilar pair of factors one member of the pair dominates the other.

Question 6.
Write any three reasons for selection of pea plants by Mendel for his hybridization experiment?
Answer:

1. It is a true-breeding plant.
2. It is a self-pollinated crop plant.
3. It has a short life span.

Question 7.
Give the schematic representation of one gene inheritance with respect to law of dominance?
Answer:
It is a cross between two parents which differ from one another with respect to single pair of contrasting characters or single character. Mendel took height of pea plant as an example and made a cross between a pure or homozygous tall pea plant and a pure or homozygous dwarf pea plant. In the generation, all plants obtained were tall since T is dominant over t (dwarfness).

When F₁ plants were self-pollinated then in the F2 generation phenotypic ratio obtained was 3:1 and genotypic ratio 1:2:1.
Parents Pure Tall Pure Dwarf
TT tt -P₁

Gametes (T)(T) (t)(t)
Self-pollination Tt × Tt.

3:1 Phenotypic ratio
ITT Pure or homozygous tall.
2Tt Hybrid tall ltt Homozygous dwarf
Phenotypic ration = 3:1
Genotypic ratio = 1:2:1

Question 8.
Describe Monohybrid cross with an example?
Answer:
It is a cross between two parents which differ from one another with respect to single pair of contrasting characters or single character.

Mendel took height of pea plant as an example and made a cross between a pure or homozygous tall pea plant and a pure or homozygous dwarf pea plant.

In the F₁ generation all plants obtained were tall since T is dominant over t (dwarfness)

When F₁ Plants were self pollinated then in the F₂ generation phenotypic ratio obtained was 3:1 and genotypic ratio 1:2:1.
Parents Pure Tall Pure Dwarf
TT tt P₁
Gametes (T)(T) (t)(t)
Self pollination Tt X Tt.

Question 9.
State and explain the law of independent assortment with the help of inheritance of 2 genes?
Answer:
When two or more than two alleles are brought together in F₁ hybrids they assort independently of one another in F₁ generation during the formation of gametes.

When a plant producing round and yellow coloured seeds is crossed with a plant producing Wrinkled and green seeds all the F, hybrids produce round and yellow coloured seeds as Yellow (Y) is dominant over green (y) and allele for round (R) is dominant over wrinkled (r).

When the F, hybrids are self crossed to obtain F₂ generation all four types of plants are obtained such as round yellow (9), round and green (3), wrinkled and Yellow’ (3), Wrinkle and green (1) in the ratio of 9:3:3:1. Genotypic ratio is 1: 2: 2: 4: 1: 2: 1: 2: 1
(Phenotypic ratio) Parental at generation.

Phenotype Round and Yellow X Wrinkled and Green
Genotype RRYY rrYY
Gametes RY rY
RrYy F₁ Generation.
All offsprings are found and yellow
RrYy X RrYy

Round and yellow – 9, Round and Green – 3 Wrinkled and yellow – 3 Wrinkled and Green – 1

Test cross (Dihybrid):
When the F₁ hybrid obtained by crossing two parents differing in 2 characters are again crossed with double recessive parent it is test cross

Genotype RrYy X rryy
Gametes RYRy ry
rY ry

Round and yellow RrYy- 4 Round and Green Rryy – 4 Wrinkled and Yellow rryy – 4 Wrinkled and green rryy – 4. Test cross-ratio. Phenotypic and Genotypic ) = 4: 4: 4: 4 or 1: 1: 1: 1.

Question 10.
Explain the following terms with an example?
Answer:
(a) Codominance: When both the alleles are equally dominant it is termed as codominance.
AB blood group consists of two alleles IA and IB and they are both present together. Both express their own types of sugar because of codominance.

(b) Incomplete dominance: When the dominant gene is not able to mask the effect of recessive gene completely it results in blending of characters. This is termed as incomplete dominance.

When a homozygous red flowered plant is crossed with homozygous white flowered plant then in F, generation all flowers are pink.

When F, plants are self pollinated then in F2 generation both phenotypic and genotypic ratios obtained are 1:2:1.
RR homozygous red X rr – P,
Gametes (R) (P) (r) (r)
Rr = F₁
Pink.

Phonotypic and genotypic ration = 1:2:1
1- Pure Red, 2 – Pink, 1- Pure white.

