September 9, 2024

Henry Le 

11th Grade

Westminster High School 

Have you ever wondered why children resemble their parents? From a regular standpoint, it makes sense that a mom and a dad come together and have a child who often shares features of both. But from a scientific point of view, what’s the reason behind this?

Cells that aren’t involved in reproduction are called somatic cells. In the case of humans, most somatic cells have 46 chromosomes, bundles of DNA containing the genetic information that encodes for our body functions and traits. However, sex cells, or gametes (sperm and egg cells), only have 23 chromosomes. This is to form a zygote (fertilized egg cell) from both parents’ gametes without it containing an abnormal number of chromosomes, which otherwise usually results in deformities or fatality for offspring. This is the case with other sexually reproducing organisms, with varying numbers of chromosome pairs.

Variations in the arrangement of DNA are called alleles, which in turn change how genetic information is expressed in an organism; An allele is essentially a version of a genetic sequence on a chromosome. Gregor Mendel is credited for his work on the laws of inheritance; Known for his pea plant experiments, his analysis of how characteristics are inherited and passed down led to a breakthrough in hereditary studies, including alleles. 

In most cases, each parent involved in sexual reproduction gives one allele to their children, where the child will have two of a certain allele. These alleles make up a genotype, an organism’s genetic information. However, alleles may be either recessive or dominant. When alleles are expressed in an organism’s phenotype (characteristics resulting from the genotype), recessive alleles will be hidden by dominant alleles, therefore causing an organism to express a dominant phenotype. In most cases, recessive phenotypes are only expressed when a genotype contains two recessive alleles. This can be illustrated in a Punnett square, which helps visualize the possible genotypes and phenotypes of offspring, as well as their probabilities, in simple Mendelian genetics. According to Mendel’s law of independent assortment, alleles that are inherited independently from another do not affect other alleles’ chances of being inherited, meaning that most alleles have predictable genotype probabilities. (However, this only applies to genes that are on different chromosomes or far apart on the same chromosome; otherwise, they are likely to be genetically linked.) In a Punnett square, dominant alleles are denoted by a capital letter, and recessive alleles are denoted by a lowercase letter. As an example monohybrid Punnett square, two parents with the Bb genotype would have 25% BB, 50% Bb, and 25% bb genotype children. Therefore, 75% of children would have a dominant genotype and 25% of children would have a recessive phenotype. In addition, Punnett squares may also represent dihybrid crosses.  For instance, an RrYy crossed with a RrYy would have a 9:3:3:1 ratio of RY, Ry, rY, and ry genotypes.

However, there are exceptions to this. If neither allele in a heterozygous genotype is dominant, incomplete dominance occurs, where both phenotypes are expressed and blended. For example, a red flower and a white flower that don’t have dominant alleles over each other may create pink flower offspring. In contrast, when both alleles are expressed equally, both phenotypes are shown simultaneously. Unlike the previous example, if the red and white flowers were to be equally expressed, offspring may have red and white petals. In addition, sex-linked alleles may affect an organism’s phenotype. During meiosis, a type of cell division for gametes, sex chromosomes are distributed. Alleles may be linked to sex chromosomes, which consequently link certain traits to different sexes. Furthermore, not all traits are determined by a single gene. Polygenic traits are influenced by multiple genes (polygenes), where polygenes all affect the same phenotype. Some examples of polygenic traits include height and eye color. This allows these traits to have a broad range rather than a few distinct results.

Though patterns of inheritance are often complex, understanding its fundamentals allows people to predict and analyze genetic outcomes and make informed decisions regarding heredity.