Mendelian Genetics

Mendelian Genetics

Genetics is the study of heredity. Heredity is the transmission of genetic characteristics from one generation to the next.

Gregor Mendel was the first to work out the principles of Genetics.

His experiments were carried out using the pea plants. This organism was a good choice for several reasons.

Advantages of Using Pea Plants for Genetic Research

1. Pea plants were commercially available, easy to cultivate, and grew rapidly.

2. Varieties of pea plants were available that bred true generation after generation. This means that each offspring was identical to the parent in the trait of interest.

3. The pea plant flower is self-pollinating. This meant that Mendel could control the genetic cross. He could open the flower bud of a plant with a selected trait and snip off the anthers before they matured. He then took pollen from the flower of a second pea plant showing a trait he wanted to cross and transferred it to the stigma of the first. In this way, Mendel could control the traits that were crossed.

Selection of Traits for Further Study

Mendel focused on the inheritance of several distinct traits. He started out with 32 traits and narrowed these down to seven characters, or pairs of traits.

The traits which Mendel studied included: seed form, seed color, flower position, flower color, pod form, pod color, and stem length. Each trait came in two distinct possibilities.

Trait	Alternatives
Seed form	round	wrinkled
Seed color	yellow	green
Flower position	axial	terminal
Flower color	purple	white
Pod form	inflated	constricted
Pod color	green	yellow
Stem length	tall	dwarf

Mendel’s experiments had three features that represented improvements over previous experiments involving genetic crosses:

1. He studied characters of pea plants that offered just two distinct possibilities. For example, if Mendel was studying seed form, the seeds were either round or wrinkled. If he was studying seed color, the seeds were either yellow or green.

2. He traced and recorded the type and number of all progeny produced from each pair of parent pea plants that he cross-bred.

3. Finally, he followed the results of each cross for two generations.

In a set of experiments on the inheritance of seed color, Mendel crossed a strain of pea plant that produced yellow seeds with a strain that produced green seeds. This type of cross, which involves only one trait at a time (seed color) is known as a monohybrid cross.

Mendel performed the cross by opening up a flower of a pea plant with a yellow seeds and removing the anthers. He then dusted the stigma of that flower with pollen from a flower from a pea plant that green seeds. The first generation is called the P1, or parental generation. The offspring from such a cross are the F1, or first filial generation.

Mendel noticed that every individual in the F1 generation produced yellow seeds. Because the yellow trait appeared and the green trait appeared hidden, Mendel called the trait that appeared in the F1 generation dominant. Mendel wondered what had happened to the green trait. Had it disappeared? To answer this, Mendel continued the cross for another generation by crossing the F1 plants.

Mendel crossed the F1 plants. It was not necessary to open up the flower and cross fertilize it with pollen from another F1 plant. He simply allowed the F1 plants to self-fertilize. The offspring from this cross constituted the F2 generation or second filial generation.

Mendel saw that the green trait reappeared in the F2 generation. Mendel called the trait that was hidden in the F1 generation but that reappeared in the F2 generation recessive.

Mendel carefully counted and recorded the number of the yellow and green seeds. Mendel’s results were 6,022 yellow seeds and 2,001 green seeds out of a total of 8,023 seeds. Reducing the numbers to a ratio by dividing by the lowest number (2,001), Mendel found that the ratio was 3 yellow: 1 green. The outward appearance of a trait (yellow or green in this example) is known as the phenotype. Therefore, the phenotypic ratio in a monohybrid cross is 3:1.

The outward appearance of a trait is known as the phenotype .

The genetic makeup of an individual is its genotype .

Mendel noted two important points from this experiment:

1. Although the green-seed trait has disappeared in F1 it reappeared in F2.

2. When the green-seed trait reappeared it was unchanged from its appearance in the P1 parent.

Mendel reasoned that the factor responsible for the green seed trait must have been present In the F1 plants but appeared hidden. Although the trait was hidden, it was not altered during its residence there. Mendel inferred that each original P1 plant contributed information for producing seed color to the F1 generation. Because the yellow trait appeared and the green trait appeared hidden, Mendel called the trait that appeared in the F1 generation (in this case yellow) dominant. The trait that appeared hidden he called recessive.

Mendel found that during the genetic crosses, the hereditary characteristics remained distinct. There had not been any blending of characteristics, as was proposed for heredity by an idea popular at that time. Mendel called these distinct factors that controlled heredity Elemente. They are now known as genes.

Mendel called upon his background in mathematics in order to explain why his genetic crosses resulted in the appearance of traits in constant proportions. Mendel deduced that the 3:1 ratio of dominant to recessive traits in the F2 generation could occur if each individual possesses only two hereditary units that supply information for each character. He realized that the genes must have occurred in the offspring in pairs, one gene inherited from each parent. These pairs of genes were separated again when the mature F1 plants produced gametes, with one gamete containing one member of the pair and the other gamete containing the other member.

