Incomplete dominance

 Works on problems of heredity have shown that the dominance is not of universal occurrence and there are many examples of incomplete dominance in which the genes of an allelomorphic pair express themselves partially when present together in the hybrid. As a result the heterozygotes (Aa) are phenotypically intermediate between two homozygous types (AA* aa).

For instance, when red snapdragon plants are crossed with white snapdragon plants, all the F₁ hy­brids have pink flowers. This third phenotype results from the heterozygote flowers having less red pigment than the red homozygotes. The breeding of the F₁ hybrids produces F₂ offspring with a phenotypic ratio of 1 red to 2 pink to 1 white. In incomplete dominance we can distinguish the heterozygotes from the two homo­zygous varieties, and the genotypic and phenotypic ratios for the F₂ generation are the same, 1:2:1. The segregation of the red and white alleles in the gametes produced by the pink-flowered plants confirms that the genes for flower color are heritable factors that maintain their identify in the hybrids; that is, inheritance is particulate.

It is incomplete dominance - the kind of inheritance of allelic genes where a cross between organ­isms with two different phenotypes (AA x aa) produces offspring with a third phenotype that is a blending (Aa) of the parental traits. Incomplete dominance is manifested when the interacting enzymes are slightly different in their activity.

In humans, traits with incomplete dominant inheritance are size of nose, salience of lips, size of mouth and eyes, distance between eyes, hair types (straight, wavy) and such hereditary disorders as Friedreich’s ataxia, cystinuria are inherited according to principle of incomplete dominance. For any character, the domi- nant/recessive relationship we observe depends on the level at which we examine phenotype; e.g., con­sider a fatal recessive Tay-Sachs disease, inherited disorder of lipid metabolism when crucial enzyme hexosaminidase does not work properly. Brain cells of Tay-Sachs babies lack a crucial lipid-metabolizing enzyme. Thus, lipids accumulate in the brain, causing the disease symptoms and ultimately leading to death.

At the organism level of normal versus Tay-Sachs phenotype, the Tay-Sachs allele qualifies as a re­cessive (aa).

At the biochemical level,however, we observe intermediate phenotype characteristic of incomplete dominance. The hexosaminidase enzyme deficiency can be detected in heterozygotes who have an activity level of the lipid-metabolizing enzyme that is intermediate between individuals homozygous for the normal allele and individuals with Tay-Sachs disease. Heterozygous individuals are genetically programmed to produce only 40-60% of the normal amount of an enzyme that prevents the disease.

Mendel's first law: Law of Segregation

Mendel did not formulate his conclusions as laws or principles of genetics, but later researchers have done so. Restating and using modern, standardized terminology, this is the information that developed and expanded from his early experiments.

1. Inherited traits are encoded in the DNA in segments called genes, which are located at particular sites (loci, singular locus) in the chromosomes. (Genes are Mendel's “factors.”)
2. Genes occur in pairs called alleles, which occupy the same physical positions on homologous chromosomes; both homologous chromosomes and alleles segregate during meiosis, which results in haploid gametes.
3. The chromosomes and their alleles for each trait segregate independently, so all possible combinations are present in the gametes.
4. The expression of the trait that results in the physical appearance of an organism is called the phenotype in contrast to the genotype, which is the actual genetic constitution.
5. The alleles do not necessarily express themselves equally; one trait can mask the expression of the other. The masking factor is the dominant trait, the masked the recessive.
6. If both alleles for a trait are the same in an individual, the individual ishomozygous for the trait, and can be either homozygous dominant or homozygous recessive.
7. If the alleles are different—that is, one is dominant, the other recessive—the individual is heterozygous for the trait. (Animal and plant breeders often use the term “true-breeding” for homozygous individuals.)

Geneticists use a standard shorthand to express traits using letters of the alphabet, upper case for dominant, lower case for recessive. Red color, for example, might be Ror r so a homozygous dominant individual would be RR, a homozygous recessive individual, rr and a heterozygous individual Rr.

Crosses between parents that differ in a single gene pair (such as those that Mendel made) are called monohybrid crosses (usually TT and tt). Crosses that involve two traits are called dihybrid crosses. Symbols are used to depict the crosses and their offspring. The letter P is used for the parental generation and the letter F for the filial or offspring generation. F1 is the first filial generation, F2 the second, and so forth.

