Form 4 Biology – GENETICS AND VARIATION -1

Genetics is a branch of science which deals with the study of inheritance and variation.

Definition of terms

1. Heredity
   

   Is a passing of features from parents to their young.
   

2. Variation
   

   Possessing of characteristics which are different from these of the parents and other offsprings.
   

3. Genotype
   

   Is the genetic constitution or make up of an organism
   

4. Phenotype
   

   Is the outward or physical appearance of an organism
   

5. Dominant gene
   

   Is a gene that prevents the expression of another gene.
   

6. Recessive gene
   

   Is a gene that is masked by another gene.
   

7. Homozygous
   

   Is a condition where by the two genes for a given trait are similar/ alike
   

8. Heterogeneous
   

   Is a condition where the two genes for a trait are different.
   

9. 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.
   

10. Trait
    

    Are characteristics inherited by individual from their parents
    

11. Allele
    

    Is an alternative form of a gene controlling the same characteristics but produce different effect
    

    Example: T-tallness and t- shortness
    

12. 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. MONOHYBRID INHERITANCE
    

edu.uptymez.com

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 prossesing 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

1. Autosomes
   
2. Heterosomes
   

edu.uptymez.com

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

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 denas 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

 
 

         NUCLEIC ACID

Nucleic acids are polymericmacromolecules, or large biological molecules, essential for all known forms of life. Nucleic acids, which include DNA (deoxyribonucleic acid) and RNA (ribonucleic acid), are made from monomers known as nucleotides. Each nucleotide has three components: a 5-carbon sugar, a phosphate group, and a nitrogenous base. If the sugar is deoxyribose, the polymer is DNA. If the sugar is ribose, the polymer is RNA.

Together with proteins, nucleic acids are the most important biological macromolecules; each is found in abundance in all living things, where they function in encoding, transmitting and expressing genetic information—in other words, information is conveyed through the nucleic acid sequence, or the order of nucleotides within a DNA or RNA molecule. Strings of nucleotides strung together in a specific sequence are the mechanism for storing and transmitting hereditary, or genetic, information via protein synthesis.

 
 

         DNA (deoxyribo nucleic acid)

– DNA has a double stranded shape or coil twisted like a ladder to form a double helix.

– DNA is the genetic material contained in the genes.

COMPONENTS OF DNA

• Deoxyribose sugar
  
• Phosphate group
  
• Organic base or Nitrogenous bases.
  

edu.uptymez.com

Nitrogenouse base

• Adenine (A)
  
• Guanine (G)
  
• Uracil (U)
  
• Cytosine (C)
  
• Thymine (T)
  

edu.uptymez.com

Functions of DNA

• There are genetic material which are responsible for genetic characteristics
  
• They assemble the amino acids to form a protein molecule
  

edu.uptymez.com

RNA (ribonucleic acid)

The RNA molecule is responsible for carrying genetic information from the DNA molecule to the ribosome which is the sight of the protein synthesis

 
 

TYPES OF RNA

Messenger RNA – carries information from the nucleus in from of base triplets.

Transfer RNA – It transfers the appropriate amino acids to the ribosome.

       

DIFFERENCE BETWEEN DNA & RNA

 
 

 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

1. The garden pea has many contrasting and easily recognized characteristics.
   
2. The hybrid obtained from the cross fertilization was fertile
   
3. The flowers of a garden pea are bi sexual and naturally self pollinated
   
4. The garden pea plant matures relatively fast producing many off springs (seeds)
   

edu.uptymez.com

           
      MENDELIAN INHERITANCE.

1. 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”
   

    
    

2. 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
   

edu.uptymez.com

• Is a chart showing the possible combination of factors among the offspring of a cross.
  

   
   

• It is used to show the formation of zygotes.
  

edu.uptymez.com

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.g: if the individual is homozygous (bb)

      

 
 

DOMINANCE

Dominance is a state of one character /gene from one parent masking the corresponding character from another parent.

 Types of dominance

1.  Mendelian inheritance – Complete dominance
   
2. Non-Mendelian inheritance – Incomplete dominance
   

                                        – Co-dominance
   

edu.uptymez.com

 
 

1. COMPLETE DOMINANCE
   

edu.uptymez.com

• 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:
  

edu.uptymez.com

1. 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
   

        
   

edu.uptymez.com

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

                                             BB Bb bb
 
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 or 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

                                                1   :   2 : 1