Inheritance 💰
No. We're not talking about that kind of Inheritance. No, this has nothing to do with Money. Just get up and go look at yourself in the Mirror. You've inherited the physical body that you see from your parents, who've inherited theirs from your grandparents and so on.
In short, this chapter is about genetic inheritance.
The Concept of Inheritance
Inheritance is the passing of traits from parents to their offspring, either through asexual reproduction or sexual reproduction. This is the process by which an offspring cell or organism acquires or becomes predisposed to the characteristics of its parent cell or organism 🧬.
Table of Definitions
| Term | Meaning |
|---|---|
| Inheritance | The transmission of genetic information ℹ️ from generation to generation 👨👩👦. |
| Chromosome | A thread of DNA 🧬, made up of a string of genes 🧬🧬 🧬. |
| Gene | A length of DNA that codes for a protein. |
| Haploid nucleus | A nucleus containing a single set of unpaired chromosomes Examples: 1. Sperm cells 2. Egg cells 🥚 |
| Diploid nucleus | A nucleus containing two sets of chromosomes Examples: 1. Zygote 2. Heart cells 💖 3. Liver cells |
| Mitosis | Nuclear division that gives rise to genetically identical cells 👥 |
| Meiosis | Reduction division in which the chromosome number is halved from diploid to haploid, resulting in genetically non-identical cells |
| Allele | Any of the alternative versions of a gene. For Eg:- TT, Tt, tt are alternative forms of the gene for height) |
| Genotype | Is the genetic make-up of an organism in terms of the alleles present. Example: To describe a person with gene for height, ‘T’ can be used. |
| Phenotype | Is the feature of an organism (in terms of words) Eg:- Height. Hair Colour, Finger Length, etc. |
| Homozygous | - Having two identical alleles of a particular gene (such as TT and tt) - Two identical homozygous individuals that breed together will be pure breeding (for example if TT breeds with TT, an offspring with TT will be born) |
| Heterozygous | Having two different alleles of a particular gene (such as Tt), not pure breeding. |
| Dominant alleles | An allele that is expressed if it is present (such as T) |
| Recessive alleles | An allele that is expressed only if the dominant allele of the gene is absent (such as t) |
| Codominance | A condition where both the alleles - dominant and recessive - have an effect on the phenotype of that plant For example, 1. CWCW produces white flowers in a plant. ⚪️ 2. CRCR produces red flowers in a plant. 🌹 3. CWCR produces pink flowers in a plant (and not white!) 🌷 |
Important Note:
Dominant and recessive gene expressions:
- TT = Homozygous dominant
- tt = Homozygous recessive
- Tt = Heterozygous
Mitosis and Meiosis
If I asked you, what is the difference between Mitosis and Meiosis, you'd probably say one of the things below:
Student 1:
Mitosis involves Nuclear division that gives rise to genetically identical cells, and Meiosis involves the Reduction division in which the chromosome number is halved from diploid to haploid, resulting in genetically different cells
Student 2:
Mitosis is spelt M.I.T.O.S.I.S
Meiosis is spelt M.E.I.O.S.I.S
They sound different, and hence
they might stand for different meanings too!
Logically both answers make sense right? But scientifically speaking the first student's answer would be considered correct by your typical CIAE examiner.
Regardless, there is more to Mitosis and Meiosis beyond just memorising the definition 👇
Comparison Between Mitosis and Meiosis
| Mitosis | Meiosis |
|---|---|
| Type of reproduction carried on is asexual | Type of reproduction carried on is sexual |
| Genetically identical chromosomes are formed 👩🎤👩🎤 | Genetically different chromosomes are formed 👨🎤🧑🎤 |
| Two diploid cells formed 🔴🔴 | Four haploid cells formed 🟠🟡🟢🔵 |
| All cells except sex cells are made by this kind of division | Only sex cells (sperm and egg cells) are made by this kind of division. |
Stem cells
When a zygote forms, it begins to divide by mitosis; forming genetically identical cells. This soon forms an embryo.
After this period, the cells from the embryo (known as embryonic stem cells) begin to take up different functions; some of them will become skin cells, some liver, some brain, etc. This is known as differentiation.
Though every cell in our body has the same genes, only some of them specifically, are expressed.
Inherited Diseases 🧬 🦠
| Inherited Disease | Notes |
|---|---|
| Cystic Fibrosis 🥵 | - In Cystic Fibrosis, the cells in the lungs make mucus more thicker than usual - It decreases the efficiency of the gas exchange surfaces due to the trapped mucus - The mucus made blocks the pancreatic duct and prevents digestive juices from flowing, affecting digestion |
| Haemophilia 🩸 | - In haemophilia, the blood in a person’s body fails to clot when there is a cut or wound. - It is caused by the inheritance of a defective gene that prevents blood clotting |
| Sickle Cell Anaemia 🩸 | Is a genetic disease where the red blood cells become sickle shaped in the absence of oxygen |
| Downs Syndrome 🧬 | - Is another genetic disorder where the chromosomes 21s fail to separate during meiosis in a woman’s ovaries. - This means that an extra chromosome 21 will be in the egg cell; if it gets fertilised, then the zygote will have 47 chromosomes instead of 46! - Children with downs syndrome are usually extremely friendly people and have heart diseases when they grow up |
Genetic Diagrams
There is a standard way of writing information such as an organism’s and its parent’s genotypes and phenotypes. Moreover, these genetic crosses help us to predict the probabilities of an offspring inheriting a particular characteristic of its parents.
