Mendelian Laws: Understanding Human Genetic Variation

Gregor Mendel's principles of inheritance, also known as Mendelism, form the cornerstone of modern genetics. Mendel's laws were first tested on pea plants and fruit flies, but evidence quickly emerged that they applied to all living things, including humans. Mendel's laws include the Law of Dominance and Uniformity, the Law of Segregation, and the Law of Independent Assortment. These laws explain the modes of inheritance of characteristic traits passed on through generations, such as eye colour in humans. Mendel's insight greatly expanded the understanding of genetic inheritance and led to the development of new experimental methods.

Recessive and dominant traits

Gregor Mendel's principles of inheritance form the cornerstone of modern genetics. Mendel's laws were first tested on pea plants and fruit flies, but evidence quickly emerged that they applied to all living things, including humans.

Mendel's principles of inheritance are also known as Mendelian inheritance or Mendelism. Mendel's findings allowed scientists to predict the expression of traits based on mathematical probabilities. Mendel's three principles of inheritance are:

• The principle of uniformity: All the progeny of a cross between two parent plants that differ by only one trait will appear identical. Mendel's experiments showed that the offspring of two different parent plants would not have a blend of the traits of each parent, but instead, would look like either one or the other parent. For example, when Mendel cross-fertilized plants with wrinkled seeds and plants with smooth seeds, the offspring had only smooth seeds.
• The principle of segregation: This principle states that the "particles" (now known as alleles) that determine traits are separated into gametes during meiosis, and meiosis produces equal numbers of egg or sperm cells that contain each allele. Mendel recognized that a recessive trait was carried down to the F2 generation from the earlier P generation. For example, when Mendel cross-fertilized plants with wrinkled seeds and plants with smooth seeds, the F1 generation looked like one parent, but the F2 generation had seeds that were either round or wrinkled. Mendel found that the seeds were expressed in a 3:1 ratio, suggesting there was a hidden recessive form of the trait.
• The principle of independent assortment: According to this principle, alleles at one locus segregate into gametes independently of alleles at other loci. Mendel tested this principle by examining the inheritance of two characteristics at once. He crossed plants that were purebred for two characteristics, such as seed color (yellow and green) and seed shape (round and wrinkled). Mendel's results showed that the various phenotypes were present in a 9:3:3:1 ratio. When he looked at each characteristic separately, he found that the proportion of each trait was still approximately 3:1, suggesting that the two traits had segregated independently.

Dominant and recessive traits are a key part of Mendel's principles of inheritance. Mendel found that some traits were dominant, meaning that if the two parents differed by only one trait, the offspring would express the dominant trait. Recessive traits, on the other hand, were only expressed if both parents contributed the same recessive trait.

Dominant and recessive traits in humans include:

• Dominant traits: Dimples, freckles, red/green colourblindness, widow's peak, PTC tasting ability, curly hair, and attached earlobes.
• Recessive traits: Blue eyes, red hair, and straight hair.

Inheritance patterns

Gregor Mendel's principles of inheritance form the cornerstone of modern genetics. Mendel's principles of heredity, also known as Mendelism, were initially derived from simple hybridization experiments with pea plants. Mendel's theories were later integrated with the Boveri-Sutton chromosome theory of inheritance by Thomas Hunt Morgan, forming the core of classical genetics.

Mendelian inheritance refers to the inheritance of traits controlled by a single gene with two alleles, one of which may be dominant to the other. Mendelian traits in humans are human traits that are substantially influenced by Mendelian inheritance. However, it is important to note that not many human traits are controlled by a single gene with two alleles, and most—if not all—Mendelian traits are also influenced by other genes, the environment, immune responses, and chance. Therefore, a minority of human traits are purely Mendelian.

Autosomal Inheritance

Autosomal traits are controlled by genes on one of the 22 human autosomes. An example of a single-gene autosomal trait is earlobe attachment, where the allele for free-hanging earlobes (F) is dominant to the allele for attached earlobes (f). Other single-gene autosomal traits include widow's peak and hitchhiker's thumb.

Sex-Linked Inheritance

Traits controlled by genes on the sex chromosomes are called sex-linked traits, or X-linked traits in the case of the X chromosome. Single-gene X-linked traits have a different pattern of inheritance than single-gene autosomal traits. This is because males have just one X chromosome, always inherited from their mother, and they pass it on to all their daughters but none of their sons.

An example of a recessive X-linked trait is red-green colour blindness, which is fairly common in males but relatively rare in females. Other examples of recessive X-linked traits include albinism and haemophilia.

Non-Mendelian Inheritance

Many traits are inherited in a non-Mendelian fashion. For example, height depends on multiple genes rather than just one, and is therefore a polygenic trait. Additionally, most traits, including all complex traits, are non-Mendelian. Some traits commonly thought of as Mendelian, such as psychiatric diseases, are also non-Mendelian.

