Principles of inheritance- Mendelian laws

CELL BIOLOGY AND GENETICS

INTRODUCTION Mendelian laws of inheritance are

statements about the way certain characteristics are transmitted from one generation to another in an organism.

The laws were derived by the Austrian monk Gregor Mendel (1822–1884) based on experiments he conducted in the period from about 1857 to 1865.

For his experiments, Mendel used ordinary pea plants.

Among the traits that Mendel studied were the color of a plant's flowers, their location on the plant, the shape and color of pea pods, the shape and color of seeds, and the length of plant stems.

Mendel's approach was to transfer pollen (which contains male sex cells) from the stamen (the male reproductive organ) of one pea plant to the pistil (female reproductive organ) of a second pea plant.

As a simple example of this kind of experiment, suppose that one takes pollen from a pea plant with red flowers and uses it to fertilize a pea plant with white flowers. What Mendel wanted to know is what color the flowers would be in the offspring of these two plants. In a second series of experiments, Mendel studied the changes that occurred in the second generation. That is, suppose two offspring of the red/white mating ("cross") are themselves mated. As a result of these experiments, Mendel was able to state three generalizations about the way characteristics are transmitted from one generation to the next in pea plants.

Gregor Mendel’s monastery garden.

Traits studied by Mendel

Dominant

Recessive

GENETIC VOCABULARY Generations:

P = parental generation F1 = 1st filial generation, progeny of the P generation F2 = 2nd filial generation, progeny of the F1 generation

(F3 and so on) Crosses:

Monohybrid cross = cross of two different true-breeding strains (homozygote) that differ in a single trait.

Dihybrid cross = cross of two different true-breeding strains (homozygote) that differ in two traits.

Allele: One of two or more forms a gene may take. Dominant: An allele whose expression overpowers the

effect of a second form of the same gene. Gamete: A reproductive cell. Heterozygous: A condition in which two alleles for a given

gene are different from each other. Homozygous: A condition in which two alleles for a given

gene are the same. Recessive: An allele whose effects are concealed in

offspring by the dominant allele in the pair.

LAW OF DOMINANCE The law of dominance is used to explain the expression of only one of the parental characters in a monohybrid cross in the F1 and the expression of both in the F2 generation. It also explains the 3:1 proportion obtained at the F2.

PREDICTING TRAITS The application of Mendel's three laws makes it possible to predict the

characteristics of offspring produced by parents of known genetic composition.

For example, the cross between a sweet pea plant with red flowers (RR) and one with white flowers (rr).

RR → R + R and rr → r + r There are, then, four ways in which those alleles can recombine, However,

all four combinations produce the same result: R + r → Rr. In every case, the gene formed will consist of an allele for red (R) and an allele for "not red" (r).

When two plants from the first generation are crossed with each other. Again, the alleles of each plant separate from each other:

Rr → R + r Again, the alleles can recombine in four ways. In this case, however, the

results are different from those in the first generation. The possible results of these combinations are two Rr combinations, one RR

combination, and one rr combination. Since R is dominant over r, three of the four combinations will produce

plants with red flowers and one (the rr option) will produce plants with white flowers.

Hence proof of law of dominance.

PICTORIAL REPRESENTATION OF LAW OF DOMINANCE

LAW OF SEGREGATION•Mendel's law of segregation is based on the fact that the alleles do not show any blending and both the characters are recovered as such in the F2 generation though one of these is not seen at the F1 stage.•For example, suppose that a pea plant contains a gene for flower color in which both alleles code for red. •One way to represent that condition is to write RR, which indicates that both alleles (R and R) code for the color red. •Another gene might have a different combination of alleles, as in Rr. In this case, the symbol R stands for red color and the r for "not red" or, in this case, white. •Mendel's law of segregation says that the alleles that make up a gene separate from each other, or segregate, during the formation of gametes such that a gamete receives only one of the 2 alleles.•That fact can be represented by simple equations, such as: •RR → R + R or Rr → R + r

PICTORIAL REPESENTATION OF LAW OF SEGRAGATION

INCOMPLETE DOMINANCEIncomplete dominance (also called partial dominance or semi-dominance) occurs when the phenotype of the heterozygous phenotype is distinct from and often intermediate to the phenotypes of the homozygous phenotypes. For example, the SNAPDRANGON flower color is either homozygous for red or white. When the red homozygous flower is paired with the white homozygous flower, the result yields a pink snapdragon flower. The pink snapdragon is the result of incomplete dominance. A similar type of incomplete dominance is found in the FOUR’O CLOCK flower wherein pink color is produced when true-bred parents of white and red flowers are crossed. In quantitative genetics, where phenotypes are measured and treated numerically, if a heterozygote's phenotype is exactly between (numerically) that of the two homozygote's, the phenotype is said to exhibit no dominance at all, i.e. dominance exists only when the heterozygote's phenotype measure lies closer to one homozygote than the other.When plants of the F1 generation are self-pollinated, the phenotypic and genotypic ratio of the F2 generation will be 1:2:1 (Red:Pink:White)

INCOMPLETE AND CO- DOMINANCE

CO- DOMINANCECo-dominance occurs when the contributions of both alleles are visible in the phenotype.For example, in the ABO blood group system, chemical modifications to a glycoprotein (the H antigen) on the surfaces of blood cells are controlled by three alleles, two of which are co-dominant to each other (IA, IB) and dominant over the recessive i at the ABO locus. The IA and IB alleles produce different modifications. The enzyme coded for by IA adds an N-acetylgalactosamine to the membrane-bound H antigen. The IB enzyme adds a galactose. The i allele produces no modification. Thus IA and IB alleles are each dominant to i (IAIA and IAi individuals both have type A blood, and IBIB and IBi individuals both have type B blood, but IAIB individuals have both modifications on their blood cells and thus have type AB blood, so the IA and IB alleles are said to be co-dominant).Another example occurs at the locus for the Beta-globin component of hemoglobin, where the three molecular phenotypes of HbA/HbA, HbA/HbS, and HbS/HbS are all distinguishable by protein electrophoresis. (The medical condition produced by the heterozygous genotype is called sickle-cell trait and is a milder condition distinguishable from sickle-cell anemia, thus the alleles show incomplete dominance with respect to anemia). For most gene loci at the molecular level, both alleles are expressed co-dominantly, because both are transcribed into RNA.

LAW OF INDEPENDENT ASSORTMENT

Mendel's second law is called the law of independent assortment.

That law refers to the fact that any plant contains many different kinds of genes.

One gene determines flower color, a second gene determines length of stem, a third gene determines shape of pea pods, and so on.

Mendel discovered that the way in which alleles from different genes separate and then recombine is unconnected to other genes.

That is, suppose that a plant contains genes for color (RR) and for shape of pod (TT).

Then Mendel's second law says that the two genes will segregate independently, as:

RR → R + R and TT → T + T

PICTORIAL REPRESENTATION OF INDEPENDENT ASSORTMENT