Down syndrome, the most common genetic condition in the united states, is also called trisomy 21. what does this mean? a. a person with down syndrome has only one copy of chromosome 21 instead of two copies. either the sperm cell or the egg cell was lacking this chromosome. b. a person with down syndrome has two copies of chromosome 21 but they're fused together. during early embryo development, these chromosomes didn't segregate properly. c. a person with down syndrome has three copies of chromosome 21 instead of two copies. an extra chromosome 21 was present in either the sperm or the egg cell. d. a person with down syndrome has three copies of chromosome 21 instead of two copies. two sperm cells simultaneously fertilized the egg cell.

C. A person with Down syndrome has three copies of chromosome 21 instead of two copies. An extra chromosome 21 was present in either the sperm or the egg cell.

Explanation:

Trisomy refers to presence of three copies of particular chromosome in genome of organisms. Since Down syndrome is caused by trisomy 21, the affected person has three copies of chromosome 21, instead of two. Here, either male or female gamete could have two copies of chromosome 21. Fusion of gamete with extra copy of chromosome 21 with a normal gamete led to development of zygote with trisomy 21.

question#1

ans:

cell is the basic unit of organization of all living things. cells are often called the building block of life and it is the smallest unit of life. the study of cell is called cell biology. cell consists of cytoplasm enclosed within membrane, which contain bio-molecules such as proteins and nucleic acid.

two third of cell is water, which means that two-third of your whole body is water. the rest is the mixture of molecules, mainly proteins, lipids and carbohydrates.

question#2

ans:

organ:

an organ is a group of tissues specialized to form a functional unit. an example is the heart in animals.

organ system:

an organ system is a group of several organs whose functions are synchronized to release an effective action in a living organism. an example is digestive system.

question#3

different systems perform different function at tissue level. for example, the respiratory system facilitates breathing. in the alveoli tissue of the lungs, the exchange of oxygen and carbon dioxide molecules between the air and the bloodstream occurs by passive transport, so that oxygen is taken in and carbon dioxide and water are removed. different systems of the body carry out different functions. for example, your digestive system is responsible for taking in and processing food, while your respiratory system working with your circulatory system is responsible for taking up oxygen and getting rid of carbon dioxide.

question#4

different systems perform different function at tissue level. for example, the respiratory system facilitates breathing. in the alveoli tissue of the lungs, the exchange of oxygen and carbon dioxide molecules between the air and the bloodstream occurs by passive transport, so that oxygen is taken in and carbon dioxide and water are removed. different systems of the body carry out different functions. for example, your digestive system is responsible for taking in and processing food, while your respiratory system—working with your circulatory system is responsible for taking up oxygen and getting rid of carbon dioxide.

question#5

our body works because of the interactions that happen between organs and organ systems.  

for example:  

the heart to regulate blood flow throughout the body. the blood carries oxygen and nutrients to the cells and carries carbon dioxide and waste away from the cells. body systems work together in maintaining a constant internal environment. when this balance is disrupted, the body systems may not function properly, and human health can suffer.   you start to feel bad.

it marks things for degradation via the proteasome. it alters there celluar location.