Biology Kenya
Genetics
Genetics
Genetics
Concepts of genetics
First law of hereditary
FIRST LAW
This law is also called Mendel’s first law of inheritance or law of segregation. The law states, “An organism’s characteristics are determined by internal factors which occur in pair”. Only one of the factors can be contained in a single gamete.
In modern terms this means that genes occurring in pairs control the characteristics of an organism but only one gene can be carried in a single gamete.
There are four main concepts in this law:
- Genes can exist in more than one form
- An organism inherits two alternative form of a gene for a particular trait, one from each parent
- During the production of gametes pair of alleles separate. Thus each gamete has one allele for each trait.
- When the two alleles in a pair are different one is dominant while the other is recessive. This condition is called complete dominance.
When inheritance of one pair of characteristics is studied at a time it is called Monohybrid inheritance.
MONOHYBRID INHERITANCE
This is an inheritance of one pair of characteristic or trait at a time.
Example 1:
Mendel selected tall plants and self pollinated them.
Consider below
chart Tall x Tall
His results were that all trees were tall.
Also he cross-pollinated the pure breed tall plant and pure breed short (dwarf) plants. Consider belowchart Tall x dwarf
All plants produced are normal and also are known as the first filial generation (F1).Hence he concluded that the tallness is said to be dominant which the gene for dwarfness is said to be recessive in the garden pea. Because the gene of dwarf are masked by the gene for tallness.Hybrid is an offspring of a cross-between parents showing unlike characteristics.Test Cross (Back cross) this is the cross that involves off springs of two different pure lines.
Interpretation of data from Monohybrid Experiments to Demonstrate Mendel’s First Law of Inheritance
Some conditions in human follow Mendelian monohybrid inheritance. Example, a condition that is associated with a simple pair of alleles and are inherited in Mendelian fashion
Examples of such conditions are:
- Albinism
- Sickle Cell anemia
- Rhesus blood group
- Haemophilia
- Achondroplasia
Patterns of Inheritance that Follow Mendel’s First Law
DIHYBRID INHERITANCE
Mendel continued to study the inheritance of two pairs of characteristics. This inheritance is known as dihybrid cross.Dihybrid Cross is the inheritance of two characteristics in which each is controlled by a different gene, different locus.
Examples of two characteristics to an organism:
- Tall with purple flower
- Dwarf with white flower
From the above experiment Mendel made the following conclusions:
- Two phenotypes in the ration 9:3:3:1 resembled one or other of the parent
- Two phenotypes did not resemble any of the parents’ phenotypes but instead had combined the characteristics of both parents
- The ratio of tall to dwarf plants was 3:1 and that of the purple-flowered plant was 3:1
Concepts of Incomplete Dominance and Co-dominance
Not all inheritance follows Mendelian fashion. Mendel only considered characteristics that were determined by single genes with two alleles in which one is dominant and the other recessive. Later research showed that in some alleles neither one is dominant over the other. That condition is known as co-dominance or incomplete dominance.
Incomplete Dominance
This is the condition in which no allele is dominant or recessive compared to the other example when red and white flowered varies of the four o’clock plants are crossed, all the plants of the F1 generation produce pink flowers.
Complete dominance
Complete Dominance is a condition in which a dominant gene completely masks recessive gene. Example: Homozygous tall plant crossed with the homozygous short/dwarf plant. chart showing Tall x Dwarf.
Patterns of Inheritance that deviates from Mendel’s First Law of Inheritance
NON-MENDEL INHERITANCE
Not all inheritance follows Mendelian fashion. Mendel only considered characteristics that were determined by single genes with two alleles in which one is dominant and the other recessive. Later research showed that in some alleles neither one is dominant over the other. That condition is known as co-dominance or incomplete dominance.
INCOMPLETE DOMINANCE
This is the condition in which no allele is dominant or recessive compared to the other
Example when red and white flowered varies of the four o’clock plants are crossed, all the plants of the F1 generation produce pink flowers.
