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The Complex Chemical Code of Life (Part 4) - Inheritance and Genes

MrElementDec 20, 2016, 9:17:47 AM
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This Blog Series will focus on the nature of DNA, going through the basic principles you learn in High School to first year University/College. This is intended as an educational series, hopefully leading to better (basic) understanding of this topic by the general public.

I will try to be to the point for sake of space and time (there's just so much), but I'll do my best to ensure I link bits of information if you're more curious - do tell me if I've missed something (which I'm sure I have, somewhere).

Enjoy!

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The image shown in the header shows the flowers of a sweet-pea, which played one of the most important roles in kicking off the field of study when now know as genetics, you'll find out more below.

Gregor Mendel and his peas: Modern genetics from an abbey garden in Austria

 

It may seem quite unlikely that the 'Father of Modern genetics' made his discoveries in an Augustinian monastery. But you'll find that some of the greatest scientific discoveries across the centuries have come from the unlikeliest of places, and people from all walks of life. Gregor Mendel was no exception.

Mendel was born in 1822, and grew up in a small farm region of Austria that is now the Czech Republic. Mendel's secondary education was tumultuous - as his parents moved him to a school in Troppau. He faced many difficulties but graduated from the school with honors in 1840.

By 1843 (age 21), Mendel entered the Augustinian monastery at the St Thomas Abbey (pictured to the right) in Brno, against the behest of his father, who wanted him to carry on his family's farm. In 1851, Mendel left the monastery and begun his two-year study in physics and chemistry at the University of Vienna, here his scientific knowledge and curiosity grew. After finishing his study, he returned to the monastery in Brno and began teaching at a local school.

It was around 1857 when Mendel began to breed pea plants in the monastery garden - and began to wonder how the individual plants inherited certain features from their parents. Thus began the ground-work that would change the way we think about how we inherit characters from our parents. It was with these peas, that Mendel deduced two laws of inheritance.

Before Mendel's different approach to how organisms inherit different features, there was a different model of inheritance: called the blending model. The blending model suggested that genetic material of both parents are "mixed," for example, the a blue and a yellow flower are supposed to produce a green one. It also predicts that over time, this will give rise to a freely uniform population of different mixes of traits  - which is something that isn't observed anywhere. This model also failed to explain how certain traits kept appearing after several generations did not have the traits.

The Law of Segregation

 When Mendel began his work, he chose characters in the plant that only had two alternative forms - such as white and purple flowers, he also made sure that the plants he was using for his research only produced offspring of the same type, which are said to be true-breeding. In his experiments, Mendel crossed the true-breeding plants that displayed opposite characteristics in what is called hybridisation. The true-breeding plants are called the P generation while the first generation of offspring are called the F1 generation; the F1 generation is then self-pollinated (or cross pollinated with another F1 individual) which produces the Fgeneration.

Mendel's model of inheritance is made of four parts:

  1. Alternative versions of genes (called alleles) account for variation that is found in inherited characters such as flower colour. In molecular biology (which didn't exist in his time), this concept can be linked to the nature of DNA and chromosomes.
  2. Each character of an organism has two copies of a gene (two alleles) that are inherited from the parents of the individual. This is one prediction that Mendel has made without even knowing chromosomes existed, nowadays we know that his prediction is true.

  3. If the two alleles are different, then the location of the gene on the genetic material of an organism is different for each allele. As such there is a dominant allele, determining the appearance of the organism, and the recessive allele which does not affect the appearance of the dominant trait.

  4. The law of segregation which states that the two alleles for a heritable trait segregates (separates from one another) when the sex cells (gametes) of an organism are formed, which leads to the alleles appearing in separate sex cells. Therefore, an egg or a sperm can only have one allele of two that are present in the normal cells of an individual.

