Genetics

(a) Offspring

(b) Alleles

(c) Recessive allele

(d) Gene

(e) Phenotype

The dominant forms of hereditary traits in humans are:

• Curly hair ✓
• Dimpled cheeks ✓
• Rolling tongue ✓

Homozygous condition

Reason — A recessive allele fails to express itself in the presence of its contrasting dominant allele. Hence, it can express itself only when it is present along with an identical recessive allele, i.e., in the homozygous condition.

Both the parents are heterozygous dominant.

Reason — Tongue rolling is a dominant trait and non-rolling is recessive. Vijay is a non-roller, so his genotype is homozygous recessive (rr), which means he received one recessive allele from each parent. Since Ajay is a tongue roller, both parents must carry the dominant allele too. Therefore, both parents are heterozygous (Rr), i.e., heterozygous dominant.

Chromosome

Reason — Chromosomes are thread-like structures that become distinct and visible only during cell division, when the chromatin condenses and coils into compact chromosomes.

50%

Reason — The female gamete carries only an X chromosome, while the male gametes are of two types — X-bearing and Y-bearing — in equal numbers. If an X-bearing sperm fertilises the egg, the child is a girl (XX); if a Y-bearing sperm fertilises the egg, the child is a boy (XY). Hence, the probability of having a daughter is 50%.

the number of chromosomes are reduced to half

Reason — Gametes such as the egg cell are formed by a reduction division, so they contain half the number of chromosomes present in the body cells. This ensures that the normal chromosome number is restored after fertilisation.

1 : 2 : 1

Reason — In a monohybrid cross, the F₂ generation shows a genotypic ratio of 1 : 2 : 1 (1 homozygous dominant : 2 heterozygous : 1 homozygous recessive), while the phenotypic ratio is 3 : 1.

X or Y chromosome

Reason — In males, the sperms are of two types — X-chromosome bearing and Y-chromosome bearing. Therefore, a single sperm contains the autosomes and either an X chromosome or a Y chromosome.

variations

Reason — Variations are the dissimilarities or differences found among the individuals of a species. These make each individual different from its parents and from one another.

(A) is true but (R) is false.

Explanation
Haemophilia is indeed an X-linked recessive disease, so the assertion is true. However, the reason is false because haemophilia affects males more commonly than females. Males have only one X chromosome, so a single recessive gene expresses itself, making them more likely to be affected.

P - When two alleles are similar, Q - Alternative forms of a gene that express the same trait in different ways.

Reason — Homozygous condition occurs when the two alleles of a character are similar (identical). An allele is one of the alternative forms of a gene that occupies the same position on homologous chromosomes and expresses the same character in two different ways.

The F₂ generation ratio of round to wrinkled seeds was 3:1.

Reason — When a pure round (RR) plant is crossed with a pure wrinkled (rr) plant, all F₁ plants are heterozygous round (Rr), not homozygous. On self-pollination, the F₂ generation shows a phenotypic ratio of 3 round : 1 wrinkled. Round is the dominant trait, so the round seed trait is not recessive.

Raj and Abhay

Reason — In a dihybrid cross, the F₂ phenotypic ratio is 9 : 3 : 3 : 1, i.e., 9 Yellow-Round : 3 Yellow-Wrinkled : 3 Green-Round : 1 Green-Wrinkled. Hence, Raj (9 yellow-round) and Abhay (3 yellow-wrinkled) are correct. Green-round is 3 (not 1) and green-wrinkled is 1 (not 3), so Sonu and Lata are incorrect.

(A) is false and (R) is true.

Explanation
The assertion is false because it was Mendel (not Darwin) who preferred pea plants, and the pea plant is bisexual (has both male and female parts in the same flower), not unisexual. The reason is true because pea plants are normally self-pollinated, which is one of the features that made them suitable for Mendel's experiments.

