Is the study of inheritance of characteristics or the study of transmission of characteristics/traits from the parents to the off springs.
|parents||Male X female|
|↓ Growth and development|
|Off springs (sexually mature individual|
Transmission of characters from the parents to off springs is by gametes. The male organism if animal contributes sperms and in plants pollen grains
The Female animal contributes ova and in plants it will contribute ovules. Animals have to mate to bring the two gametes together.
In human beings, they have sexual intercourse, in plants the two gametes come together by pollination (self/cross pollination). When the female and male gametes fuse, fertilization is said to have occurred. Fertilization is the fusion of the male and female nuclei to form a zygote.
In animals the zygote is formed in the fallopian tube and it begins to undergo growth (growth is an irreversible increase in the size of an organism.) Growth occurs by cell division.
The type of cell division which leads to growth is mitosis. The zygote also undergoes development. Development is the change in shape and form. Growth and development eventually lead to a sexually mature organism.
Sexual maturity in flowering plants is evidenced by the on set of flowering. In female animals its evidenced by ovulation and in male animals by sperm development.
Gametes are formed by meiosis (meiosis is the type of cell division resulting in formation of gametes and takes place in the reproductive cells while mitosis is the cell division that leads to growth of an organism and occurs in the non reproductive cells/somatic cells/body cells
Examples of somatic cells;
Liver cells, check cells, ovary cells, testis cells etc
Examples of reproductive cell;
Sortollic cells in the testis which form sperms
Follicles in the ovary that give rise to ova
With in the cell nucleus is the observable genetic material known as the chromosomes. In body cells chromosomes are found in pairs (Half from the female parent and half from the male parent.) In reproductive cells/gametes, they occur in a single set called haploid and represented by n but in body cells they occur in a double set called diploid and represented by 2n.
These are threadlike structures found in the nucleus of the cell and they contain the genetic material responsible for inheritance. They form the physical basis for inheritance since their structure can be observed under a high power microscope.
Each chromosome is made up of two longitudinal strands called the chromatids.
Each chromatid has a double helical DNA molecule. The two chromatids are held together by a structure called the centromere. During cell division, the spindle fibres are attached to the centromeres.
The chromosomes are present in pairs. The pairs are called the homologous pairs (they must be similar in structure and also have the same chemical composition). A species will always have the same number of chromosomes. This is called the chromosome number and it will always be an even number. This number is called the diploid number. During gamete formation, the homologous chromosomes separate and the gametes will have only half the number of chromosomes. This number is called the haploid number. Thus the somatic or the vegetative cells of all organisms are diploid and the gametes are haploid.
Number of chromosomes per cell nucleus varies from species to species. In man there are 46 chromosomes per nucleus (23 pairs) of somatic cells and 23 chromosomes in the gametes.
Is the chemical compound responsible for inheritance of characters.
DNA is one of the nucleic acids and the other is RNA.
DNA: Deoxyribo nucleic acid (less oxygen)
RNA: Ribo nucleic acid (more oxygen)
Both are similar in composition but different in structure.
Chemical composition of DNA
- Contains a 5 carbon sugar (ribose sugar) in its structure and there fore has five corners.
- It has a nucleic acid. An example is the phosphoric acid
- Nitrogen/organic base is also present. There are four types namely
i) Adenine ii) Thymine iii) Cytosine iv) Guanine.
- In both DNA and RNA adenine, Cytosine and Guanine are found.
- Thymine is found in DNA while Uracil is found in RNA.
The following combine;
- Adenine (A) + Thymine (T) for DNA or Adenine (A) + Uracil (U) in case of RNA.
- Cytosine (C) + Guanine (G)
Always the pairing of bases as indicated above is due to the matching of their structure (i.e complimentary base pairing rule)
Formation of the DNA molecule
Formed from nucleotide units to form long chains
It’s the ability of DNA to produce a copy of its self.
DNA replication occurs in three stages and its catalyzed by a series of enzymes.
i) Twisted strands un wind giving two strands. The strands are still joined.
ii) Weak bonds between the nitrogen bases will be broken down to give two separate strands.
iii) One strand will induce the formation of another strand which is complimentary to it and the same thing will happen to the other strand.
