Lesson OverviewLesson Overview
11.1 The Work of11.1 The Work of
Gregor MendelGregor Mendel
THINK ABOUT IT
What is an inheritance?
It is something we each receive from ourparents—a contribution that determines ourblood type, the color of our hair, and somuch more.
The Experiments of Gregor Mendel
Where does an organism get its uniquecharacteristics?
An individual’s characteristics aredetermined by factors that are passed fromone parental generation to the next.
The Experiments of Gregor Mendel
Every living thing—plant or animal, microbe orhuman being—has a set of characteristicsinherited from its parent or parents.
The delivery of characteristics from parent tooffspring is called heredity.
The scientific study of heredity, known asgenetics, is the key to understanding what makeseach organism unique.
The Experiments of Gregor Mendel
The modern science ofgenetics was founded byan Austrian monk namedGregor Mendel.
Mendel was in charge ofthe monastery garden,where he was able to dothe work that changedbiology forever.
The Experiments of Gregor Mendel
Mendel carried out hiswork with ordinary gardenpeas, partly because peasare small and easy togrow. A single pea plantcan produce hundreds ofoffspring.
Today we call peas a“model system” or
“model organism.”
The Experiments of Gregor Mendel
Scientists use modelsystems because they areconvenient to study andmay tell us how otherorganisms, includinghumans, actually function.
The Experiments of Gregor Mendel
By using peas, Mendelwas able to carry out, injust one or two growingseasons, experiments thatwould have beenimpossible to do withhumans and that wouldhave taken decades—ifnot centuries—to do withother large animals.
The Role of Fertilization
Mendel knew that the male part of each flowermakes pollen, which contains sperm—theplant’s male reproductive cells.
The Role of Fertilization
Similarly, Mendel knew that the female portionof each flower produces reproductive cellscalled eggs.
The Role of Fertilization
During sexual reproduction, male and femalereproductive cells join in a process known asfertilization to produce a new cell.
In peas, this new cell develops into a tinyembryo encased within a seed.
The Role of Fertilization
Pea flowers are normally self-pollinating, whichmeans that sperm cells fertilize egg cells fromwithin the same flower.
A plant grown from a seed produced by self-pollination inherits all of its characteristics fromthe single plant that bore it. In effect, it has asingle parent.
The Role of Fertilization
Mendel’s garden had several stocks of pea plantsthat were “true-breeding,” meaning that they wereself-pollinating, and would produce offspring withidentical traits to themselves.
In other words, the traits of each successivegeneration would be the same.
A trait is a specific characteristic of an individual,such as seed color or plant height, and may varyfrom one individual to another.
The Role of Fertilization
Mendel decided to “cross” his stocks of true-breeding plants—he caused one plant toreproduce with another plant.
The Role of Fertilization
To do this, he had to prevent self-pollination. Hedid so by cutting away the pollen-bearing maleparts of a flower and then dusting the pollenfrom a different plant onto the female part ofthat flower, as shown in the figure.
The Role of Fertilization
This process, known as cross-pollination,produces a plant that has two different parents.
Cross-pollination allowed Mendel to breedplants with traits different from those of theirparents and then study the results.
The Role of Fertilization
Mendel studied seven different traits of peaplants, each of which had two contrastingcharacteristics, such as green seed color oryellow seed color.
Mendel crossed plants with each of the sevencontrasting characteristics and then studied theiroffspring.
The offspring of crosses between parents withdifferent traits are called hybrids.
Genes and Alleles
When doing genetic crosses, we call the originalpair of plants the P, or parental, generation.
Genes and Alleles
Their offspring are called the F1, or “first filial,”generation.
Genes and Alleles
For each trait studied in Mendel’s experiments,all the offspring had the characteristics of onlyone of their parents, as shown in the table.
Genes and Alleles
In each cross, the nature of the other parent,with regard to each trait, seemed to havedisappeared.
Genes and Alleles
From these results, Mendel drew two conclusions.His first conclusion formed the basis of our currentunderstanding of inheritance.
An individual’s characteristics are determined byfactors that are passed from one parentalgeneration to the next.
Scientists call the factors that are passed fromparent to offspring genes.
Genes and Alleles
Each of the traits Mendel studied was controlled byone gene that occurred in two contrasting varieties.
These gene variations produced differentexpressions, or forms, of each trait.
The different forms of a gene are called alleles.
Dominant and Recessive Traits
Mendel’s second conclusion is called the principleof dominance. This principle states that somealleles are dominant and others are recessive.
An organism with at least one dominant allele for aparticular form of a trait will exhibit that form of thetrait.
An organism with a recessive allele for a particularform of a trait will exhibit that form only when thedominant allele for the trait is not present.
Dominant and Recessive Traits
In Mendel’s experiments, the allele for tallplants was dominant and the allele for shortplants was recessive.
Dominant and Recessive Traits
In Mendel’s experiments, the allele for tall plants wasdominant and the allele for short plants was recessive.Likewise, the allele for yellow seeds was dominant overthe recessive allele for green seeds
Segregation
How are different forms of a gene distributed tooffspring?
During gamete formation, the alleles foreach gene segregate from each other, sothat each gamete carries only one allele foreach gene.
Segregation
Mendel wanted to find out whathad happened to the recessivealleles.
To find out, Mendel allowed allseven kinds of F1 hybrids toself-pollinate. The offspring ofan F1 cross are called the F2generation.
The F2 offspring of Mendel’sexperiment are shown.
The F1 Cross
When Mendel compared the F2plants, he discovered the traitscontrolled by the recessivealleles reappeared in thesecond generation.
Roughly one fourth of the F2plants showed the traitcontrolled by the recessiveallele.
Explaining the F1 Cross
Mendel assumed that adominant allele had masked thecorresponding recessive allelein the F1 generation.
The reappearance of therecessive trait in the F2generation indicated that, atsome point, the allele forshortness had separated fromthe allele for tallness.
Explaining the F1 Cross
How did this separation, or segregation, ofalleles occur?
Mendel suggested that the alleles for tallnessand shortness in the F1 plants must havesegregated from each other during theformation of the sex cells, or gametes.
Let’s assume that eachF1 plant—all of whichwere tall—inherited anallele for tallness from itstall parent and an allelefor shortness from itsshort parent.
The Formation of Gametes
The Formation of Gametes
When each parent, or F1adult, produces gametes,the alleles for each genesegregate from oneanother, so that eachgamete carries only oneallele for each gene.
A capital letter represents adominant allele. A lowercaseletter represents a recessiveallele.
Each F1 plant in Mendel’scross produced two kinds ofgametes—those with theallele for tallness (T) andthose with the allele forshortness (t).
The Formation of Gametes
The Formation of Gametes
Whenever each of twogametes carried the t alleleand then paired with the othergamete to produce an F2plant, that plant was short.
Every time one or moregametes carried the T alleleand paired together, theyproduced a tall plant.
The F2 generation had newcombinations of alleles.