Consider below chart Red x White
INHERITANCE OF ABO BLOOD GROUPS
The entire human population falls under four main blood group that are –A, B, AB and O.Allele A and B are condomint white allele O is recessive to both A and B.
Example: parents with heterozygous blood group A and B have off spring with blood group A, B, AB and O as illustrated in the following cross.
Phenotype Blood group A x Blood group Bchart
Sex determination in humans
The Mechanism of Sex Determination and Inheritance
Human beings have 46 chromosomes (23 pairs of homologous chromosome). In every body cell of these, two are sex chromosomes while 44 are referred to as Autosome.
By definition:
Sex determination refers to the interpretation between male sex and female sex.
A diagrammatical representation of human sex determination is shown below.chart showing cross Male x Female
Linkage
The Concept of Sex Linked, Sex Limited and Sex Influenced Characters
This refers to the tendency in which one chromosome carries other genes.
Unlike other chromosomes in which each of the homologous chromosomes carries gene for the same characteristics, X and Y do not carry the same gene.
Consequences of Sex Preference and Sex Selection
Sex preference and selection is the tendency of people to like one type of sex more than the other. This tendency is very common in African countries and some parts of Asia.
Some people in a family prefer having boys than girls while others prefer girls to boys. Those who prefer boys do so in a belief that boys will perpetuate the linage and take care of the parents when females are living far away with their husbands. Those who prefer girls argue that, girls are kind and merciful; therefore they can take care of their parents in old age.
The sex preference and selection is influenced by a number of socio-cultural factors. Some of the factors include the following:
- Manpower Generation: Some societies prefer boys to girls because they generate wealth upon getting married. A family will get a lot of cattle or money as a bridal price.
- Generation and protection of wealth: Some societies prefer girls more than boys because girls will prefer to have more sons than girls so that they can somehow benefit indirectly through their son.
- Land ownership In some societies a woman cannot own land thus prefers more sons than daughters because they can benefit from the sons.
The concept of Variation
Variation, in biology, refers to any difference between cells, individual organisms, or groups of organisms of any species caused either by genetic differences (genotypic variation) or by the effect of environmental factors on the expression of the genetic potentials (phenotypic variation). Variation may be shown in physical appearance, metabolism, fertility, mode of reproduction, behavior, learning and mental ability, and other obvious or measurable characters. If you consider almost any characteristic, you will find differences between various people (or other animals or plants) in a population.
Variations among Organisms
Genetic variation describes naturally occurring genetic differences among individuals of the same species. This variation permits the flexibility and survival of a population in the face of changing environmental circumstances. Consequently, genetic variation is often considered an advantage, as it is a form of preparation for the unexpected. The variation between different species is always greater than the variation within a species.
Genetic variations are caused by differences in number or structure of chromosomes or by differences in the genes carried by the chromosomes. Eye colour, body form, and disease resistance are genotypic variations. A variation cannot be identified as genotypic by observation of the organism. Breeding experiments must be performed under controlled environmental conditions to determine whether or not the alteration is inheritable.
Environmentally caused variations may result from one factor or the combined effects of several factors, such as climate, food supply, and actions of other organisms. These variations do not involve any hereditary alteration and in general, are not transmitted to future generations.
The Meaning of Continuous and Discontinuous Variations
Types of variation
Variations are classified either as continuous, or quantitative (smoothly grading between two extremes, with the majority of individuals at the centre, as height in human populations); or as discontinuous, or qualitative (composed of well-defined classes, as blood groups in man). A discontinuous variation with several classes, none of which is very small, is known as a polymorphic variation. The separation of most higher organisms into males and females and the occurrence of several forms of a butterfly of the same species, each coloured to blend with a different vegetation, are examples of polymorphic variation.
