SSLC-BIOLOGY-CHAPTER-1-NON-MENDELIAN INHERITANCE-EXTRA THOUGHTS

• Beyond Mendel's Laws Gregor Mendel is known as the "Father of Genetics". He discovered how traits are inherited through his experiments on pea plants. 
• He introduced the concepts of dominant and recessive genes. Thus forming the basis of what we now call Mendelian inheritance. His work, mainly with pea plants, led to what we call Mendel's Laws of Inheritance

Mendel's Laws of Inheritance:

Law of Dominance: 

• When two different alleles for a trait are present in a pair (heterozygous condition), one allele (the dominant one) expresses itself, while the other (the recessive one) remains hidden. Let's take the example of the height of a pea plant.

T= Tall (dominant)

 t = Dwarf (recessive)

• Here, in the Tt condition, the plant becomes Tall because the dominant T masks the effect of recessive t.

Law of Segregation:

• During the formation of gametes (sex cells), the two alleles for a trait separate (segregate) from each other. Hence, each gamete carries only one allele for each gene. This means that the paired alleles for a trait do not stay together when gametes like sperm or egg are formed. Instead, they separate so that each gamete gets only one allele. When fertilisation occurs, the offspring gets one allele from each parent, restoring the pair. Consider a pea plant with genotype Tt (Tall and dwarf alleles).

• During gamete formation, the alleles T and t segregate into different gametes. So, half of the gametes carry T, and half carry t. When fertilisation occurs, the offspring can have genotypes TT, Tt, or tt depending on the combination. This is why, under Mendelian inheritance, breeding a tall (T) plant with a dwarf (t) plant does not produce an intermediate-height plant. Instead, the dominant trait (Tall) is expressed.

Non-Mendelian inheritance 

• Non-Mendelian inheritance refers to any pattern of inheritance that does not follow the straight forward rules laid out by Mendel. 
• According to Mendel's laws, traits are passed from parents to offspring in predictable ways. 
• Like one trait dominating over the other in a heterozygous individual. However, as science advanced and more organisms were studied, scientists noticed that not all traits followed these simple patterns.
• In fact, many traits in living organisms are influenced by multiple genes, the environment, or even by interactions between genes. 
• Non- Mendelian inheritance helps explain why some children have features that cannot be easily predicted from their parents' traits.
•  For example, human skin colour, blood types, and even certain genetic disorders do not follow Mendel's dominant- recessive model.
• These patterns include incomplete dominance, co-dominance, multiple alleles and polygenic traits.

Incomplete dominance

• Imagine that you have two buckets of paint, one red and one white. If you mix them completely, you will get pink paint. In genetics,incomplete dominance is a bit like that!

What is incomplete dominance? 

• In incomplete dominance, when you have two different versions of a gene (alleles), neither allele completely takes over the other. Instead, they mix together, and you see a blend of the two traits in the offspring. Neither parent's trait is powerful enough to completely mask the other. This results in a "blended" inheritance, where no trait is entirely dominant or recessive.
• Difference between the two In complete dominance, one allele is like a "boss." If you have one dominant allele, that trait will always show up, even if there's a recessive allele present. For example, if brown eyes are dominant over blue eyes, a person with one brown eye allele and one blue eye allele will still have brown eyes. The brown allele completely hides the blue allele.
• In incomplete dominance, there is no "boss". Both alleles contribute to the final look, creating something new that's in between the 2 parents.

Example of incomplete dominance 

• Snapdragon flowers: If you cross a red snapdragon flower (RR) with a white snapdragon flower (rr), their offspring would not be red or white. Instead, all the baby snapdragons will have pink flowers (Rr). The red and white get mixed to make pink.

If you then breed two of these pink snapdragons, you'll get red, pink, and white flowers in the next generation in a 1:2:1 ratio.

Another example

Hair type in humans:

• If one parent has curly hair and the other has straight hair, their child might have wavy hair. 
• Wavy hair is a mix, as it is not completely curly or straight. Why does it happen?
• Incomplete dominance happens because the protein or enzyme made by one allele isn't fully active enough to produce the full trait on its own. When there are two different alleles, both contribute to the trait, resulting in a characteristic that is a blend of the two.

Co-dominance

Imagine that you have a paint set. If you mix blue and yellow, you will get green. 

That is how most genes work - one trait might hide another, or they might blend together. 

But what if you mix blue and yellow and get patches of pure blue and patches of pure yellow side-by- side?

 That is what co-dominance is all about!

