MGSLG. (2020). Module 1 Unit 2

How to teach the different kinds of monohybrid crosses

  • Learners MUST understand the link between meiosis and genetics.
  • During the crossing over in prophase I of meiosis, chromosomes share information and then during metaphase I, separate randomly. 
  • This determines the combination of chromosomes and genes that you have as an individual. Genetics determines individual variation (to be different) and survival of the fittest. 
  • Learners MUST have a clear understanding of the genetic terminology in order to study genetics and answer genetic problems.
  • Mendel’s Laws are very important - understand the concepts of dominance and how this plays a role in monohybrid crosses (mono = one = one characteristic or trait).
  • Be aware of confusing the word ‘cross/ crossing’ with ‘crossing over’ in Meiosis.  You cross individuals and calculate the chances of a characteristic or trait being in the offspring. Learners must be clear of the difference between these two terms.
  •  Questions on blood group inheritance and sex determination are often asked. The more examples of genetic crosses that they do, the better they will do.
  • Pedigree diagrams are a popular way to express family history and are often asked in exams.  Make sure they know how to answer them
  • There are basically FOUR types of monohybrid crosses:
    • Homozygous Dominant x Homozygous recessive: HH x hh
    • Heterozygous x Heterozygous: Hh x Hh
    • Heterozygous x Homo recessive: Hh x hh
    • Heterozygous x Homo dominant: Hh x HH
  • In the notation of the genotype the dominant allele represented by a CAPITAL LETTER must always be written first e.g. Gg and NOT gG


Important terms and concepts

Biological term

Description

Albinism

The condition that results from the absence of skin pigmentation

Alleles

Two alternative forms of a gene at the same locus

Artificial selection/selective breading

The breeding of organisms over many generations in order to achieve a desirable phenotype

 

Biotechnology

The use of biological processes, organisms or systems to improve the quality of human life

Clone

A copy of an organism that is genetically identical to the original organism

Cloning

The process by which genetically identical organisms are formed using biotechnology

Co-dominance

The type of inheritance where both alleles are equally dominant and both express themselves equally in the phenotype.  E.g. A white cow  crossed with a black bull will produce a calf with black and white patches

Complete dominance

The type of inheritance where the dominant allele masks the expression of the recessive allele in the heterozygous condition

Chromatin network:

Visible as thread-like structures in the nucleus of an inactive cell

 

Chromosome:

A structure made up of two chromatids joined by a centromere that carries the hereditary characteristics within the DNA

 

Dihybrid cross

A genetic cross involving two different characteristics e.g. shape and colour of seeds

Dominant allele:

An allele that masks or suppresses the expression of the allele partner on the chromosome pair and the dominant characteristic is seen in the homozygous (e.g.: TT) and heterozygous state (e.g.: Tt) in the phenotype.

 

Gene

A segment of DNA/a chromosome that codes for a particular characteristic

Gene mutation:

 

A change of one or more N- bases in the nuclear DNA of an organism. 

Genetic variation:

This includes a variety of different genes that may differ from maternal and paternal genes resulting in new genotypes and phenotypes.

 

Genotype

This is the total genetic composition of an organism. It is the information present in the gene alleles, for example BB, Bb or bb.

 

Genome

The complete set of chromosomes in the cell of an organism

Haemophilia

A sex-linked genetic disorder characterised by the absence of a blood-clotting factor

Heterozygous

An individual having two non-identical alleles for a characteristic

Homologous structures

Similar structures on different organisms that suggest they have a common ancestor

Homozygous:

When two alleles that control a single trait (on the same locus) are identical.

Hypothesis

A tentative explanation of a phenomenon that can be tested and may be accepted or rejected

Incomplete dominance

The type of inheritance where both alleles express themselves in such a way that an intermediate phenotype is formed. E.g. A white flowering plant crossed with a red flowering plant will produce a pink flowering plant.

Locus:

The exact position or location of a gene on a chromosome.

 

Mendel’s Law of Dominance

When two individuals with contrasting pure breeding  characteristics are crossed, the individuals of the first generation (F1) will ALL resemble the parent with the dominant characteristic.

Mendel’s Law of Independent Assortment

    

Alleles of a gene for one characteristic segregate independently of the alleles of a gene of another characteristic. The alleles for the two different genes will therefore come together randomly during gamete formation. This is also known as random assortment.

Mendel’s Principle of Segregation

During gametogenesis the two alleles of a gene separate so that each gamete will receive one allele of a gene for a specific characteristic/trait.  

Monohybrid cross

A genetic cross involving one characteristic e.g. colour of seeds

Mutation

A sudden change in the sequence/order of nitrogenous bases of a nucleic acid

Multiple alleles: 

When there are more than two possible alleles for one gene locus. e.g. blood groups

Phenotype:

This is the external, physical appearance of an organism. The phenotype is determined by the genotype. (phenotype, when both recessive gene alleles are present e.g.: bb)

Pedigree diagram

A diagram showing the inheritance of genetic disorders over many generations

Population

A group of organisms of the same species living in the same habitat at the same time

Recessive allele:

An allele that is suppressed when the allele partner is dominant.  The recessive trait will only be expressed/seen if both alleles for the trait are homozygous recessive e.g.:  tt