Question 11.
Briefly mention the contribution of T.H Morgan in genetics?
Answer:
(a) They could be grown easily in the laboratory.
(b) They have a short life span
(c) A single mating can produce a large members of progenies.
(d) They exhibit sexual dimorphism i.e., clear differentiation between male and female sexes.

Question 12.
Who had proposed the chromosomal theory of inheritance?
Answer:
This theory was proposed by waiter Sutton and Theodore Bovery in 1902.
(a) A child has blood group O if the father has blood group A and mother blood group B, work out the genotypes of parents and genotypes of other offsprings.
Father × Mother
A B
Genotype of father I^A I^A I^B I^B
I^A I^B AB

Heterozygous state
Father I^Ai × I^Bi
Mother. I^AI^B, I^Ai, I^Bi, ii O
A B
Other offsprings can have genotype AB, A B, other than O.

Question 13.
How is sex determined in human beings?
Answer:
In humans, XX-Xy type of Sex determination is found. Females are homogametic with XX chromosome. Ma- leas are heterogametic as 50% of gametes carry the ‘Y’ chromosome and other 50% of gametes carry Y chromosome.

Parents Female Male
AA+XX AA+XY
Gametes A+X A+X A+Y
Offspring AA+XX AA+XY
Female Male.

Question 14.
What is male heterogamety? Give an example.
Answer:
In humans XX-XY type of sex determination is found. Females are homogametic with XX chromosome. Males are heterogametic as 50% of gametes carry ‘ Y’ chromosome and other 50% of gametes carry X chromosome.

Question 15.
What is pointmutation? Give on example?
Answer:
When mutation arises due to change in a single pair of DNA it is termed as point mutation Ex;- Sickle cell anaemia, Genetic disorders.

Pedigree analysis: Analysis of traits in several of generations of a family is called pedigree analysis. In human . genetics pedigree study provides a strong tool which is utilized to trace the inheritance of a specific trait, abnormality or disease. Some of the standard symbols used in pedigree analysis are as follows Image

Broadly genetic disorders are of 2 types.

1. Mendelian disorders.
2. Chromosomal disorders.

Mendelian disorders are mainly determined by alteration or mutation in a single gene. They are Haemophilia, cystic fibrosis, sickle celled anemia, color blindness, phenylketonuria. By pedigree analysis, one can understand whether the trait is dominant or recessive.

X-linked recessive gene shows transmission from carrier female to male progeny.

Haemophilia: It is also called bleeder’s disease. It is a sex linked recessive disease which shows transmission from female which is involved in clotting of blood.
In this particular disease a simple cut will result in continuous oozing of blood. The heterozygous female carrier will transmit thetdisease to her sons.
Female becoming haemophiliac is rare Queen victoria was carrier of the disease.

Sickle celled: This is a autosome-linked recessive trait transmitted from parents to the offspring when both partners are carrier for the gene or heterozygous. The disease is controlled by a single pair of alleles HbA and Hbs. Only homozygous individuals show the diseased phenotype. The defect is caused by the substitution of glutamic acid by valine at the sixth position of B globin chain of the haemoglobin molecule.
The mutant hemoglobin molecule undergoes polymerization under low O₂, tension causing change in the shape of RBC from biconcave disc to elongated sickle like structure.

Phenylketonuria: This error of metabolism is also due to autosomal recessive trait. The affected individual lacks an enzyme that convents amino acid phenylalanine to tyrosine. As a result phenyl alanine is accumulated and converted into phenyl pyruvic acid and other derivatives accumulation of these in brain results in mental retardation. They are excreted through urine because of poor absorption by kidney.

Chromosomal disorders: The chromosomal disorders are caused due to absence or excess or abnormal arrangement of one or more chromosomes.
Aneuploidy is the failure of segregation of chromatids during cell division cycle resulting in the gain or loss of a chromosome.

1. Down’s syndrome: This is caused due to the presence of an additional copy of chromosome no. 21 (Trisomy of 21). This disorder was described by Langdori down. 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, mental development is retarded.
2. Klinefelter’s syndrome: It is due to the presence of an additional copy of X chromosome resulting into karyotype of 47, XXY. Such an individual has overall masculine development (development of breast, i.e. Gynaecomastia is also expressed. Such individuals are sterile.
3. Turner’s Syndrome: This is caused due to absence of one of the X chromosome i.e 45 with XO, such females are sterile as ovaries are rudimentary besides other features including lack of secondary sexual charaters.

