Question 5.

What is a test-cross, and how is it used to determine the genotype of an organism?

Solution:
A test-cross is an experimental cross used to determine the genotype of an organism displaying a dominant phenotype. The organism with the dominant phenotype could be either homozygous dominant or heterozygous for the trait. By crossing it with a homozygous recessive organism, the genotype of the dominant phenotype organism can be inferred.

Design of a Test-Cross:

1. Identify the Organisms:
   • Dominant Phenotype Organism (unknown genotype): This organism shows the dominant trait but could be either homozygous dominant (AA) or heterozygous (Aa).
   • Recessive Phenotype Organism (known genotype): This organism is homozygous recessive (aa).
2. Perform the Cross:
   Cross the dominant phenotype organism (A) with the recessive organism (aa).
3. Analyze the Offspring:
   • If the dominant phenotype organism is homozygous dominant (AA): $$\text{Cross: } AA \times aa$$ All offspring will exhibit the dominant phenotype.
   • If the dominant phenotype organism is heterozygous (Aa): $$\text{Cross: } Aa \times aa$$ The offspring will show a phenotypic ratio of 1:1:
     • 50% will exhibit the dominant phenotype.
     • 50% will exhibit the recessive phenotype.

Example:
Assume we have a plant with a dominant phenotype for flower color (e.g., purple flowers), and we want to determine its genotype. We cross it with a plant that has recessive flowers (e.g., white flowers). The test-cross will help us determine whether the purple-flowered plant is homozygous dominant or heterozygous based on the phenotypes of the offspring.

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Question 6.

Using a Punnett Square, work out the distribution of phenotypic features in the first filial generation after a cross between a homozygous female and a heterozygous male for a single locus.

Solution:
Consider a cross in guinea pigs between a heterozygous male with a black coat color (Bb) and a homozygous female with a white coat color (bb). The male yields two types of gametes, B and b, whereas the female yields one type of gamete only, b. The resulting F1 generation will have a 1:1 ratio of black and white coat color.

Male (Bb)	Female (bb)	Gametes	Phenotype in F1
B (black)	b (white)	Bb	Black coat
b (white)	b (white)	bb	White coat

Thus, the F1 generation will have a 1:1 ratio of black to white coat color.

Phenotype: Tall and green plant – 3

Dwarf and green plant – 1

Question 8.

Two gene pairs show independent assortment. What will be the phenotypic ratio in the F2 generation?

Solution:
For two gene pairs showing independent assortment, the F2 generation will exhibit the following phenotypic ratio:

$$9 : 3 : 3 : 1$$

This ratio represents:

• 9: Dominant for both traits.
• 3: Dominant for the first trait, recessive for the second trait.
• 3: Recessive for the first trait, dominant for the second trait.
• 1: Recessive for both traits.

Example:
Consider a cross between two heterozygous pea plants (YyRr x YyRr), where Y = Yellow seeds (dominant) and R = Round seeds (dominant). The F2 generation will show:

• 9 Yellow Round seeds (YYRR, YyRR, YYRr, YyRr).
• 3 Yellow Wrinkled seeds (YYrr, Yyrr).
• 3 Green Round seeds (yyRR, yyRr).
• 1 Green Wrinkled seed (yyrr).

Question 9.

Briefly mention the contribution of T.H. Morgan to genetics.

Solution:
The contributions of T.H. Morgan to the field of genetics are as follows:

• He proposed and established that genes are positioned on chromosomes.
• He discovered the basis for variations resulting from sexual reproduction.
• He introduced the concept of linkage and distinguished between linked and unlinked genes.
• He stated the chromosomal theory of linkage.
• He conducted studies on sex-linked inheritance.
• Morgan proposed a chiasma-type hypothesis, demonstrating that chiasma causes crossing over.
• He observed that the frequency of recombination between two linked genes is directly proportional to the distance between them.
• He proposed the theory of inheritance.
• He developed the methodology for chromosome mapping.
• He conducted studies on mutation.

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Question 10.

What is pedigree analysis? Suggest how such an analysis can be useful.

Solution:
A pedigree is a record of the inheritance of a specific genetic trait over two or more generations, typically presented in the form of a diagram or family tree. Pedigree analysis involves the study of several generations of a family to trace the inheritance patterns of particular traits, and is often applied to human beings and domesticated animals.

The usefulness of pedigree analysis includes:

• Tracing Inheritance: It serves as a powerful tool to trace the inheritance of specific traits, diseases, or abnormalities.
• Genetic Counseling: It is helpful for genetic counselors in advising couples about the likelihood of having children with genetic disorders such as color blindness, hemophilia, thalassemia, or sickle cell anemia.
• Identifying Origins: The analysis can indicate the origin of a trait and its transmission through ancestors.
• Applying Mendel’s Principles: It helps demonstrate that Mendel’s principles can be applied to human genetics with some modifications, such as considering quantitative inheritance and sex-linked traits.
• Advising on Marriages: It provides reasoning for why marriages between close relatives can be harmful.
• Medical Research: It is valuable in extensive research within the field of medical science.

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Question 11.

