Genetics — Some Basic Fundamentals

Mention if the following statements are True or False:

(i) Genetics and heredity are the same thing.

(ii) "Like begets like", this applies only to animals.

(iii) The entire human population shows variations.

Answer

(i) False
Corrected statement — Genetics and heredity are related but not the same thing. (Heredity means the transmission of genetically based characteristics from parents to offspring whereas Genetics is the study of heredity.)

(ii) False
Corrected statement — "Like begets like", this applies to all organisms.

(iii) True

Which of the following in humans are established genetic traits ? (Tick-mark the correct ones) in the box provided.

(i) Capacity to be a good cricketer

(ii) Curly hair

(iii) Left-handedness

(iv) Quality of voice

(v) Red-green colour blindness

Answer

Following are the established genetic traits in humans:

• Curly hair
• Left-handedness
• Red-green colour blindness

Mention the following:

(i) Total number of pairs of chromosomes in each body cell in humans ............... .

(ii) Number of pairs of autosomes in one cell of humans ............... .

Answer

(i) 23

(ii) 22

A certain couple got only four daughters in a row and no son. Does it mean that the husband does not produce Y-bearing sperms? Explain.

Answer

No, it does not mean that the husband does not produce Y-bearing sperms. During conception, half of the sperm released are X-bearing and the other half are Y-bearing. In this case, only the X-bearing sperm fertilized the wife's eggs in each instance. This can happen due to random chance, as the probability of having a male or female child is approximately 50% for each pregnancy.

Write the basic unit of heredity ?

Answer

Gene is the basic unt of heredity.

Define the following terms :-

(i) Allele

(ii) Dominant gene

(iii) Genotype

(iv) Phenotype

(v) Recessive gene

Answer

(i) Alternative forms of a gene, occupying the same position on homologous chromosomes and depicting different forms of the same character are called alleles.

(ii) The gene which can super-rule or mask the effect of its allele is called Dominant gene.

(iii) The set of genes present in the cells of an organism is called its genotype.

(iv) The observable characteristics which are genetically controlled are called as phenotype.

(v) The form of allele which remains submissive in presence of dominant form and expresses only in homozygous condition is called recessive gene.

Mention the number of paired homozygous chromosomes in

(i) Human female

(ii) Human male

Answer

(i) 23

(ii) 22

Reason — The term used in the above question should be "homologous". All the 23 pairs of chromosomes in female are homologous while in male only 22 pairs are homologous because one pair (sex chromosomes-XY) is different in size and gene sequence.

Can there be a heterozygous recessive? Explain.

Answer

No, there can never be a heterozygous recessive. In case of a heterozygous condition, there is one dominant gene and one recessive gene. The character of dominant gene will be expressed as it will mask the expression of recessive gene. Therefore, the recessive gene will always express in homozygous condition and there cannot be a heterozygous recessive.

List any four traits in humans which you can easily study just by observing and making family charts.

Answer

Eye colour (Brown/Blue), Hand (Left/Right Handedness), Earlobe (Free/Attached) and Cheeks (Dimpled/Normal).

Who discovered for the first time the basic principles of genetics?

Answer

Gregor John Mendel

Give the common and scientific names of the organism on which Gregor Mendel had worked.

Answer

Common name — Garden pea
Scientific name — Pisum sativum

Distinguish between the following pairs:

(i) Monohybrid and Dihybrid cross

(ii) Genotype and Phenotype

(iii) Character and Trait

(iv) Dominant and Recessive Allele

(v) Autosomes and Sex chromosomes

Answer

(i) Difference between Monohybrid and Dihybrid cross:

(ii) Difference between Genotype and Phenotype:

(iii) Difference between Character and Trait:

(iv) Difference between Dominant and Recessive Allele:

(v) Difference between Autosomes and Sex chromosomes:

Write two characters of pea pod with their alternative traits.

Answer

Two characters of pea pod with their alternative traits are :

Which one of the following is the phenotypic monohybrid ratio in F₂ generation?

1. 3 : 1
2. 1 : 2 : 1
3. 2 : 2
4. 1 : 3

Answer

3 : 1

Reason — The phenotypic monohybrid ratio in F₂ generation is 3:1 because 3 of the offsprings have dominant allele and one is homozygous recessive.

If a pure tall plant is crossed with a pure dwarf plant, then offspring will be

1. all tall
2. all dwarf
3. 3 tall 1 dwarf
4. 50% tall 50% dwarf

Answer

all tall

Reason — Tallness is a dominant character which will be expressed in heterozygous condition.

The 9 : 3 : 3 : 1 dihybrid ratio is due to

1. segregation
2. crossing over
3. independent assortment
4. homologous pairing

Answer

independent assortment

Reason — The 9 : 3 : 3 : 1 dihybrid ratio is due to independent assortment as the allele of one character is independent of other.

A plant with green pods and smooth seeds with genotype Ggss will give rise to the following gametes:

1. Gg and Ss
2. Gs and ss
3. Gs and gs
4. Gg and gs

Answer

Gs and gs

Reason — Only two types of gametes are possible i.e. Gs and gs, as gene 's' is in homozygous condition. This is in accordance to the law of independent assortment.