Combination of X and Y chromosome from either of its parents

Reason — A male offspring has the constitution XY. He receives the X chromosome from his mother and the Y chromosome from his father. Thus, the male offspring inherits a combination of X and Y chromosomes, one from each parent.

Linkage

Reason — Linkage is the tendency of genes to remain together in their parental combination. It is an exception to the law of independent assortment.

X-linked genes

Reason — A father passes his Y chromosome to his sons and his X chromosome only to his daughters. Therefore, X-linked genes are never passed from father to son.

9 : 3 : 3 : 1

Reason — In a dihybrid cross, the F₂ generation shows a phenotypic ratio of 9 : 3 : 3 : 1, which is explained by the law of independent assortment.

Recessive

Reason — A recessive allele fails to express itself in the presence of its contrasting dominant allele. Its effect is masked by the dominant allele.

rr

Reason — Tongue rolling is a dominant trait and non-rolling is recessive. A person who cannot roll his tongue expresses the recessive trait, which is possible only in the homozygous recessive condition (rr).

Genes

Reason — A gene is the unit of inheritance which controls the expression of a character. Chemically, a gene is a linear segment of DNA.

3 : 1

Reason — Purple flower colour is dominant over white. When a pure purple (PP) plant is crossed with a pure white (pp) plant, the F₁ plants are all purple (Pp). On self-pollination, the F₂ generation shows a phenotypic ratio of 3 purple : 1 white.

Out of a pair of contrasting alleles present together, only the dominant allele is able to express itself while the recessive remains suppressed.

Reason — According to the law of dominance, when a pair of contrasting characters are present together, only the dominant character expresses itself in the F₁ generation while the recessive character remains suppressed.

It is a sudden change in one or more genes in an organism's cells which is heritable.

Reason — Mutation is a sudden random change in the genetic material (genes) of a cell that may cause it to differ in appearance or behaviour from the normal type. It alters the hereditary material and hence is heritable.

Present on chromosomes

Reason — Genes are located on chromosomes. A gene is a linear segment of DNA that occupies a specific position on the chromosome.

Either X or Y chromosome

Reason — In males, two types of sperms are produced — X-chromosome bearing and Y-chromosome bearing. Hence a single sperm contains either an X chromosome or a Y chromosome, along with the autosomes.

(i) Phenotype

(ii) Recessive gene (recessive allele)

(iii) Alleles

(iv) Genes

(v) Gene

(vi) Heterozygous

(vii) Recessive gene (recessive allele)

(viii) Haemophilia

(ix) Homozygous

(i) True

(ii) False
Corrected statement — Mendel observed seven traits in pea plant.

(iii) False
Corrected statement — Pea plants can self-pollinate.

(iv) True

(v) False
Corrected statement — Hypertrichosis of the pinna is generally treated in school genetics as a Y-linked (holandric) condition, not an X-linked condition.

(vi) True

(vii) False
Corrected statement — Chromosomes other than the pair of sex chromosomes are called autosomes.

(viii) False
Corrected statement — The theory of Inheritance of Acquired Characters was proposed by Lamarck.

(i) Genes are located on the chromosomes as linear segments of DNA.

(ii) Alleles are located at the same position (loci) on a pair of homologous chromosomes.

(a) Dominant

(b) Recessive

(c) Dominant

(d) Recessive

(e) 3 : 1

Sex linkage is the inheritance of those genes (characters) which are located exclusively on the sex chromosomes (X or Y chromosome). Such genes are inherited along with the sex of the individual.

The allele that fails to express itself in the presence of its contrasting dominant allele is called the recessive allele.

Phenotype is the physical expression of a trait (character) of an organism, e.g., tall pea plant.

When the two alleles of a character are similar (identical) in a diploid condition, the individual is said to be homozygous for that character.

A gene is the unit of inheritance which controls the expression of a character in cooperation with its allele, other genes and the environment. Chemically, a gene is a linear segment of DNA.

