Chromosomes, genes, and DNA

Key definitions

Chromosomes are made of DNA, which contains genetic information in the form of genes.
A gene is a length of DNA that codes for a protein.
An allele is an alternative form of a gene.

Inheritance is the transfer of genetic information from generation to generation.
- These characteristics are controlled by genes, similar to coded instructions.
- People who belong to the same family often resemble each other. Characteristics or traits (e.g. physical appearance, personality, talents) are often shown by people in different generations of a family. These people have inherited the feature from their parents.
The study of inheritance is called genetics.

Genes from one generation are transmitted to the next in the gametes.
At fertilization, the gametes fuse to form a zygote, which contains the genetic information from both parents.
- A new individual grows from the zygote. Half of its genetic information derives from its male parent and half from its female parent.

Chromosomes are made of DNA, which contains genetic information in the form of genes.
- Textbook definition: “A chromosome is a thread of DNA carrying genetic information in the form of genes.”
These chromosomes are visible through a microscope during cell division.
Most cells have nuclei containing chromosomes.
The diploid number is the total count of chromosomes in a standard body cell of an organism, consisting of two complete sets (one set from each parent).
- There are 46 chromosomes in the nucleus of a human cell. This is the diploid number.
Chromosomes can be sorted into 23 pairs based on their size and shape. In each pair, one of the chromosomes have been inherited from the male parent and one from the female.
When unraveling a chromosome, it would form a very long thread. This thread is made up of DNA (deoxyribonucleic acid).
- The DNA is wound around molecules of protein. During cell division, the DNA and protein are packed very tightly together and the chromosomes are coiled up.
- At other times, they are uncoiled so the cell can use the information in the DNA.

As mentioned before, the human body’s cells contain 23 pairs of chromosomes. One pair is the sex chromosomes.
A female is XX, so all her eggs carry an X chromosome.
A male is XY, so approximately half of his sperm carry an X chromosome and half carry a Y chromosome.

Egg from mother	Sperm from father	Chromosomes of child	Sex
X	X	XX	Female
X	Y	XY	Male

This table shows that the father’s sperm determines the sex of the child:
- an X-carrying sperm produces an XX female
- a Y-carrying sperm produces an XY male.

A gene is a length of DNA that codes for a protein.
Each chromosome has many genes along its length, although the real number depends on its length and the length of the genes.
- Some of our chromosomes are much longer than others, therefore containing lots of genes.
Each gene is a unit of inheritance in that it codes for a specific protein. It is a chemical code that the cell interprets as an instruction to make a protein molecule.
- There is a gene for each protein, such as haemoglobin, amylase, lipase, etc.
Humans have between 20,000 and 25,000 different genes.
The proteins produced by cells influence how the body works and what it looks like.
- Some features that genes control are influenced by the environment.
  - For example, your height is determined by both genes and the environment (the environment being your diet and how much you exercise).
- Other genes are not affected by the environment.
  - For example, your blood group is determined by a gene and the environment has no effect on this.