Continuous variation
This type of variation exhibits a wide range of differences for the same characteristics, from one extreme end to the other. Characteristics showing continuous variation vary in a general way, with a broad range, and many intermediate values between the extremes. As a matter of fact, if you consider a large enough sample from a population, perhaps plotting frequency as a histogram or as a frequency polygon, you will find that most of the values are close to the average (mean), and extreme values are actually rather rare. Examples of continuous variations in human beings include weight, height and complexion. Height is an example of continuous variation. People vary in height from very short to very tall, with many intermediate heights.
Discontinuous variation
Discontinuous variation is a type of variation that shows sharp differences among individuals of a species, with no intermediate forms.
Individuals fall into a number of distinct classes or categories. This is based on features that cannot be measured across a complete range. A person either has the characteristic or not. There is no intermediate condition.
The ability to roll the tongue (one is either tongue roller or non tongue roller), fingerprints, sex (one is either male or female) and the ABO blood group system where one can only have blood group A, B, AB or O. and blood groups. In plants, a pawpaw tree is either male or female. These characteristics can be explained much more easily by simple rules of genetics and are less likely to be affected by other factors. Discontinuous variations are unchangeable and unaffected by the external environment.
Difference between Continuous and Discontinuous Variation
Some of the major differences between continuous and discontinuous variations in inheritance are as follows:
Continuous Variations:
- The variations fluctuate around an average or mean of species.
- Direction of continuous variations is predictable.
- They are already present in the population.
- Continuous variations are formed due to chance segregation of chromosomes during gamete formation, crossing over and chance pairing during fertilization.
- They can increase adaptability of the race but cannot form new species.
- Continuous variations are connected with the mean or average of the species by intermediate stages.
- The continuous variations are also called fluctuations.
- When represented graphically, continuous variations give a smooth bell shaped curve.
- They are very common.
- Continuous variations do not disturb the genetic system.
Discontinuous Variations:
- A mean or average is absent in discontinuous variations.
- The direction of discontinuous variations is unpredictable.
- Discontinuous variations are new variations though similar variations might have occurred previously.
- Discontinuous variations are produced by changes in genome or genes.
- Discontinuous variations are the fountain head of continuous variations as well as evolution
- These variations are not connected with the parental type by intermediate stages.
- Discontinuous variations are also known as mutations or sports.
- A curve is not produced when discontinuous variations are represented graphically.
- These variations appear occasionally.
- They disturb the genetic system of the organism.
Causes of Variation among Organisms
Variation can be due to inheritance, and also to environmental factors such as climate and diet.
Genetic causes of variation (inherited variation)
Some variation within a species is inherited. Variation in a characteristic that is a result of genetic inheritance from the parents is called inherited variation. Each egg cell and each sperm cell contains half of the genetic information needed for an individual. When these join at fertilisation a new cell is formed with all the genetic information needed for an individual. Examples of inherited characters in humans include eye colour, hair colour, skin colour and lobed or lobeless ears.
Gender is inherited variation too, because whether you are male or female is a result of the genes you inherited from your parents.
Genetic variation can be caused by mutation (which can create entirely new alleles in a population), random mating, random fertilization, and recombination between homologous chromosomes during meiosis (which reshuffles alleles within an organism's offspring). Some of these variation causes are explained in detail below:
Independent assortment of homologous chromosomes
This occurs at the time of gamete formation. At the time of gamete formation during meiosis, the parental chromosomes separate at random hence forming different gametes with different chromosomes. This independent assortment gives a wide variety of different gametes and hence individuals.
Crossing-over
Chromosomal crossover (or crossing over) is the exchange of genetic material between homologous chromosomes that results in recombinant chromosomes during sexual reproduction. Crossing over and random segregation during meiosis can result in the production of new alleles or new combinations of alleles. Portions of paired chromosomes may be exchanged to form new chromosomal and gene combinations in gametes resulting in new trait combinations in offspring.
Non-disjunction
Non-disjunction results into doubling of the chromosome number due to failure of chromosomes to segregate during meiosis. This leads to increase in cell size and subsequent increase in size of various parts of the organism, hence variation.
Mutations
Practical adaptations of genetics
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