The disease colour blindness is a :

1. Recessive X-linked disease
2. Dominant Y-linked disease
3. Recessive Y-linked disease
4. Dominant X-linked disease

Answer

Recessive X-linked disease

Reason — The disease colour blindness is due to recessive gene that occurs on the X-chromosome.

The physical expression of a character is called :

1. Genotype
2. Alleles
3. Phenotype
4. Traits

Answer

Phenotype

Reason — Phenotype are the observable characteristics that are genetically controlled.

Purity of gametes is linked to :

1. Law of dominance
2. Law of independent assortment
3. Law of segregation
4. Law of limiting factor

Answer

Law of segregation

Reason — Two alleles of a character are independent and separate during formation of gametes. They do not mix with each other or lose their identity.

The phenotypic dihybrid ratio of F₂ generation is

1. 9:3:1:1
2. 9:1:3:1
3. 9:1:3:3
4. 9:3:3:1

Answer

9:3:3:1

Reason — The phenotypic dihybrid ratio of F₂ generation is 9:3:3:1 due to the law of independent assortment.

In the human male, a sperm contains autosomes and :

1. X and Y chromosomes
2. Only X chromosome
3. X or Y chromosome
4. Only Y chromosome

Answer

X or Y chromosome

Reason — In the human male, a sperm contains autosomes and either an X chromosome or a Y chromosome.

If the mother is normal and the father is haemophilic, then their two daughters will be :

1. Normal
2. Haemophilic
3. Carrier
4. None

Answer

Carrier

Reason — Since mother is normal, one of the X chromosome in daughters will be normal. Hence they will be carrier due to X chromosome received from father.

Match the terms in column I with their the explanations in column II.

Answer

Name any two genetic diseases in humans.

Answer

Colour blindness and Haemophilia are two genetic diseases in humans.

Which one of the following genotypes is homozygous dominant and which one homozygous recessive in regard to tongue rolling:
Rr, rr, RR?

Answer

Homozygous recessive – rr
Homozygous dominant - RR

Rewrite the correct form of the statement by changing the first or the last word only:

(a) Duplicated chromosomes remain attached at a point termed as centrosome.

(b) The full complement of DNA of an organism is termed as genetics.

(c) The inheritable feature of an organism is termed as heredity.

(d) Terminal flower position is a dominant trait of pea flower.

(e) Alternative forms of a gene are called traits.

Answer

(a) Duplicated chromosomes remain attached at a point termed as centromere.

(b) The full complement of DNA of an organism is termed as Genome.

(c) The inheritable feature of an organism is termed as character.

(d) Axillary flower position is a dominant trait of pea flower.

(e) Alternative forms of a gene are called Allele.

Among lion, tiger and domestic cat, all the three have the same number of 38 chromosomes, yet they have different appearances. How do you account for such differences?

Answer

All species have a fixed number of chromosomes. However, the characteristics of species including physical appearance, body functions, behavior, etc. are not simply the outcome of chromosome number, but these are the result of the units called genes which the chromosomes carry. The lion and the cat have the same number of chromosomes (38). Yet one is distinct from the other in body size, appearance, colour, behavior, etc. All such characteristics of an organism are the result of the genes located on the chromosomes.

List any three features of garden pea with their dominant and recessive traits.

Answer

Explain why generally only the male child suffers from colour blindness and not the female?

Answer

Colour blindness is caused due to recessive genes, which occur on the ‘x’ chromosomes. Males have only one X chromosome. If there is recessive gene present on X chromosome, then the male will suffer from colour-blindness. Females have two X chromosomes. It is highly impossible that both the X chromosomes carry abnormal gene. Hence, if one gene is abnormal and since it is recessive, its expression will be masked by the normal gene present on the other X chromosome. Due to this reason, females are unlikely to suffer from colour-blindness.

Define the following terms:

(a) Pedigree chart

(b) Variations

(c) Mutation

Answer

(a) Pedigree chart — A pedigree chart is a diagram that shows the occurrence and appearance or phenotypes of a particular gene or organism and its ancestors from one generation to the next. In the pedigree chart, males are shown by squares and females by circles.

(b) Variations — The small differences among the individuals of the same species are called variations.

(c) Mutation — Mutation is a sudden change in one or more genes, or in the number or in the structure of chromosomes. Mutation alters the hereditary material of an organism's cells and results in a change in certain characters or traits for example :

1. Sickle cell anaemia
2. Radioactive radiations

State the three Mendel's laws of inheritance.

Answer

Mendel's law of inheritance are as follows —

1. Law of Dominance — Out of a pair of contrasting characters present together, only one is able to express itself while the other remains suppressed. The one that expresses is the dominant character and the one that is unexpressed is the recessive one.

2. Law of Segregation (also called the law of purity of gametes) — The two members of a pair of factors separate during the formation of gametes. They do not blend but segregate or separate into different gametes. The gametes combine together by random fusion at the time of zygote formation.

