The Rules of

Modern Genetics

Look around the room. You may share certain traits with another student, but you probably do not look exactly like anyone else. Have you ever wondered how that happened?

What is genetics?

Genetics is the scientific study of heredity

• When studying genetics all traits are represented by 2 letters or 2 alleles.

• Traits are either dominant or recessive.

• All capital letters represent a dominant trait

• All lowercase letters represent a recessive trait Tt

Represents a trait for height

Try these examples:

1. Trait: height of pea plants T represents tall

t represents short

2. Trait: pea plants flower color

F represents purple

f represents white

Tt will be tall

TT will be tall

tt will be short

Tall is Dominant

Short is recessive

Ff will be purple

FF will be purple

ff will be white

Purple is Dominant

White is recessive

Mendelian Genetics

• The first clues to understanding genetics came from Gregor Mendel.

• Austrian priest

• began his work in the 1860’s

• Performed the first experiments in genetics.

Father of Genetics

Mendel’s Experiments

• Mendel performed experiments on garden pea plants to help him understand the process of heredity.

• He decided to cross tall pea plants with short plants to study the trait of height.

• Other than height, Mendel studied six other traits in garden peas: seed shape, seed color, pod shape, pod color, and flower position.

Growing up on his family farm, Mendel learned a lot about cultivating flowers and fruit trees.

Mendel’s Results

From doing thousands of breeding experiments like those with peas and others with parakeets, the following rules were deduced:

Rule #1

For a given trait, there can be more than one form of the gene present.

EXAMPLE: height of pea plants.

Tall is dominant over short

T=tall plants and t=short plants

• A tall plant can be represented as: T T, or Tt • All short plants must be represented as: tt

Rule #2

Each different form of a gene is called an allele.

EXAMPLE:

In parakeets, the gene for green is an allele and the gene for yellow is an allele.

Rule #3

For most organisms, an offspring receives one gene from its father and one gene from its mother

Rule #4

• EXAMPLE: • Trait: Dimples • D = dimples present• d= no dimples

• If the following were true about 2 parents, would they have a child with dimples or not?

• DD mother• DD father

If both parents carry the same allele for a trait, that is the only trait that will be passed on.

The child would have dimples!!

Rule #5

If a parent carried two different alleles, that parent can pass on either allele.

EXAMPLE: Trait: shape of eyes B represents almond shape b represents round shape

If the following were true about 2 parents what shape of eyes would their offspring have?

Bb mother bb father

The child could be

Bb or bb

Rule #6

Some alleles are hidden or silent in combination with other alleles.

Where B= cleft chin (dominant) b= no cleft chin (recessive)

* Bb = phenotype will be cleft * BB = phenotype will be cleft * bb = phenotype will be NO cleft

Rule #7

Alleles that are hidden are called recessive alleles.

Rule #8

Bb Pp Tt Ff

Alleles that are never hidden when present are called dominant alleles.

Genotype

The word genotype is used when you are talking about the type of genes an organism actually has.

EXAMPLE:

Two parakeets could have the same colors but different genes so their genotype would be different while their phenotype would be the same.

Gg

GG

gg

REMEMBER!!There are two genes for each trait, one from each parent, called alleles.When the dominant gene is present. it always hides the recessive gene.Dominant genes are represented as upper case

lettersRecessive genes are represented as lower case

lettersPhenotype is how an organism looksGenotype is the genetic makeup of the organism

Try This:

In parakeets,

• green is dominant and is represented by G.

• Yellow is recessive and is represented as g.

For the following genotypes list the phenotype:

a) GG

b) Gg

c) gg

Pedigree chart

A pedigree is like a family tree where squares represent males and circles represent females.