Definition of terms
Heredity
Is a passing of features from parents to their young.
Variation
Possessing of characteristics which are different from these of the parents and other offspring.
Genotype
Is the genetic constitution or make up of an organism
Phenotype
Is the outward or physical appearance of an organism.
Dominant gene
Is a gene that prevents the expression of another gene.
Recessive gene
Is a gene that is masked by another gene.
Homozygous
Is a condition where by the two genes for a given trait are similar/ alike
Heterogeneous
Is a condition where the two genes for a trait are different.
Gene
Is a part of chromosome that carries the genetic material called DNA. Are also referred to as nucleotide chemical units of inheritance arranged along the chromosomes.
They are called hereditary factors. Each gene occupies a specific location called LOCUS.
Trait
Are characteristics inherited by individual from their parents
Allele
Is an alternative form of a gene controlling the same characteristics but produce different effect
Example: T-tallness and t- shortness
MONOHYBRID CROSS
Are offspring produced by crossing two individual with different character
E.g. homozygous green podded plant (GG) and homozygous yellow podded plant (gg)
13. FIRST FILLIAL GENERATION (F1)
Is the first generation of offsprings produced after crossing the parental genotypes.
14. SECOND FILLIAL GENERATION (F2)
Are offsprings produced by selfing the F1 generation
15. HYBRID.
Is an offspring of a cross between parents showing unlike characters. The characteristic exhibited by a hybrid is influenced by the dominant gene.
15. MONOHYBRID INHERITANCE
This is inheritance of one pair of contrasting (different characteristics e.g height where an individual is either tall or short).
16. DIHYHIBRID INHERITANCE
This is inheritance of two pairs of characteristics
Example: - pure tall pea plant with colours flowers and dwarf pea plant possessing white flowers.
17. EPISTASIS
It is the interaction between the two different known as allelic dominant genes
18. PEDIGREE
Is the historical or ancestral record of individuals shown in a chart, table or diagram
19. CHROMOSOMES
They are thread like structures found in the nucleus of the cell they are only visible when a cell nucleus is about to divide. Every nucleus of the cell of the same species has a constant number of chromosomes e.g.
Drosophila has 8 chromosomes, fruit fly pea plant has 40chromosomes sheep has 56 wheat has 14 chromosomes maize has 20 chromosomes.
Each member of the chromosome pair is known as homologous chromosome
Types of chromosomes
There are two types of chromosomes in the human body
i. Autosomes
These are also known as autosomal chromosomes. They carry all genetic information except that of sex. In humans autosomes are 44 in numbers forming 22pairs
ii. Heterosomes
These are also known as sex chromosomes these chromosomes determine the sex of the organism in humans. One pair is responsible for the determination of sex
DIPLOID AND HAPLOID NUCLEI
Diploid nucleus has the chromosomes occurring as homologous pair e.g 23 pairs in the human this is denoted as 2n diploid nuclei are found in the gametes
Haploid nuclei have only one set of unpaired chromosomes. In 23 chromosomes are there haploid nuclei are denoted as n diploid cells are formed after fertilization
GENETIC MATERIALS
Genes are nucleotide chemical units of inheritance arranged along the chromosome and are capable of being replicated and mutated.
Each gene occupies a specific location on a chromosome this location is known as locus (plural is loci) each chromosome contains many genes.
Homologous chromosomes when paired together will have similar or different genes called alleles.
An alleles is an alternative form of gene controlling the same character out producing different effects. The gene can control color of the skin
THE NUCLEIC AIDS, TYPES OF HEREDITARY MATERIALS
There are two types of nucleic acids:-
(a) Ribonucleic acid, RNA.
(b) Deoxyribonucleic acid, DNA.
CHEMICAL NATURE OF NUCLEIC ACIDS:-
Chemically nucleic acids are composed of the following:-
i. Pentose sugar
ii. Nitrogenous(organic bases)
iii.Phosphate group
iv. Chemical bonds
Protein coat
Pentose sugar.
This is a five carbon sugar.
In RNA, there is ribose sugar where as in DNA, there is deoxyribose sugar.
2. Nitrogenous (organic) base
There are two groups of organic bases:
(a)Purine bases-
These include:
(i) Adenine (A)
(ii) Guanine (G)
(b)Pyrimidine bases-
These include
(i) Thymine (T)
(ii) Cytosine (C)
(iii) Uracil (U)
Note that;
Thymine is a DNA pyrimidine while Uracil is an RNA pyrimidine. No uracil in DNA nor is
there thymine in RNA
3. Phosphate group:-
This is derived from phosphoric acid and it is this group that makes compounds (DNA and RNA)
Acidic in nature.
The three components are combined by condensation reactions to give a nucleotide. By a similar
Condensation reaction a dinucleotide is formed and continued condensation reaction leads to the
Formation of a polypeptide. The main function of nucleotides is the formation of nucleic materials
RNA and DNA which have vital roles in protein synthesis and heredity.