3. Law of Independent Assortment — When there are two pairs of contrasting characters, the distribution of the members of one pair into the gametes is independent of the distribution of the other pair.

Does the sex of the child depend on the father or is it just a matter of chance? Discuss.

Answer

The sex of the child depends upon the kind of sperm that fertilises the egg. The egg contains only one X chromosome, but half of the sperms released into the genital tract of the female during coitus are X-bearing and the remaining half are Y-bearing. It is simply a matter of chance as to which category of sperm fuses with the ovum:

• If the egg (X) is fused by X-bearing sperm, the resulting combination is XX i.e. female constitution and the child produced is a female (Daughter).
• If the egg (X) is fused by Y-bearing sperm, the resulting combination is XY i.e. male constitution and the child produced is a male (Son).

Distinguish between the following pairs:

(a) Karyotype and Karyokinesis

(b) Autosomes and Sex chromosomes

(c) Homozygous and Heterozygous chromosomes

Answer

(a) Difference between karyotype and karyokinesis —

(b) Difference between autosomes and sex chromosomes

(c) Difference between homozygous and heterozygous chromosomes

In a certain species of animals, black fur (B) is dominant over brown fur (b). Predict the genotype and phenotype of the offspring, when both parents are 'Bb' or have heterozygous black fur.

Answer

In the question it is given that B refers to black fur and b refers to brown fur. So the genotype and phenotype of the offspring will be:

Genotype — 1 (Homozygous Black Fur) : 2 (Heterozygous Black Fur) : 1 (Homozygous Brown Fur)

Phenotype — 3 (Black Fur): 1 (Brown Fur)

Two pairs (A and B) of rabbits were crossed as given below:

(a) Can you tell which coat colour (black or white) is dominant?

(b) Is the coat colour sex-linked?

Answer

(a) Black

(b) No

Make a Punnett square and find out the genotypic and phenotypic ratios of F₁ and F₂ generations in the progeny of a genetic cross between:

(a) A pure tall (TT) pea plant with a pure dwarf (tt) pea plant.

(b) Red flower variety of pea (RR) with white flower variety of pea (rr).

Answer

(a) A pure tall (TT) pea plant with a pure dwarf (tt) pea plant:

F₂ Generation:

Genotype - 1 (Homozygous tall) : 2 (Heterozygous tall) : 1 (Homozygous dwarf)

Phenotype - 3 (Tall) : 1 (Dwarf)

(b) Red flower variety of pea (RR) with white flower variety of pea (rr):

F₂ Generation:

Genotype — 1 (Homozygous red) : 2 (Heterozygous red) : 1 (Homozygous white)

Phenotype — 3 (Red) : 1 (White)

Mendel crossed a homozygous pea plant having round seeds (RR) with a homozygous pea plant having wrinkled seeds (rr). He got different results. On the basis of it, answer the following questions:

(a) Which character of seed is studied in the experiment?

(b) Which of the above two traits is dominant ?

(c) Write the phenotype and genotype of F₁ offspring.

(d) Mention and state the Mendel’s law shown in the above cross.

(e) Make a Punnett square for F₂ generation when two plants of F₁ offspring are crossed with each other.

(f) Write the phenotypic ratio of F₂ progeny.

(g) What will be the genotypic ratio of F₂ offsprings ?

(h) What are the two traits of seed colour ? Also mention which is dominant and recessive ?

(i) Write the scientific name of garden pea.

(j) Write two main features of pea plant due to which Mendel had selected it for his hybridisation studies.

Answer

(a) Shape of seed

(b) Round seed shape is dominant.

(c) Phenotype — Round seed
Genotype — Rr

(d) Law of dominance, which states that out of a pair of contrasting characters present together, only one is able to express itself while the other remains suppressed.

(e)

(f) Phenotypic ratio — 3:1

(g) Genotypic ratio — 1:2:1

(h) The two traits of seed colour are:

1. Yellow (dominant)
2. Green (recessive)

(i) Pisum sativum

(j) The two main features due to which Mendel selected pea plants are :

1. Many varieties were available in alternative forms of a character.
2. The life span of pea plants is short and many generations can be obtained and studied in less time.

A homozygous plant having round (R) and yellow (Y) seed is crossed with another homozygous plant having wrinkled (r) and green (y) seeds. Answer the following questions :

(a) Give the genotype of the F₁ generation.

(b) Mention the phenotype of the F₁ offsprings.

(c) Give the possible combinations of gametes that can be obtained from F₁ hybrids.

(d) Give the dihybrid phenotypic ratio and the phenotype of the offsprings of the F₂ generation when two plants of F₁ generation are crossed.

(e) Name and state the law which explains the dihybrid phenotypic ratio.

Answer

(a) Genotype : RrYy

(b) All Round and yellow seeds

(c) RY, Ry, rY, ry

(d) 9:3:3:1

(e) Law of Independent Assortment, which states that the distribution of the alleles of one character into the gametes is independent of the distribution of the alleles of the other character.