What kinds of crosses did Mendel make to conclude that factors/genes segregate? First of all, he made certain that the plants that he planned to use in the experiment were pure line for the trait—that is, that they bred true for the trait for two or more years. (Peas are self-pollinated so he simply grew the plants and examined their offspring.) Other experimenters omitted this step, which confounded their results. Mendel then made a series of monohybrid crosses for each of the seven traits he had identified using parents of opposite traits—tall (TT) vs. dwarf (tt), yellow seed (YY) vs. green (yy) seed, round seed (RR) vs. wrinkled (rr), and so forth. (He, of course, did not symbolize them with letters, but he did know that seeds from his tall pure-line plants would always produce tall plants, seeds from the dwarfs would always produce dwarf plants, and so on.)

 

Mendel then let the F1 plants self-pollinate: Tt × Tt and in the F2 generation counted the numbers of individuals with each of the traits. For the tall × dwarf crosses he got 787 tall plants and 277 dwarf plants (6,022 yellow seeds and 2,001 green seeds, and so forth).

 

An easy way to determine the possible gene combinations is to construct a Punnett square, a grid in which all the possible gametes from one parent are listed on one side and those from the second parent across the top. Combine the gametes from the side and the top in the squares, and all of the possible gamete combinations are diagrammed. The previous cross in a Punnett square would look like this:

 

You can see from the Punnett square that three of the four gamete combinations will contain at least one dominant allele (T) and that there is only one chance out of four that the recessive (t) can be expressed. Mendel's experimental results fit the phenotypic probability ratio of 3:1. The genotypic ratio, which Mendel didn't know about, is not 3:1, but 1:2:1. That is, 1 homozygous dominant (TT):2 heterozygous dominants (Tt):1 homozygous recessive (tt). The Punnett square shows only thepossible combinations, not the actual. It provides an easy way to visualize theprobabilities of a certain combination occurring. In some inherited traits, whether the allele comes from the male or the female parent can make a difference, but in most traits such information does not matter.

After making monohybrid crosses for all the traits and finding that the ratios always approximated 3:1, although the actual numbers of plants and offspring for each cross varied, Mendel concluded that the traits must be carried in pairs that segregate (separate) when gametes are formed. This conclusion is now known as Mendel's first law, the Law of Segregation.

To confirm his hypothesis, he made another kind of cross, a backcross, which mates an offspring with one of its parents. Mendel backcrossed his F2 tall plants to the dwarf parent and got half tall plants, half dwarf, a 1:1 ratio. If he had backcrossed to the tall parent, what would the ratio have been? Right, all tall; that's why breeders today maketest crosses back to the homozygous recessive parent to see if their phenotypically dominant individuals are homozygous or heterozygous.

 

General Characteristics of Viruses

 General Characteristics of Viruses

Viral structure: Typical viral components are shown in Fig. 2. These components are a nucleic acid core and a surrounding protein coat called a capsid. In addition some viruses have a surrounding lipid bilayer membrane called an envelope.

Fig. 2. The components of helical virus

A.  Nucleic acid

• Viral genomes are either DNA or RNA (not both)

• Nucleic acid may be single- or double-stranded

Fig. 3. Types of virus genomes 

B.  Capsid

• Protein coat

• Protection of Nucleic Acid

• Provides Specificity for Attachment

• Capsomeres are subunits of the capsid

Fig. 4. Capsid structure

C.  Envelope

• Outer covering of some viruses

• Envelope is derived from the host cell plasma membrane when the virus buds out

• Some enveloped viruses have spikes, which are viral glycoproteins that project from the envelope

• Naked (non-enveloped) viruses are protected by their capsid alone

Fig. 5. Enveloped helical virus

 

 

2.  Size of viruses:

• Determined by electron microscopy

• Ranges from 20 to 14000 nm in length

Fig. 6. Size of different viruses

 

3.  Shape of viruses:

 

Four basic morphologies

• Icosahedral - efficient means to conserve and enclose space; form capsomers (planar faces formed by association of proteins) 

• Helical - capsid is shaped like a hollow protein tube 

• Enveloped - outer covering derived from the host cell's nuclear or plasma membrane and often possessing spikes or peplomer projections involved in attachment and entry into a host cell sometimes via their enzymatic activity 

• Complex symmetry - viruses that fit neither of the above categories or which may employ portions in combination, e.g., bacteriophage

Fig.7. Types of viral symmetry

 

4.  Host Range: The specific types of cells a virus can infect in its host species represent the host range of the virus.

 

• Animal virus

• Plant virus

• Bacterial virus (bacteriophage)

Host range is determined by attachment sites (receptors)   

 

Important points to remember: 

• VIRION – a complete single viral particle

• Obligatory intracellular parasites

• Contain DNA or RNA

• Do not undergo binary fission

• Sensitive to interferon

• Contain a protein coat

• Some are enclosed by an envelope

• Some viruses have spikes

• Most viruses infect only specific types of cells in one host

• Host range is determined by specific host attachment sites and cellular factors (receptors)

• Viruses replicate through replication of their nucleic acid and synthesis of the viral protein.