Structure of a typical nucleotide:-
4. Chemical bonds:-
There are two types of chemical bonds:-
Phosphodiester bonds
These hold the nucleotides together.
Hydrogen bonds
These hold together the complementary base pair in DNA as well as RNA.
5. Protein coat:-
The DNA of the eukaryotes has a histon protein coat over its surface.
(A) RIBONUCLEIC ACID (RNA)
Chemical nature:-
Ribonucleic acid is chemically composed of the following substances:-
(a) Pentose sugar – This is a 5 carbon sugar called ribose.
(b) Phosphate group- derived from phosphoric acid.
(c) Organic (nitrogenous) bases – These are of two types.
(i) Purines – These are Adenine (A) and Guanine (G).
(ii) Pyrimidines - These are Uracil (U) and Cytosine.
(d) Chemical bonds: These are of two types:-
(i) Phosphodiester bonds – Which hold the nucleotides together.
(ii) Hydrogen bonds – Which hold together the complementary base parts in tRNA
molecule.
Diagram to show structure of RNA:
Role of RNA.
.
The role of RNA is situational:
1. In the presence of DNA, RNA in collaboration
Controls heredity.
Controls protein synthesis.
2. In the absence of DNA, RNA alone.
Controls heredity.
Controls protein synthesis.
TYPES OF RNA
According to function and location in the cells, there are three types of RNA:-
Messenger RNA (mRNA).
This is the type of RNA formed from one of the strands of DNA in the process
Called transcription.
Role of mRNA:-
Messenger RNA carries the genetic code from DNA in the nucleus to the ribosome in the cytoplasm. This genetic code contains the information about the types of amino acids that should be joined together to form a protein molecule.
(b) Ribosomal RNA (rRNA).
Ribosomal RNA (rRNA) or soluble RNA constitutes about 80% of the total RNA in the cell. Ribosomal RNA is synthesized by a special DNA found in the nucleolus at a special region called a nucleolar organizer. It makes a bulk of the ribosome.
Role of rRNA
(i) It is an integral part of the ribosome i.e large proportion of the ribosome is made up on rRNA.
(ii) It attracts other types of RNA i.e mRNA and tRNA towards the ribosome during protein synthesis.
(c) Transfer RNA (tRNA)
This constitutes about 15% of the total RNA in the cell.
Structurally, tRNA is a clover – lead shaped molecule with a folded loop – like chain.
The looping of the chain, results into pairing of the folded of organic bases. Hence the formation of hydrogen bonds.
The molecule has got four active / recognition sites.
The upper site recognizes an amino acid, whereas the lower side (Anticodon) recognizes the mRNA. One of the sides recognizes the ribosome whereas the other one recognizes in enzymes, amino – acyl tRNA synthetase
.
Role of tRNA
The role of tRNA is to carry the activated amino acids from various parts of the cytoplasm to their binding site, the ribosome.
(B) DEOXYRIBONUCLEIC ACID (DNA
Chemical nature:-
DNA is chemically composed of the following substances:
Deoxyribose sugar:
This is a pentose (5 – carbon) sugar.
(ii) Organic or nitrogenous bases
(a) Purine bases – These are Adenine (A) and Guanine (G).
(b) Pyrimidine bases –
BASE PAIRING RULES:
Since DNA is double stranded molecule, the bases on the two strands appear in pairs being held together by the hydrogen bonds.
The strands run in opposite directions, that is are Antiparallel.
The base pairing rules make the chains, Complementary.
According to Watson - Crick modal of DNA structure, a purine pairs with a pyrimidine.
The rules are that:
(a) Adenine pairs with thymine and the two bases are held together by two hydrogen bonds
(b) Cytosine pairs with guanine and the two bases are held together by three hydrogen bonds.
(iii) Phosphate group. Divided from phosphoric acid.
(iv) Protein - Over the surface of DNA, there is a histone protein coat.
(v) Chemical bonds -
(a) Phosphodiester bonds
(b) Hydrogen bonds
Diagrammatic structure of DNA:
Figure 1Untwisted DNA strand and Double helix structure of DNA
Role of DNA in protein synthesis.
There are two types of chemical bonds.
– These hold the nucleotides together.
– These hold the complementary base parts together.
This role of DNA is that, it instructs the cell of the types of amino acid that should be
initiated to form a protein molecule. That is the message contains the information about
the types of amino acids that should be joined up forming the protein molecules.
Qn:-One of the characteristics of DNA as a hereditary material; is that it is metabolically
very stable. State the features of DNA that account for this metabolic stability
Answer;-
The features of DNA account for this metabolic stability include the following:-
(a) Possession of a histone protein coat
(b) The helical nature, increases mechanical strength.
(c) The chemical bonds i.e. hydrogen and phosphodiester bonds, increase mechanic strength.
DNA REPLICATION
DNA replication is a process whereby the exact copies of DNA (replicable) are produced by the old DNA molecules.