After replicating, two new DNA molecules similar to the original DNA molecule are formed.
Why is DNA suitable for inheritance?
i) Because of its ability to replicate
ii) DNA is capable of carrying large a mount of genetic information
iii) DNA is a very stable chemical and therefore can not easily be changed.
Cell division is a process which leads to cell multiplication.
It occurs in both plants and animals. Original cells which undergo division are known as parent cells and the new on ones resulting from division are known as daughter cells.
There are two types of cell division i.e Mitosis (mitotic cell division) which occurs in somatic cells and Meiosis (meiotic cell division) which occurs in reproductive cells.
Stages of mitosis
1) Interphase (resting stage of the parent cell) During this stage the following happens to prepare a cell for nuclear division)
In this stage the cell builds up energy reserve in form of ATP
It also builds up food/nutrient reserve
Replication of DNA also takes place in the chromosomes. i.e the amount of DNA is doubled
There is synthesis/replication of new cell organelles/structures eg mitochondria, endoplasmic reticulum, centrioles, chloroplasts etc
Chromosomes become visible as long thin entangled threads.
The nucleolus begins to shrink and centrioles move to the opposite ends of the cell
Chromosomes shorten and they can be seen to comprise of 2 chromatid joined at the centromere
Nuclear membrane breaks up
Mendel’s contribution in genetics:
He was an Austrian and by practice he was a monk. He carried out experiments about inheritance in plants over 120 years ago.
The 1st experiment he carried out was referred to as monohybrid inheritance. The experiment considered one type of contrasting characters at a time. Hybrid is as a result of crossing between two different characteristics. In the first experiment mendel used pure breeding seeds
E.g tall plants crossed with tall plants Tall off springs only (no short plants)
He planted garden pea (Pisum sativum). The garden pea showed a variety of characteristics e.g colour of flowers, colour of pods, height of stems, nature and texture of pods, and shaped of pods.
The pattern of transmission of different characteristics was interesting eg when a plant showing one set of characteristic is cross pollinated with that showing opposite characteristic, the first generation off spring will be showing one parent’s characteristic.
When the first generation plants are self pollinated, a mixture of both parental characteristics is shown.
Parents: Tall cross pollinated with short
1st generation off springs: All tall
1st generation off springs self pollinated
2nd Generation off springs: ¾ tall and ¼short.
For colour of pods
Parent plants Green pods X Yellow pods
Gametes pollen grains ovules
1st Generation off springs green coloured pods
The seeds from the first generation are planted again and after flowering, self pollination was carried out.
1st Generation off springs Green pods X (self pollinated) green pods
Gametes pollen grains ovules
2nd Generation off springs ¾ green pod ¼ yellow pod plants
A mixture of green poded and yellow poded plants was got.
Mendel referred to what is responsible for the characteristic as genes carried by chromosomes.
A gene is a unit of inheritance:
There fore, A gene responsible for green pods is dominant (green pod is a dominant character) and is represented by letter G and yellow pod is a recessive character and the gene is represented by letter g.
Green pods X yellow pods
Parents GG gg
1st gen Gg Gg Gg Gg All off springs green poded
1st generation off springs self pollinated.
Parents Green pods X Green pods
Genotype Gg Gg
2nd gen GG gg Gg Gg
(GG, Gg, Gg) = ¾ 2nd generations plants with greens pods and (gg) = ¼ 2nd generation plants with yellow pods.
Tallness is dominant character and shortness is recessive character.
Let the gene for tallness be T
Let the gene for shortness be t
There fore the genotype for the tall plant is TT and for the short plant is tt
Parents Tall plant X Short plant
Genotype T T t t
1st generation Tt Tt Tt Tt First generation plants, all tall
First generation off springs self pollinated
1st generation Tall plant X Tall plant
Genotype Tt Tt
2nd gen TT tt Tt Tt
(TT, Tt, Tt) = ¾ 2nd generation off springs tall and tt = ¼ 2nd generation off springs short.
Working out fertilization using the punnet/chi square
- 1. Genotype: is the genetic make up of an organism. From the illustrations above (Tallness), we see 3 genes TT, Tt and tt which gives 1:2:1 as the genotypic ratio.
