Download Mendel`s Discoveries

Mendel’s Discoveries
Slide 2
Gregor Mendel was an Austrian monk who lived in the middle of the 19th
century. A fair amount about Mendel’s life is known from history, including the fact that
some of his lowest grades received in school were in biology. Despite his academic
shortcomings, however, Mendel’s work as a scientist uncovered some of the most
fundamental concepts of genetics, and provided a basis for many of the major
advancements in biology that were to come in the following century.
Slide 3
Over a period of about nine years, Mendel performed breeding
experiments on bean plants by collecting data on the hereditary patterns of a number of
different characters of the plants, such as the color of fruits and flowers, the shape of
seeds and fruits, and the size of plants. Mendel’s studies involved crossing bean plants
that were true-breeding for different forms of various characters. For example, one of
Mendel’s crosses involved breeding plants that always produced smooth seeds with
plants that were true for wrinkled seeds. The outcome of this cross is shown in the
illustration. Mendel called his true-breeding plants the parental generation, or P. Crosses
between two parents result in the first filial generation, or F1, for short. These F1 progeny
were then allowed to self-fertilize, producing a second filial generation, or F2. Mendel
measured the characters under study as they occurred in both the F1 and F2 generations,
and then used a fairly simple mathematical explanation to describe the results that he
found. As we will discuss in the next few slides, Mendel’s results led to the discovery of
two fundamental laws that govern heredity not just in bean plants, but in all sexually
reproducing organisms.
Slide 4
Mendel’s first experiments described the heredity patterns of single
characters, such as flower color, or seed shape. Crosses performed to follow the
inheritance of a single character are called monohybrid crosses. Invariably in these
crosses, the F1 generation displayed only one trait, or version of the character, that was
present in the parental generation. For example, when Mendel crossed purple-flowered
plants with white-flowered plants, only purple-flowered plants were observed in the F1
generation. When plants with smooth seeds were crossed with plants with wrinkled seeds,
only smooth-seeded plants were observed in the F1 generation. Mendel called the traits
that appeared in the F1 generation dominant traits. The traits that did not appear in the F1
generation were called recessive traits.
When Mendel allowed the F1 generation to self-pollinate, the recessive traits reappeared
in the resulting F2 generation. However, as shown in the table, the dominant and
recessive traits always appeared in the ratio of three dominant phenotypes to one
recessive phenotype.
Slide 5
Mendel explained these results by reasoning that each plant had two units
of inheritance for any given trait. Plants could then theoretically have three possible
combinations for a character - they could have two dominant units, two recessive units, or
a dominant and a recessive unit. Furthermore, when the adult plants formed gametes
through the process of meiosis, each of the units separated so that each gamete carried
only one unit. This means that when gametes from separate parents combined and
developed into a new plant, the offspring would have one unit of inheritance for a
character from each of its parents.
Today, we call Mendel’s units of inheritance genes, and the different versions – dominant
and recessive – different alleles of the same gene. As you follow through this lesson, note
that different genes are denoted by letters. The dominant allele of a gene is represented by
a capital letter, while the recessive allele is denoted by a lower-case letter. For example,
the gene for flower color is represented by the letter ‘w’. The dominant allele that results
in purple color is a capital W; the recessive form that corresponds to white color is a
lower case w. Individuals with two of the same alleles are termed homozygous, and are
represented as WW or ww, while individuals with different alleles are termed
heterozygous, and are represented as Ww. Mendel’s true-breeding plants, therefore, were
homozygous for certain characters, such as flower color. When these homozygous
parental plants produced gametes, all of the gametes from a given parent contained the
same allele. When gametes from two different parents combined, the resulting offspring,
were heterozygous. In appearance, however, only the dominant trait was observed,
because the F1 plants carried the dominant allele, which was expressed over the recessive
allele.
Slide 6
When Mendel’s F1 generations produced gametes, equal numbers of
gametes contained dominant and recessive alleles. When these gametes combined, there
would be an equal chance of each of the following allelic combinations in the offspring
genotype: dominant-dominant, dominant-recessive, recessive-dominant, and recessiverecessive. However, the physical appearance of the F2 plants would be observed in the
ratio three dominant to one recessive, because three of the possible allele combinations
contain dominant alleles, one contains only recessive alleles. This three to one ratio is the
ratio which Mendel observed.
At this point, you should note that the physical appearance, or phenotype, of an organism,
is determined by its genetic makeup, or genotype. In addition, different genotypes, such
as the homozygous dominant and heterozygous conditions, can lead to the same
phenotype.
Slide 7
The results from Mendel’s first experiments led to the formation of
Mendel’s first law, the law of segregation. The law of segregation states that during the
formation of gametes, the alleles of a gene separate so that each gamete only receives one
allele for each gene.
Slide 8
Mendel’s second set of experiments involved measuring the heredity
patterns of two characters at a time. This type of cross is called a dihybrid cross. For
example, Mendel crossed plants that were true-breeding for seeds that were both round
and yellow with plants true-breeding for green and wrinkled seeds. The results Mendel
obtained were similar to those of his first experiments, although they were a little more
difficult to interpret due to the inclusion of two characters rather than just one. In the F1
generation, all of the plants displayed both of the dominant traits found in the parental
generation. In our example, the F1 generation all produced smooth, yellow seeds.
In the F2 generation, Mendel always found that the traits in question appeared together in
a 9:3:3:1 ratio. In other words, out of every sixteen F2 plants measured, nine had both
dominant traits, six had only one of the dominant traits, and one displayed both recessive
traits.
Slide 9
To explain the results of his dihybrid crosses, Mendel proposed that not
only did the alleles of each of the two genes segregate during gamete formation, as he
had shown in his first experiment, but the two genes assorted independently of each other
as well. For example, take a look at the F2 results in the illustration. If you add up only
the round seeds compared to the wrinkled seeds, you will find 12 round and 4 wrinkled
seeds. This, of course, is a three to one ratio, as is expected from Mendel’s first law. The
same is true of the yellow and green seeds. However, when the characters of seed shape
and color are considered together, the resulting 9:3:3:1 ratio can only be explained if the
two genes for these characters have behaved independently of each other during gamete
formation. When this occurs, gametes with all possible allele combinations (RY, Ry, rY,
and ry) are formed with equal probability. When these gametes then combine to form
offspring, the offspring are produced in a 9:3:3:1 phenotypic ratio.
Slide 10
The results of Mendel’s dihybrid crosses led to the formulation of
Mendel’s second law, called the law of independent assortment. This law states that
during gamete formation, the alleles of different genes assort independently of one
another. Due to this, any gamete may receive any combination of the alleles present in
the organism. Recall that Mendel’s first law, the law of segregation, dealt with the
segregation of different alleles of the same gene. The law of assortment describes the
assortment of the alleles of different genes.
Slide 11
Mendel’s work helped lay the foundation for modern genetics, and his
discoveries and conclusions have largely stood the test of time. However, as we will see
in the following lesson, there are a number of exceptions to Mendel’s second law, and a
number of examples where the phenotype of an organism is not so clearly defined by
strictly dominant and recessive alleles of genes. In fact, many researchers consider it
amazing that all seven of the characters that Mendel chose for study displayed such clear
patterns of dominant-recessive heredity. The exceptions to Mendel’s rules, of course,
have presented some of the major challenges to geneticists since Mendel’s discoveries.