All questions of Theory of Evolution for Year 10 Exam

The DNA in the nucleus of a cell is the information source for making proteins. If the information is changed, different proteins will be made. The basic event in reproduction is the creation of a DNA copy. Cells use chemical reactions to build copies of their DNA. This creates two copies of the DNA in a reproducing cell and they need to get separated from each other. DNA copying is accompanied by the creation of an additional cellular apparatus, and then the DNA copies separate, each with its own cellular apparatus.

A) 50%

Explanation:

A heterozygous tall plant has the genotype Tt (T for the tall allele and t for the dwarf allele). A homozygous dwarf plant has the genotype tt. When these two plants are crossed, the possible genotypes of the progeny can be determined using a Punnett square.

Tt (heterozygous tall plant)
T | t
---------
t | Tt | tt
t | Tt | tt

As we can see, there are 4 possible outcomes: 2 Tt (tall) and 2 tt (dwarf). So, the proportion of dwarf progeny (tt) is 2 out of the 4 possibilities, which is equal to 50%.

The genotypic ratio in the F2 generation of a monohybrid cross is 1:2:1, corresponding to homozygous dominant, heterozygous, and homozygous recessive individuals, respectively.

Understanding Mendel's F2 Generation
Mendel's experiments with pea plants laid the foundation for genetics, especially in understanding inheritance patterns.
Single Trait Inheritance
When studying a single trait, Mendel observed the inheritance of dominant and recessive alleles. In his experiments, he crossed homozygous parents:
- P Generation: One parent with two dominant alleles (AA) and another with two recessive alleles (aa).
- F1 Generation: All offspring (Aa) displayed the dominant trait.
F2 Generation Results
When F1 plants were self-fertilized, the F2 generation emerged, revealing a classic 3:1 phenotypic ratio of dominant to recessive traits:
- Phenotypic Ratio: 3 dominant (AA or Aa) : 1 recessive (aa)
However, when considering the genotypes:
- Genotypic Ratio: This includes:
- 1 homozygous dominant (AA)
- 2 heterozygous (Aa)
- 1 homozygous recessive (aa)
Thus, the genotypic ratio for the F2 generation is:
- 1 AA : 2 Aa : 1 aa
Correct Answer: 1:2:1
This corresponds to option 'B'. The 1:2:1 ratio illustrates the distribution of genotypes resulting from the segregation of alleles during meiosis.
Conclusion
Mendel's work exemplifies the foundational principles of genetic inheritance, with the F2 generation showcasing the predictable ratios of genotypes and phenotypes, crucial for understanding heredity.

Understanding Human Sex Chromosomes
In humans, the determination of biological sex is primarily influenced by the composition of sex chromosomes. Let's break down the key aspects:
Sex Chromosome Composition
- Females (XX): Women have two X chromosomes, which are denoted as XX. This configuration is essential for typical female development and functions in reproduction.
- Males (XY): Men possess one X and one Y chromosome, represented as XY. The presence of the Y chromosome triggers male biological development and characteristics.
Significance of the Y Chromosome
- The Y chromosome carries genes that are critical for male sex determination and sperm production. One of the key genes on the Y chromosome is the SRY gene, which initiates the pathway for male sex differentiation.
Why Other Options are Incorrect
- Option A (Both males and females have two X chromosomes): This statement is false because only females have two X chromosomes. Males have one X and one Y.
- Option B (Males have two Y chromosomes): This is incorrect. Males have one Y chromosome, not two. Having two Y chromosomes is not viable in humans.
- Option D (Sex chromosomes do not affect sex determination): This statement is misleading. Sex chromosomes are crucial for determining biological sex in humans.
Conclusion
Thus, the correct answer is option 'C': Females have two X chromosomes (XX), and males have one X and one Y chromosome (XY). This chromosomal structure is fundamental to our understanding of human biology and reproduction.

In a dihybrid cross, Mendel observed a phenotypic ratio of 9:3:3:1, showing the independent inheritance of two traits (e.g., seed shape and color).

Mendel's success in discovering the laws of inheritance can be attributed to several factors, but the most important one is his approach to experimentation. His approach was unique and different from the contemporary methods of experimentation.

Consideration of one character at a time:
Mendel focused on studying one trait at a time, rather than studying multiple traits simultaneously. This allowed him to maintain a clear focus on the traits he was studying and avoid confusion caused by multiple traits. He chose to study pea plants, which have seven distinct traits that can be easily identified and studied. By choosing to study one trait at a time, Mendel was able to identify patterns of inheritance that were not noticeable before.

Qualitative analysis of data:
Mendel used a qualitative analysis of data, which means he focused on identifying the presence or absence of a trait rather than measuring the quantity of the trait. This approach allowed him to identify clear and distinct patterns of inheritance. He counted the number of offspring with a particular trait and used statistical analysis to determine the ratios of dominant and recessive traits in the offspring.

Observation of distinct inherited traits:
Mendel observed seven distinct inherited traits in pea plants, including seed color, seed shape, flower color, and plant height. He carefully observed the traits and recorded his findings. This allowed him to identify patterns of inheritance that were not noticeable before.

His Knowledge of biology:
Mendel had a good understanding of biology, which helped him to interpret his findings and draw conclusions about patterns of inheritance. He was familiar with the concepts of genetics, heredity, and variation. This knowledge allowed him to design experiments that were effective in studying inherited traits.

In conclusion, Mendel's success can be attributed to his focus on studying one trait at a time, qualitative analysis of data, observation of distinct inherited traits, and his knowledge of biology. These factors allowed him to discover the laws of inheritance, which revolutionized the field of genetics.

Mendel performed cross-breeding experiments on garden pea (Pisum sativum).

 Although he studied the inheritance of seven different pairs of contrasting characters in this plant, he considered only one pair at a time. 

He crossed two pea plants having contrasting characters (e.g., tall and dwarf pea plants) by artificial pollination and obtained the hybrids. 

The resulting hybrid plants were then crossed with each other. He obtained the data from these crosses and analyzed the results carefully.

Role of Chromosomes in Sexual Reproduction
Chromosomes play a crucial role in sexual reproduction, ensuring genetic diversity while maintaining the stability of DNA within species. Here’s how they contribute:
1. Genetic Material Organization
- Chromosomes are structures that organize and package DNA, making it possible for genetic information to be efficiently replicated and distributed during cell division.
- Each species has a specific number of chromosomes that carry genes, which are the units of heredity.
2. Halving Chromosome Numbers in Germ Cells
- During sexual reproduction, germ cells (sperm and egg) undergo meiosis, a special type of cell division that reduces the chromosome number by half.
- This reduction is essential so that when fertilization occurs, the resulting organism has the correct diploid number of chromosomes.
3. Genetic Variation
- Chromosomes facilitate genetic variation through processes like crossing over and independent assortment during meiosis.
- This variation is vital for evolution and adaptation, allowing populations to respond to environmental changes.
4. Stability of DNA
- The maintenance of chromosome structure is crucial for the stability of DNA across generations.
- Proper chromosome alignment and segregation during cell division prevent mutations and ensure that genetic material is accurately copied and passed on.
Conclusion
In summary, while chromosomes indeed play a role in combining genetic material and ensuring stability, the correct answer to the role of chromosomes during sexual reproduction is more aligned with maintaining stability in DNA rather than solely transferring dominant traits or merely combining genes. Hence, option 'D' is the most fitting choice in this context.