TT and tt are known as homozygous genes
They are called so because they were formed from fusion of the same gene.
TT is homozygous dominant (tall) and tt is homozygous recessive (short)
TT and tt are pure breeds.
Tt is known as heterozygous (tall) created from different genes fusing together. Its not a pure breed.
- 2. Phenotype: is the external expression of a gene present in an organism. When expressing its self its known as an allele which is a short of allelomorph.
When not expressing its self, its simply termed as agene.
A dominant gene is one that over shadows a weaker gene known as a recessive gene.
A recessive gene is one that is over shadowed by a dominant gene.
This happens when both the recessive and dominant genes for a particular trait/ characteristic are present in an organism i.e heterozygous (Tt )
- 3. Filial generation: The off springs that result from fusion of gametes in various generations eg
F1 Generation 1st generation.
F2 Generation 2nd generation.
F3 Generation 3rd generation.
Its used to determine the genotype of either homozygous dominant (TT, GG, HH) or Heterozygous (Tt, Gg, Hh). Since genotypes TT and Tt both produce tall plants, its not possible to know from the phenotype whether the tall plants are homozygous dominant or heterozygous.
The test or back cross is done by crossing the tall plants from the F1 generation with a true recessive plant (tt).
The proportion of tall and short off springs in F2 will determine the genotype in the tall F1 plants.
In case of homozygous dominant (TT), when crossed with homozygous recessive, the off springs are 100%tall
Parents (F1) Tall plant X Short plant
Genotype T T t t (Homozygous recessive)
2nd generation Tt Tt Tt Tt 2nd generation plants, all tall
An in case of Heterozygous (Tt)
Parents (F1) Tall plant X Short plant
Genotype T t t t (homozygous recessive)
2nd generation Tt tt tt Tt F2 generation plants, 50% tall and 50% short
This is a condition where genes controlling contrasting characteristics have equal influence when in heterozygous genotype. Such gene are said to be co-dominant genes.
E.g in hibiscus plants genes responsible for the red and white flower colours are co-dominant.
If a plant with red flowers is cross pollinated with that of white flowers, what are the possible genotype and phenotype of the F1 off springs?
Let the gene responsible for red flower colour be R and the gene for white flower colour be W. There fore the Genotype for plant with red flowers is RR and for the plant with white flowers is WW
Parents Red flower X White flower
Genotype R R W W
1st generation RW RW RW RW First generation: all pink flowered plants
F1 off springs are self pollinated, state the possible genotype, phenotype, genotypic ratio and phenotypic ratio
F1 off springs Pink flowered plant X Pink flowered plants
Genotype R W RW
2nd generation RR WW RW RW
Genotype = RR, RW and WW
Genotypic ratio= 1:2:1
Phenotype = Red flower plant (RR), Pink Flower Plants (RW and RW) and white Flower plant (WW)
Phenotypic ratio 1:2:1
Blood groups in human beings
A and B are co-dominant genes but dominant over O
|A||AA or AO||Blood group A|
|B||BB or BO||Blood group B|
|A and B||AB only||Blood group AB|
What is the possible genotype of off springs from a marriage between a man of blood group A and a woman of blood group B?
Possible genotype of the father: AA and AO and that of the mother: BB and BO
Parents Mother X father
Genotype B B A A
F1generation AB AB AB AB
|Blood group B||Blood group A|
|BB||AA →||all AB|
|BO||AA →||AB, AB, AO,AO|
|BB||AO →||AB, AB, BO, BO|
|BO||AO →||AB, BO, AO, O|
There fore the possible blood groups of the F1 children are: A, B, AB and O
- In a mixed day school, Angela got pregnant and she is of blood group B, Kapere a fellow student was accused to be responsible for her condition, which he denied. Angela gave birth to bouncing baby boy of blood group O. As an investigation was done Kapere was un cooperative and his blood group would not be discovered, but both his parents were of blood group A. Work out to find whether kapere would be the likely father of the baby.
- A woman of blood group A claims that a man of blood group AB is the father of her child. A blood test reveals that the child’s blood group is O. is it possible that the woman’s claim is correct? Could the father have been of blood group B? Explain your reasoning.