Significance of DNA replication:-
(i) Since it occurs prior to the nuclear division, DNA replication ensures that all newly formed cells have the same amount of DNA.
(ii) It ensures sameness and constancy of hereditary materials of the cells.
(iii) Occasional mistakes during DNA replication, results into genetic variations hence evolution.
(iv) If evidence mistake attracts uracil instead of thymine. RNA is constructed not DNA. This occurs when the enzyme fails to recognize the methyl group of uracil.
MECHANISM OF DNA REPLICATION:-
The two strands of a DNA unwind and separating thus acting as temperature to which a complementary set of nucleotides would attach by base pairing. In this way each original molecule of DNA give rise to two copies with identical structures.
In the presence of ATP an enzyme DNA polymerase links free DNA to form complementary bases. The unwinding of DNA, double helix is controlled by the enzyme helicase. DNA polymerase then move along the strand resulting formation of complementary bases and hence a free nucleotide and finally extending new stand of DNA. As the enzymes continue to move along one base at a time, the new DNA strand grows. This is called continuous replication in which one strand is copied before another strand.
The formation (copying) of another strand involves movement of DNA polymerase away from unwinding enzyme. This results in the small gaps being left at some points along the newly constructed DNA stand. These gaps are then sealed by an enzymes DNA ligase. This is called
Discontinuous replication.
Semi – conservative replication
In this method of replication, each newly formed double helix retains (conserves) of the two strands of the original DNA double helix.
That is in each of the newly constructed DNA molecules, there is an old and new strand.
Illustration:-
1. A representative portion of DNA, which is about to undergo replication is shown.
2. DNA polymerase causes the two strands of the DNA to separate.
3. The DNA polymerase completes the splitting of the strand. Meanwhile free nucleotides are attracted to their complementary bases.
4. Once the nucleotides are lined up joined together. The remaining unwinded bases continue to attract these complementary nucleotides.
5. Finally the nucleotides are joined to form a complete polynudeotide chain. In this way two identical strands of DNA are formed. This original DNA material, this method of replication is called Semiconservative method.
DIFFERENCE BETWEEN DNA & RNA
DNA
RNA
Has a deoxyribose sugar
Has a ribose sugar
Has a double stand
has a single stand
Found in the nucleus, mitochondria and chloroplast
Found in nucleus and cytoplasm.
Has organic bases, cytosine, guanine adenine and thymine
Has organic bases, cytosine guanine, adenine and uracil
It is more stable
Less stable
Has high molecular mass
It has low molecular mass.
It is constant within a cell
The amount of RNA is variable
PRINCIPLES OF INHERITANCE.
Concept of inheritance.
Historical background of genetics
Father of genetics is Gregory Mendel
Mendel’s experiment
Mendel has selected garden pea plants [Pisum sativa]
REASONS FOR SELECTING Pisum sativa.
The garden pea has many contrasting and easily recognized characteristics.
The hybrid obtained from the cross fertilization was fertile
The flowers of a garden pea are bi sexual and naturally self-pollinated
The garden pea plant matures relatively fast producing many off springs (seeds).
SOME OF THE CHARACTERISTICS THAT HE STUDIED WERE:
Height of the stem tall or dwarf
Texture of the seed coat – smooth or wrinkled
Colour of flowers- green or yellow
Position of pods – axial or terminal
MENDEL WAS SUCCESSFUL DUE TO THE FOLLOWING REASONS
He chose to study a single character at a time (monohybrid inheritance). He later studied two characters at a time (dihybrid inheritance)
Each character he chose was expressed in two clearly contrasting forms without intermediate.
He qualified his results that is he counted and recorded the number of offsprings bearing each trait.
WHY MENDEL’S WORK REMAINED UNNOTICED?
His statistical calculations was complicated and not easily understood
His position as a priest and a teacher was not in his favour, as it would have been if he belonged to a university.
He was ahead of his time, and published his work in a local journal that has limited circulation.
He published his work in local journal that had limited circulation.
Note: He presented his work before the natural history society at Burn and was published in 1866 but remained unnoticed until 1900.
MENDELIAN INHERITANCE.
LAW OF SEGREGATION
It states that “characteristics of an organism are controlled by internal factors (genes) occurring in a pair is carried in each gamete”
LAW OF INDEPENDENT ASSORTMENT
“Each of the 2 alleles of one gene may combine randomly with either of the alleles of another gene independently”
PUNNET SQUARE
Is a chart showing the possible combination of factors among the offspring of a cross.
It is used to show the formation of zygotes.
Female gametes are placed on the right while male gametes are placed on the left side.
Male
Female
Example:
A cross between homozygous tall (TT) and homozygous dwarf (tt) plant can be illustrated as follows:
Let assume tall is male and dwarf is female
TEST CROSS
A cross used to cross an individual of unknown genotype with a homozygous recessive individual
Example: - A homozygous dominant individual (TT) will phenotypically appear the same.