Alternative forms of the same gene are now known as alleles.

We know today that this separation of the alleles of the gene pair would occur during the type of division known as meiosis.

When the alleles of a gene pair are the same, for example two genes for yellow, the organism is said to be homozygous for that particular trait. When the individual has two different alleles for a particular trait it is said to be heterozygous for that trait.

An individual could have two of the same alleles for a dominant trait. For example, a pea plant could have two copies of the gene for yellow. This plant would breed true for yellow generation after generation. Such an organism is known as homozygous dominant. Or a pea plant could have two copies of the gene for the recessive trait green. Such an organism is known as homozygous recessive.

The recessive allele will separate from its dominant partner when gametes are again formed. Only if two recessive alleles come together – one from the female gamete and one from the male – will the phenotype then show the recessive trait.

Another possibility is that the individual could carry two alleles of the gene pair that are different. If one allele is dominant to the other, the trait controlled by the dominant gene would appear and the trait controlled by the recessive gene would be hidden.

The Law of Segregation

The results of Mendel’s experiments on dominant and recessive inheritance led to what is now known as Mendel’s first law: The Law of Segregation. According to this law, individuals carry two discrete hereditary factors (genes) for each trait. One allele for each trait is received from each parent. The alleles come together in the zygote as the sperm and egg fuse during the process of fertilization. The alleles separate again when the mature F1 plants produce gametes. During meiosis these two alleles segregate, or become separated, from each other. One allele for every character is then incorporated into each maturing gamete and is transmitted during fertilization in an unaltered state to the next diploid generation.

The hypothesis that every individual carries pairs of factors for each trait and that the members of the pair segregate during the formation of gametes is known as Mendel’s first law, or the principle of segregation.

A Cross using Genetic Shorthand and a Punnett Square

Let’s perform the same cross above but this time lets use genetic shorthand and a

Punnett square.

We are going to cross a pea plant that produces yellow seeds with a pea plant that produces green seeds.

Yellow X Green

The pea plant that produces yellow seeds breeds true for the yellow seeded trait and is homozygous for yellow. Let’s let Y stand for the yellow gene. There are two copies in the homozygous yellow parent plant so its genetic makeup is YY. Let’s let y stand for the recessive green trait. Again there are two copies, so this parent is yy. The cross becomes: YY X yy The first generation is called the P1 or parental generation.

P1 YY X yy

A YY parent can only produce gametes with a Y gene. Let’s say that this is a male gamete. A yy parent can only produce gametes with a y gene. Let’s say that this is a female gamete. When a sperm cell containing a Y gene fertilizes an egg cell containing a y gene, the result is a zygote that is Yy.

A cross of a YY parent with a yy parent results in offspring that are Yy. The generation resulting from this cross is the F1 or first filial generation. All of the offspring have two different alleles for seed color. They are heterozygous. Because yellow is dominant to green, the outward appearance of the individuals, or their phenotype will be yellow. The genetic makeup or genotype of these individuals is Yy.

The cross is continued by allowing the heterozygous F1 individuals to self-fertilize. The result of this cross is the same as if pollen from one Yy individual were dusted onto the stigma of a different Yy individual.

F2 Yy X Yy

To show the results of this cross, a Punnett Square is used. For a monohybrid cross such as this, a box with four squares is used. On the top of the box the alleles present in the male gametes are shown. Because the alleles separate as the gamete is formed, one column is headed with Y, and the other column is headed with y. The alleles present in the female gametes are placed on the side of the box, a Y for one row and y for the other row.

Y y

Y y	YY	Yy
	Yy	yy

The result of this cross include: one YY individual. This homozygous individual is yellow. There are two heterozygotes (Yy). Because yellow is dominant, they appear yellow. There is one yy. Because it is homozygous and two recessive genes occur together, its appearance is green. Based on outward appearance, there are three yellow individuals for every one green. The phenotypic ratio is

3 Yellow: 1 Green.

Based on genetic makeup, there is one homozygous dominant YY, there are two heterozygotes Yy, and one homozygous recessive yy. The genotypic ratio is

1 YY: 2Yy: 1yy.

Exercise

Perform a genetic cross of pea plants with purple flowers with pea plants with white flowers. Use W for the purple trait, and w for the white trait. Show the results for the F1 and F2 generation.

P1

F1

A Test Cross

When Mendel carried out a cross of two plants that were heterozygous for yellow seeds, he found that in the F2 generation, there were three yellow-seeded plants to 1 green-seeded plant. Although the yellow-seeded plants had seeds that all looked alike, Mendel realized that genetically, there were two kinds of yellow-seeded plants. In order to test this explanation, Mendel performed what is known as a test cross. A test cross is a cross between an individual with the dominant phenotype (but unknown genotype, that is either homozygous dominant or heterozygous) with a homozygous recessive. The result of the cross will reveal whether the parent with the dominant trait was homozygous or heterozygous for the trait.