Question 16.
Which amino acid replaces glutamine at the 6th position of beta chain of haemoglobin?
Answer:
Valine.

Question 17.
Explain: (1) Down’s syndrome, (2) Klinefleter’s?
Answer:
1. Down’s syndrome: This is caused due to the presence of an additional copy of chromosome no. 21 (Trisomy
of 21). This disorder was described by Langdori down. 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, mental development is retarded.

2. Klinefelter’s syndrome: It is due to the presence of an additional copy of X chromosome resulting into karyotype of 47, XXY. Such an individual has overall masculine development (development of breast, i.e. Gynaecomastia is also expressed. Such individuals are sterile.

Question 18.
What is pedigree analysis?
Answer:

Question 19.
Write short note a linkage and recombination.
Answer:

• Physical association of genes on a chromosome is termed as linkage.
• The more closely the genes are found on chromosames greater is the linkage less closely the genes are found on chromosome lesser is the linkage.
• Original needed (O.S of Human Health and dieases strategies in improvement of crop).

Question 20.
Define the terms linkage and recombination.
Answer:
Physical association of genes on a chromosomes is termed as linkage.

The more closely the genes are found on chromosomes greater is the linkage less closely the genes are found on chromosome lesser is thf linkage.

Original needed (O.S of Human Health and dieases strategies in improvement of crop).
Recombination describes non parental gene combination.

→ Principles of Inheritance of Variation:

• Genetics is a branch of biology that deals with inheritance as well as variation of characters from parents to offspring.
• Gregor Mendel is also known as the father of genetics. He conducted hybridization experiments in garden pea plants (possum sativum).

He selected pea as his experimented plants because.

1. It is a true-breeding plant.
2. It is a self-pollinated crop plant.
3. It has several contrasting forms.
4. It has a short life span.

Pea plant exists in several contrasting states and they are as follows.

→ Some Important Terminologies in Genetics
(a) Gene – Gene is a basic unit of heredity.
(b) Allele – Genes that code for a pair of contrasting traits are known as alleles.
(c) Homozygous – When identical genes control a pair of characters it is termed as homozygous.
Ex: With respect to height of plant pure tall is represented as TT and pure dwarf TT.
(d) Heterozygous – When unidentical genes control a pair of contrasting characters it is heterozygous EX: Tt.
(e) Phenotype – The external morphological appearance of a character is termed as phenotype.
(f) Genotype – The genetic make up or genetic constitution of a particular individual is genotype.
(g) Test cross is a cross between Fj hybrid and its recessive parent.
(h) Pack cross is a cross between Fj hybrid with any one of its parents.
(i) Dominant – In a pair of dissimilar factor one dominates the other as in Fj it is called as dominant factor.
(j) Recessive – In a pair of dissimilar factor the factor which remains suppressed it termed as recessive.
(k) Co-dominant alleles,: When two different alleles are together both the alleles express their character side by side.

→ Monohybrid Cross:

• It is a cross made between two parents with respect to a single character.
• When a tall plant is crossed with a dwarf plant, all the F₁ hybrids are tall because tall is dominant.
• When the F₁ hybrids are self crossed in the F₂ generation there are 3/4th (75%) of tall plants and 1 /4th (25%) of the dwarf plants in the ration of 3:1 which is the phenotypic ratio.
• The genotypic ratio is 1:2:1 where 1 plant is pure or homozygous tall, 2 plants are heterozygous tall and 1 is pure dwarf. Parent Generation (P) Homozygous (Pure) × Homozygous (Pure)
• Tail Pea plant Dwarf pea plant

Image

→ Phenotypic Ratio:
3 Tall Plants: 1 Dwarf plant
Genotypic ration
1 Pure tall: 2 heterozygous tall: 1 pure dwarf.

Mendel Gave 2 laws.

1. Law of Dominance.
2. Law of Segregation.

→ Law of Dominance:

1. Characters are controlled by discrete units called factors.
2. Factors occur in pairs.
3. In a dissimilar pair of factors, one member of the pair dominates (dominant) the other (recessive).