• Viruses do not multiply in chemically defined media

• All ss-RNA viruses with negative polarity have the enzyme transcriptase (RNA dependent RNA polymerase) inside virions.

• Retroviruses and hepatitis B virus contain the enzyme reverse transcriptase.  

Ecological Pyramids

Ecological Pyramids: In a food chain, producers and consumers at different trophic level are connected in terms of number, biomass and energy. These properties reduces from producers to consumers and representing these parameters for food chain gives a pyramid with a broad base and a tapering apex (Figure 39.6). Ecological pyramids can be of three types:

(a)     Pyramid of Numbers

(b)     pyramid of biomass

(c)     pyramid of energy

Example of inverted ecological pyramid is provided by parasitic food chains (Figure 39.7). A single mango tree supports large number of birds, which in turn supports a large number of parasites like lice and bugs. Hyperparasites, such as bacteria and fungus are the greatest in the number and occupy the top of the inverted pyramids.

 

Flow of energy in food chain: Sun is the ultimate source of energy on earth and plants utilizes it to produce food for rest of the member of the ecosystem. Only the 1% of the total energy fall on green part of leaves is changed into the potential energy of the organic substances, the rest of the energy dissipates as heat. To explain the flow of energy, lindermann proposed the law of ten per cent law. This law proposed that during transfer of food energy from one trophic level to the other, only 10% is stored at higher trophic and the rest 90% is lost in respiration, decomposition and waste in the form of heat (Figure 39.8). For example, 5000 jules fall on leaves, it will convert only 50 jules into the chemical form (food). It will be eaten by rabbit, he will get only 5 jules (10% of 50 jules) on next trophic level. Rabbit will be consumed by carnivorous, and they can be able store only 0.5 jules (10% of 5 jules).

Ecological Equilibrium: Ecosystem always remains in the state of equilibrium. The equilibrium is dynamic is nature and biotic components appear and disappear time to time due to their death or predator. In addition, decomposer converts the complex organic matter of dead plant and animals into the simple inorganic substances. These simple inorganic substances pass through the soil, plants and animals in a cyclic manner, and this keeps the life going on in an ecosystem. Thus, both biotic and abiotic components are in a dynamic state.

 

ECOLOGY TERMINOLOGY

•  Ecology is a branch of biology concerned with the study of the interactions of living organisms with each other and with their environment.
• An ecosystem is a community of organisms that interact with their environment.
• Biosphere is a region of the earth where life can exist.(atmosphere, hygrosphere, lithosphere)
• A habitat is a place where an organism lives.
• An abiotic factor is anything that is non-living and has an effect on living organisms in an ecosystem. The two main types are:

1. Climatic factors are weather conditions that have an effect on living organisms in an ecosystem.
2. Edaphic factors are anything relating to the soil or geology of land that have an effect on living organisms in an ecosystem.

• A biotic factor is anything that is living and has an effect on living organisms in an ecosystem. (e.g presence of predator, presence of pathogenic organisms).
• Pathogenic: capable of producing disease.
• A grazing food chain is a relationship of the sequence of predator-prey relationships in an ecosystem.
• A food web consists of two or more interconnected food chains.
• An ecological pyramid of numbers shows the numbers of organisms at each trophic level in a food chain. (May be upright, partially upright or inverted in shape.
• Niche refers to the functional role an organism plays in its habitat.
• A population is a group of organisms living in a habitat that belong to the same species.
• A community is a group of organisms living in a habitat that belong to many different species.
• Competition is the struggle between organisms for a resource that is in limited supply.

1. Contest competition is the direct fight between two organisms for a resource that is in short supply. (e.g. two stags fighting for a mate)
2. Scramble competition is the struggle amongst a number of organisms for a resource that is in short supply. Each organism gets a small share of the resource. (e.g. a pack of vultures competing for a portion of the kill made by a large predator)

• A resource is a stock or supply (such as food) that can be drawn on.
• Predation is the catching, killing and eating of another organism.
• Symbiosis is the biological relationship in which two species live in close proximity to each other and interact regularly in such a way as to benefit one or both of the organisms.
• Mutualism is when both of the organisms benefit from the presence of each other, e.g. N2-fixing bacteria that live in root nodules of legume plants (such as peas) assimilate NO3- from N2.
• Parasitism is where one organism, called the parasite, lives in or on another organism, called the host, and the host is harmed. (e.g. aphids are parasites of plants, athletes foot and mosquitoes)