Cleft chin, which is also referred to as dimple chin, is an inherited trait. HealthHearty provides information that will help you understand the genetic factors of a cleft chin.
Did You Know?
Cleft chin finds a mention in the Online Mendelian Inheritance in Man (OMIM), which is a compendium of human genes and genetic phenotypes. OMIM is a part of the National Institute of Health’s NCBI network of life sciences databases that enlists more than 10,000 human genetic variants.
Our physical appearance is determined by our genes, which are segments of deoxyribonucleic acid (DNA) involved in producing a polypeptide chain. The color of our eyes, texture and color of hair, hairline, shape of facial features, presence of dimples on the cheeks, dimple chin, freckles, shape of the body parts, etc., are all determined by our genes. The transmission of the genetic characteristics or traits from the parents to the offspring is referred to as heredity. Genotype, which means the genetic makeup of a cell or organism (information or instructions within the cell), is responsible for phenotype, which means the observable traits or expression of information within a gene.
Chromosomes
Every cell in the human body contains a nucleus, which in turn contains thread-like strands called chromosomes. Normally, each cell in the human body has 23 pairs of chromosomes. Out of these, 22 pairs of chromosomes are referred to as autosomes.
The members of each of these 22 pairs are homologous. Both the members of a pair have the same genes (but could be in different allelic forms). The 23rd pair, which is the sex chromosome, differs in males and females. The sex cell (sperm cell/egg cell) has one set of chromosomes. While males have one X and one Y chromosome, women have two X chromosomes. When an egg cell and sperm cell fertilize, the fertilized egg cell contains 23 pairs of chromosomes.
While one chromosome in each pair comes from the mother, the other comes from the father. Thus, at the time of conception, both the mother and father contribute a copy of each gene. The combination of chromosomes determines the traits that the offspring would inherit. This is the reason why we look somewhat similar and different from our siblings.
Genes and Alleles
All the cells contain the same DNA, and every time a cell divides, the original genetic material is copied. Genes are basically DNA segments that store the information needed for the cell to assemble proteins. These proteins are responsible for the specific physical traits. The term ‘locus’ refers to the location of a particular gene on the chromosome. Homologous chromosomes have the same gene at the same locus, although they may carry different forms of that gene. These variations are called alleles. A variation in protein activity or expression causes a different phenotype. Alleles have minor differences in their sequence of DNA bases, and are responsible for producing alternative forms of a particular trait. Alleles can be dominant or recessive. A dominant allele is a variation that produces the same phenotype, whether its paired allele is identical or different. On the other hand, a recessive allele shows or produces the phenotype only if its paired allele is identical.
One can have two dominant alleles, one dominant and one recessive allele, or two recessive alleles. A dominant allele is represented with a uppercase letter, whereas the lowercase equivalent of that letter is used for representing a recessive allele. If both alleles are dominant or one allele is dominant and the other one is recessive, there’s a great likelihood of that particular trait being present in the individual. The allele that is dominant suppresses or masks the effect of the recessive allele. Thus, expression of recessive alleles occurs in the absence of dominant trait at the locus, which means that only recessive alleles are present on both the homologous chromosomes. Mostly, dominant alleles are able to produce an enzyme or functional protein that is essential for the expression of traits, while recessive alleles are unable to do so. In order to possess a recessive trait, one must inherit the recessive allele from both parents.
If we use the letter C for the dominant allele for cleft chin, then the recessive allele (a chin without a dimple) will be represented by the letter c. So, those who have two identical alleles (CC or cc) are homozygous for this trait, while those with different alleles for that gene (Cc) are heterozygous. While the genotype comprises the paired alleles that one possesses for a particular trait, the observable trait itself is referred to as the phenotype.
Factors Affecting the Expression of Cleft Chin Phenotype
The genetic makeup of the offspring or the probabilities of phenotype and genotype ratios can be determined using a Punnett square to some extent. It is a tool that evaluates the frequency of the occurrence of particular genotype and phenotype in the offspring. It can be used for predicting genotypes for one or more traits. If it is used for predicting genotype for one trait, such as a cleft chin, the square will have two rows and two columns. On the top of the square, the two alleles of one parent are written, whereas the two alleles of the other parent are written on the left side.
If both parents are heterozygous, (Cc x Cc) C and c would be written on the top and the left side of the square. The combination within the square in the first row would be CC and cc, and in the second row would be Cc and cc. So, according to the Punnett square, the genotypes of the offspring from this cross between two heterozygous parents are CC, Cc, and cc (1:2:1 ratio). Thus, if both parents have one dominant allele and other recessive allele (Cc x Cc), then the children have 75% chance of developing cleft chins.
If both parents are homozygous and have dominant alleles for cleft chin (CC x CC), there’s a 100% chance of their children having cleft chins. If one of the parent is homozygous to the trait (CC) and the other is recessive (cc), even then there’s a 100% chance of children developing cleft chins.
If one of the parent is heterozygous to the trait (Cc), while the other is homozygous (cc), then the Punnett square will have Cc, cc, Cc, and cc in the two rows. Thus, there is a 50% chance of the children developing cleft chins.
If the aforementioned theory is taken at face value, then one would wonder, why a child doesn’t have a cleft chin in spite of both parents having a cleft chin, or another child has a cleft chin even if his/her parents don’t. Well, one must understand that phenotypic expression can be affected by something called variable penetrance. Variable penetrance refers to the influence of environmental factors or modifier genes on the phenotypic expression of a dominant trait. For example, the cleft chin could occur, if the fusion of the lower jaw is affected by the fetal environment. Moreover, sometimes the presence of genes called modifier genes could suppress or alter the phenotypic expression of the dominant cleft chin gene. Under such circumstances, the bones could fuse, which in turn would cause a smooth chin, even if both the parents have a cleft chin.
On a concluding note, cleft chin genetics is not that simple. Other factors can influence the probability of a cleft chin in a child whose parents have a cleft chin. It is believed that recessive traits can skip one or more generations. It was in 1941 that Lebow and Sawin first suggested that cleft chin was a genetic character. This conclusion was based on data from a single family. While they suggested that cleft chin was a recessive trait, other studies suggest that cleft chin is a dominant trait. Though there might be contradictory studies, cleft chin is looked upon as an example of variable penetrance. Therefore, cleft chin must not be looked upon as a determinant of paternity.