Law of Segregation: This law is based on the fact that alleles do not show any blending both the characters are recovered as such in the F₂ generation.
The factors or alleles of a pair segregate from each other such that a gamete receives only one of the two factors.

Test Cross: It is the cross made between the F₁ hybrid, and its recessive parent to confirm whether the hybrid is homozygous or heterozygous.

When the test cross is conducted 50% of the plants are tall and other 50% of the plants are dwarf in the ration of 1:1 this confirms that F₁ hybrids are heterozygous.

Tall Dwarf
(Tt) 1:1 (tt)

→ Significance of Test Cross:
1. It helps to know whether the organism is homozygous or heterozygous for a trait.
Law of Independent Assortment: When two pairs of traits are combined in a hybrid segregation of one pair of characters is independent of other pair of characters.

→ Dihybrid Cross:
When a plant producing round and yellow colored seeds is crossed with a plant producing wrinkled and green seeds all the F, hybrids are round and yellow only. This indicates that alleles for yellow (Y) is dominant over green (y) and allele for round (R) is dominant over wrinkled.
When the hybrids are self ’crossed to obtain F₂ generation all four types of plants are obtained such as round yellow (9), and green (3), wrinkle and Yellow (3), Wrinkle and green (1) in the ratio of
9:3:3:1. Genotypic ratio is 1: 2: 2: 4: 1: 2: 1: 2: 1 (Phenotypic ratio) Parent at generation.

Phenotype Round and Yellow X Wrinkled and Green
Genotype RRYY rrYY
Gametes RY rY
RrYy F₁ Generation.
All off springs are found and yellow

RrYy X RrYy

Round and yellow – 9, Round and Green – 3 Wrinkled and yellow – 3 Wrinkled and Green – 1
Phenotypic ratio: 9:3:3:1
Genotypic ratio: – 1:2:2:4:1:2:1:2:1

→ Phenotypic Ratio:
ITT Pure or homozygous tall
2Tt Hybrid tall
1tt Homozygous dwarf
Phenotypic ration = 3:1
Genotypic ratio = 1:2:1

→ Deviations From Mendelian Principle Incomplete Dominance:
When the dominant gene is not completely dominant over the recessive gene it results in production of intermediate character or blended inheritance. Dog flower, or Antirrhinum is an example of incomplete dominance. When the red-colored flower were crossed with the white colored flower it resulted in production of pink colored flowers in the F₁ generation.

When F₁ plants were self-pollinated than in F₂ generation both phenotypic and genotypic ratios obtained are 1:2:1.
Parents RR homozygous red X rr – P,
Gametes (R) (P) (r) (r)
Rr = F1
Pink.

Phonotypic and genotypic ration =1:2:1
1- Pure Red, 2 – Pink, 1- Pure white.

→ Codominance:
When the alleles for 2 contrasting characters are brought together in heterozygous conditions both alleles express their character together and it is termed codominance.
Ex: ABO blood group in man

ABO blood group is controlled by gene 1. The plasma membrane of red blood cells has sugar polymers that protrude from its surface and the kind of sugar is controlled by the gene.

Gene I has 3 alleles I^A, I^B, and i. The alleles I^A and I^B produce a different type of sugar while it does not produce any sugar. When IA and IB are both present they express their own types of sugars because of codominance Hence red blood cells have both A and B types of sugars.

Multiple alleles – More than 2 i.e., the alleles govern the same character it is termed as Multiple alleles.

A single gene product may produce more than one effect. For example starch synthesized in pea seeds is controlled by one gene which has two alleles (B and b). Starch is synthesized effectively by homozygotes BB and therefore large starch grains are produced. On the other hand, bb homozygotes have lesser efficiency in starch synthesis and produce smaller starch grains. After maturation of the seeds BB seeds are round bb are wrinkled. Starch grains produced is of intermediate size in Bb seeds.

→ Chromosomal Theory of Inheritance:

1. This theory was proposed by Walter Sutton and Theodore Bovery in 1902.
2. They worked out the chromosome movement during meiosis.
3. Chromosome movement was parallel to the behavior of genes and he used chromosome movements to explain Mendel’s Law.
4. Sutton and Boveri argued that the pairing and separation of a pair of homologous chromosomes would lead to the segregation of a pair of factors’ they carried.