For example, a tall pea plant is bred with a dwarf one. What do you think will its offspring be?
| Parental phenotype | Tall plant | Dwarf plant |
| Parental genotype | TT | tt |
| Gametes | T | t |
Non Expressed Recessive Genes
‘Tt’ means that all the offsprings born would be tall, and they would be having the ‘hidden’ or non expressed recessive gene in them. In simple words, the plant offspring is a ‘carrier’ of the gene for dwarfness.
To get dwarf plants, the offsprings in the above example can be bred amongst themselves; this would give rise to 25% of the offsprings being dwarf.
| Parental Genotypes |
Parent 1 | ||
| T | t | ||
| Parent 2 | T | TT (Tall) |
Tt (carrier) |
| t | Tt (carrier) |
tt (Dwarf) |
|
Sex determination
Similarly, using genetic diagrams can be useful to determine the sex (gender) of an offspring.
| Parental Genotypes |
Parent 1 👩 |
||
| X | X | ||
| Parent 2 👨 |
X | XX 👧 |
XX 👧 |
| Y | XY 👦 |
XY 👦 |
|
Sex Linkage
A sex-linked characteristic is one in which the gene responsible for a particular function is located on a sex chromosome, which makes it more common in one sex than the other.
- The sex chromosomes X and Y not only determine your gender; they contain other genes as well.
- The X chromosome has a larger chromatid than the Y chromosome. Thus the genes present on the X chromosome (apart from the gene for your sex) will not be present on the Y chromosome.
- This means that if an X chromosome has a gene for colour blindness 👁, The Y chromosome will not have the gene; this can lead to a male having colour blindness being born as the X chromosome is dominant and there is no recessive allele on the Y chromosome.
- Similarly when an X chromosome (from a female) with a gene for colour blindness fuses with another X chromosome (from a male) with a recessive allele for the same gene, an offspring who is the carrier of colour blindness is born.
Hence colour blindness is more common due to these reasons in males rather than in females as:-
- There is less chance of a recessive allele being expressed in a female (XX) because the other X chromosome may carry the dominant form of the allele.
- The male chromosome doesn’t have a recessive allele.
The 5 possible phenotypes and their genotypes for red-green colour blindness are:
- XBXB : Woman with normal vision
- XBXb: Woman with normal vision (who is a carrier)
- XbXb: Woman with red-green colour blindness
- XBY: Man with normal vision
- XbY: Man with red-green colour blindness
Inheritance of sex linked characteristics
Let us see a much more simplified genetic diagram that will help explain the inheritance of sex linked characteristics.
In the following case, a male without red-green colour blindness (XBY) mates with a female with its carrier gene (XBXb). Let’s see what will happen now!
| Parental Genotypes |
Parent 1 👩 |
||
| XB | Xb | ||
| Parent 2 👨 |
XB | XBXB (Female without Red-Green Colour Blindness) |
XBXb (Female with Red-Green Colour Blindness Recessive Allele) |
| Y | XBY (Male without Red-Green Colour Blindness) |
XbY (Male with Red-Green Colour Blindness) |
|
The genetic diagram predicts that about half of their male children will have red-green colour blindness; whereas all of their female children will have normal vision.
DNA and Protein Synthesis
A DNA molecule is made up of 2 strands of nucleotides.
- Proteins are made up of long chains of amino acids.
- The sequence of these amino acids in a protein molecule determines the final shape of the molecule.
- This shape affects the way the protein works.
- This can control metabolic reactions in the body.
- This then affects the way the organism’s body works.
DNA and the genetic code
The genetic code is carried by a long and complicated molecule called Deoxyribonucleic acid or DNA. DNA is responsible in the production of proteins (see protein synthesis)
- The DNA is found in the nucleus of all cells. It is formed into X-shaped structures called In human diploid cells (except for eggs and sperm), there are 46 chromosomes. These are divided into 23 pairs.
- A chromosome is made up of two chromatids held together in the middle by a centromere.
- This DNA strand looks a bit like a ladder twisted into a double helix structure. The rungs of the ladder are made up of pairs of base molecules with shapes complimentary to each other.
- It is the order of the bases that carry the actual genetic code.
- There are only 4 different types of bases:
- Adenine (A),
- Cytosine (C),
- Guanine (G)
- Thymine (T)
- As mentioned above, the pairs of bases have shapes complimentary to each other (like an enzyme and its substrate!)
- Adenine and Thymine always join together (AT or TA)
- Cytosine and Guanine always join together (CG or GC)
- The genetic code contained in our chromosomes is used to make new cells by making proteins.
Passing on the code
The code is passed on to the new cells using either of two processes, mitosis or meiosis.
(See mitosis and meiosis)
Protein synthesis
Protein synthesis is all about how a protein is made in reality with the ‘synthesis’ (joining) of amino acid molecules.
Here is the process:
- DNA is found in the nucleus
- Protein synthesis happens on the ribosomes, in the cytoplasm
- An mRNA (Messenger RNA 🦅) is used as a mode of transport from the DNA to the ribosomes.
- The DNA in the nucleus unwinds
- This produces a length of mRNA which has bases complementary to the DNA’s length.
- When the mRNA finishes copying the base sequence, it moves out of the nucleus through a nucleus pore.
- It then attaches itself to a ribosome
- As the mRNA molecule passes through the ribosome, a strand of amino acid begins to join simultaneously.
- The ribosome links amino acids exactly in the right order to make the desired protein.
Note: when the mRNA passes through the ribosome, it doesn’t ‘make’ the amino acids but simply join them to form a protein.
(it is like joining a necklace of fallen beads!)
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Aniruddha Jaydeokar