GENÉTICA: PRINCIPIOS DE LA HERENCIA MENDELIANA, GENOTIPO Y FENOTIPO

[Music]

Hello,

welcome to Zona Fisio. My name is

Sergio Trujillo, a doctor, professor, and lover

of human physiology. This is the

first genetics video from Zona Fisio,

with the aim of covering the

necessary knowledge to understand

human physiology and pathophysiology.

In this video, we will talk about the

first Mendelian principles of

inheritance, as well as answer the question of the

difference between genotype and phenotype. Let's begin.

As an introduction, we will present

the basic postulates of Mendelian inheritance.

Mendel definitely marked a turning point

in genetics. He proposed the

existence of inheritance, what we

know today as chromosomes. How did he

achieve this? By experimenting with the

hybridization of peas. Mendel spent a

lot of time crossing different types of

peas and observing the

changes that appeared generation after

generation.

The first step in

Mendel's experimental approach is the

monohybrid cross, which consists of crossing two

purebred peas with different

characteristics,

let's say one green with parents of the

same color and one yellow with the

same characteristics.

Mendel noticed... The

resulting generation of peas was all

yellow,

so he crossed them again.

This is where Mendel

began to delve into the concepts of

inheritance when he noticed that 25

percent of the second generation

was green. This is

where the concepts of

homozygotes and heterozygotes are introduced. Mendel

realized that when

crossing peas with

different characteristics, there was a

loss of purity, and in

subsequent generations, the peas would not

all be the same color.

Mendel understood this phenomenon as

factors in pairs, associating that

peas had genetic traits in

pairs, one from each parent, and that there were

three possible combinations. In

the example of the peas: yellow (

both parents), green (both

parents), and yellow (one parent

while the other is green). Along with

this concept, he also understood the

existence of dominance and

resistance, meaning that when two

different factors responsible for a

given trait are present in an

individual, one of the factors dominates

over the other, as

seen in our example with the

yellow color. It would be the dominant one since it

manifested in 75% of the crosses of the

first generation. Finally, Méndez

suggested a segregation in the

genetic characters. He stated that

in the formation of gametes, the

paired factors separate or

segregate randomly in such a way that each

gamete receives one or the other with equal

probability. This means that the

parent will have a 50% probability

of inheriting either the dominant

or recessive character from the other parent. Let's

summarize the three concepts in the

following diagram. We will consider the capital

letter Y as the

yellow peas and the lowercase letter

as the green peas. The principle of the

paired factor is demonstrated with the letters:

double capital 'a', one capital and one

lowercase 'a', and double lowercase 'a'. These

being the three possibilities of

parents that we mentioned

earlier. We will see the concept of dominance

with the capitalization of the

letter. Yellow is the dominant one, and that is why we

have assigned it the

capital letter, while green is

recessive and has the lowercase letter. Very well,

what would happen if we graphically simulated

Méndez's experiment? First,

We would cross two purebred peas,

these being double a capital a and double a

lowercase a to refer to yellow and

green respectively. We will use the

following table to understand the

concept of segregation. This tells us

that the parent would have a 50

% probability of inheriting the

dominant trait and 50% the recessive one.

So, under this law, the traits would

remain, and the offspring would all be either

a capital a or lowercase a. As we can

observe after the combinations,

what color would the peas be?

Exactly yellow, since this is the

principle of dominance: to manifest itself

despite the existence of another trait. As a

result, we have that there is a 100

% probability of having

offspring that are either a capital a or lowercase a, that is,

heterozygotes.

Very well, now what would happen if we cross

this first generation? Let's see, we

can realize that the

traits are now quite different

due to segregation. We obtained one

pea with double a

capital a trait, two with one capital

and one lowercase trait, and one more with

double a lowercase trait. So, what does this mean? It

means that after the cross of the

first generation, there is a There is a 25%

probability that the pea will be

yellow with homozygous traits (i.e.,

two capital 'a's), a 50% probability that it will be

yellow with heterozygous traits (

i.e., one capital and one lowercase 'a'),

and a 25% probability that it will be green with

homozygous traits (i.e., two lowercase 'a's). It is

important to remember that if an

offspring expresses the

recessive trait, it

will necessarily be homozygous, since if it were

heterozygous, the dominance would

prevail.

Having understood this, the concepts of

genotype and phenotype are very simple.

Genotype refers to the collection of

genes of an individual or also to the

two alleles inherited from a

particular gene. Phenotype simply

constitutes the observable traits of an

individual. In our example, the genotype would

consist of the traits two capital 'a's,

one capital 'a', one lowercase 'a',

and two lowercase 'a's, while the

phenotype would be the characteristics that

these traits would produce, such as the

color yellow or green. In the

human context, the genotype is a bit more

complicated than capital or

lowercase letters, since it involves names.

and the locations of specific genes on

a chromosome of the human genome,

while the phenotype would be all

the physical and

functional characteristics such as

eye and hair color, height, build, facial

features, among many others.

In conclusion, Mendel laid the foundation for us to

later understand

how inheritance works: the

existence of a dominant

and inherited genotype and the phenotype that will be manifested by these

factors.

Therefore, the genotype will be the

genetic content that dictates the characteristics

of the individual and that will contribute part of the

genotype of their offspring, while

the phenotype will be the

visible characteristics, whether dominant or recessive,

dictated by the genotype.

Very well, this relates to the

present video. I invite you to

subscribe to the channel so you don't

miss any of the academic videos we

will be uploading. My name is

Sergio Trujillo, and these are

[Music]

and

[Music]