BACK CROSS
In the crossing of individual of unknown genotype with the homozygous parent.
This is another form of test cross, but the difference is that in test cross, it is crossed with any individual while in back cross with a parent. E.g: if the individual is homozygous (bb)
NOTE:
An individual which has identical alleles in corresponding loci on a pair of homologous chromosomes for example TT (T for tall) or (tt for dwarf) is called a HOMOZYGOTE.
The state of an individual having similar alleles is called HOMOZYGOUS. An individual with dissimilar alleles is called a HETEROZYGOTE.
The state of having similar alleles is called HETEROZYGOUS for example Tt. An individual with two dissimilar alleles which are dominant is said to be a HOMOZYGOUS DOMINANT for example TT while the one with two similar recessive alleles is said to be a HOMOZYGOUS RECESSIVE. E.g. tt.
DOMINANCE
Dominance is a state of one character /gene from one parent masking the corresponding character from another parent.
TYPES OF DOMINANCE
Mendelian inheritance - Complete dominance
Non-Mendelian inheritance - Incomplete dominance
Co-dominance
COMPLETE DOMINANCE
Is the dominance where by one gene masks the expression of the other gene.
A dominant gene always masks a recessive gene when the two occur together.
EXAMPLE:
A man homo zygote for brown iris marries a women who has blue iris. Show the results of F1. What colour would the iris of the cross between 2 members of F1?
Solution: -
The gene for brown iris is completely dominant over gene for blue iris in woman.
Let gene for brown be B and b for blue
Genotypes - All are Bb.
Phenotype - All have brown iris.
Selfing F1
Genotypes - BB,Bb,bb
Phenotypes - 3 Brown iris, 1 brown iris
Genotypic ration - 1 : 2 : 1
Phenotypic ratio - 3:1
2. A pure purple flowered pea plant was crossed with pure white pea plant. Offsprings for F1 were phenotypically all purple flowered plants when F1 was selved a mixture of purple pea flowered and white pea plant were produced at an approximate ratio of 3:1
Solution: -
Let gene for purple be P and white be p
Genotypes : all are Pp
Phenotypes : all have purple flower
Self F1
Genotype - PP, Pp, pp
Phenotypic ratio - 3 : 1
Genotypic ratio - 1 : 2 : 1
2. INCOMPLETE DOMINANCE
In incomplete dominance there is no dominant or recessive gene, but both express themselves equally. It results in a heterozygous individual which does not resemble any of the heterozygous individual which does not resemble any.
Example: -
1. A red flowered rose was crossed with white rose and all members of F1 were pink. When pink were selfed, a mixture of red, pink and white flowered plants were obtained.
Solution:-
Let, R – Red , G – White
Genotypes : all are RG
Phenotype : all are pink
Genotypes - RR, RG, GG
Genotypic ratio - RR : RG : GG
1 : 2 : 1
Phenotypic ratio - 1 red : 2 pink : 1green
3. CO-DOMINANCE
In co- dominance genes from both parents are dominant and are phenotypically expressed in the offspring.
Example: A red cow is mated with white bull. In F1 generation all of offsprings have equal patches of red and white fur. Therefore neither red nor white gene is dominant over the other such cattle and called Roan.
When a roan cow is mated with roan bull, offsprings may be red, roan or white mated in the ratio of 1 : 2 : 1
Let;
W - white bull
R – Red cow
Genotypes – all are RW
Phenotype – all are Roan
Phenotypes ratio - 1 Red : 2 Roan : 1 White
Genotypic ratio - RR : RW : WW
GENETICS AND VARIATION -2
SIMPLE MENDELIAN TRAITS
The following are example of Mendelian’s traits in man
ALBINISM
Albinism is absence of pigmentation melanin in human skin/ animals or plants. This pigmentation is responsible for dark colour of the skin. As a result the person has white hair, pink eyes and light skin. In plant are characterized by lack of chlorophyll
It is controlled by a recessive gene. Human showing this disorder must be homozygous recessive. Heterozygous are normal but career.
Examples
What will be the result of normal man who married an albino woman?
Solution: -
Let gene for normal be A and Albino be a
Phenotype - all are normal (Heterozygous)
2. What would be the result of a cross between heterozygous parents?
Genotype – AA, Aa and aa
Phenotype – normal man, carrier and albino
3. What would be the result of crossed between heterozygous parent with an albino parent.
Solution: -
Gene :Aa - heterozygous parent
aa - albino parent
Genotypes - Aa and aa
Phenotypes - half normal/ carries and albinos.
4. What would be the result of crossed between heterozygous parent and homozygous nomal parent
Solution:-
Heterozygous Aa, homozygous AA
Genotypes - AA, Aa
Phenotypes - all are normal (normal, carriers)
2. ACHONDROPLASIA
Achondoplasia is a disorder that is characterized by a shorted body, legs and hands. It is controlled by a dominant gene. Individuals with this disorders are homozygous dominant or Heterozygous. Homozygous recessive are perfectly normal
Examples: -
What would be the result of a normal man who married an achondroplasiawoman.
