Principles Of Inheritance & Variation

The inheritance of genes follows outlines – individuals that reproduce maintain characteristics from their gene pool that are carried onto to their offspring. Chromosomes carry gens in a lined arrangement that is common for organisms of the same species. Alleles segregate during meiosis permitting new blends of characteristics to be designed by the union of gametes. Biologists have established methods for non-natural guidance of genes, cells and organism development.

A gene is an inheritable code that decides a certain characteristic.

What You (Should) Know By Now:

(Skip to What You Don’t Know)

Mendel’s Laws of Inheritance

1. Law of Dominance (Dominate vs Recessive Alleles):

Things to remember:

(i) Traits are decided by “discrete units” called alleles.

(ii) Alleles work in pairs to decide the trait expression – phenotype.

(iii) In an unlike pair of alleles one allele will dominate over the other to decide the phenotype. The decisive allele is the Dominant Allele and the other allele is the Recessive Allele.

When two parents contain pure genotypes (homozygous) but for opposing traits, in a cross of those parents, only one arrangement of the possible traits will develop in the next generation. Every offspring of this blend will have one dominant trait and one recessive allele – this will lead to the physical expression of the dominant allele over the recessive allele. The trait expressed is the dominant phenotype. The phenotype trait that is NOT expressed in the hybrid is called recessive.

In the above example a green pod – GG – is crossed with a yellow pod – gg –

G is the dominant allele for Green colour,

g is the recessive allele for Yellow colour,

the possible resultant genotypes all show the same phenotype – green coloured pods but contain a heterozygous genotype – Gg (Dominant Green with recessive Yellow)

 

2.Law of Segregation

As the gametes (eggs or sperm) develop in the parent bodies, the two alleles in control of a particular characteristic separate from one another. Alleles for a trait are then paired with another allele for the same trait but from the other parent – this happens upon fertilization which helps in producing the genotype for the traits of the offspring.

The two alleles that decided a certain phenotype in the parent will separate during Meiosis to form haploid gametes with only one of the two alleles.

This law is built on the observation that the alleles do not show any merging or fusion within themselves. Both alleles work together but are separable in following generations.

 

 

3.Law of Independent Assortments

Alleles for diverse traits are dispersed to gametes (and by extension, to offspring) independent to each other. Which entails that the development of chance arrangements of chromosomes during meiosis and of genes on either homolog of a pair of chromosomes is based upon basic laws of probability and the passage of each trait is independent of its allele pair.

Example: the trait for height of the plant and the trait for shape of peas do not depend on each other. These traits are independent of each other and can independently and randomly be distributed into the sperm or egg cells.

 

 

Anh this video for an explanation of the Chromosomal Theory of Inheritance

This is how to draw Punnett Squares

What You Don’t Know:

Crossing Over in meiosis

In a pair of homologous chromosomes, one strand is from your father while the other is from your mother and the preparation and separation of these chromosomes happens during meiosis.

During the prophase of meiosis I is when this process of crossing over occurs, and these homologous chromosomes are split and passed onto two daughter cells.

Crossing over is the interchange of genetic code between two homologous chromosomes to give rise to recombinant chromosomes, in order to achieve the most possible variation for the survival of the offspring. In prophase I, homologous chromosomes pair with each other and form connecting bridges along gene positions that align. Following this there is a switch in pieces of code along the length of the chromosome. These linkages formed between the two homologs are known as chiasmata.

Why is Crossing Over Important?

Crossing over helps to bring about random shuffling of genetic material during the process of gamete formation. This results in formation of gametes that will give rise to individuals that are genetically distinct from their parents and siblings.

This genetic variation is required to increase the ability of a population to survive. A greater genetic diversity would reduce the chances of inheritance of deleterious traits in the population, and therefore, help increase the fitness of the individuals of a population.

An increased genetic variation would also mean a greater variation in susceptibility to diseases. So, if there were to be an epidemic of a disease, this variability would prevent the whole population from being wiped out.

Another benefit of genetic variation is that some traits that would increase an individual’s ability to survive may be introduced in the population.

You can look through this article if you’d like to get into the nitty-gritties of crossing over

Cystic Fibrosis

	C	c
C	CC (normal)	Cc (heterozygous normal)
c	Cc (heterozygous normal)	cc (Cystic Fibrosis)

“Cystic fibrosis is a progressive, genetic disease that causes persistent lung infections and limits the ability to breathe over time.” – Cystic Fibrosis Foundation

Caused due to a recessive allele from the CFTR (Cystic Fibrosis transmembrane conductance regulator) gene, it is the most common genetic disease in Europe. Located on the 7^th chromosome, the recessive gene causes the faulty expression of chloride ion channels involved in the secretion of sweat, mucus and digestive juices. Therefore, if both parents are heterozygous for the CF gene, then their off-springs would have a 25% chance of developing Cystic Fibrosis

You can read more about Cystic Fibrosis at the Cystic Fibrosis Foundation website.

Sex-linked diseases

Haemophilia is a Genetic disease linked to the Y chromosome and hence only affects men.

Genetics is a constantly advancing and changing field in terms of correlations, discoveries and techniques.

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Problems remembering? We got you fam.