Introduction to Heredity

(Mendelian Genetics)

Early times

By 1800, people had a relatively poor

knowledge of how characteristics were

passed on in a family.

It was obvious that children resembled

their parents, but that was about it – the

mechanism of heredity was not readily

apparent. (Don’t forget, there was no

knowledge of mitosis, meiosis, or DNA.)

Early hereditary “theories”

Seeds

Mixing of male and female semen

Spermists

Ovists

Pangenesis

Blending

Gregor Mendel

The basic laws of

heredity were first

formed during the mid-

1800s by an Austrian

botanist monk named

Gregor Mendel.

Because his work laid the

foundation to the study of

heredity, Mendel is

referred to as “The Father

of Genetics.”

Mendel grew up on a small farm in what is today the Czech Republic.

In 1843, Mendel entered an Augustinian monastery.

He studied at the University of Vienna from 1851 to 1853 where he was influenced by a physicist who encouraged experimentation and the application of mathematics to science and a botanist who aroused Mendel’s interest in the causes of variation in plants.

These influences gelled in Mendel’s experiments.

Mendel’s Pea Plants

• Mendel based his laws on his studies of garden

pea plants.

• Pea plants were a great choice for many

reasons:

• they weren’t mice or humans

• they self-pollinated

• they were inexpensive, easily grown, and

easily taken care of and controlled

• they had quick maturation

• they were easily quantified (counted)

• they demonstrated numerous obvious,

contrasting traits

Mendel’s Seven Traits

Mendel’s Experiments Mendel noticed that some plants always produced offspring

that had a form of a trait exactly like the parent plant. He

called these plants purebred plants. For instance, purebred

short plants always produced short offspring and purebred tall

plants always produced tall offspring.

X

Purebred Short Parents

Purebred Tall Parents

X

Short Offspring

Tall Offspring

Mendel’s Experiments

According to the prevailing theory (blending):

X

Mendel’s First Experiment Mendel crossed purebred plants with opposite forms of a

trait. He called these plants the parental generation , or P

generation. For instance, purebred tall plants were crossed

with purebred short plants.

Parent Tall

P generation

Parent Short

P generation

X

Offspring Tall

F1 generation

Mendel observed that all of the offspring grew to be tall

plants. None resembled the short parent. He called this

generation of offspring the first filial , or F1 generation,

From the results of this test, and many more

like it using the two purebred forms of the

contrasting traits, Mendel formulated his

Law of Dominance.

Law of Dominance : in a cross involving

two contrasting purebred traits, the one that

is shown is dominant, the other recessive.

• For example, tall is dominant and dwarf is

recessive.

Mendel’s Second Experiment Mendel then crossed two of the offspring tall plants produced

from his first experiment.

Tall

F1 generation

X

3⁄4 Tall & 1⁄4 Short

F2 generation

Mendel called this second generation of plants the second

filial (F2) generation. To his surprise, Mendel observed

that this generation had a mix of tall and short plants. This

occurred even though none of the F1 parents were short.

Parent Plants Offspring

Mendel’s second law, the Law of Segregation, has

three parts. From his experiments, Mendel

concluded that:

1. Plant traits are handed down through

“factors” in the sperm and egg.

2. Because offspring obtain hereditary factors from

both parents, each plant must contain two “trait

factors” for every trait.

3. The factors in a pair segregate (separate)

during the formation of sex cells, and each

sperm or egg receives only one member of the

pair.

Dominant and Recessive Mendel went on to reason that one trait factor in a pair

may mask, or hide, the other factor. For instance, in his

first experiment, when he crossed a purebred tall plant with

a purebred short plant, all offspring were tall. Although the

F1 offspring all had both tall and short factors, they only

displayed the tall factor. He concluded that the tallness

factor masked the shortness factor.

Today, scientists refer to the “factors” that control traits as

genes. The different forms of a gene are called alleles.

Alleles that mask or hide other alleles, such as the “tall”

allele, are said to be dominant.

A recessive allele, such as the short allele, is masked, or

covered up, whenever the dominant allele is present.

Homozygous Traits What Mendel referred to as a “purebred” plant we now know

this to mean that the plant has two identical alleles for a

particular trait. For instance, a purebred tall plant has two tall

alleles and a purebred short plant has two short alleles. The

modern scientific term for “purebred” is homozygous.

X

Short Offspring

short-short short-short short-short

Short Parents

According to Mendel’s Law of Segregation, each parent

donates one height allele to the offspring. Since each parent

had only short alleles to donate, all offspring will also have

two short alleles (homozygous) and will therefore be short.

Hybrid (Heterozygous) Traits In Mendel’s first experiment, F1 offspring plants received one

tall allele and one short allele from the parent plants. Therefore,

all offspring contained both alleles, a short allele and a tall

allele. When both alleles for a trait are present, the plant is said

to be a hybrid for that trait. Today, we call hybrid alleles

heterozygous.

Parent Tall

P generation

Parent Short

P generation

X

Offspring Tall

F1 generation

short-short

short-tall short-tall tall-tall

Although the offspring have both a tall and a short allele, only

the tall allele is expressed and is therefore dominant over short.

Later Experiments

Mendel wondered what would happen if

more than one trait was considered at a

time, for example if he crossed a plant

purebred for both smooth and yellow seeds

with one purebred for both green and

wrinkled seeds.

His results were quite convincing...

Are Different Characters Like

Color and Shape Inherited

Together or Inherited

Independently?

Mendel performed dihybrid

crosses to find out.

Mendel’s conclusion:

Different characters are

inherited independently.

Law of Independent Assortment Mendel’s third law, the Law of Independent Assortment,

states that pairs of alleles separate independently of each

other in the production of sex cells. For instance, consider

an example of the following allele pairs:

According to Mendel’s Law of Independent

Assortment, the allele pairs will separate during

the formation of egg or sperm cells. The plant

will donate one allele from each pair. The plant

will donate either a yellow or green seed allele,

either a yellow or green pod allele, and a

wrinkled or round seed allele. It will always

donate a wrinkled pod shape. The donation of

one allele from each pair is independent of

any other pair. For example, if the plant

donates the yellow seed allele it does not mean

that it will also donate the yellow pod allele.

Mendel Showed...

1. Each parent contributes one trait factor

(allele) of each trait shown in the offspring.

2. The two members of each allele pair

segregate from each other during gamete

formation.

3. Blending didn’t work.

4. Males and females contribute equally to

offspring traits.

5. Acquired traits weren’t inherited.

Later On...

Letters of the alphabet used for alleles.

• Ex. T = tall, t = dwarf

New vocab. genotype (genetic makeup

of an organism), phenotype (physical

makeup of an organism)

Punnett squares used to predict crosses.