When someone says how much two siblings “look alike,” or are surprised by how much two siblings “look nothing alike,” they are unknowingly commenting on the concept of heredity. Heredity is the passing of traits from parents to offspring. Traits are definable features that make each organism unique from all other organisms. Traits can be physical, behavioural, or even a predisposition to certain diseases.
Traits are carried and passed down from one generation to the next by genes. Genes are segments of DNA molecules, which are packaged as distinct structures called chromosomes. A gene occurs at a specific location on a chromosome. The principles that explain how genes are transmitted from parent to offspring were first discovered by Gregor Mendel, an Austrian monk and scientist, who crossed pea plants. His research revealed concepts that became the foundation of genetics or the study of heredity. Mendel‘s work is summarized in two laws.
- Mendel’s law of segregation: Genes determine traits; genes exist in more than one form called alleles. An organism has two alleles for each trait, one allele from each parent. For each pair of alleles, one form is dominant and one form is recessive. Within a pair, alleles can be the same or different. If they are different, the dominant allele will mask the recessive allele.
- Mendel’s law of independent assortment: Genes act independently of one another. Pairs of alleles separate and unite without affecting the way other allele pairs separate and unite. There is now evidence supporting some exceptions to this law.
The cellular processes that are involved in heredity are meiosis and fertilization. Meiosis produces cells or gametes that are involved in the reproduction of an organism. Female gametes are called egg cells; male gametes are called sperm cells. Meiosis is a type of cellular division where one cell with 46 chromosomes will undergo two divisions to produce 4 cells with 23 chromosomes .During meiosis, allele pairs are split during the second division. Fertilization is the process of producing offspring where two gametes, egg and sperm, unite to merge single alleles into a pair for each trait.
The cycle of meiosis and fertilization explains how genes physically move from parent to offspring. Genes are then expressed as traits as the organism grows from a single cell into a multicellular organism. Every gene that an organism carries is referred to as the genotype. The phenotype is the expression of the genes as traits .Phenotypes result from the interaction of the genotype with the environment.
The possible genotypes of the offspring of two individuals with known genotypes can be predicted with a Punnett square. A Punnett square is made by creating a grid where the alleles for the mother’s trait are placed in the top row of the grid; the alleles for the father’s trait are placed in the left column of the grid. A capital letter is used to indicate the dominate allele and a lowercase letter is used for the recessive allele. The combinations of the mother’s and father’s alleles are written in the intersecting squares as in a multiplication table. The resulting graph will show all the possible genotypes from the alleles and the probability that each genotype will occur in offspring. When two alleles are the same, the individual is said to be homozygous. When two alleles are different, the individual is said to be heterozygous. If a trait is recessive, the alleles must be homozygous for it to be expressed. If the trait is dominant, the alleles can be either heterozygous or homozygous dominant for the trait to be expressed.
The dominant-recessive relationship is only one possible interaction between genes and inherited traits. Some genes have more than two alleles. In this system, alleles can exhibit codominance or incomplete dominance. This is the relationship that determines the blood type in humans. Blood types in humans are controlled by 3 alleles: A, B, and O. A and B are dominant over the recessive O, but when the A and B occur together, they are codominant resulting in a unique blood type.
Another occurrence that determines how genes are expressed is the presence of inherited genes on sex chromosomes. Alleles that occur on the X and Y sex chromosomes can be affected by the sex of the organism. A trait that occurs on a sex chromosome is said to be sex-linked. Males are heterozygous (XY) and females are homozygous (XX) for the sex chromosomes. There are X-linked traits and Y-linked traits. The Y-linked traits are only inherited by male offspring. X-linked traits are more often expressed in the male population because a male that has an X-linked recessive allele for a trait cannot receive the X-linked dominant allele, as they only have one X chromosome. The trait will, therefore, be unmasked and expressed in the recessive state. Color-blindness and hemophilia are two traits that are sex-linked.
Humans, dating back to ancient times, had a general understanding of heredity based on observations that certain qualities in plants and animals were advantageous and could be encouraged through selective breeding and farming techniques. But it was Mendel, who first began to unravel the mystery of how two parents with blue eyes can produce a child with brown eyes.