We can display these key points in a Punnett square , a useful table that helps us predict the inherited characteristics of offspring, based on dominant and recessive alleles. Mendel noticed that the purple flowers (P) were the dominant alleles and the white flowers (p) were recessive. In the Punnett square below, you'll be able see the relationship between the dominant and recessive alleles in a monohybrid cross: 

The Law of Independent assortment

The Law of Segregation that came from Mendel's experiments followed only a single characteristic of an individual. As a result, the F1 progeny (genetic information of the individuals) were all considered to be monohybrids; breeding these individuals results in a monohybrid cross. The second law of inheritance came about because Mendel began to study two characteristics. For example:

  • The peas (seeds) may be yellow or green in colour. 
  • And the peas are either smooth/round or wrinkled.

From previous tests, Mendel knew that yellow peas (denoted by Y) were a dominant trait, and the allele for green coloured peas (y) was recessive; round peas were dominant (R) while wrinkly peas were recessive (r). This type of cross is called a dihybrid cross. Crossing a true breeds of both alleles was one of the ways Mendel figured out that genes independently assort when gametes are formed. See the Punnett square below:

The Law of Segregation only applies to genes located on chromosomes that aren't related, to each other or are found very far apart to each other on the same chromosome. 

Probability and genetics

Even genetics is influenced by the laws of probability and chance, as such the multiplication rule applies to the probabilities of a trait being inherited. For example the probability that a two coins are heads when two coins are flipped is: 1/2 x 1/2 = 1/4. This same rule can be applied to the Punnett square that I showed first. So each outcome would be 1/4 for the F1 generation; using the addition rule, the F2 generation can be calculated: 1/4 + 1/4 = 1/2.

Using the Punnett square above, we can predict the probability of certain traits being present. For example:

Probability of YYRR = 1/4 (probability of YY) * 1/4 (RR) = 1/16

Probability of YyRR = 1/2 (Yy) * 1/4 (RR) = 1/8

You can try some combinations for yourself, if you're up to it! You'll soon learn how the probability of a certain trait being inherited to an offspring.

Degrees of Dominance

Mendel's peas always showed one colour over the other because these genes were examples that showed complete dominance over another gene. For some Genes however, another phenomenon may occur - incomplete dominance. The classic example is when red snapdragons are crossed with white snapdragons, a pink snapdragon is produced. In other words, the phenotype (how a gene is expressed physically) is somewhere between red and white. Another form is codominance, where the two alleles are expressed separately, at the same time; for instance, both red and white petals are present in a flower,

Pleiotropy occurs when a gene has multiple phenotypic effects; so the gene codes for more than one physical attribute, for example, the red blood cells of humans display multiple blood types. See below:

Epistasis occurs when the expression of one gene alters the expression of another, found somewhere else. The best example for this is found in labradors where the allele combination of BB or Bb or Bb is effected by the expression of the E/e genes. You'll see that the colour of the labs changes depending on the E/e combination. See the table below and notice the pattern:    

Polygenic Inheritance is the idea that the expression of some genes can't be classified as binary (like purple/white flowers), but are expressed in graduations along a continum (like a spectrum of phenotypic characteristics). These types of characteristics are called quantitative characters;  this type of variation indicates that polygenic inheritance is occuring; where an additive affect of two or more genes affects a single chracteristic that is displayed. Examples include height and skin colour.

Nature and Nuture

The phenotypic character of some genes can depend on the environment as well as the genetics. For example, a tree has it's own phenotype, however it's leaves will vary in size, shape and green-colour because of the amount of wind exposure, rain and sun (drought etc.). The same is true is humans, characteristics such as height can be affected due to nutrition, and skin colour is influenced by the amount of sun exposure (just don't stay out too long!). It can be said that the phenotypic characters that organisms display isn't completely locked into place, but some characters instead inlcude a whole genotypic range, and is broadest of polygenic chracters; geneticists call these chracteristics multifactorial, as they contain many different factors that influence how they appear, such as the environment, which all collectively affect the phenotype.

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In the next part I'll begin going into the basics of the chromosonal basis of inheriting chracteristics.

Until next time!

To Part 3

 

 

To Part 5