Mutation is a sudden random change in the genetic material of a cell that may cause it to differ in appearance or behaviour from the normal type. For example, sickle-cell anaemia is caused by a gene mutation.

Differences between Test cross and back cross are:

Differences between Monohybrid and dihybrid cross are:

Differences between Law of segregation and law of independent assortment are:

Differences between Pure and hybrid strains are:

Differences between Genotype and phenotype are:

The unit factor of heredity is the gene. It is the unit of inheritance which controls the expression of a character and is passed on from parents to offspring. Chemically, a gene is a linear segment of DNA.

A true breeding plant is one which is homozygous (pure) for a character and produces offspring of the same type, generation after generation, on self-pollination. For example, a pure tall plant (TT) always produces tall offspring.

When a pure red (RR) plant is crossed with a pure white (rr) plant, all F₁ plants are red (Rr). On self-pollination of F₁ plants, the F₂ generation is obtained as shown below:

F₂ Generation:

Genotypic ratio — 1 (RR) : 2 (Rr) : 1 (rr), i.e., 1 : 2 : 1

Phenotypic ratio — 3 (Red) : 1 (White), i.e., 3 : 1

The scientific name of the pea plant used by Mendel is Pisum sativum.

Among the given traits, wrinkled seeds and yellow pod are recessive. (Axillary flower position is a dominant trait, while terminal flower position is recessive.)

Homologous chromosomes are a pair of chromosomes that are similar in size and shape, carry genes for the same characters at the same positions (loci), and receive one member from each parent. The alleles of a character occupy the same position on such homologous chromosomes.

The person will have black hair. Since the gene for black hair is dominant over the gene for red hair, the person is heterozygous (one gene for black hair and one for red hair). The dominant black hair gene expresses itself while the recessive red hair gene remains suppressed. Hence, the person's hair colour will be black.

A duplicated chromosome consists of two identical sister chromatids joined together at a point called the centromere.

The atomic explosion released large amounts of radiation. This radiation caused mutations, i.e., sudden changes in the genetic material (genes and chromosomes) of the plants and animals. Since mutations are heritable, these changes were passed on to the offspring, leading to persisting deformities in their bodies over generations.

When two heterozygous round (Rr) plants are crossed:

(a) Phenotypic ratio — 3 (Round) : 1 (Wrinkled), i.e., 3 : 1

(b) Genotypic ratio — 1 (RR) : 2 (Rr) : 1 (rr), i.e., 1 : 2 : 1

(c) Wrinkled seed is the recessive trait.

Mendel's first law is the Law of Dominance. It can be shown by a monohybrid cross between a pure tall (TT) and a pure dwarf (tt) pea plant. In the F₁ generation, all plants are tall (Tt), showing that tallness is dominant over dwarfness while the recessive character (dwarfness) remains suppressed.

Phenotype of F₁ — 100% Tall
Genotype of F₁ — 100% Tt (Heterozygous)

Mendel's law of independent assortment states that when more than one pair of contrasting characters are brought together in an individual, the distribution of the alleles of one character (controlling one character) is free to separate from the distribution of any other pair of alleles (controlling another character).

(i) Mutation — Mutation is a sudden random change in the genetic material of a cell that may cause it to differ in appearance or behaviour from the normal type. For example, sickle-cell anaemia is caused by a gene mutation.

(ii) Monohybrid cross — A monohybrid cross is a cross in which only one pair of contrasting characters is taken into consideration at a time. For example, a cross between a pure tall and a pure dwarf pea plant.

(iii) Homologous Chromosomes — Homologous chromosomes are a pair of chromosomes that are similar in size and shape and carry genes for the same characters at the same positions, one being received from each parent.

(iv) Phenotype — Phenotype is the physical expression of a trait (character) of an organism, e.g., tall pea plant.

The dihybrid phenotypic ratio is 9 : 3 : 3 : 1.