Solution: -
Genes for normal man - aa
Genes for achondroplasia women - AA
Phenotypes - All are anchondroplasia
Genotypes - Aa
2. What would be the result of a cross between an achondroplasia woman who is homozygous and achondroplasia man who is heterozygous?
Solution: -
- AA, Aa
Phenotypes - All achondroplasia
3. What would be the result of a cross between heterozygous parents?
Solution: -
Phenotypes - 3 Achondroplasia, 1 Normal
Genotypes - AA, Aa, aa
Phenotypic ratio - 3: 1
Genotypic ratio - 1 : 2 : 1
3. HAEMOPHILIA
Haemophilia is a hereditary trait characterized by delayed blood clotting. The result is prolonged bleeding even small injuries can lead to death. The haemophilic girl rarely live beyond puberty because of excessive menstrual bleeding. It causes high mortality rate.
It is controlled by recessive gene. Heterozygous are normal/carries but homozygous individuals are haemophilic.
Worked example: -
If a normal man married a haemophilic woman, the offsprings would be
Solution: -
Let genotype for the man X H Y and woman X h Y h
A haemophilic man will be X h y
Haemopholic female will be X h X h
H – not suffering from haemopholic while h – haemopholic
4. COLOUR BLINDNESS
Is the hereditary trait characterized by inability to detect certain colours of the spectrum. The common colour blindness is inability to distinguish between red from green.
It is controlled by a recessive gene. Homozygous individual are colour blind while heterozygous are normal or carrier.
e.g . If a colour blindness man marries a nomal woman, the offspring will be as follows.
Let B – normal
b - Colour blind
5. SICKLE CELL DISEASE
This is a genetic disorder which makes the red blood cell acquire sickle shape under certain conditions. It may occur when the person is attacked by certain diseases. e.g.malaria. Also when oxygen tension in the atmosphere is very low. The sickled cells ability to carry oxygen is reduced
It is controlled by a Recessive gene. Homozygous individuals are sickled cell while heterozygous individuals are normal/ carriers.
NOTE:
HbA – perfect normal
HbS - sickle cell trait
* If a carrier man marries a carrier woman the offspring will be - sickle cell anaemia
6. TONGUE ROLLING
This is a hereditary trail which is characterized by rolling a tongue into a U – shape. It is controlled by a dominant gene. Heterozygous and homozygous individuals are tongue rollers. Recessive are not tongue rollers.
TRAITS/ DISORDERS AND THEIR CONTROLLED GENE
DOMINANT GENE
RECESSIVE GENE
Achondroplasia
Haemosphilia
Tongue rolling
Colour blind
Night blindness
sickle cell
Brown iris
Blue iris
Having more than 5 fingers & toes
Normal night vision
Albinism
Normal number of finger and toes.
HOW TO SOLVE GENETIC PROBLEMS BY USING PUNNET SQUARE
In human beings normal skin pigment (melanin) is dominant over albinism. An albino male mates with a heterozygous female. If the female gives birth to 6 fraternal twins what will be the propaple genotypic and phenotypic ratio of the offspring?
Solution: -
i) Let letter A - dominant gene
a - recessive gene (albinism)
• Write the genotypes of the parents
(male) aa x Aa (female)
ii) Use these genotype to complete the punnet square
iii)Summarize the genotypic and phenotypic ratios
Genotypic ration - Aa : aa = 1Aa: 1aa
Phenotypic ration - ½ normal skin pigmented : albino = 1:1
2. In human beings normal skin pigment is dominant (A) over albinism (a) one couple with normal pigment mate and produce six fraternal twins. Out of 6, 4 have normal skin pigment and 2 are albino. What are the genotypes of the parents?
Solution: -
Write complete/partial parents genotypes and offspring
Parents A
Four normal skin offspring A
Since normal skin is dominant, each of parent and 4 children must have at least one dominant gene
Since albino gene is recessive, 2 albino offspring are homozygous recessive (aa)
Two albino offsprings (aa)
A - (Normal skin parent)x A - (normal skin parent)
A - (4 normal offspring aa - (2 albino offspring)
Since one gene for albino comes from each parent. Therefore each parent is heterozygous (Aa)
RHESUS FACTOR
About 85% of the human population has a gene located on the chromosomes number one that produces a function protein called ANTIGEN & (Rhesus factor)
Individuals with rhesus factor are rhesus positive (Rh+) and the remain 15% do not have this factor are rhesus negative (Rh-). Rh+ is dominant over Rh-.
Rhesus antibody is normally absent in plasma of human blood. The Rh- people produce this antibody if Rh+ blood is transfused to them. These Rh+ antigens react with rhesus antibody causing agglutination. The present or absent of Rh factor gives the blood groups the + or – signs.
