Genetics

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Genetics
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 Genetics is the study of heredity and variations in organism. Through this statement there are two common words that are heredity and variation.Heredity is the passing on of characteristics from parents to the off spring.Variation is the observable differences in organisms from the same species.The hereditary characteristics are passed from parents to their offspring through distinct units called genes.Genes are hereditary materials or factors, which determine a specific characteristic or trait in an organism.

COMMON TERMS USED IN GENETICS
1. First filial generation (F1)
This is arising from the crossing of two pure breeds.
2. 
Second filial generation (F2)
This is the generation obtained by crossing individuals of the first filial generation.
3. Genotype
This is the genetic makeup of an organism or an individual
4. Phenotype
This is the outward appearance determined by a gene
5. Haploidy (n)
Having one set of unpaired chromosomes in the nucleus
6. Diploidy (2n)
Having two sets of homologous (similar) chromosomes in the nucleus
7. Alleles
It is the alternative number of genes in the nucleus or in the same position
8. Homolozygosity
This control of a character by two different alleles for example TT, BB or bb
9. Heterozygosity
This control of a character by two different alleles example Tt, Bb etc
10. Dominance
This is a condition where an allele can express itself in the presences of other alleles. Example: ‘Tt’ where ‘T’ is the dominant that expresses its effect in the presence of ‘t’.
11. Recessiveness
A condition where an allele can only express itself when they are in homozygous form but does not express its effect on the presence of other alleles.
12. Mutation
A sudden random change in the genetic make up of a cell, causing it and all cells derived from it to differ from normal cells.
13. Mutagen
This is an agent capable of increasing the rate of mutation in an organism like formaldehyde and nitrous acid.
14. Selfing
It s the crossing offspring od the same pair of parents

GENETIC MATERIALS
Hereditory characteristics are passed from parents to their offspring through distinct units called genes.There are a lot of genes in an organism’s body. Genes are arranged in a linear manner, making chromosomes.Chromosomes are thread like structures found in the nuclei of all body cells. Gene is made up of chemical substances called Nucleic acid.There are two types of nucleic acids found in cells, these are:
  • Deoxyribonucleic acid (DNA)
  • Ribonucleic acid (RNA)
These acids are made up of building blocks called nucleotides. Each nucleotide consists of three molecules linked together, that is a pentose sugar, phosphoric acid and organic base.

DNA
DNA is called the “molecule of life”. This is because it determines the physical and behavioural characteristics of an organism. The DNA determines example the colour of your hair, eyes, skin, ears and nose, height, ability or inability to roll the tongue all.The DNA is made up of many nucleotides that make the double stranded.

STRUCTURE OF DNA
DNA is a double stranded helical (spiral) molecular chain of a nucleic found within the nucleus of a cell.By “double stranded helical” it means that the DNA consists of two strands, which twist around each other in a spiral fashion.The DNA is made up of many nucleotides forming a polynucleotide chain. Polynucleotide means many nucleotides.The polynucleotide chain runs in the opposite direction. Each chain is joined to the other by pairs of bases. There are four bases namely Guanine (G), Cytosine , Adenine (A) and Thymine (T)[Image: STRUCTURE%20OF%20DNA.jpg]

Note:Guanine and adenine are collectively called purines while cytosine and thymine are collectively called pyrimidine.Guanine pairs with cytosine and adenine pairs with thymine.DNA plays a key role in inheritance because it replicates itself during mitosis and meiosis. DNA also undergoes changes and has genetic information the characteristics of a species.

RNA
The chromosomes replicate during cell division. The replication occurs during mitotic and meiosis cell divisionsThe chromosomes determine the type of protein synthesized. The genes determine the actual characteristics of the organisms. In protein synthesis deoxyribonucleic acid acts as a template for the formation of ribonucleic acid (RNA).