This ratio is explained by the Law of Independent Assortment, which states that when more than one pair of contrasting characters are brought together in an individual, the distribution of the alleles of one character is free to separate from the distribution of any other pair of alleles (controlling another character).

Mendel's law of segregation states that the paired factors (alleles) responsible for a character segregate (separate) into gametes and are recombined at fertilisation. Since each gamete carries only one factor for a character, the gametes are always pure. Hence, it is also called the law of purity of gametes.

Mendel's law of dominance states that when a pair of contrasting characters are present together, only one character is able to express itself in the F₁ generation while the other remains suppressed. The character which is expressed is called the dominant character, while the suppressed character is called the recessive character.

(i) (a) F₁ generation — Cross between pure tall (TT) and pure dwarf (tt):

All F₁ plants are tall (Tt).

(b) F₂ generation — Self-pollination of F₁ (Tt × Tt):

(ii) Phenotype of F₂ generation — Tall and Dwarf plants (3 Tall : 1 Dwarf).

(iii) F₁ generation:
Phenotypic ratio — 100% Tall
Genotypic ratio — 100% Tt (Heterozygous tall)

F₂ generation:
Phenotypic ratio — 3 (Tall) : 1 (Dwarf)
Genotypic ratio — 1 (TT) : 2 (Tt) : 1 (tt)

Haemophilia is an X-linked recessive disease found in humans. Individuals suffering from this disease lack a factor responsible for the clotting of blood, so bleeding does not stop easily. Since it is a recessive character, it is masked in the heterozygous condition. A mother may carry the disease (carrier) and can transmit it to 50% of her sons, even if the father is normal. The gene for haemophilia is located on the X chromosome.

(i) The mother is heterozygous (carrier) for the trait. She carries the recessive gene for colour blindness on one of her X chromosomes but is not colourblind herself.

(ii) One son and one daughter are colourblind in the family.

(iii) The gene for colour blindness is located on the X chromosome.

The answer to part (ii) is based on the two filled (affected) symbols shown in the chart — one son and one daughter.

(i) Genotype of F₁ — TtRr
Phenotype of F₁ — Tall plants with red flowers

(ii) The possible combinations of gametes from the F₁ hybrid (TtRr) are: TR, Tr, tR, tr

(iii) Dihybrid ratio — 9 : 3 : 3 : 1
Phenotypes of F₂ offspring:

• 9 Tall with red flowers
• 3 Tall with white flowers
• 3 Dwarf with red flowers
• 1 Dwarf with white flowers

(i) Cross between two heterozygous round plants (Rr × Rr):

(ii) Phenotype ratio — 3 (Round) : 1 (Wrinkled)
Genotype ratio — 1 (RR) : 2 (Rr) : 1 (rr)

(iii) In human males, the sex chromosomes are X and Y (XY).
In human females, the sex chromosomes are X and X (XX).

(iv) Mutation — Mutation is a sudden random change in the genetic material of a cell that may cause it to differ in appearance or behaviour from the normal type. For example, sickle-cell anaemia is caused by a gene mutation.

(v) The gametes of human beings contain 23 chromosomes (i.e., half the number present in the body cells).

(i) Genotype of F₁ — TtRr
Phenotype of F₁ — Tall plants with red flowers

(ii) The possible combinations of gametes from the F₁ hybrid (TtRr) are: TR, Tr, tR, tr

(iii) Mendel's law of independent assortment states that when more than one pair of contrasting characters are brought together in an individual, the distribution of the alleles of one character is free to separate from the distribution of any other pair of alleles (controlling another character).

