INTRODUCTION TO GENETICS

Heredity and Genetics

What is Inheritance?

Every living organism has a set of characteristics inherited from its parent(s)

Scientists wanted to know what makes each species unique and through careful observations and experiments, the science of genetics formed Genetics is the study of heredity

Genetics

Before genetics was understood, scientists could see that traits were passed from parent to offspring

It was not until the mid-1800s that anyone attempted to explain what was going on

Gregor Mendel (1822-1884), an Austrian monk was the first to describe how traits were passed from one generation to the next

Mendel’s Experiments

Mendel used ordinary pea plants in which he knew that part of the flower produces pollen (similar to sperm in animals) and part produces egg cells

After fertilization took place (when both reproductive cells fuse), a new seed would form with a small embryo inside

Pollination

In order to reproduce, flowering plants go through the process of pollination

Pollen from the stamen, the male flower structure, is transferred to the pistil, the female structure

In nature, plants usually pollinate themselves-called self-pollination (pollen from a flower will land on the pistil of the same flower)-this produced clones

Mendel’s Pea Plants

Mendel used pea plants in his experiments because they grow and reproduce quickly and they have several traits to observe Plant height Seed color Seed shape Seed coat color Pod Shape Pod Color Flower position

Mendel also used plants that were true-breeding True breeding means

that the plants would produce clones of themselves

Mendel’s Cross

In Mendel’s experiments, he crossbred two different plants

He did this by transferring the pollen of one plant, to the pistil of another

When Mendel started his experiments he decided to name each generation His first plants he worked with (the parents) he

termed the P generation The first generation of offspring was called the F1

The second generation was called the F2 and so on

The First Experiment

Mendel’s first experiment was crossing a tall pea plant with a short pea plant (P generation)

His first generation of offspring (F1) were all tall plants (offspring with genetically different parents are called hybrids)

He then allowed his F1 generation to self pollinate

The F2 generation had many tall plants, but also some short plants

Genetic Factors

Mendel realized that the tall F1 generation had “factors” (genetic codes) for both tall and short plants

However, he realized that the factors for shortness were hidden by the factors for tallness

Mendel concluded that plant height (and other inherited characteristics) must be controlled by pairs (or alternate forms) of factors

Inherited Characteristics

Mendel determined one of the factors is dominant (visible) while the other factor is recessive (hidden)

Organisms that have two factors that are different for a given characteristic or trait are called heterozygous

Organisms that have two factors that are the same for a given trait are called homozygous

Back to the Experiment

Mendel’s original P generation were homozygous , one parent had two factors for being tall, while the other parent had two factors for being short (both being homozygous)

The factors that Mendel discovered are now called alleles

Genes

Genes are sections of DNA that are found on the chromosome and each gene carries instructions for making specific proteins

Inherited traits can be determined by one or many genes

An alternate form of a gene is called an allele and for each trait, an individual receives one allele from each parent

Alleles

An individual’s genotype is a description of an individual’s particular combination of alleles (genetic makeup-we CANNOT know this by looking at someone!!)

An individual’s appearance is called a phenotype (which is determined by the dominant allele) Meaning if you inherited one dominant allele for

dimples, you would have dimples even if you also inherited a recessive allele for the trait

Probability

The likelihood that an event will occur is called probability For example: think about flipping a coin, you have two

outcomes, either the coin will land heads or tails. The probability of getting either one is 50% or 1 in 2 chances.

The principles of probability can be used to predict the outcomes of genetics crosses-this is done by using the diagram called a Punnett Square

Punnett Squares

A punnett square is a table the predicts the likelihood/probability that a trait will show up in an offspring

It will show all probable combinations of alleles that are possible

Using this table, dominant alleles are represented by a capital letter with its matching recessive allele as a lowercase letter

The letter usually represents the dominant allele Ex: Tall (T) and short (t)

Example

An example of a cross will be with Mendel’s pea plants, for example the dominant allele of tallness is represented by a T while the recessive is represented by a t

In Mendel’s initial experiment with plant height he dealt with crossing a homozygous dominant (TT) with a homozygous recessive plant (tt) Remember HOMO means SAME!!!

Punnett Squares

The parents genotypes (genetic makeup) will be located at the top and left side of the punnett square (in this case TT on the left and tt across the top)

Each box gets a letter-but make sure you DO NOT split the parent’s genotype between the top and side, i.e. one parent is on the top, the other is on the left

When determining the offspring, you combine the top letter with the one of the left (if there is a dominant gene-that ALWAYS goes first!!!)

P Generation

t t

T Tt Tt

T Tt Tt

Determining Probability with Punnett Squares

With the previous cross you can deduce that the genotypic ratio is 4/4 or 100% that the offspring will be Tt

You can also determine the phenotypic ratio, as it will be the same 4/4 or 100% tall

Self pollination

After Mendel crossed the P generation, he allowed the F1 offspring to self pollinate (again meaning one plant’s pollen would fertilize with itself on the pistil)

F1 generation

T t

T TT Tt

t Tt tt

Probability from F1 Offspring

The probabilities for this second cross is much different than the first

The genotypic ratio is now 1:2:1 (one TT, two Tt and one tt)

The phenotypic ratio is 3:1 (three tall to one short)

Review

1. What genotypic ratio did Mendel find in his F2 generation? Phenotypic?

2. What are dominant alleles? Recessive?3. Why were true-breeding plants important

for Mendel’s experiments?4. What is probability? How are these

principles used to predict the outcomes of genetic crosses?

5. What are Punnett squares?6. What is a genotype? Phenotype?

More Review

7. Different forms of a gene are called…?8. Organisms that have two identical alleles

for a particular trait are said to be….?9. If a homozygous tall pea plant is crossed

with homozygous short pea plant ….. The recessive trait seems to disappear The offspring are of medium height No hybrids are produced All the offspring will be short

Definitions

Make sure you are able to define the following (WITHOUT LOOKING!!!) Gene Allele Homozygous Heterozygous Dominant Recessive Gamete Genetics