GENETIC SEGREGATION: WHAT IT CONSISTS OF AND EXAMPLE - BIOLOGY - 2025

The genetic segregation is the distribution of genes from parents to children during meiosis. A gene can be defined as a portion of DNA that codes for some specific phenotype: it can be a protein or a gene involved in cell regulation.

Genes are physically located on chromosomes, highly organized entities of DNA and proteins where genetic information is stored. At the time of reproduction, these hereditary factors must be separated and passed on to the descendants.

The experiments carried out by Gregor Mendel allowed us to understand the process of segregation, explained in its well-known laws.

What does it consist of?

Genetic segregation is the separation and transfer of genes to progeny and occurs during the process of cell division by meiosis. Chromosome segregation is the basis of this concept.

Mendel's First Law

According to the principle of segregation or first law enunciated by Gregor Mendel, organisms have two alleles for a certain character.

An allele is a form or a variant of a gene. For example, we can hypothetically have one allele for blonde hair and another allele for brown hair. The alleles are usually denoted with capital letters for the dominant and lowercase for recessive.

According to the first law, each gamete (egg or sperm) in its formation process receives one or the other of these alleles. At the time of fertilization, a diploid organism is formed again with one allele received from each parent.

One of the most relevant conclusions from this experience is to note that genes are discrete particles that independently segregate from parent to child.

Before Mendel, erroneous hereditary principles were handled and it was thought that genes behaved like fluids that could mix with each other, losing the initial variability.

Mendel's second law

In a second round of experiments, Mendel added another morphological character to the study. Now, individuals with two characteristics (for example, plants with round and yellow seeds versus plants with wrinkled and green seeds) were crossed and their offspring counted.

After analyzing the data, Mendel was able to conclude that each character behaved independently. This principle can be summarized as follows: each hereditary characteristic is distributed independently.

Linkage and segregation groups

It is now known that Mendel evaluated characters in his experimental plants (seed roughness, stem height, etc.) that were physically located on separate chromosomes.

When loci (the place that genes occupy on chromosomes) are contiguous or adjacent on a chromosome, they are quite likely to segregate together in what is known as a "linkage group."

Consequences of segregation

When the zygote receives two equal alleles from its parents, the organism is homozygous for the character studied. If both alleles are dominant, it is called homozygous dominant and denoted AA (or any other letter, both capitalized).

In contrast, if both alleles are recessive, it is a recessive homozygous and is indicated by lowercase letters: aa.

It is also possible that the offspring inherits a dominant and a recessive allele. In this case, it is heterozygous and is indicated by the capital letter at the beginning, followed by the lower case: Aa.

The phenotype - or the observable characteristics of an organism - depends on its genotype and the environment. If the genotype is AA or aa they simply express the trait they determine; in the case of heterozygotes, the trait expressed is that determined by the dominant allele.

The latter is true only in case dominance is complete; There are also other cases, such as incomplete dominance or codominance.

Meiosis

Meiosis is the phenomenon of cell division that occurs in the germ lines of organisms to give rise to haploid gametes from diploid cells.

Meiosis begins with DNA replication and subsequently occurs the rounds of chromosome segregation called meiosis I and II.

Meiosis I is a reductional step of the process, in this stage the transformation to haploid cells occurs. To achieve this, homologous chromosomes pair up (in prophase) and segregate into different cells (in anaphase) in a random way.

Furthermore, in meiosis I a process called recombination or meiotic crossover occurs, where the exchange of genetic material occurs between the non-sister chromatids of the homologous chromosomes. For this reason, the gametes produced are all different from each other.

During crossover, a region called the chiasm appears that holds the chromosomes together until the spindle separates them.

When recombination is not performed properly, errors in segregation can occur, resulting in the development of an organism with chromosomal defects.

For example, Down syndrome occurs due to improper segregation in which the body carries three chromosomes (and not two) in the twenty-first pair.

Example

Flowers on pea plants

Pea plants of the Pisum sativum species may present flowers with purple petals and in other individuals they may be white. If two pure lines of these two variants are crossed, the resulting first filial generation exhibits only purple flowers.

However, the white character has not disappeared in these individuals. It is not observable because it is masked by the dominant allele that is related to the color purple.

Using the aforementioned nomenclature, we have that the parents are AA (purple) and aa (white).

The first filial generation is composed only of plants with purple flowers and, although phenotypically they look the same as one of their parents (AA), they differ in their genotypes. The entire first generation is heterozygous: Aa.

These heterozygous individuals produce four types of gametes: the female A and gametes and the male A gametes already in identical proportions.

To ensure that the alleles appear in pairs and that they secrete at meiosis, it is necessary to cross heterozygous purple individuals with individuals bearing white flowers.

Although it seems to be an identical cross to the initial one, the result is different: half of the individuals have white flowers (genotype aa) and the other half have purple flowers (Aa).

References

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