Thomas Hunt Morgan and his colleagues gave the experimental clarification of the chromosomal theory of inheritance. Morgan worked with tiny fruit flies drosophila melanogaster. He selected Drosophila for his experiments as:

1. They complete their life cycle in about two weeks.
2. A single mating could produce a large number of progeny flies.
3. Clear cut differentiation of sexes – male and female flies are easily distinguishable.
4. It shows lot of varations.

→ Linkage and Recombination:
Morgan carried out several dihybrid crosses in Drosophila to study sex linked genes. Morgan hybridized yellow bodied, white eyed females to brown bodied red eyed males and inter crossed their progeny he observed that the two genes did not segregate independently of each other and the F₂ ratio deviated significantly from the 9:3:3:1 ratio.

Morgan and his group knew that the genes were located on the X chromosome and they saw that when the two genes in the dihybrid cross are situated on the same chromosome the proportion of parental gene combinations were much higher than the non-parental type.

The term linkage was coined to describe the physical association of genes on a chromosome.

Recombination describes non-parental gene combinations. Morgan and his group found that when genes were grouped on the same chromosome some were tightly linked (i.e. they showed low recombination while others were loosely linked i.e. showed high recombination.

When genes of white and yellow were tightly linked they showed only 1.3% recombination while white and miniature showed 372% recombination.

→ Sex Determination:
Chromosomal mechanism of sex determination can be traced to some experiments carried out in insects. Henking (1891) found out a specific nuclear structure in spermato genesis in a fe insects.

50% of sperm received this structure after spermatogenesis where as the other 50% of sperm did not receive at. Henking gave the term ‘X’ body to it X body was later termed as X chromosome.

→ Grass Hopper XO Sex Determination:
Here males have only one X chromosome besides the auto somes where as females have a pair of X chromosomes. Xx – Xy type of sex determination is found in insects and man. Among males an X chromosome is but its counterpart is distinctly smaller and callec Y chromosome. Females have a pair of X chromosomes. B oth males and females bear same member of autosomes plusXX.

In human beings also XX – Xy type is found. Male heterogamety is observed where in some gametes have only X chromosome and some have Y chromosome.
XX XY Parents
Gametes (x) (x) (x) (y)
(xx) (xy)

→ Female off spring Male offspring:
In humans out of 23 pairs of chromosomes present 22 pairs are exactly same in both males and females and are autosomes.

A pair of X chromosmes are present in the female where as presence of X and Y chromosome are determinant of male characteristics.

During spermatogenesis among males, two types of gametes are produced 50% of the total sperm produced . carry X chromosome where as rest 50% has Y chromosome besides the autosomes.

Females however produce only one type of ovum with an X chromosome. In case the ovum fertilizes with a – sperm carrying X chromosome the zygote develops into a female (XX) and the fertrilization of ovum with Y chromosome carrying sperm results in male offspring.

Thus it is evident that it is the genetic make up of sperm that determines the sex of the child.

It is thus unfortunate in our society women are blamed for giving birth to female child.

Mutation: It is a phenomenon which results in alteration of DNA sequences and results in changes in the genotype and phenotype of the organism. Mutation also arises due to change in a single base pair of DNA. This is known as point mutation. A classic example of such mutation is sickle celled anaemia.

Deletions and insertion of base pairs of DNA causes frame shift mutation.
Sex determination in Birds.
Here it is ZZ-ZW type.
Here females have two sex chromosome one is of Z type and another is of W type males have a pair of Z chromosomes besides autosomes.

→ Sex Determination in Honeybee:

1. The female is diploid having 32 chromosomes.
2. Male is haploid having 16 chromosomes.
3. An offspring formed from fertilization of a sperm and egg develops into either queen or worker which are fe-male.
4. An infertilized egg develops into a male (drone) i.e. by means of parthenogenesis.
5. This method of sex determination is called haplodiploid sex determination.
6. Male produce sperms by mitosis and gemale produce egg by meioses.
7. Hence male honey bee donot have father and thus cannot have sons but have grandson.

Parents Female × Male
32 × 16
Meiosis Mitosis
16 16 16
16 32
Male Female.

2nd PUC Biology Important Questions with Answers