20. How are our traits determined?

Contents of the chapter

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20.1 Genes contain genetic information
20.2 Alleles are dominant or recessive
20.3 The inheritance of alleles can be described in a diagram
20.4 Properties of a single gene
20.5 How can parents with freckles have a child without freckles?
20.6 Genes affect many of our traits
20.7 Acquired traits
20.8 The function of genes is complex
20.9 Sex-linked inheritance
20.10 Mutations are errors in genetic information
20.11 Genes and diseases

20.1 Genes contain hereditary information

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Children inherit traits from their parents. More specifically, traits themselves are not inherited, but the genes that provide the traits are. A certain piece of DNA strand makes up a gene. There are thousands of genes in each chromosome. In total, humans are estimated to have more than 20,000 genes. The exact figure is still unknown.

People usually have two copies of each chromosome. One copy is inherited from their mother (via the egg) and the other from their father (via the sperm).

There are two forms of each gene, one located in the chromosome inherited from the father and the other in the chromosome inherited from the mother. These gene forms may be the same or different from each other. Changes (or variations) in the gene for that characteristic cause different kinds of gene forms. Each variation of a gene is called an allele. The two alleles that form a gene pair are inherited, with one allele coming from each parent. Alleles interact with each other in different ways. These two copies of the gene contained in your chromosomes influence the way your cells work.

If an individual inherits completely identical alleles that affect a particular trait from his parents, the individual is said to be homozygous for that trait. If, on the other hand, the gene forms or alleles inherited from the parents are different from each other, the individual is considered heterozygous in terms of the trait. The location of each gene in a chromosome is called a locus.

A cell has two copies of a chromosome. If they are carrying the same allele (same variety of the gene) then they are homozygous for that gene. If they are different, the chromosome pair is heterozygous.

20.2 Alleles are dominant or recessive

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The effect of alleles on the inheritance of a trait can be either dominant or recessive. The gene form of the dominant allele masks the effect of a recessive allele. Thus, an individual receives two alleles from his parents that affect each trait, which can be either identical or different from each other. An individual can therefore have two dominant alleles, two recessive alleles, or one dominant and one recessive allele for a gene.

The trait produced by the dominant allele always appears in the individual. This is true whether the individual is heterozygous and homozygous. The recessive form of the trait, in turn, is manifested only if it is homozygous.

For example, the composition of a human's ear wax can be of two types: dry or moist. The dominant allele of the trait causes an individual to have moist ear wax, whereas the recessive allele causes the individual to have dry ear wax. If an individual inherits a recessive allele (a) from both parents, their ear wax is dry because their inheritance is (aa). If, on the other hand, one or both parents pass on the dominant allele (A), the individual has moist ear wax because their inheritance is (Aa) or (AA).

The composition of ear wax
Individual	Genotyoe	Phenotype
Homozygous (dominant allele)	AA (i.e., an individual has inherited an A allele from both parents)	moist ear wax
Heterozygous	Aa	moist ear wax
Homozygous (recessive allele)	aa	dry ear wax

20.3 The inheritance of alleles can be described in a diagram

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The transmission of a hereditary trait from one generation to another can be illustrated by making a crossover diagram. The trait is denoted by a letter selected according to the recessive allele. Alleles that confer a trait are marked with either uppercase or lowercase letters.

The dominant allele is marked with an upper-case letter, whereas the recessive allele is marked with a lower-case letter. Thus, for example, the dominant gene that causes curly hair could be labeled A. In this case, the curly-haired individual may inherit the alleles AA or Aa. A straight-haired individual, on the other hand, has inherited the alleles aa.

20.4 Properties of a single gene

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Only a few hereditary traits are determined by a single gene. These properties appear as clear either/or traits.

Examples of hereditary traits determined by a single gene are the type of an individual's earlobe, freckles and the individual's ability to roll their tongue.

The colour of an individual's eyes is affected by several genes. However, it can be thought that brown-eyedness is dominant and blue-eyedness is a recessive trait.

Which features of a girl’s face are dominant and which are recessive?

20.5 How can parents with freckles have a child without freckles?

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Freckles are small dark patterns visible on a person's face. The occurrence of freckles is caused by a dominant allele that can be denoted by the letter F. In contrast, the recessive allele that causes the individual not to have freckles can be denoted by the letter f.

The genome of a person with freckles is either FF (homozygous) or Ff (heterozygous). The notation Ff means that one homologous chromosome has the allele F, whereas the other chromosome has the allele f.

If the mother's genome is Ff, the F allele is transferred to some of the mother's egg cells, whereas the f allele is transfered to others. The resulting oocytes can be labelled as follows: F-egg and f-egg.

Similarly, if the paternal inheritance is Ff, the father's sperm cells can be either F-sperms or f-sperms.

If an f-sperm fertilizes an f-egg, the child will inherit the alleles ff. Because there is no F allele in the genome that causes freckles, the child does not develop freckles.

