Inheritance
Inheritance is the transmission of genetic information from generation to generation.
Genetic information is stored in the the form of DNA within the cell nucleus. Here are some key definitions that you must be aware of:
- Chromosome is a thread like structure of DNA
- A gene is a length of DNA which codes for a specific protein
- An allele is a version of a gene
For example, imagine a gene that codes for eye color. There are variations in this gene called alleles. One allele for this gene may code for brown eyes, whereas another allele may code for blue eyes.
Sex inheritance
Normal human cells have 23 pairs of chromosomes. These called known as diploid cells.
One of the pairs of chromosomes codes for sex inheritance.
Sex inheritance depends on the presence of X and Y chromosomes.
- Males have one X chromosome and one Y chromosome (XY)
- Females have two X chromosomes (XX)
DNA structure and function
The function of DNA is to control cell function by controlling the production of proteins i.e. enzymes, antibodies, cell receptors, etc.
The DNA is made of a double helix backbone, which are connected by pairs of bases.
- Adenine always pairs with thymine
- Cytosine always pairs with guanine
The sequence of bases along a particular length of a DNA strand codes for the manufacture of a specific protein.
Consider the particular length of DNA (gene) in the diagram above. The code within that length is ATTCGAA. This base sequence within the gene codes for the specific types and order of amino acids that become joined to form a protein.
All body cells contain the same genes, but not all genes are expressed because the cell only makes certain proteins that it really needs.
Protein manufacture
As we’ve discussed above, a gene is a particular length of a DNA molecule containing a specific sequence of bases. This base sequence codes for which amino acids should be joined in what order, to build a particular protein.
DNA held within the nucleus of the cell. Let us consider a particular gene (gene A) that makes a certain protein (protein A).
- A molecule called mRNA ‘copy’ the DNA base sequences found within gene A – This is called transcription
- mRNA molecule leaves the nucleus
- mRNA passes through the ribosomes in the cytoplasm
- Ribosomes ‘read’ the base sequences and assemble various amino acids in a specific order based on the base sequences – This is called translation
Diploid vs haploid
As mentioned before, a normal human cell contains 23 pairs of chromosomes. This particular number is extremely important for normal bodily function. Cells like this are called diploid cells, and are majority of the cells in the human body.
Gametes i.e. sperm cells/egg cells, have only a single set of 23 chromosomes. This is half the normal amount in order to maintain 46 chromosomes after fertilization.
In summary:
- Diploid cells have 23 pairs of chromosomes
- Haploid cells have a single set of 23 chromosomes
Cell division
Mitosis
Mitosis is the nuclear division giving rise to genetically identical cells. This is very important for various bodily functions:
- Growth
- Repair
- Cell replacement
Imagine diploid cell A (46 chromosomes) undergoing mitosis. If this cell were to simply divide in half, then it the resulting cells will only have 23 chromosomes.
Therefore, the cell duplicate its chromosomes before mitosis occurs (i.e. from 46 to 92) in order to maintain the chromosome number after the cell division occurs.
Meiosis
Meiosis is a type of nuclear division giving rise to cells that are genetically different. This is a reduction division to form haploid cells which produce gametes.
Essentially, diploid cell A (46 chromosomes) will form haploid cells with 23 chromosomes.
Monohybrid inheritance
Important definitions
- Genotype = The genetic make up of an organism in terms of the alleles present
- Phenotype = Observable features of an organism
- Alleles = Variations of a given gene
- Homozygous = Two identical alleles of a particular gene
- Heterozgous = Two different alleles of a particular gene
- Dominant allele = Allele that is always expressed if present
- Recessive allele = Allele that is only expressed if the dominant allele is not present
Monohybrid crosses
A monohybrid cross is a genetic mix between two individuals who have homozygous genotypes, or genotypes that have completely dominant or completely recessive alleles, which result in opposite phenotypes for a certain genetic trait.
Eye colour
There are different alleles that code for eye colour
- Blue eye allele (b) is recessive
- Brown eye allele is dominant (B)
A brown eyed individual can therefore have two possible genotypes: BB or Bb
A blue eyed individual can only have one possible genotype: bb
Example 1 – Homozygous recessive (bb) X Heterozygous(Bb)
Consider a brown eyed person with genotype BB having a child with another person with blue eyes genotype bb
You can use the punnet square to look at all the possible genotypes of the child.
In this scenario, the chance of a blue eyed child is 2/4 and the chance of a brown eyed child is 2/4 , therefore the ratio is 1:1
Example 2 – Homozygous recessive (bb) vs Homozygous dominant (Bb)
Consider a brown eyed person with genotype Bb having a child with another person with blue eyes genotype bb
In this scnerario, the chance of a blue eyed child is 0/4, and the chance of a brown eyed child is 4/4.
Example 3 – Heterozygous (Bb) X heterozygous (Bb)
Consider a brown eyed person with genotype Bb having a child with another person with brown eyes genotype Bb
In this case, the chance of a blue eyed child is 1/4, and the chance of a brown eyed child is 3/4. The ratio of blue:brown is 1:3
Co-dominance
Co-dominance is when a pair of alleles are neither dominant to one another. Both alleles can therefore impact the phenotype.
Blood type is a good example of co-dominance. The alleles of bloodtypes are: IA, IB and IO.
- IA and IB are co-dominant
- IA and IB are dominant over IO
Combination of these alleles can therefore results in blood groups A, B, AB, and O.
Sex-linked characteristics
Sex linked characteristics are characteristics in which the gene responsible is located on the sex chromosome, and therefore making it more common in one sex than the other.
Red-green colour deficiency
The colour deficient gene is an abnormal gene found in the X chromosome (Xc).
A male (with XY chromosomes) with these gene will inevitably have colour deficiency because males only have one X chromosome, so this abnormal gene will always be expressed.
Females on the other hand, may have the colour deficient gene but have normal colour vision. This is because females have two X chromosomes (XX).
The normal colour vision allele dominates the abnormal allele, so as long as the female has one normal X chromosome then they will have good colour vision.
To summarize:
- Male (XY) = Normal colour vision
- Male (XcY) = Colour deficient
- Female (XX) = Normal colour vision
- Female (XcX) = Normal colour vision i.e. carrier
- Female (XcXc) = Colour deficient