Genetic Inheritance: Principles, Laws, and Applications
Genetics is a branch of biology that studies the transmission of hereditary traits and genes. Genetic inheritance refers to the series of mechanisms that transmit traits from an individual to its descendants.
Genetic Inheritance: Core Concepts
Genes and Hereditary Traits
The following are some of the basic concepts of genetic inheritance:
- Hereditary traits are the morphological or physiological characteristics of an individual that can be transmitted to their descendants, such as eye color.
- A gene is a fragment of DNA that transmits this information.
Key Concepts in Genetic Inheritance
- An allele is a variant that determines a particular trait in an individual. Diploid (2n) species have two alleles for each gene, one on each homologous chromosome. These alleles can be the same or different.
- A locus (plural: loci) is the fixed position of a gene on a chromosome. It is found in the same place on each homologous chromosome.
Other Important Aspects of Inheritance
- An individual is homozygous or purebred for a certain trait when both alleles for that genetic trait are the same. An individual is heterozygous or hybrid when the alleles for the genetic trait are different.
- Alleles can be dominant or recessive. The dominant allele, represented by a capital letter, prevents the recessive allele, represented by a lowercase letter, from being expressed.
- A genotype is an individual’s set of genes in its chromosomes.
- A phenotype is an individual’s set of observable traits.
Mendel’s Laws of Inheritance
Mendel’s First Law: Principle of Uniformity
When two purebred individuals (homozygotes) are crossed, all their first filial generation (F1) offspring have the same genotype and phenotype.
This law is also called the Principle of Uniformity. All the offspring will exhibit the same phenotype, either matching the dominant parent or presenting the recessive phenotype if both parents are recessive.
- One parent is a dominant homozygote, and the other is a recessive homozygote.
- During meiosis, each parent contributes a gamete with one chromosome. The gametes from each parent will be uniform, as they possess only one type of allele.
- After fertilization, the offspring inherit a diploid set of chromosomes, receiving one allele from each parent, making them heterozygous.
- The phenotype of all offspring is identical to that of the dominant parent.
Mendel’s Second Law: Principle of Segregation
Each parent produces two different types of gametes: some carry the allele R and others the allele r.
During fertilization, any gamete from one parent can join any gamete from the other parent.
The phenotype of the offspring can be two different types: purple or white. The genotype can be dominant homozygous, heterozygous, or recessive homozygous.
Mendel’s Third Law: Independent Assortment
When two individuals with two or more different traits are crossed, each trait is transmitted independently from the others.
Mendel’s third law, the Principle of Independent Assortment, states that during gamete formation, the alleles for different genes segregate independently of one another. This is observed when crosses are made to follow the transmission of two different traits simultaneously, typically starting with two homozygous parents that are dominant and recessive for both traits.
Beyond Mendel: Complex Inheritance Patterns
The traits that Mendel used in his experiments were controlled by alleles that were either completely dominant or recessive. However, many traits are controlled by more than one allele, leading to proportions of traits in offspring that differ from Mendel’s findings. Inherited traits can be broadly categorized into two types:
- Discontinuous traits are clearly differentiated and have few alternatives.
- Continuous traits have a continuous variation, with many varieties and very small differences between individuals.
Intermediate Inheritance and Codominance
- Intermediate inheritance: The phenotype of heterozygotes is a mixture of the phenotypes of purebreds.
- Codominance: The phenotype of heterozygotes is not a mixture of the purebreds; instead, both parental phenotypes manifest simultaneously.
Continuous traits follow quantitative inheritance, which produces numerous phenotypes that have slight variations. Some examples include height, weight, or skin color.
Blood Group Systems
The HLA System
The HLA system (Human Leukocyte Antigen) is a group of antigens found on the membrane of lymphocytes and other cells in the human body. It is a crucial part of the human immune system, recognizing and rejecting antigens foreign to the organism. For example, when a donor and recipient of a transplant are not compatible, the transplanted organ is rejected by the recipient’s immune system.
There is a great variety of antigens in this system, which are controlled by a large number of different alleles.
The ABO Blood Group System
The ABO blood group system classifies human blood into four types. These types depend on the presence or absence of specific glycoproteins on the membrane of red blood cells. Our plasma may recognize certain carbohydrates in the membranes of red blood cells as antigens.