Co-dominance

In co-dominance, both alleles are dominant. They both express themselves fully and equally in the organism. The roan coat in cows is one of the best and easiest examples to understand co- dominance.

To understand co-dominance, imagine that we are breeding a red cow and a white cow. We will use the following symbols for the alleles.

R=Allele for red coat colour

W = Allele for white coat colour.

The R and W alleles are co-dominant. A cow with the RW genotype will express both red and white coat colours simultaneously. This means that the cow will have red and white hairs mixed together all over its body, giving it the typical roan look.

Q: How do alleles control the expression of coat colour in cattle

• The alleles provide specific instructions to the cells responsible for producing pigment. These cells are primarily within the hair follicles. It is a complex interplay of several genes and their various alleles. It leads to a wide range of coat colours in different cattle breeds.

HUMAN BLOOD GROUP-AB GROUP

• You might have heard about different blood groups like A, B, AB, and O. This is another fantastic example of co- dominance. 
• Your blood type is determined by specific markers on the surface of your red blood cells. The gene for blood group has three possible alleles: IA, 1B, and i. 
• IA produces 'A' markers. IB produces 'B' markers. i produces no markers (it is recessive).
•  If you inherit an IA allele from one parent and an 1B allele from the other, your blood type will be AB. 
• This means that your red blood cells will have both 'A' markers and 'B' markers on their surface. Both alleles express themselves equally. Neither one is hiding the other.

MULTIPLE ALLELISM

Noormally, when we talk about a gene, we think of just two possible forms (alleles), like "tall" or "short," and "red" or "white." But for some genes, there can be three or more different alleles existing in the population for that single trait. It is important to remember that any individual will have only two alleles for that gene- one from each parent. But within a whole group of individuals, you might find many different versions of that gene.

Think of it like this - Two Alleles (Normal): Imagine a vending machine that only sells Coke and Pepsi. You can only choose one of the two.

Multiple Alleles:

• Now, imagine a vending machine that sells Coke, Pepsi, Sprite, Fanta, and Miranda. You can still only pick two drinks, but there are many more options available in the machine itself.

• The best and most common example of multiple alleles is the ABO blood group system in humans. You might know that your blood type is A, B, AB, or O. This is determined by a single gene that has three different alleles.
• IA allele: This allele codes for the presence of a specific marker called 'A antigen' on the surface of your red blood cells.
• IB allele: This allele codes for the presence of a different marker called 'B antigen' on the surface of your red blood cells.
• i allele: This allele codes for no antigen on the surface of your red blood cells. JA, 1B, and i.

Here are the possible genotypes (allele combinations) and their resulting phenotypes (blood types)

Multiple alleles are important because they, 

Increase genetic variation 

• They add more variety to a I population, which can be beneficial for survival as environments change. 

Explain complex traits 

• They help explain how traits E like blood type have more than just two simple outcomes.

Are common in nature 

• Many traits in plants, animals, and humans are controlled by genes with multiple alleles, not just two.

Polygenic inheritance

The word "poly" means "many," and "genic" refers to "genes." So, polygenic inheritance occurs when I multiple genes work

I together to control a single trait. Consider it like a team E project. Instead of one student doing all the work, E the whole group helps out. The final project's quality depends on everyone's I combined effort.

In polygenic inheritance several different genes (let's say Gene A, Gene B, Gene C, and so on) are all involved in determining one specific trait. Each of these genes I might have its own alleles, and each allele contributes a small "bit" to the final outcome. The effects of all these genes (and their I alleles) add up to create the final appearance or characteristic of the trait. Unlike traits controlled by a single gene (which often have clear categories, like "purple flowers" or "white flowers"), polygenic traits show a continuous range of variation. This means that you see many different

I shades or levels of the trait, not just a few distinct options.

Human skin colour  

• Skin colour isn't just "dark" or "light." There is a wide E spectrum of skin tones. This is because several different genes control the amount and type of melanin pigment produced in your skin. Each E gene contributes a small amount to the overall darkness or lightness, leading to countless variations in skir colour.

Human height. 

• There are not just two genes that decide if someone is tall or short. Instead, many different genes contribute to how tall a person grows. Some genes might affect bone growth, others muscle development and so on. The combination of all the alleles from these many genes determines your final height. That's why you see people of such different heights.

Eye colour -

• Although some charts simplify it, human eye colour is polygenic. It is not controlled by a single gene for "blue" or "brown." Severa genes work together to determine the amount and location of pigment in the iris, resulting in blue, green, hazel, brown, and many I shades in between.