STRUCTURE OF RNA
RNA consists of only a single strand of polynucleotide. The polynucleotide is made up of many nucleotides. Each nucleotide consists of a nucleobase, ribose sugar and phosphate group.The RNA sugar is ribose and not deoxyribose. Its nucleotides contain only one of four bases that are:
  • Guanine (G)
  • Cytosine ©
  • Adenine (A)
  • Uracyl (U)
NB: Uracyl replaces the thymine of DNA. So in this the adenine can pair with uracil while the Guanine pairs with thymine.[Image: STRUCTURE%20OF%20RNA.jpg]The RNA is involved in protein synthesis. There are various types of RNA:
  • Messenger (mRNA)
  • Transfer (tRNA)
  • Ribosomal (rRNA)
PRINCIPLES OF INHERITANCE
Gregor John Mendel advanced the principles of inheritance. In 1856 – 1863 Mendel grew and tested some 29,000-pea plants. From these studies, he formulated the law of segregation and the law of assortment.After his work on peas, Mendel began to experiment with honeybees. However, he failed to produce a clear picture of their heredity because of difficulties in controlling the mating behaviour of queen bees.Mendes works was largely criticized and generally rejected during his lifetime. It was only after his death that his work gained broad recognition. He is now considered the father of modern genetics, Mendel died on January 6th, 1884.Mendel chose the garden peas because of the following reasons:
  • It is self-pollinating but can be cross-pollinated
  • It matures very fast
  • It produces many seeds and hence many off springs
  • It has several physical properties
Some of the characteristics that be studies were:
  • Height of the stem-tall or dwarfs
  • Texture of the seed coat – smooth or wrinkled
  • Colour of flowers – purple or white
  • Colour of pods – green or yellow
  • Position of flowers – axial or terminal
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 belowchart Tall x TallHis 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 dwarfAll 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.

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:
  1. Genes can exist in more than one form
  2. An organism inherits two alternative form of a gene for a particular trait, one from each parent
  3. During the production of gametes pair of alleles separate. Thus each gamete has one allele for each trait.
  4. When the two alleles in a pair are different one is dominant while the other is recessive. This condition is called 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

MONOHYBRID INHERITANCE IN HUMAN
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 fashionExamples of such conditions are:
  • Albinism
  • Sickle Cell anemia
  • Rhesus blood group
  • Haemophilia
  • Achondroplasia
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:
  1. Two phenotypes in the ration 9:3:3:1 resembled one or other of the parent
  2. Two phenotypes did not resemble any of the parents’ phenotypes but instead had combined the characteristics of both parents
  3. Ratio of tall to dwarf plants was 3:1 and that of purple flowered plant was 3:1

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 otherExample when red and white flowered varies of the four o’clock plant are crossed, all the plant of the F1 generation produce pink flowers.Consider belowchart 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
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 SEX-LINKED CHARACTERSThis 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.Examples of sex-linked disorders are haemophilia and colour blindness.

HAEMOPHILIA
This is a hereditary sex-linked condition in which the blood of a human being takes a long time to clot. The recessive gene located on the X-chromosome causes it.Consider belowIn inheritance of haemophilia is outlined below by H and the gene with haemophilia are represented by h.chart normal female x normal male

INHERITANCE OF COLOUR BLINDNESS
Colour blindness refers to the inability to distinguish between certain colour especially red and green colour.Also this brought by the recessive gene located on the X chromosomes. This disorder caused when these formation of three or more than of cone.Consider belowThe gene with colour blindness is represented with small c while the gene with normal vision is represented with capitalchart Phenotype normal female x normal maleExample2Parent phenotype carrier female x colourblind maleTherefore the female have the possibility of either being carrier or colour blind while males have the possibility of either being normal or color blind.

SEX-LIMITED CHARACTERISTICS
These are characters that are restricted to only one sex, either male or female. Genes for such characters may be carried on autosomes.On the sex-limited characters are the long hair of male lion and comb plumage of hens.

