Chapter 9: Introduction to Genetics



Chapter 9: Introduction to Genetics

Section 1: The Work of Gregor Mendel

The Work of Gregor Mendel

Biological inheritance, or _________________________, is the key to differences between species

Heredity is much more than the way in which a few characteristics are passed from one generation to another

Heredity is at the very center of what makes each species unique, as well as what makes us human

The branch of biology that studies heredity is called _______________________

Early Ideas About Heredity

Until the 19th century, the most common explanation for family resemblances was the theory of ___________________________________________

1 Because both male and female were involved in producing offspring, each parent contributed factors that were “blended” in their offspring

But, in the last century biologists began to look at the details of heredity

They began to develop a very different view

The work of the Austrian monk _____________________________________ was particularly important in changing people’s views about how characteristics are passed from one generation to the next

Gregor Mendel

Born in 1822 to peasant parents in what is now the Czech Republic

Entered a monastery at the age of __________

Four years later he was ordained a priest

In 1851, Mendel was sent to the University of Vienna to study science and mathematics

He returned 2 years later and spent the next 14 years teaching high school

In addition to his duties, Mendel was in charge of the monastery ________________________

1 This is where he did his work that revolutionized biological science

From his studies, Mendel had gained an understanding of the sexual mechanisms of pea plants

Pea flowers have both __________________________________

Normally, pollen from the male part of the pea flower fertilizes the female egg cells of the very same flower

1 ____________________________________

1 Seeds produced by self-pollination inherit all of their characteristics from the single plant that bore them

Mendel learned that self-pollination could be prevented

He was able to pollinate the two plants by dusting the pollen from one plant onto the flowers of another plant

1 _____________________________________

1 Produces seeds that are the offspring of two different plants

2 Mendel was able to cross plants with different characteristics

Mendel started his studies with peas that were ____________________________

1 If they were allowed to self-pollinate, the purebred peas would produce offspring that were identical to themselves

1 These purebred plants were the basis of Mendel’s experiments

In many respects, the most important decision Mendel made was to study just a few isolated ________________, or characteristics, that could be easily observed

He chose __________ different traits to study

By deciding to restrict his observations to just a few traits, Mendel made his job of measuring the effects of heredity much easier

Genes and Dominance

Mendel decided to see what would happen if he crossed pea plants with different characters for the same trait

A character is a form of a trait

For example, the plant height trait has two characters: _________________________________

Mendel crossed the tall plants with the short ones

From these crosses, Mendel obtained seeds that he then grew into plants

These plants were _____________________, or organisms produced by crossing parents with different characters

What were those hybrid plants like?

Did the characters of the parent plants blend in the offspring?

To Mendel’s surprise, the plants were not half tall

Instead, all of the offspring had the character of only one of the parents _______________________________________________

The other characteristic had apparently disappeared

From this set of experiments, Mendel was able to draw two conclusions

1 Individual factors, which do not blend with one another, control each trait in a living thing

1 Merkmal – German for character

2 Today, the factors that control traits are called ________________

3 Each of the traits Mendel studied was controlled by one gene that occurred in two contrasting forms

4 The different forms of a gene are now called _________________

2 Principle of ________________________________

1 Some alleles are __________________________, whereas others are __________________________

Segregation

Mendel did not stop his experimentation at this point

What happened to the recessive characters?

To answer this question, he allowed all seven kinds of hybrid plants to reproduce by self-pollination

1 __________________________________

1 Purebred parental plants

2 __________________________________

1 First filial generation

3 __________________________________

1 Second filial generation

The F1 Cross

The results of the F1 cross were remarkable

The recessive characters ___________________________ in the F2 generation

This proved that the alleles responsible for the recessive characters had not disappeared

Why did the recessive alleles disappear in the F1 generation and reappear in the F2?

Explaining the F1 Cross

Mendel assumed that the presence of the dominant tall allele had masked the recessive short allele in the F1 generation

But the fact that the recessive allele was not masked in some of the F2 plants indicated that the short allele had managed to get away from the tall allele

1 ______________________________

1 During the formation of the reproductive cells, the tall and short alleles in the F1 plants were segregated from each other

The possible gene combinations in the offspring that result from a cross can be determined by drawing a diagram known as a _____________________________

Represent a particular allele by using a symbol

Dominant = _______________________________

Recessive = _______________________________

Punnett squares show the type of reproductive cells, or ____________________, produced by each parent

Punnett square results are often expressed as ___________________

_______________________________

1 Physical characteristic

_______________________________

1 Genetic makeup

_______________________________

1 Two identical alleles for a trait

2 Purebred

______________________________

1 Two different alleles for a trait

2 Hybrid

Independent Assortment

After establishing that alleles segregate during the formation of gametes (reproductive cells), Mendel began to explore the question of whether they do so independently

In other words, does the segregation of one pair of alleles affect the segregation of another pair of alleles?

For example, does the gene that determines whether a seed is round or wrinkled in shape have anything to do with the gene for seed color?

The Two Factor Cross: F1

In this cross, the two kinds of plants would be symbolized like this:

1 Round yellow seeds

1 ________________

2 Wrinkled green seeds

1 ________________

Because two traits are involved in this experiment, it is called a two-factor cross

The plant that bears round yellow seeds produces gametes that contain the alleles R and Y, or RY gametes

The plant that bears wrinkled green seeds produces ry gametes

An RY gamete and an ry gamete combine to form a fertilized egg with the genotype RrYy

Thus, only one kind of plant will show up in the F1 generation – plants that are heterozygous, or hybrid, for both traits

Remember that the concept of dominance tells us that the dominant traits will show up in a hybrid, whereas the recessive traits will seem to disappear

This cross does not indicate whether genes assort, or segregate independently

However, it provides the hybrid plants needed for the next cross – the cross of F1 plants to produce the F2 generation

The seeds from the F2 plants will show whether the genes for seed shape and seed color have anything to do with one another

The Two Factor Cross: F2

What will happen when F1 plants are crossed with each other?

If the genes are not connected, then they should segregate independently, or undergo __________________________________________

This produces four types of gametes RY, Ry, rY, and ry

Mendel actually carried out this exact experiment

1 Concluded that genes could segregate independently during the formation of gametes

2 In other words, genes could undergo independent assortment

A Summary of Mendel’s Work

Mendel’s work on the genetics of peas can be summarized in four basic statements:

1 The factors that control heredity are individual units known as genes. In organisms that reproduce sexually, genes are inherited from each parent.

2 In cases in which two or more forms of the gene for a single trait exist, some forms of the gene may be dominant and others may be recessive.

3 The two forms of each gene are segregated during the formation of reproductive cells.

4 The genes for different traits may assort independently of one another.

................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download