The Basics of Genetics

1. Traits are passed or inherited from one generation to the next.

What part of your cells contain your genetic instructions?

Your genetic instructions are located in the nucleus of every cell, stored within chromosomes.

How will your traits be passed on to your future children?

Fathers pass genetic instructions to offspring through sperm cells.

Mothers pass genetic instructions to offspring through egg cells.

Chromosomes: How many? Where are they?

A human body cell contains 46 Chromosomes, which are stored inside every cell’s nucleus.

Human sex cells (sperm or egg) contain 23 chromosomes each.

2. Traits of an organism are controlled by genes.

A gene is a section of a chromosome, that codes for a specific trait.

Chromosomes are made of tightly wound strands of DNA and proteins.

gene

3. Organisms inherit genes in pairs, one from each parent.

Human sex cells (sperm or egg) contain 23 chromosomes each.

When the egg is fertilized, the new baby will have 46 chromosomes in each of its cells, which is the correct number of chromosomes for a human.

Sexual Reproduction (2 parents) creates unique offspring.

In plants, male chromosomes are passed through pollen grains to female egg cells. Once fertilized, these eggs will develop into seeds.

The new plants will receive a combination of traits from both parents.

4. Some genes are dominant, while others are recessive.

“Stronger” traits are called dominant.

“Weaker” traits are called recessive.

Geneticists use symbols (letters) to represent the different forms of a gene.

Dominant traits are represented by a capital letter.

Yellow seeds are dominant……..

Y

Recessive genes (for the same trait) are represented by THE

SAME lower case letter.Green seeds are

recessive……….

yDominant (yellow) = Y

Recessive (green) = y

In pea plants, tall is dominant over short. The letter used to represent the tall gene is T.

The short gene is represented by t.

Tall =T

Short = t

Tall =T

Short = t

In humans, brown eyes are dominant over blue.

Brown = B

Blue = b

5. Dominant genes hide recessive genes when both are inherited by an organism.

Y + y = yellow seeds (yellow is dominant)

T + t = tall plant (tall is dominant)

B+ b = brown eyes (brown is dominant)

A PUREBRED or Homozygous organism have two of the same genes

for a trait.

TT = purebred TALL tt = purebred SHORT

A HYBRID or Heterozygous organism has two different genes for

a trait.

Tt = hybrid TALL (tall is dominant.)Bb = hybrid for black feathers (black is dominant.)

Will a Hetrozygous/Hybrid human with the genes Bb have brown or

blue eyes?

Bb = Brown eyes The dominant gene (brown), will be expressed

in a hybrid.

Why must all blue eyed people be Homozygous/Purebred for that trait?

If the dominant gene is present, it will always be expressed.

The only possible gene combination for blue eyes is

bb.

6. Some genes are neither dominant nor recessive. These genes show

incomplete dominance. If an organism shows

INCOMPLETE DOMINANCE for a trait, the hybrid will show a blend of the two traits.

In humans, hair texture is such a trait.

Because neither curly or straight hair is recessive, no lower case letters are used.

CC = curly hair

CS = wavy hair

SS = straight hair

These basic laws of inheritance control the traits of all living

organisms on Earth.