SEX PREFERENCE AND SELECTION AND ITS CONSEQUENCE IN SOCIETY
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:
  1. 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 bridal price.      
 b. 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.       
c. Land ownership

In some societies a woman cannot own land thus prefers more sons than daughters because they can benefit from the sons.  
GENETIC DISORDERS

The meaning of Genetic Disorders:This refers to an abnormality, which results from problems in the genes of an organism, and it is inherited.This means that the genetic disorder is caused by the change in the gene or chromosomes due to an error in the person’s genetic materials.ORGenetic disorders are malfunctioning of the body’s physiological mechanisms due to changes on gene or chromosomes; for example the change in number of chromosomes from 46 to 47 chromosomes or below that hence leading to genetic disorders.Genetic disorders includes:
  • Down’s Syndrome or Mongolism/Mongolia
  • Turner Syndrome
  • Super male and Super female
  • Haemophilia
  • Colour blindness
NOTE: The changes in the genes and chromosomes may be caused by different factors, which lead to genetic disorder. The sudden change in genetic materials of the cell that may cause it to differ from other cells is known as mutation.An organism is affected by mutation, which occurs naturally at a low rate. A number of factors may contribute to mutation. Such factors include various chemicals and radiation example X-rays.Mutation can be due to a change in gene itself i.e. this is called point mutation only or in the arrangement of gene chromosome. Other causes of mutation include addition or loss of chromosomes and duplication of genes.There are two types of mutation including chromosomal mutation and gene mutation. Both leads to the genetic disorder is to lead to change in gene and chromosome.Chromosomal mutation leads to genetic disorder like Down’s syndrome, Turner Syndrome, Klinefelter’s Syndrome. It affects the appearance or the number of chromosomes. Gene mutation leads to genetic disorders like Haemophilia and Colour blindness, which affects the genes.

Down’s syndrome or Mongolism:This is a chromosomal abnormality in which there are three copies of chromosome number 21 instead of the usual 2. The person with Down’s syndrome therefore has 47 chromosomes in his/her body cells. The affected individuals have a short broad face, slanted eyes, short fingers and weak muscles. Such individuals are usually mentally retarded.The presence of the extra chromosome on chromosome 21 is known as trisomy. The extra chromosome on chromosome 21 is due to its failure to separate during meiosis. This is known as non-disjunction. This non-disjunction occurs when homologous chromosome fail to separate in meiosis II of the egg. Therefore after the fertilization, chromosome 21 will contain 3 (i.e. the 2 which failed to separate plus the one from the father) instead of normal two (i.e. each from one parent). In some few cases non-disjunction may occur in the father’s sperm.

Causes of Down’s syndrome
An extra chromosome on chromosome 21, thus making three instead of two, causes this disorder. The extra chromosome on chromosome 21 is caused by failure to separate during meiosis.

Effects of Down’s syndrome
People with down’s syndrome are very susceptible to diseases including heart diseases. They thus die young, mostly not more than 30 years. They may also suffer discrimination in those societies that consider it as a curse or something very unusual.

Turner’s syndrome (XO)
This is a genetic disorder of female (women) caused by absence of second sex chromosome. Such women are XO, rather than the normal XX chromosome. In this disorder there are only 45 chromosomes, the female lacks secondary sexual feature, small uterus, the internal genitals never mature and therefore she is sterile. This disorder is characterized by lack of ovaries and menstrual cycle.

Causes of Turner’s syndrome
This genetic disorder is caused by absence of X chromosome in normal XX chromosome. This occurs when the second X chromosome lacks i.e. make XO instead of normal XX chromosome.

Effects of Turner’s syndrome
  • The women who suffer from Turner’s syndrome are abnormally short; their ovaries usually do not develop and hence are infertile.
  • They may also suffer mental abnormalities, which may lead to difficulty in learning
  • Missing one copy of this gene causes short stature and skeletal abnormalities in women with Turner’s syndrome. This is due to the researchers who identified one gene called SHOX that is important for bone development and growth, hence when missing it will cause skeletal abnormalities.
Super Male and Super Female
This is the genetic disorder caused by the non-disjunction of sex chromosomes, leading to a male having an extra Y chromosome (XYY) that is a super male. Also a female having an extra X chromosome (XXX) that is a super female; hence both male and female have 47 chromosomes instead of 46.Individuals with an extra X or Y chromosome appear very tall but in most cases they look normal and do not show any physiological or medical abnormalities.Men with the XYY syndrome were previously thought to be overly aggressive and more likely to become criminals. These original stereotypes came about because several researchers in the 1960s found a number of men with XYY syndrome in prisons and mental institutes. Since then, broader, less biased studies have been done on males with XYY and females with XXX syndrome.

