Chromosomes and Genes

Basic life constituents

Integral Clinic in Assisted Reproduction in Mexico

An expert team in fertility procedures

In this space we aim to bring you essential information with the objective of familiarizing you in a brief and simple way with this thrilling world.

What are they?

The key of life is located in the nucleus of the cell, which is the center that regulates all of its activities, there are 46 pairs of chromosomes (23 maternal and 23 paternal), only ovules and spermatozoids have 23 chromosomes respectively. Women have two X chromosomes and men have one X and one Y chromosome, which respectively are formed by a substance called DNA or Deoxyribonucleic Acid which is constituted by approximately 30,000 genes, dedicated to determine the characteristics that will make each human being unique and unrepeatable. Heritage is a phenomenon in which living beings transmit their characteristics to their heir. When we mention physical characteristics we are denoting the PHENOTYPE as referred in genetic language; these characteristics are contained in our genetic code, which is referred as GENOTYPE in genetic language. The GENOTYPE determines how the biochemical and physic characteristics of a being will be. We used to believe that minuscule tissues and organs from both father and mother were inherited and once they joined, the characteristics of the new being were determined. Half way through the XIX Century, the scientist Gregor Johann Mendel stablished the genetic heritage mechanisms, still current and formaly known as “Mendel Laws” identified as the GENETIC basis.

Chromosomes relevance

Here an example: let’s take a chromosome from the father and take a chromosome from the mother, together they form one of the 23 pairs of chromosomes, each chromosome is filled with lots of genes, but we’ll center our attention in one GENE which is responsible of the eyes’ color, the gene that codifies the color of the eyes, will always be on the Chromosome 6, to define a place or position in a chromosome we will use the term LOCUS, if the father has blue eyes and the mother has brown eyes, this difference between the LOCUS on the father’s and the mother’s chromosomes will be known as ALLELE or ALLELIC VARIATION and is the origin or the different color options.

Brown eyes


allelic variations


Blue eyes

The color of the eyes of this couple’s offspring will rely on each of the genes, which are classified as follows:

DOMINANT GENES (they dominate)

RECESIVE GENES (they need two copies of the same allele – which is one the variant of the gene)

If in the Family of one or both Parents blue eyes have predominance, this will be the DOMINANT GENE and that’s the way it will manifest, a baby with blue eyes will be born.

Another basic concept that will help us to understand the subject is:

HOMOZYGOUS: which means that both alleles are the same (both Parents have blue eyes)

HETEROZYGOUS: which means that both alleles are different (father has blue eyes and mother has Brown eyes)

An individual can either be homozygous for blue eyes or heterozygous if carries a dominant brown eye allele and a recessive blue eye allele.

When a spermatozoid and an ovule reach each other producing the beginning of a pregnancy, they form a new cell containing 46 chromosomes (23 from the mother and 23 from the father).

MEIOSIS: process by which the spermatozoids diffusing mother cells divide in half their genetic material and shift from having 46 pairs of chromosomes to 23 chromosomes.

During this process mistakes can be present, which will produce spermatozoids with an abnormal chromosomal provision giving as a result sterility, abortion or failure of the IVF (In Vitro Fecundation) techniques. A study of the MEIOSIS will provide information from all the chromosomes.

Through the performance of a CARIOTYPE study we are able to know if there is any chromosomal alteration, this study is targeted for all people concurring with repetitive abortion, congenital syndrome and infertility confirmation, IVF failure and aged women.

The defects of the chromosomes and genes can be due to: Numeric alterations, the normal number of chromosomal copies should be 2, if there is less or more it causes an alteration, like in Down’s syndrome example, which has 3 copies of the 21st chromosome. Monogenic alterations, which consist in a mistake or mutation in one single GENE. Structure alterations, in which the chromosomal content is not normal, either because there is a fragment missing or it is misplaced.

Alterations in Cariotype or in Meiosis do not have a treatment, but the prognosis of fertility can be improved by selecting the embryos by Preimplantation Genetic Diagnosis (PGD) which allows transferring the better pre-embryo through the study of its GENOMA.

In Fertility Center Cancun, our specialists will be at your disposition to extend this information and that way we can start clearing out your questions step by step, get close to us, it will be a pleasure to help you.