Cleft chin is mentioned in the online Mendelian inheritance in man (OMIM), which is a compendium of human genes and genetic phenotypes.
As you all know that our physical appearance is determined by our genetic makeup, by segments of DNA involved in producing a polypeptide chain. For example- the color of our eyes, texture, and color of hair, hairline, the shape of facial features, the presence of dimples on the cheeks, dimpled chin, freckles, the shape of the body parts etc., all are determined by our genes. Transmission of the genetic characteristics or traits from the parents to the offspring is called heredity. The genetic makeup of an individual is responsible for the phenotype or traits and expression of that person.
Genes and alleles –
All the cells in an organism contain the same DNA, and every time a cell divide the original genetic material is copied, a gene is a storehouse of the information of the organism, they are basically DNA segments that codes for the proteins and these proteins responsible for specific physical traits. There is a particular place on the chromosome where the gene is located called “locus”. In the homologous chromosomes, there are same genes at the same locus; they may carry different forms of a gene known as alleles. If there is any variation in protein activity or expression than it will cause a different phenotype. In alleles, there is a minor difference in the DNA sequence and that cause the alternative forms of a particular trait. Alleles can be dominant or recessive.
There are two alleles, one dominant and one recessive allele or maybe both are recessive allele in a person. If both alleles are dominant or one dominant and other recessive than there is a great possibility of having that particular trait in an individual, the dominant allele reduces the effect of the recessive allele.
If we use the letter C for the dominant allele for the cleft chin, then the recessive allele will be (a chin without dimple) represented by letter c, so in case of identical alleles the situation will be CC or cc and in heterozygous will be Cc.
Factors affecting the expression of the cleft chin phenotype –
The genetic makeup of the child or the phenotype and genotype can be determined using a Punnett square; it is a tool that calculates the frequency of the presence of particular genotype or phenotype in the child. It can be used to evaluate the phenotype for one or many traits.
In case of cleft chin if both the parents are heterozygous, (Cc, Cc) then according to Punnett square, the genotype of the offspring from this cross between two heterozygous parents will be in 1:2:1, thus if both parents have one dominant allele and other recessive then the child has 75% chance of having cleft chin.
If both the parents are homozygous for the dominant allele than there will be 100% chance of having the cleft chin trait in the children.
If one parent is heterozygous (Cc) and other is homozygous (cc), then there is 50% chance of having the cleft chin trait.