19. Heredity
Contents of the chapter
19.1 What is heredity?
Heredity, also known as inheritance, is the passing on of traits from parents to their offspring. The field of biology that studies heredity is called genetics.
We all belong to a chain of generations. The genes we possess come from our parents, who have inherited their genes from their parents. Genes may give us traits that have been seen in our grandparents. But we are also more than the sum of our genes. Our being is also affected by our living environment, social relationships and experiences.
Heredity refers to the transmission of genes from one generation to the next. The genetic material contains instructions about the development of the individual from a fertilized egg to an adult, as well as information on what the individual's hereditary characteristics are. Genetic material also directs the development, growth and function of cells themselves. The genome allows a fertilized egg to develop into an individual of a particular species. The individual carries genes typical of its species, but more specifically, it carries the genes of its parents.
19.2 Chromosomes contain DNA
Genetic information is contained in a molecule called DNA.
DNA is a long, very tightly twisted molecular strand. If the DNA strands of a single cell were opened, they would be almost two meters long.
Each chromosome contains one DNA molecule. Chromosomes are thread-like molecules that carry hereditary information about everything from the individual's height to their eye color. Chromosomes are located in the nuclei of cells, from where they direct cell function.
Chromosomes can be observed with a light microscope, by staining a cell during its division stage. At that point, the chromosomes are packed so tightly that the power of the light microscope is sufficient to view the chromosomes.
Chromosomes can be arranged in pairs based on their properties. These chromosome pairs are identical and therefore known as homologous chromosomes. A human has 23 homologous chromosomes, of which 22 pairs are numbered chromosomes, called autosomes. The final pair consists of the sex chromosomes, X and Y. In other words, humans have 46 chromosomes in each cell - except for gametes, which differ slightly from other cells in terms of their size and structure.
While most cells in our body have two copies of each autosome, sperm and egg cells carry only one copy of each autosome. When a sperm fertilizes an egg, the embryo now contains two copies of each autosome, one from the father and one from the mother. Consequently, each person has two copies (alleles) of every gene carried on an autosome: one inherited from their father and one from their mother.
The X and Y chromosomes are known as the sex chromosomes. Women have two copies of the X chromosome, one from their father and one from their mother. Men have one X chromosome, from their mother, and one Y chromosome, from their father.
19.3 Gender is determined by sex chromosomes
In normal cell division, i.e. when new cells are formed in muscle or skin tissue, chromosomes are duplicated and replicated. The resulting cell has the same number of chromosomes as the dividing 'parent cell'. The resulting new cells are exact copies of the previous cells.
During fertilization, an ovum and a sperm cell fuse together, forming a zygote. But how can the zygote have the correct number of chromosomes?
Germ cells have only half the number of chromosomes as a diploid cell one of each pair. They are considered haploid (n) cells. In a human egg or sperm cell, there are 23 chromosomes, one of which is a single X or Y chromosome.
The number of chromosomes is reduced from 46 to 23 during the process of meiosis. The fertilization of the egg by a sperm cell restores the diploid number of 46 chromosomes.
In a female, the primary oocyte is diploid, containing 44+XX chromosomes. However, when the primary oocyte undergoes its first meiotic division in order to form the secondary oocyte and a polar body, the secondary oocyte is haploid and contains 22+X chromosomes.
The primary spermatocyte contains 44 + XY chromosomes. As a result of meiotic division, two types of sperm can be produced: those with the chromosomes 22 + X and those with the chromosomes 22 + Y.
The sex of an individual is determined when a sperm cell fertilizes an egg.
If an egg is fertilized by a sperm cell with an X chromosome, the developing individual becomes a female.
Similarly, if the sex chromosome of the fertilizing sperm cell is Y, the developing individual becomes a male.
19.4 The distribution of chromosomes in gametes is random
When germ cells form, the homologous chromosomes are randomly shared into two daughter cells. In this case, each germ cell and the embryonic stem cell initiating the development has a different combination of chromosomes inherited from the parents. This creates genetic differences in children of the same parents.
Siblings may differ very much in terms of their inherited traits, such as hair quality, eye colour. Similarly, siblings can also differ from each other in terms of traits that are also influenced by external factors, such as height or intelligence. The random distribution of alleles or gene forms also results in "uneven" heritability of inherited diseases between individuals.
19.5 Genotype and phenotype
During fertilization, half of the father's genome is mixed with half of the mother's genome in order to form the diploid genome of the new individual. The result is the individual's genetic make-up or genotype. The genotype is completely unique to each individual, a one-of-a-kind combination of parental genetic material. The only exception to this rule is identical twins that originate from a single fertilized egg. Their genetic material is completely identical.
Studies have shown that identical twins can develop differently if they grow in different environments. For example, nutrition, its quality and quantity, as well as the stimuli from the growing environment, can form different kinds of individuals from the same genotype. Although individuals have the same genotype, they may have unique ways of expressing that genotype. The individual expression of a genotype is called a phenotype.
A phenotype consists of an individual's observable traits. Some traits are determined by the genotype, while other traits are determined by environmental factors. Heredity defines the boundaries within which an individual’s characteristics can change, and the environment modifies the characteristics within those boundaries.
Environmental factors include not only nutrition, but also, for example, the chemicals to which an individual is exposed to during their development and growth, as well as their personal experiences. These factors affect the development of an individual right from fertilization.