Chromosomes and Inheritance
The Chromosome Theory of Inheritance
In 1915, zoologist T.H. Morgan confirmed the Chromosome Theory of Inheritance, which explains Mendel’s laws.
On chromosomes, genes are aligned in a specific place or locus. The two alleles that determine a trait are located on each of two homologous chromosomes. When a cell prepares to divide, homologous chromosomes duplicate, resulting in four chromatids. During meiosis, each of the four daughter cells (gametes) produced receives only one chromatid from each chromosome. Genes located on the same chromosome are usually transmitted together, but crossovers between homologous chromatids can produce different combinations.
Chromosome Structure Types
Chromosomes can be classified by the position of their centromere:
- Metacentric: The arms are more or less equal.
- Submetacentric: One arm is slightly shorter than the other.
- Acrocentric: One arm is much shorter than the other.
- Telocentric: One arm is almost non-existent.
Sex Determination and Inheritance Patterns
Sexual reproduction is possible because individuals in a species can produce gametes. Gender differentiation occurs during embryonic development, when gonads (in animals) or gametangia (in plants) are formed. These are reproductive organs that appear in the adult stage of an organism.
In some cases, males and females of a species can differ in size, color, and other traits. This phenomenon is called sexual dimorphism.
XX/XY Sex Determination System
Females have two identical sex chromosomes called X chromosomes. Males have one X chromosome and one Y chromosome.
The Y chromosome almost exclusively contains genes that control the development of male sexual characteristics. The X chromosome carries genes that are not solely related to sex determination. This system determines the sex of mammals and some fish and amphibians.
XX/XO Sex Determination System
In this system, there are only X chromosomes: the female has two, and the male has one.
This system determines the sex of some insects, such as grasshoppers and crickets. The male has one chromosome less than the female.
Males are responsible for determining the sex of offspring; they can produce gametes with an X chromosome or gametes that only contain autosomes (no sex chromosome).
ZZ/ZW Sex Determination System
In this system, males have two identical sex chromosomes (ZZ), and females have two different chromosomes (ZW). The transmission of gender is similar to the XX/XY system.
This system determines the sex of birds, butterflies, and some species of fish.
Sex-Linked Inheritance Examples
Examples of sex-linked traits include haemophilia (a disease where the blood doesn’t coagulate), color blindness, auricular hypertrichosis (excessive hair in the ears), and ichthyosis (rough, dry, and scaly skin).
Haemophilia: A Sex-Linked Disorder
Haemophilia is characterized by the absence of one of the factors that contributes to blood coagulation. For a person with this disease, even a small blood vessel rupture can lead to serious bleeding.
This disease is associated with a recessive allele on the X chromosome, meaning women can be carriers without suffering from it. Men, having only one X chromosome, either suffer from the disease or do not.
In the past, children with haemophilia rarely lived long enough to have children themselves.
Sex-Influenced Inheritance
While sexual dimorphism refers to general differences between sexes, sex-influenced inheritance describes how certain traits are expressed differently based on an individual’s sex.
Examples include antlers in male deer, manes in male lions, or alopecia (baldness) in men. This occurs because the same allele on genes located on autosomes can be dominant in one sex and recessive in the other. This phenomenon is known as sex-influenced dominance.
Applications of Genetic Inheritance Laws
We can use the laws of inheritance to solve different types of problems:
- Identify genotypes from known phenotypes.
- Predict the results of a cross and calculate the probabilities of different types of offspring, often used in farming to improve species.
- Discover parental genotypes from the phenotypes of their offspring.
- Calculate the inheritance model of a certain trait.
- Genes or fragments of DNA are often inherited together and serve as genetic markers that can be used for paternity tests, forensic science, cattle breeding, food science, and biodiversity studies.
Pedigree Charts in Genetics
Pedigree charts trace the transmission of a trait over several generations. This helps scientists determine if:
- Certain diseases or physical traits are hereditary.
- Diseases are more common in one sex.
- The alleles responsible are dominant or recessive.
- Offspring are more or less likely to suffer from a disease or inherit a trait.