The table below shows the reactions of blood types with and without Rh factor.
KEY: (√) - No agglutination
(x) - Agglutinatio
WORKED EXAMPLE
A Rh+ man marries a woman who is Rh- and produces 10 children, what will be the phenotypes of the children
SEX INHERITANCE
Sex is a phenotypic character, it is dependent upon the genotype and environment. In sexually reproducing organisms, each individual is a product of a male and a female. Each individual receives an equal number of chromosome from male and female body. Fo example each individual receives 23 chromosomes from the mother and 23 from the father.
In many species female chromosomes (sex) are XX and male are XY
The chromosomal mechanism of sex determination varies in different organisms
Example: -
Organisms
Gametes
Zygotes
Ova
Sperm
Female
Males
Drosophila, Human beings, Grasshoppers, Birds, Moths, Butterflies
XX
XX
XY
XY
XO
XX
2X
2X
XY
XY
XO
2X
SEX DETERMINATION AND INHERITANCE
Sex of a child (man) is determined by sex chromosomes. Human being have 46 chromosomes (23 pairs of homologous chromosomes) in every body of these, 2 are sex chromosomes while 44 are referred to as autosomes. Autosomes determine physical characteristics such as height and body size. There are two types of sex chromosomes which are X and Y. These chromosomes determine the sex of a child.
- The male carries X and Y chromosomes which are different in shape and size and are said to be Heterogametic. The male genotype is XY.
- The female carries two X chromosomes which are similar in shape and size and are said to be Homogametic.
- A sperm (male gamete) has either an X or Y chromosomes while the ovum (female gamete) always contain the X chromosomes.
- Secondary sexual characteristics of females are controlled by genes on the X chromosomes.
- Male secondary sexual characteristics are controlled by genes on the Y chromosomes.
- The sex of a child is a matter of chance and depends on whether the sperm that fertilizes the ovum carries a Y or a X chromosomes. The chances of a baby being a girl or a boy are 50:50.
- Maleness depends upon the presence of Y chromosomes and Femaleness depends upon the absence of the Y chromosomes.
Sex determination in human.
The ratio of boys to girls is 1:1. This means that the probability of getting a boy or a girl is 50%.
SEX - LIMITED CHARACTERS.
These are characters that are restricted to only one sex, either males or females.
Examples of sex-limited characters;-
i. Growth of facial hairs (Beard and Moustach) in males.
This develops as a result of production of male hormonies. The gene for beards growth is also present in females but it is not expressed.
ii. Baldness in males.
iii. Breast development in females (lactation).
iv. Long hairs of male lions (Male: lion, Female: lioness)
v. Comb plumage of hens (Male: cork, Female: hen)
vi. Hairy ears and nose is a common characteristics among males especially those of Asiatic descent.
The fact that the characteristics are only present in the males, suggests that the gene responsible for the trait is located on the Y chromosomes.
SEX - INFLUENCED CHARACTERS.
Are the characters that are expressed as dominant in one sex and recessive in the other. These are characters or traits that tend to be more conspicuous in one sex than the other. An example of sex - influenced characters is the presence or absence of horns in some breeds of sheep.
- The horned condition behaves as dominant in males but as recessive in females.
- The hornless state is dominant in the female sex but recessive in the male.
Note: The dominance difference of sex-influenced characters is mainly the result of hormonal interaction with the genotype.
SEX PREFERENCE AND SEX SELECTION.
- Sex preference is favouring one sex (gender) and not the other.
- Sex selection means choosing the sex (gender) of the baby to have.
- Therefore, sex preference and selection result into people to like one type of sex more than other. This tendency is very common in African countries and some parts of Asia. Basically, both males and females are equal and depend on each other in many aspects of life. However, there has been a tendency of some people to prefer one type of sex over the other. Some people in families prefer having boys than girls while others prefer girls over boys.
- Those who prefer boys do so in a belief that boys will perpetuate the lineage 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 mercy, therefore they can take care of their parents at old age.
SOCIO - CULTURAL FACTORS THAT INFLUENCE SEX- PREFERENCE AND SEX SELECTION.
(i) Man power generation.
Some societies, especially pastoralists prefer boys over girls because boys help in animal grazing.
(ii) Generation and protection of wealth.
In some societies girls are more preferred than boys because they generate wealth upon getting married. A family will get a lot of cattle or money as a bride price.
(iii) Land ownership.
In some societies a woman can not own land, thus females prefer to have more sons than girls so that they can somehow benefit indirectly through their sons.
Conclusion;-
- Sex preference and selection have negative impact as it may result into in equality and discrimination. In many societies, sex preference and selection has led to boys being educated and given ample time to play and learn while girls stay at home and do house chores.
- Government and NGO'S have to take measures to rectify the situation.
SEX LINKAGE
Sex linked genes carried on sex chromosomes but have nothing to do with sex. Traits whose expression is governed by sex linked traits are called sex linked traits .