(iv) The phenotypes of the F₂ offspring are:

• Tall with red flowers
• Tall with white flowers
• Dwarf with red flowers
• Dwarf with white flowers

(v) Phenotypic ratio in F₂ generation — 9 : 3 : 3 : 1

(i) When a pure axial (AA) plant is crossed with a pure terminal (aa) plant:
Genotype of F₁ — Aa
Phenotype of F₁ — Axial flowers (all)

(ii) Cross between two heterozygous axial plants (Aa × Aa):

(iii) Phenotypic ratio — 3 (Axial) : 1 (Terminal)
Genotypic ratio — 1 (AA) : 2 (Aa) : 1 (aa)

(iv) Mendel's law of dominance states that when a pair of contrasting characters are present together, only one character (the dominant character) is able to express itself in the F₁ generation while the other (the recessive character) remains suppressed.

(i) (a) Axial flowering

Reason — Axial flowering is dominant over terminal flowering in pea plants.
Terminal flowering is a recessive trait and appears only in the homozygous recessive condition. White flower colour is a recessive trait and is unrelated to the given cross on flower position. Green seed colour is a different character studied by Mendel and is not related to flower position.

(ii) (b) Aa

Reason — Crossing a homozygous dominant parent (AA) with a homozygous recessive parent (aa) produces all heterozygous (Aa) offspring.
F₁ offspring inherit one allele from each parent and cannot be homozygous dominant. F₁ offspring receive one dominant allele from the dominant parent, so they cannot be homozygous recessive. F₁ offspring are Aa in a monohybrid cross between pure dominant and pure recessive parents.

(iii) (d) 3 : 1

Reason — Self-fertilisation of heterozygous F₁ plants (Aa × Aa) produces a phenotypic ratio of 3 dominant : 1 recessive.
2:1 is not the phenotypic ratio obtained in a Mendelian monohybrid cross. 2:1:1 is incorrect as monohybrid cross gives only two phenotypes, not three. 1:3 is incorrect as the dominant trait appears more frequently than the recessive trait in the F₂ generation.

(iv) (b) Law of Dominance

Reason — The dominant trait is expressed in the F₁ generation while the recessive trait is masked.
Law of Independent Assortment is incorrect as this law applies to the inheritance of two or more pairs of contrasting traits, not a single trait. Law of Segregation is incorrect as this law explains the separation of alleles during gamete formation, whereas the given observation mainly demonstrates dominance. Law of Purity of Gametes in incorrect as this is another name of law of segregation.

(v) (a) Terminal flowering

Reason — Terminal flowering is the recessive homozygous (aa) trait that disappears in F₁ and reappears in F₂.
Axial flowering is incorrect because axial flowering appears in both AA and Aa plants, so it is not exclusively a homozygous feature. Big flowers is incorrect because flower size is not the character involved in this cross. Bright coloured flowers is incorrect because flower colour or brightness is unrelated to the given trait of flower position.

(i) Genotype of F₁ — GgIi
Phenotype of F₁ — Green pods which are inflated
Cross-pollination has occurred to produce the F₁ generation.

(ii) Phenotypic ratio of F₂ generation — 9 : 3 : 3 : 1

(iii) The possible combinations of gametes from the F₁ hybrid (GgIi) are: GI, Gi, gI, gi

(iv) Mendel's law of segregation (law of purity of gametes) states that the paired factors (alleles) responsible for a character segregate into gametes and are recombined at fertilisation. Each gamete carries only one factor for a character, so the gametes are always pure.

(v) The scientific name of the plant used by Mendel is Pisum sativum.

(i) When a homozygous tall (TT) plant is crossed with a homozygous dwarf (tt) plant:
Genotype of F₁ — Tt
Phenotype of F₁ — Tall (all)

(ii) Cross between two heterozygous tall plants (Tt × Tt):

(iii) Phenotypic ratio — 3 (Tall) : 1 (Dwarf)
Genotypic ratio — 1 (TT) : 2 (Tt) : 1 (tt)

(iv) Mendel's law of dominance states that when a pair of contrasting characters are present together, only one character (the dominant character) is able to express itself in the F₁ generation while the other (the recessive character) remains suppressed.

(v) A dihybrid cross is a cross in which two pairs of contrasting characters are taken into consideration at a time.