Stem cells/meristematic cells

Undifferentiated cells that can develop into any cell type

Theory

Explanation of an observation that is supported by facts, models and laws


Strategies to teach terminology

  1. In every lesson identify new terms/concepts and write it on the board
  2. Learners will take down terms/concepts at the back of their notebooks noting the correct spelling
  3. Learners must define/write down the meaning of these words from listening to the educator’ lesson/finding meaning from the dictionary or textbook
  4. Break down the concept/term where possible- give the meaning of the prefix and suffix e.g. photo (light) synthesis (to build up)
  5. Use the concept in a sentence
  6. Educators must check that learners have done the above, on a daily basis e.g. asks any learner to define a concept
  7. By the end of the year ALL learners have a comprehensive GLOSSARY of ALL terms /concepts
  8. ASSESSMENT: Biological terms to be included in all daily assessment tasks. Develop crossword puzzles. (Use various websites from internet e.g. eclipse)
  9. Learning terminology also helps in answering MCQs and matching questions, etc.

Genetics and inheritance

The principles of heredity

If a tall plant (dominant trait) is crossed with a short plant (recessive trait) a genetic cross could be written as follows:         

P1  Phenotype: Tall plants x short plants
Genotype: TT x tt
Meiosis
Gametes: T, T x t, t  ( Mendel’s principle of segregation)
Fertilisation

 

T

T

 t

Tt

Tt

 t

Tt

Tt

 

 

 

 

F1 Genotype:  100 %  Tt  (heterozygous tall)  
Phenotype: 100% Tall (Mendel’s Law of Dominance)
Note : that the F1 offspring have  characteristics from both parents but in the phenotype, all display the dominant characteristic.
The offspring of the F1 (Tt) grow and mature to become P2. The offspring of P2 are known as F2.

P2 Phenotype: Tall plants x Tall plants
Genotype: Tt  x  Tt
Meiosis
Gametes: T, t x T, t
Fertilisation

 

T

t

 T

TT

Tt

 t

Tt

tt




F2  Genotype: 1 TT  : 2 Tt : 1 tt  homozygous tall : heterozygous Tall :  homozygous short 
Phenotype: 75% tall : 25% short

1. TYPES OF DOMINANCE:

1.1 EXAMPLES OF MONOHYBRID CROSSES:
There are basically FOUR types of crosses. We will use one general trait e.g. hair colour: 

  • B = brown hair colour (dominant trait)
  • b = blonde hair colour (recessive trait)

CROSS EXAMPLE 1:  (Homozygous dominant x Homozygous recessive)
P1 (first parent generation) Phenotype: Brown x blonde 
Genotype: BB x bb
Meiosis
Gametes: B, B x b, b
Fertilisation

 

B

B

b

Bb

Bb

b

Bb

Bb

       

 



F1  (first filial generation = first offspring)
Genotype: Bb
Phenotype: 100% brown
            

CROSS EXAMPLE 2:  (Heterozygous x Heterozygous)                       

P1 Phenotype: Brown x Brown
Genotype: Bb x Bb
Meiosis
Gametes:  B, b x B, b                  
Fertilisation

 

B

b

B

BB

Bb

b

Bb

bb

       

 

 


F1 Genotype: BB : Bb Bb : bb   
1 : 2 : 1        
Phenotype: 75% brown and 25% blond

CROSS EXAMPLE 3:  (Homozygous dominant x Heterozygous)                                                                 

P1 Phenotype:  Brown x Brown
Genotype: BB x Bb
Meiosis
Gametes: B, B x B, b 
Fertilisation 

 

B

B

B

BB

BB

b

Bb

Bb

       

 

              

 

F1  Genotype: BB BB : Bb Bb
1 : 1
Phenotype: 100% brown 

CROSS EXAMPLE 4: (Homozygous recessive x Heterozygous)                       

P1 Phenotype: Blonde x Brown
Genotype: bb x Bb
Meiosis
Gametes: b, b x B, b
Fertilisation

 

b

b

B

Bb

Bb

b

bb

bb


F1  Genotype: Bb Bb : bb bb
1 : 1
Phenotype: 50% brown and 50% blond

1.2: Incomplete dominance

In this kind of dominance none of the two alleles of a gene are dominant over one another resulting in an intermediate phenotype in the heterozygous condition. In flowers this type of dominance could be viewed in flower colours.

For example a red flower is crossed with a white flower and the alleles are incomplete dominant. The cross for this type of dominance will be as follows:

Colour key:      R (red) W (white)

P1  Phenotype:  red x white
Genotype: RR x WW 
Meiosis
Gametes: R ,R, x W, W
Fertilisation
F1  Genotype: 4:4 RW
Phenotype: 100% pink
Another example could be found in humans:
Curly hair (CC) x Straight hair (SS) = Wavy hair (CS)

1.3: Co-dominance

In this kind of dominance both alleles of a gene are equally dominant whereby both alleles express themselves in the phenotype in the heterozygous condition. 

For example a red flower is crossed with a white flower and the alleles are co-dominant. The cross for this type of dominance will be as follows:


Colour key: R (red) W (white)
P1   Phenotype: red x white
Genotype: RR x WW 
Meiosis
Gametes: R ,R, x W, W
Fertilisation
F1  Genotype: 4:4 RW
Phenotype: 100% Red with white markings/ white with red markings

Another example in humans is: Blood groups




Last modified: Wednesday, 17 March 2021, 12:54 PM