If a child has freckles, he or she has inherited an F allele from at least one of his or her parents.

20.6 Genes affect many of our traits

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An individual's height is determined by many genes.

Most hereditary traits are determined by more than one gene. These include traits such as height, skin colour, intelligence and musicality. For example, skin colour is affected by at least four alleles in different chromosomes.

The expression of genes in an organism can be influenced by their environment. Here, too, human height is a good example. Genes that influence the individual's height are inherited from its parents, but nutrition and the amount of sleep can affect the individual’s final height. Similarly, the individual's environment and experiences have a significant impact on their intelligence.

Genes also affect other human mental traits. An individual's personality is formed in the brain, and the individual's genes direct their brain development.

20.7 Acquired traits

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A human has a myriad of different traits, only some of which are hereditary.

Traits that are not inherited are called acquired traits. For example, your ability to read this text is an acquired trait. Acquired traits are not coded in the DNA. As a result, they cannot be passed down to offspring during reproduction.

The woman in the picture has inherited the genes that have resulted in the development of biceps. However, large, trained biceps are an acquired trait, which is not inherited from parents.

20.8 The function of genes is complex

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Genes determine a person's inherited traits. They contain the building blocks for proteins that guide the person's individual development. In practice, one gene directs the production of one protein.

Tens of thousands of different proteins, such as hormones, enzymes, antibodies or other vital building blocks, are formed in the human body according to the instructions stored in the individual's genes. For the human body to function, the production and function of all these different proteins must be in order.

The function of genes is complex and still needs to be explored. It has been found that although every human cell contains all human genes, the active and functional genes in each cell are specific to that cell type. For example, a muscle cell contains genes that control hair growth, but these genes do not work. Similarly, all cells have genes that guide the early development of an embryo, but they do not function after the embryo has developed into a fetus and an adult.

Determining hereditary traits can be complicated, because their expressions are influenced by many hormones and other factors. For example, baldness is a dominant feature caused by a protein produced by alleles. However, this allele that affects baldness only works in the "right environment," that is, when an individual has enough testosterone in their body. For this reason, a woman can carry the baldness allele, but will not become bald because of it.

20.9 Sex-linked inheritance

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Some traits are inherited through sex chromosomes. An example of such a feature is the red-green colour perception defect or color blindness.

Color blindness is caused by a mutation that influences the individual's the X chromosome. X-linked color blindness is a recessive trait.

Males inherit their single X chromosome from their mothers. As a result, they can become color blind if their signle X chromosome has the color perception defect. In females, the colour perception defect manifests itself only when it is inherited from both parents. For this reason, red-green blindness is much more common in men than in women.

Image on the right: Can you see the number four in the image?

20.10 Mutations are errors in genetic information

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Mutations occur all the time during cell division and gene replication. Most mutations are not visible. If the mutation occurs during normal cell division or in a part of the genome that is not used in that cell, it will not manifest itself. However, if the mutation occurs in germ cells or in a portion of the genome that is used in that cell type, it may manifest itself. If a mutation occurs in gametes or their stem cells, it is also passed on to the next generation.

A mutation can occur within a single gene, in a chromosome, or across an entire chromosome system. In a gene mutation, the structure of the DNA changes and at the same time there may be a change in the protein produced by the gene, or a completely different protein is formed. Such a change can be harmless, causing, for example, in an individual having different colour eyes. If a gene mutation occurs in a gene encoding an enzyme or hormone that is vital to cell function, it can result in serious illness or even death.

If a mutation changes the structure of a chromosome, we speak of a chromosomal mutation. The most severe chromosomal mutations tend to be eliminated in the initial stages of fetal development, resulting in spontaneous miscarriage. Even mild chromosomal mutations usually cause some form of disease.

If the number of chromosomes changes because of the mutation, we speak of a chromosome system mutation. The best-known example of a chromosome system mutation is Down syndrome. An individual with Down syndrome has three copies of chromosome 21 instead of the usual two. The syndrome results in a characteristic facial appearance, poor muscle tone, many structural and functional problems and varying levels of developmental disability. On the other hand, many people with a Down syndrome have a sunny and positive personality.

Down syndrome is caused by an extra chromosome 21.

20.11 Genes and diseases

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Genetic research has found several genes that either cause hereditary disease or predispose to disease. Today, about 5,000 diseases caused by a single gene are known.

These genes can be either dominant or recessive. They can also be sex-linked and jump over generations. Thus, the inheritance of genes may not always be quite as simple and schematic as shown in the examples above.

Genes can influence the prevalence of various illnesses, such as coronary artery disease, allergies, asthma and certain types of cancer. Fortunately, the onset of these diseases can be influenced by your lifestyle choices.

Human health is affected by heredity and the environment. For example, getting the flu can be prevented with good hygiene.