How is sex determined in human beings?

Solution:
The chromosomal mechanism for the determination of sex in human beings follows the XX-XY genotype system. In humans, each cell nucleus contains 23 pairs of chromosomes, or 46 chromosomes in total, of which 22 pairs are autosomes, and the 23rd pair consists of sex chromosomes.

• Females are homomorphic, possessing two identical sex chromosomes (XX). They are homogametic, producing only one type of egg (22 + X).
• Males are heteromorphic, with two different sex chromosomes (XY). They are heterogametic, producing two types of sperm – (22 + X) and (22 + Y).

Fertilization Process:

• If a sperm containing an X chromosome fertilizes the egg, the developing offspring will be female (XX).
• If a sperm containing a Y chromosome fertilizes the egg, the offspring will be male (XY).

Thus, the sex ratio in the progeny is 1:1. The mechanism of sex determination in humans is known as heterogametic, which can be either male heterogamety (as in humans) or female heterogamety in other species.

Question 12.

A child has blood group O. If the father has blood group A and the mother’s blood group is B, work out the genotypes of the parents and the possible genotypes of the other offspring.

Solution:
A set of three alleles – $I^A$, $I^B$, and $i$ – control the blood group characteristics in humans. The alleles $I^A$ and $I^B$ are equally dominant, while allele $i$ is recessive to the other alleles. The table below shows the possible genotypes and corresponding blood groups:

Genotype	Blood Group
$I^A I^A$, $I^A i$	A
$I^B I^B$, $I^B i$	B
$I^A I^B$	AB
$ii$	O

Thus, if the father has blood group A and the mother has blood group B, then the possible genotype of the parents will be as follows:

• Father’s Genotype: $I^A i$ (since the child has blood group O, the father must carry the recessive allele $i$).
• Mother’s Genotype: $I^B i$ (similarly, the mother must carry the recessive allele $i$).

Possible Genotypes of Other Offspring:

Given the genotypes of the parents ($I^A i$ for the father and $I^B i$ for the mother), the possible genotypes and blood groups of their other offspring could be:

Offspring Genotype	Offspring Blood Group
$I^A I^B$	AB
$I^A i$	A
$I^B i$	B
$ii$	O

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A cross between heterozygous parents will produce progenies with AB blood group (IA IB) and O group (ii).

Question 13.

Explain the following terms with an example.

(a) Co-dominance

(b) Incomplete dominance

Solution:

(a) Co-dominance:
Co-dominance occurs when both alleles in a heterozygote are fully expressed, leading to a phenotype that shows both traits simultaneously without blending. An example of co-dominance is the ABO blood group system in humans, where the alleles $I^A$ and $I^B$ are co-dominant. Individuals with the genotype $I^A I^B$ express both A and B antigens on the surface of their red blood cells, resulting in an AB blood type.

(b) Incomplete Dominance:
Incomplete dominance is a type of inheritance where neither allele is completely dominant over the other, resulting in a heterozygous phenotype that is a blend of the two parental traits. An example is seen in the flower color of Mirabilis jalapa (the four o’clock plant). When a red-flowered plant (RR) is crossed with a white-flowered plant (WW), the F1 generation produces pink flowers (RW), which is an intermediate phenotype between the red and white parental traits.

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Question 14.

What is point mutation? Give one example.

Solution:
A point mutation is an abrupt change in the gene structure due to the alteration of a single base pair in the DNA sequence, such as through inversion or substitution, without affecting the reading frame of the succeeding bases.

Example:
Sickle cell anaemia is an example of a point mutation. This genetic disorder is caused by the substitution of a single nitrogen base, guanine, with adenine at the sixth codon of the β-globin chain of the haemoglobin molecule. This mutation leads to a change in the shape of red blood cells (RBCs) from the normal biconcave disc to an elongated, sickle shape. The sickle-shaped RBCs are less flexible, more rigid, and tend to stick to blood vessel walls, leading to blockages that slow or stop blood flow, which causes the symptoms of sickle cell anaemia.

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Question 15.

Who proposed the chromosomal theory of inheritance?

Solution:
The chromosomal theory of inheritance was independently proposed by Theodore Boveri and Walter Sutton in 1902.

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Question 16.

Mention any two autosomal genetic disorders with their symptoms.

Solution:
Autosomal genetic disorders are caused by defects in genes located on autosomes (non-sex chromosomes). Two examples of autosomal genetic disorders are Down’s syndrome and sickle cell anaemia.

1. Down’s Syndrome

Symptoms:

• Flat hands and short neck
• Broad forehead
• Partially open mouth with a furrowed tongue
• Mongolian-type eyelid fold and stubby fingers
• Stunted psychomotor, physical, and mental development
• Heart deformities and other organ deformities
• Underdeveloped genitalia and gonads

2. Sickle Cell Anaemia

Symptoms:

• Red blood cells (RBCs) change shape from a typical biconcave disc to a sickle shape under low oxygen tension.
• Sickle-shaped RBCs are rapidly destroyed, leading to anaemia.
• These altered RBCs can cause blockages in blood vessels, reducing or halting blood flow, leading to complications.