Causes of Super male and Super female
This genetic disorder is caused by the presence of extra Y chromosome (XYY) or X chromosome (XXX). This causes changes in chromosomes from normal 46 to 47 chromosomes.

Effects of Super male and Super female
Normally the male with XYY chromosomes and the female with XXX chromosome or syndrome may be taller than average, they appear very tall in some cases.Also the people with XYY and XXX syndrome they have an increased risk for learning difficulties especially in reading and speech.

Klinefelter’s syndrome (XXY)
This is a non-disjunction or genetic disorder resulting from failure of the XY or XX chromosomes of the gametes to separate and hence being inherited together. The victim has 47 chromosomes instead of 46. He is a male due to the presence of Y chromosome but he develops female secondary features.For examplechart diagrams showing Klinefelter’s syndrome

Causes of Klinefelter’s syndrome (XXY)
This genetic disorder is caused by failure of the XY or XX chromosomes of the gamete to separate, hence makes XY or XX chromosome to be inherited together. Hence results from 47 chromosomes instead of normal 46 chromosomes.

Effects of Klinefelter’s syndrome
The individual with Klinefelter’s syndrome usually has low intelligence;Male with Klinefelter’s syndrome are typically tall and may have small testes and some breast development, although this is not necessarily obvious. They may also have difficult in learning and are usually infertile.The genetic disorders may be caused by the chromosome mutation or gene mutation. Also the genes can be inherited together hence lead to genetic disorders. When genes that are inherited together they are said to be linked that is known as sex linkage. This sex linkage includes Haemophilia and Red-Green colour blindness.

Haemophilia
This is the hereditary disorder whereby blood clotting is delayed caused prolonged bleeding. People suffering from haemophilia may bleed for more than two hours following an injury. Bleeding may occur either in skin, muscles even in joints due to even a minor injury. Haemophilic girls are very rare; their chance to survive beyond puberty is minimum due to excessive bleeding during menstruation.

Causes of Haemophilia
It is caused by a recessive allele “h” carried on the X chromosome. It is easier for a haemophilic son to be produced from normal parents because a carrier female with single haemophilic allele on one of X chromosomes appears phenotypically normal. When she marries a normal man, there is a chance that they may get a haemophilic son.For example: A cross between two normal parents a carrier female and normal male.Chart diagram showing the above crossFrom the cross:
  • All F1 female are phenotypically normal
  • 50% of F1 male are haemophilic
Effects of Haemophilia
This causes the failure of blood clotting hence it leads to death. For the female an individual could not survive beyond the puberty age due to excessive bleeding during menstruation.

Colour Blindness (Red-Green Blindness)
This is a condition in which one fails to distinguish red from green colour. It is a sex linked hereditary disorder whereby an individual fail to distinguish red from green. As for Haemophilia, colour blindness is controlled by recessive gene, hence for the disorder to be expressed phenotypically it must be present in homogenous form in female, though for male a single allele is enough for the disorder to be expressed phenotypically.

Causes of Colour Blindness
This disorder as for haemophilia is caused by a recessive gene/trait carried by X chromosome.For example: If a heterozygous normal female (carrier) marries a normal male, the possibility of getting the colourblind son is expected.Let,- XcXc   - carrier female- XcY - normal male- XcY   - colourblindnesschart diagram of cross between carrier female and normal male

Effects of Colour Blindness
This disorder is characterized by the difficulty in distinguishing Red from Green colour that may lead to accidents in traffic lights, when an individual will fail to determine Red light from Green light

 
(This post was last modified: 03-17-2016 10:56 AM by MyElimu.)
03-17-2016 10:47 AM
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