Genetics — Some Basic Fundamentals

A is True and R is False.

Explanation
Genes do not carry hereditary information from offspring to parents. Instead, genes are the carriers of hereditary information from parents to offspring.

A is False and R is True.

Explanation
Karyokinesis is the division of nucleus during cell-division. Karyotype is a picture showing all chromosome pairs arranged according to their size and shape.

Both A and R are False.

Explanation
Dominant alleles can express themselves in homozygous as well as in heterozygous condition. Dominant alleles suppress recessive alleles and do not allow them to express in heterozygous condition.

A is True and R is False.

Explanation
The phenotype refers to the observable characteristics or traits of an organism.

A is False and R is True.

Explanation
A daughter can resemble her mother as well as her father because she receives genes from both the parents.

(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

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.

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).

(a) Difference between karyotype and karyokinesis —

(b) Difference between autosomes and sex chromosomes

(c) Difference between homozygous and heterozygous chromosomes

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.

all tall

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

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.

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.

Recessive X-linked disease

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

Phenotype

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

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.

9:3:3:1

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

X or Y chromosome

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

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.

Carrier daughters and normal sons

Reason — Since the mother is normal so the daughters will be carrier due to the X chromosome received from father but sons will be normal.

(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

Following are the established genetic traits in humans:

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

(i) 23

(ii) 22

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.

Gene is the basic unt of heredity.

(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.

(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.

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.

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

Gregor John Mendel

Common name — Garden pea
Scientific name — Pisum sativum

(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:

Two characters of pea pod with their alternative traits are :

(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.

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.

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.

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)

(a) Black

(b) No

(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)

(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) 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.

Colour blindness and Haemophilia are two genetic diseases in humans.

Homozygous recessive – rr
Homozygous dominant - RR