One kind of colour blindness is an example of sex linked trait in human beings located on the X – chromosome. Example of other linked are haemophilia (bleeder’s diseases).
PEDIGREE
Pedigree is a diagram or a family tree showing genetic relationships among a set of individuals, normally with respect to a specific genetic trait.
In a pedigree, a circle represents females while males are represented by a square. Parents are linked by horizontal marriage lines. For example a marriage between two heterozygous for the albino trait can be represented as shown below.
VARIATION
The difference that exist between living organisms is called Variation. It is the possession of characteristics which are different from the parent and other offspring.
TYPES OF VARIATION.
Continuous variation(quantitative variation)
Is the variation which show intermediate form between any two extremes i.e there is no clear cut distinction between two extremes.
Example in group length ranges from shortest to tallest with several intermediaries continuous variation arises from interaction between genes and environment.
Discontinuous variation(qualitative variation)
Is the variation which show clear cut distinction from one form to another form.
Example: -
In human population an individual is either a male or a female, ability to roll the tongue, albinism, blood group (A,AB,O) and rhesus factor.
Environment does not influence the characteristics that show discontinuous variation.
Example blood group can not be altered by environment.
DIFFERENCES BETWEEN CONTINUOUS AND DISCONTINUOUS VARIATION
CONTINUOUS VARIATION
DISCONTINUOUS VARIATION
Does not show clear cut distinction
Shows clear cut distinction
Show intermediate forms between the two extremes
No intermediate forms
Characters are influenced by the environment
Characters are not influenced by the environment
CAUSE OF VARIATION
Environment Factors
Food – lack of food of a certain diet leads to deficiency diseases such as Kwashiokor. Lack of enough food causes starvation. Also pathogens causes diseases in organism making the individual different from the normal ones.
2. Genetic factors
(a)Meiosis– during meiosis there is segregation of different gametes.
This reduces the chance of pairs of chromosomes producing a wide variety of different gametes. This reduces the chance of individuals being the same.
(b) Fertilization – during fertilization the nuclei of male and female gametes fuse.
This permits parental genes to be brought together in different combinations.
This may lead to desirable and undesirable qualities of parents be combined in the offspring.
(c) Mutation- This is a sudden change in gene which can be inherited are caused by mutagens as x rays, cosmic rays, chemicals as mustard gas. The individual is called a mutant after undergoing mutation and appears different from the rest of the population.
3. Migration
As species are not normally informally distributed but occurs in small isolated population called demes. If members from the deme migrate and mate with members of another deme the offspring that results have characteristics that are different from those of both parents.
TYPES OF CHARACTERS
Acquired characters
These are traits an individual develops as a result of adaptation to the environment. Example: - Walking style. They are never inherited and are also know as no-heritable characteristics
2. Inherited characters
Are traits passed on from parents to the offsprings through sexual reproduction.Are also called heritable characteristics.
Difference between acquired and heritable
ACQUIRED CHARACTERISTICS
HERITABLE CHARACTERISTICS
1.Are due to the environment
2.Can not reappear in offspring.
3.Sometimes are changeable in life time (one way lose weight)
1.Are due to genes
2.Re-appear in offspring
3.Mainly unchangeable in life time (height
GENETIC DISORDERS.
MUTATION
Mutation are changes in the genetic material in the gametes.
• It includes appearance of new characters that have never been before in that population
• Individuals who undergone mutation are called Mutants
•Mutation can be due to
Change in a gene itself
Change in arrangement of genes
Loss of chromosomes (due to unbalanced meiosis)
• Mutation can be caused by agents known as Mutagens
X-rays
Cosmic rays
Heavy metal (lead & mercury)
TYPES OF MUTATION
Gene mutation
Chromosomal mutation
GENE MUTATION
Gene mutation occur as a result of altering the chemical structure of genes
• There is a change in the sequence of nucleotides is the segments of DNA corresponding to one gene. This in turn alters the sequence of amino acids required in synthesis of a particular protein.
• The protein formed will be different from the normal ones and produce a profound effects on both the structure and development of an organism Example: sickle cell, dwarfism.
TYPES OF GENE MUTATION
Substitution
Insertion
Deletion
Inversion
i. SUBSTITUTION
This is the replacement of one or more portions of a gene with a new one. E.g. A thymine (T) on ATA on the DNA molecule is replaced by cytocise (C) and result to ACA on the DNA
This is examplified in sickle cell anaemia only one nucleotide is changed. This kind of mutation involving the change of one nucleotide is called Point Mutation.
ii. INSERTION
This involves adding a new portion of a gene to an existing one. Example: If the base Guanine (G) is inserted between two Adenine result into AGA which does not code for any amino acid.
iii. DELETION
Deletion is the remove of a portion of a gene Example: -If base Guanine (G) is deleted in a base triplet CGC resulting into alteration of base sequence reducing the number of amino acids.
iv. INVERSION
A portion of DNA strand cuts and rotates through 180° the inversion results in alteration of the base sequence at this part.