The genotype and phenotype

Gregor Mendel, a nineteenth-century Moravian monk, is known as the "Father of Genetics" for his work on the basic patterns of inheritance. Mendel's laws of inheritance are a type of biological inheritance that follows the principles he originally proposed in the 1860s.

Mendel discovered that genetic characteristics have alternate forms, or alleles, with one allele typically being dominant over the other. The phenotype reflects the dominant allele. For example, in Mendel's experiments, when he crossed purebred white-flowered pea plants with purple-flowered plants, the offspring in the first generation (F1) all had purple flowers. Thus, he called this trait dominant. When he allowed self-fertilization in the F1 generation, he obtained both colours in the F2 generation with a 3:1 ratio of purple to white flowers.

The dominance of one allele over another is not always complete, as observed in Mendel's experiments with four o'clock plants. In this case, the F1 hybrids had an intermediate phenotype that was a blend of the two homozygous genotypes. This is known as incomplete dominance.

Mendel's laws of inheritance include the Law of Dominance and Uniformity, the Law of Segregation of Genes, and the Law of Independent Assortment. These laws describe how alleles are passed from one generation to the next and how they interact to determine an organism's phenotype. The Law of Dominance and Uniformity states that an organism with at least one dominant allele will display the effect of the dominant allele. The Law of Segregation of Genes states that during gamete formation, the alleles for each gene segregate from each other, so each gamete carries only one allele for each gene. Finally, the Law of Independent Assortment proposes that alleles for separate traits are passed on independently of one another.

Mendel's laws were first tested in pea plants and fruit flies, but they were quickly found to apply to all living things, including humans. Examples of Mendelian inheritance in humans include eye colour, where brown eyes are dominant over blue, and various genetic disorders such as alkaptonuria, albinism, and Huntington's chorea.

Mendel's three laws

Gregor Mendel is recognised as the Father and Founder of genetics. He formulated three laws of inheritance, also known as Mendel's Principles of Heredity, by studying the common pea plant. Mendel's laws apply to all living things, including humans.

Mendel's first law is the Law of Segregation. This states that the two alleles of a gene separate during the formation of gametes. In other words, the two copies of each chromosome will be separated from each other, causing the two distinct alleles located on those chromosomes to segregate from one another. This means that during the production of gametes, two copies of each hereditary factor segregate so that offspring acquire one factor from each parent.

Mendel's second law is the Law of Independent Assortment. This states that a pair of traits segregates independently of another pair during gamete formation. In other words, the traits inherited through one gene will be inherited independently of the traits inherited through another gene because the genes reside on different chromosomes that are independently assorted into daughter cells during meiosis.

Mendel's third law is the Law of Dominance. This states that some alleles are dominant while others are recessive. An organism with at least one dominant allele will display the effect of the dominant allele. During gamete formation, the alleles for each gene segregate from each other so that each gamete carries only one allele for each gene.

Mendel's laws of inheritance are also known as Mendelism. They were first proposed in 1865 and 1866 and later popularised by William Bateson. Mendel's findings allowed scientists to predict the expression of traits based on mathematical probabilities.

Non-Mendelian inheritance

While Gregor Mendel's principles of inheritance form the cornerstone of modern genetics, there are many exceptions to his laws, referred to collectively as non-Mendelian inheritance. Mendel himself warned that his patterns might not apply to all organisms or traits, and indeed, many organisms have traits that do not follow his principles.

Mendel also focused on traits determined by a single gene, but some traits, such as height, depend on multiple genes. Traits dependent on multiple genes are called polygenic traits. Polygenic traits often show a wide range of phenotypes. For example, at least four different genes control skin colour in humans.

Genes located on the same chromosome that have not crossed over before segregation into the gametes will be inherited together due to genetic linkage. This is an exception to the Mendelian rule of independent assortment.

Extranuclear inheritance, also known as cytoplasmic inheritance, is a form of non-Mendelian inheritance discovered by Carl Correns in 1908. Correns observed that leaf colour in Mirabilis jalapa was dependent only on the genotype of the maternal parent. Later research identified DNA in the chloroplasts as responsible for this unusual inheritance pattern.

Genomic imprinting is another example of non-Mendelian inheritance. Genes for a given trait are passed down to progeny from both parents, but these genes are epigenetically marked before transmission, altering their levels of expression. For example, in mice, the insulin-like growth factor 2 gene undergoes imprinting, with the maternal Igf2 gene imprinted, resulting in dwarfism in the offspring.

Mosaicism is another form of non-Mendelian inheritance. Individuals who possess cells with genetic differences from the other cells in their body are termed mosaics. Mosaicism can result from a phenomenon known as X-inactivation, where one of the two X chromosomes present in female mammals is inactivated following fertilisation. Calico cats, which are almost all female, demonstrate one of the most commonly observed manifestations of this process.

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