Basics of DNA, Genes, Chromosomes, Heredity and Traits

DNA:

Instructions providing all the information necessary for a living organism to grow and function reside in the nucleus of every cell. The instructions come in a form called DNA. DNA encodes a detailed set of plans, like a blueprint, for building different parts of the cell.

The DNA molecule comes in the form of a “double helix”, built with the four-letter DNA alphabet: A, C, T, and G. A always pairs with T, and C always pairs with G.

G = Guanine

A = Adenine

T = Thymine

C = Cytosine

The DNA strand is made of letter, which form words, which then form sentences. These “sentences” are called genes.

Genes:

Genes are made of DNA. One strand of our DNA contains many genes. Genes are the directions for building all the proteins that make our bodies function. All these genes are needed to give instructions for how to make and operate all parts of our bodies. Proteins enable a cell to perform specific functions, such as working with other groups of cells to make hearing possible.

Chromosomes:

Each cell in our body contains a lot of DNA, thus chromosomes are used to compact DNA so that it can fit into a cell.

Packaging of DNA into a chromosome:

1. Double helix of DNA

2. DNA is wrapped around proteins

3. Proteins are packed tightly together until they form a chromosome

Heredity:

The passing of traits from parents to child is the basis of heredity.

How do we get traits from our parents?

Humans have two complete sets of 23 chromosomes (2 x 23 = 46 total)

Each child receives half of its chromosomes from the mother and half from the father.

When parents conceive a child, they each contribute one complete set to the child. This one set can contain chromosomes from both of the parent’s two sets.

Since the parents contribute chromosomes randomly to each new child, every child inherits a unique set of chromosomes and thus has a unique combination of traits.

Traits:

A trait is a notable feature or quality in a person. Each of us has a different combination of traits that makes us unique.

Physical trait:

Characteristics of one’s physical makeup e.g. eye colour, hair colour, height etc.

Behavioural traits:

Characteristics of the way one acts e.g. sheepdog’s herding instinct, a retriever’s desire to fetch etc.

Predisposition to a medical condition:

An increased risk of getting a certain type of disease is also a type of trait that can be passed from parent to child. E.g. sickle cell anemia, cystic fibrosis, heart disease, cancer, certain mental illnesses etc.

Genes encode the instructions that define our traits. The environment we live in also helps to define our traits. For example, while a person’s genes may specify a certain hair colour, exposure to chemicals or sunlight can change that colour.

Set of genetic information for each form - Allele

E.g. Straight thumb allele - “H” (dominant)

Hitchhiker’s thumb allele - “h” (recessive)

Each of us has two alleles for the thumb extension trait. We all have one of 3 possible combinations:

H + H = H

h + h = h

H + h = H --> The dominant gene will be exhibited as the trait

Two of the same allele for a trait - homozygous

Two different alleles for a trait - heterozygous

How are traits inherited?

Mother and father each have two alleles for the thumb extension trait. When they have a child, they pass one of their alleles to their child. The child’s trait is determined by the alleles he/she receives from his/her parents. Each child from these parents can receive a different combination of alleles.

Are all traits inherited this simply?

No. The thumb-extension trait is a well-defined physical trait, however most traits are more complex and thus cannot be tracked through generations this easily.

Alleles can work together to produce incomplete dominance e.g. crossing a red carnation and a white carnation can produce a pink one.

Single-gene traits:

Traits influenced by just one gene. They are rare.

Complex traits:

Traits shaped by more than one gene, sometimes many.

Notes taken from http://learn.genetics.utah.edu/content/begin/tour/ while completing the tour :