Using the example of a cross of yellow-seeded with green-seeded plants, let’s assume the parent chosen from the F2 generation of plants with the yellow-seeded phenotype was homozygous (YY). The test cross would be:

YY X yy

y y

Y	Yy	Yy
Y	Yy	Yy

All of the individuals resulting from the cross of an individual homozygous for yellow seeds with an individual that is homozygous recessive (green-seeded) will appear yellow. The parent with the unknown genotype was YY.

Now consider the cross in which a heterozygous individual is chosen by chance and used for the cross:

Yy X yy

y y

Y	Yy	Yy
y	yy	yy

In this case there are two yellow-seeded individuals and 2 green-seeded individuals, a ratio of 1 yellow-seeded to 1 green-seeded. The parent with the unknown genotype was Yy.

The Principle of Independent Assortment

Mendel also performed genetic crosses between individuals that differed in two characteristics, for example seed shape and seed color. Such a cross is known as a dihybrid cross. An example of such a cross is a cross in which one parent had seeds that were round and yellow and the other parent had seeds that were wrinkled and green.

Before he performed this cross, Mendel wondered: Would the round seeded trait stay together with yellow, and would the wrinkle-seeded trait stay with green or would new combinations such as round, green or wrinkled, yellow appear?

P1 RRYY X rryy

F1 RrYy

To carry out the cross of two F1 hybrids to produce the F2 generation, a Punnett square with 16 boxes is required.

RY Ry rY ry

RY	RRYY	RRYy	RrYY	RrYy
Ry	RRYy	RRyy	RrYy	Rryy
rY	RrYY	RrYy	rrYY	rrYy
ry	RrYy	Rryy	rrYY	rryy

Analysis of the results showed that new combinations of the round and yellow and wrinkled and green traits appeared. Although there were combinations that were like the original parents, Round Yellow and Wrinkled Green, there were new combinations including Round Green and Wrinkled Yellow. These traits sorted themselves out as if they were independent of one another. From this, Mendel formulated his second law the Principle of Independent Assortment .

The Principle of Independent Assortment states that members of each pair of genes are distributed independently when the gametes are formed.

This was Mendel’s second law.

The phenotypes that resulted from the cross included the following:

1. Round Yellow

RRYY, RRYy, RrYY, RrYy, RRYy, RrYy, RrYY, RrYy, RrYy,

Altogether there were 9 Round Yellow

2. Round Green

RRyy, Rryy, Rryy

Altogether there were 3 Round Green

3. Wrinkled Yellow

rrYY, rrYy, rrYy

Altogether there were 3 Wrinkled Yellow

4. Wrinkled Green

rryy

There was one Wrinkled Green

Phenotypic Ratio from a Dihybrid Cross

The phenotypic ratio from a dihybrid cross was 9:3:3:1.

Incomplete Dominance

Scientists who performed genetic crosses after Mendel discovered a type of inheritance that at first seemed to violate the principles of Mendelian Genetics. This type of inheritance, known as incomplete dominance seemed to involve a blending of characteristics, which had not been seen in the crosses that Mendel performed. Incomplete dominance can be seen in the cross of red and white snapdragon flowers.

When a true-breeding red-flower strain of snapdragon (RR) is crossed with a true-breeding white-flower strain (rr), the flowers produced in the F1 plants are pink not red.

P1 RR X rr

Red White

F1 Rr

Pink

F2

R r

R	RR red	Rr Pink
r	Rr pink	rr white

In the F2 generation one-half of the plants have pink flowers, one-fourth have red flowers, and one-fourth have white flowers. This 1:2:1 phenotypic ratio directly reflects the genotypic ratio 1RR:2Rr:1rr. The results show that the red gene is incompletely dominant over the white, causing a plant with Rr alleles to be pink rather than red.

Incomplete dominance is a genetic situation in which both alleles of a heterozygous pair exert an effect, jointly producing a phenotype intermediate between the two.

Mutation: One Source of Genetic Variation

Mutation is a change in the chemical structure of a gene or in the physical structure of a chromosome.

A point mutation alters the properties of a single gene and creates new alleles.

Changes that alter the structure of chromosomes called chromosomal mutations involve rearrangement of blocks of genes in the chromosome, not alteration at a point in one gene.

Gene deletion is the complete removal of a gene from a chromosome.

Duplication is the repetition of a section of a chromosome.

Inversion is ‘flipping over” of a section of a gene, or rotation of the section of the gene 180º.

Translocation is the movement of a gene or a group of genes to a completely different location on the chromosome or on a different chromosome.

Corrected 11/17/14

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