Test: Genes and Inheritance - Year 9 MCQ

Chromosomes are the thread-like structures found in the nucleus of a cell that play a crucial role in controlling the activities of the cell. They carry genetic information and are composed of DNA, which contains the instructions necessary for the growth, development, and functioning of living organisms. Interestingly, the number of chromosomes varies between species; for instance, humans have 46 chromosomes, while fruit flies have only 8.

Dark-colored peppered moths became more common during the Industrial Revolution because they had better camouflage against the soot-darkened tree trunks, making them less visible to predators. As pollution from coal-burning factories killed the light-colored lichens on tree bark, the lighter moths became more easily spotted and were more likely to be eaten. This example of natural selection illustrates how environmental changes can drive shifts in species populations over time.

Human gametes, which include sperm and egg cells, contain 23 chromosomes. This is half the number found in most body cells, which contain 46 chromosomes organized into 23 pairs. The reduction in chromosome number is essential for sexual reproduction, as it ensures that when the sperm and egg combine during fertilization, the resulting zygote has the correct diploid number of 46 chromosomes.

All of the above options describe types of variation among individuals of the same species. Genetic variation arises from differences in DNA sequences, while environmental variation occurs due to differences in environmental conditions. Phenotypic variation encompasses observable traits influenced by both genetic and environmental factors, highlighting the complexity of how traits are expressed in organisms.

Natural selection leads to the adaptation of a species by increasing the survival and reproductive success of individuals with advantageous traits. Over time, these traits become more common in the population as those individuals pass on their beneficial genes to their offspring. This process illustrates how species can evolve and become better suited to their environments. An interesting fact is that natural selection acts on existing variations within a population, rather than creating new traits outright.

The combination of two X chromosomes (XX) results in a female individual. In contrast, an XY combination results in a male. The sex chromosomes determine the biological sex of an individual, and this process begins at fertilization when the sperm cell contributes either an X or a Y chromosome to the egg cell, which always contributes an X chromosome.

Fertilization is the process where a sperm cell merges with an egg cell to form a zygote. During this process, the nuclei of both gametes combine, restoring the diploid number of chromosomes. This zygote then undergoes multiple divisions, leading to the development of a new organism. An interesting fact about zygotes is that they represent the very first stage of development for all multicellular organisms, setting the foundation for all subsequent growth and development.

DNA is primarily structured as a double helix, resembling a twisted ladder. This configuration is crucial for its function, as it allows for the compact storage of genetic information and the precise replication of DNA during cell division. Each strand of the helix consists of nucleotides, which encode the genetic instructions necessary for life. Interestingly, the double helix model was first proposed by James Watson and Francis Crick in the 1950s, revolutionizing our understanding of genetics.

The environment interacts with genetic factors to influence the variation of traits in organisms. While genetic differences provide the potential for variation, environmental factors, such as nutrition and habitat conditions, can affect how these traits are expressed. For example, a goat's body size can be influenced by both its genetic makeup and the amount of food it consumes. This interplay demonstrates the complexity of trait development in living organisms.

A potential consequence of a species failing to adapt to environmental changes is extinction. If a species cannot adjust to new conditions or if their habitat is altered beyond their capacity to survive, their population may decline to the point of complete extinction. For example, rapid climate change poses a significant threat to many species today, as their ability to adapt may not keep pace with the speed of environmental shifts. This highlights the urgent need for conservation efforts to protect vulnerable species in changing ecosystems.