Multiple allele is a situation where by more than two alleles are controlling a certain characteristic. For example alleles A, B and O control the ABO blood group system in man.
Other conditions in Man transmitted in mendellian fashion. (Monohybrid inheritance in man)
- 1. Albinism
Is a condition which results when the pigment for normal skin colour fails to form and this due to a recessive gene a.
Characteristics of albinism
White skin, Pink eyes and Golden hair.
To obtain an albino the child must receive recessive genes from both parents. This implies that an albino is homozygous recessive.
Gene for normal skin pigment: A and gene for Albino: a
Homozygous dominant: (AA) normal skin colour
Heterozygous: (Aa) Normal skin colour
Homozygous recessive: (aa) Failure of formation of normal skin pigment (albino)
To get a child who is an albino:
I) Both parents must be carriers
Parents Mother (carrier) X father (carrier)
Genotype A a A a
AA aa Aa Aa
AA: normal skin colour
Aa: Normal skin colour but carrier
II) One parent is a carrier and the other is an albino
Parents Mother (albino) X father (carrier)
Genotype a a A a
Aa aa Aa aa
Aa: normal skin colour but carrier
From the above two marriages, the mode of transmission of genes is similar to the mendellian fashion.
- 2. Sickle cell anaemia
In this condition the person doesn’t possess bi concave shaped red blood cells but the shape is like that of a new moon. A person with sickle cells doesn’t have a large surface area so that sufficient oxygen can be transported through haemoglobin found in a normal person. People with sickle cell anaemia have short breath, tend to sleep when tired and have retarded growth.
Gene S is responsible for abnormal Haemoglobin.
Dominant gene H is responsible for normal haemoglobin.
NB: this is not an example of incomplete dominance.
Homozygous dominant HH: Normal haemoglobin
Heterozygous HS: sickle cell carrier (shows mild signs but never gets attacks)
Homozygous recessive SS: sickler
Sex is determined by a special type of chromosome found in the sperms and the ova and they are termed as sex chromosomes. In man, since one chromosome is X and the second is Y, they may be referred to as heterozomes and those that are similar are autosomes. Ova can only carry the X chromosome; Sperms may either carry the X chromosome or Y chromosome. The sex of the child depends on which sperm fertilizes the egg. If its an X sperm, the off spring is XX (girl) and if it’s the Y sperm, the off spring is XY (boy). Each off spring has characteristics limited to it. These are termed as sex limited characteristics
|Have a penis||Have a clitoris|
|Have beards||No beards|
|Have narrow tips and nipples||Have wide hips and breasts|
Parents Male X female
Genotype X Y X X
XX XX XY XY
Sex linkage: sex linked genes
Is a condition where the genes controlling a trait/characteristic have to be transmitted on a sex chromosome. Such traits are referred to as sex linked characters controlled by sex linked genes. Most of the sex linked genes are recessive and commonly found on the X chromosome and in rear cases on the Y chromosome
When the X chromosome has a recessive gene in males, normally the Y chromosome is empty. Since the chance for the Y chromosome to be empty is common, there fore males can inherit a recessive gene from a carrier mother and inherits an empty Y from the father and becomes a sufferer.
There fore sex linked characters are common in males than females. (in most cases females end up as carriers if they have a sex linked gene)
Examples of sex linked genes:
- Haemophilia: Simply means failure of blood to clot such that a person bleeds for a long time.
XHXH-Normal female homozygous dominant:
XHXh-Carrier female (heterozygous)
XhXh – Female Haemophilia sufferer
XHY –normal male
XhY-Male Haemophilia sufferer
- Colour blindness: is the inability to distinguish between primary colours ie red, green and blue.
XCXC-Normal female homozygous dominant:
XCXc-Carrier female (heterozygous)
XcXc – Colour blind female
XCY –normal male
- Premature balding
- Browning of teeth
- Porcupine man: the growth of thick hair at the entrance of the auditory canal. Its suspected to be associated with the Y chromosome because its found only in male.
- Explain what is meant by sex linked characters
- List examples of sex linked characters found in man
- In a marriage, a woman who is heterozygous for the sex linked character of colour blindness marries a colour blind man. What would be the result of this marriage?
Show the working if the sons from this marriage got married to carrier females, what would be the result?