Example: -A base triplet CTA can have its base thymine (T) and Adenine (A) cut and rotated. The result is CAT which is different from amino acid.
2. CHROMOSOMES MUTATION
Chromosomes mutation involves changes in the structure of the chromosomes. During meiosis homologous chromosomes interwine at several points called chiasmata and create opportunity for various changes on the chromatids leading to mutation.
TYPES OF CHROMOSOME MUTATION
Deletion
Duplication
Inversion
Trans location
Non-disjunction
Polypoidy
DELETION: This occurs when a portion of the chromosome breaks off and fails to reconnect to any of the chromatids, The result is the loss of genetic materials.
Deletion can be caused by error in chromosomal crossover during meiosis. These causes serious genetic deceases
2. DUPLICATION
This occurs when a portion of the chromosome replicates itself adding extra length. The result is addition of a set of genes which is a duplication
This may result to over emphasizing of a trait in an organism.
3. INVERSION
This occurs when a middle piece of the chromosomes break and rotates at 180° and rejoins the chromatid. This has the effect of reversing the gene sequence.
4. TRANS LOCATION
This occur when a portion of one chromosome breaks off and becomes attached to another chromatid of non-homologous pair. The result is transfer of genes from one pair of homologous chromosome to another.
5. NON-DISJUNCTION
This kind of chromosomal mutation is caused by addition or loss of one or more chromosomes. This occurs during meiosis where homologous chromosomes fail to separate. This results in some gametes having more chromosomes that others.
Example of non – disjunction
(a) DOWN’S SYNDROME
This is caused by presence of an extra chromosome number 21 individuals with this defect have a total of 47 chromosomes they have
Resistance to infection
Mentally retarded
Have thick tongue
Short body
Also children of old parents (above 40 years woman and 55 man) have increased chance of Down’s syndrome.
(b) KLINEFELTER’S SYNDROME
This is caused by failure of X chromosome to separate during the process of egg formation. An individual with this condition has two X chromosome and one Y chromosome (XXY). They are – outwardly male but may also have female characteristics.
(c) TURNER’S SYNDROME
This is an individual with 45 (44 + x 0) chromosome in a cell instead of 46 (44 +xx). Individual with this condition have one X and no Y i.e (XO) they individual is sterile and abnormally short female.
6. POLYPLOIDY
Occurs if the whole set of chromomes doubles after fertilization, where the spindle fail to be formed and the cell does not divide.It is rare in animals but common in plants
7. SICKLE CELL ANAEMIA
Sickle cell anaemia is an example of gene mutation. The normal haemoglobin is entirely replaced by an abnormal haemoglobin known as haemoglobin S
In sickle cell anaemia, the glutamic acid is replaced by another amino acid, the valine forming a haemoglobin s denoted by Hbs. Normal haemoglobin is denoted by HbA.
Haemoglobin S begins to crystallize when Oxygen concentration falls and causes red blood cell to assume the shape sickle. Half the number of red blood cell is sickle.
GENETIC COUNSELLING
Genetic information is used to advice couples who have hereditary disorders about chances of children inheriting the disorders. Genetic information could also be used in choosing marriage partners
GENETIC ENGINEERING
This is the alteration of the structure of DNA by man.
• Genetic engineering enables man to carry out research.
Manufacture protein (insulin)
Improve animal and plant breeds
Correct genetic disorders
Genetic engineering is the technique of changing the genotypes of an organism. It involves inserting genes from one organism into the chromosomes of another organisms. Once inserted the foreign genes work as if they were in the organism they were taken from.
APPLICATION OF GENETICS
1. MEDICINE
Genetics engineering has enabled biologists to program and make useful substance. For example the gene in man that produces insulin was inserted into escherichiacolia for producing pure insulin in large quantities.
• Human growth hormone ha also been made by using bacteria which the proper gene has been added.
• Also blood clotting factors such as fibrinogen needed by haemophiliacs are produce.
Vaccines from viruses are produced.
2. Biological warfare
Genetic engineering can help humans to produce biological weapons i.e. Anthrax and Vibrio cholera
3. Agriculture
• It is common for farmers to select and plant seeds from the healthiest and high yielding varies of plants with the aim of improving desirable traits as high fruits and crop production.
• Also genetics has enabled the beginning of selective breeding. Selective breeding is the crossing of animals or plants that have desirable traits to produce offspring that have a connection of the parents’ desirable characteristics
• Also the knowledge of genetics developed in breeding which involves crossing relatively individuals to maintain desirable traits. The various breeds of cattle, dogs, pigeons, chicken and maize, sugarcan and goats are a result of in breeding
4.Genetic disorder
Pregnant women can be informed about the deformation of the fetus
It can help in the modification of disordered genes
Dangers of genetic engineering
The outcome of genetic engineering can be weird out of our imagination
Production of new pathogens accidentally or deliberately