Mendelian Inheritance: Multifactorial Traits Explained

Gregor Mendel, a 19th-century Moravian monk, is known as the Father of Genetics for his pioneering work in understanding the process of inheritance. Mendel's Laws of Inheritance, formulated through experiments on pea plants, include the Law of Dominance, the Law of Segregation, and the Law of Independent Assortment. These laws describe the basic patterns of inheritance, such as the stable transfer of genes from parents to offspring and the dominance of certain traits. While Mendel's laws are significant, they do not apply to all traits and have exceptions, including multifactorial or polygenic traits.

Polygenic traits are influenced by multiple alleles at different loci

Mendel's laws of inheritance do not apply to multifactorial traits, which are influenced by multiple factors, including genetic and environmental factors. Polygenic traits, also known as multifactorial traits, are influenced by multiple alleles at different loci.

Polygenic inheritance refers to the inheritance of a trait governed by more than one gene. In polygenic inheritance, the trait is produced from the cumulative effects of many genes, in contrast to monogenic inheritance, where the trait results from a single gene or gene pair. Polygenic traits are influenced by multiple alleles at different loci, and the expression of the phenotype is a combination or accumulation of traits from both parents.

An example of a polygenic trait is human height. There are around 400 genes that influence height, and environmental factors such as diet and hormones also play a role. The combined size of all body parts from head to foot determines an individual's height, and each body part is, in turn, influenced by numerous genes. This results in a continuous gradation in the expression of height, with individuals exhibiting a range of heights from very short to very tall.

Another example of a polygenic trait is skin pigmentation. In humans, skin colour is influenced by around 60 loci, and the environment can also impact the expression of skin pigmentation genes. The greater the number of alleles associated with darker skin colour, the darker the skin colour, and vice versa for lighter skin colour.

Polygenic traits do not follow the patterns of Mendelian inheritance as they are influenced by multiple genes and environmental factors. Instead, polygenic traits exhibit continuous variation and are challenging to predict. Statistical analysis can provide estimates of population parameters for polygenic traits, and they typically follow a normal distribution curve.

Mendel's Law of Dominance: only the dominant trait appears in the phenotype

Mendel's Law of Dominance states that in a cross of parents with pure, contrasting traits, only one form of the trait will appear in the next generation. In other words, offspring that are hybrids for a trait will only exhibit the dominant trait in their phenotype.

Mendel's discovery of this law came about through his experiments with pea plants. He noticed that when he crossed pure tall plants with pure short plants, all the new pea plants (referred to as the F1 generation) were tall. Similarly, when he crossed pure yellow-seeded pea plants with pure green-seeded pea plants, the F1 generation were all yellow-seeded. The same pattern occurred with other traits, such as green pea pods crossed with yellow pea pods, and round seeds crossed with wrinkled seeds.

Mendel observed that when the parent plants had contrasting forms of a trait, the phenotypes of the offspring resembled only one of the parents with respect to that trait. This led him to conclude that there was a factor that made pea plants tall, and another factor that made them short. He further deduced that when these factors were mixed, the tall factor seemed to dominate the short factor.

In modern terms, we now refer to these ""factors" as alleles or genes. In the case of pea plant height, there is a gene in the DNA that controls plant height, with one form of the gene (allele) coding for tall, and the other for short. Mendel's Law of Dominance can be summarised as follows: offspring that are hybrids will always show the dominant trait in their phenotype.

However, it is important to note that Mendel's laws, including the Law of Dominance, have some exceptions. For example, in cases of incomplete dominance, the F1 hybrids may exhibit an appearance that is intermediate between the phenotypes of the two parental varieties. This phenomenon has been observed in crosses between two four-o'clock (Mirabilis jalapa) plants, where the flowers of heterozygous plants have a phenotype somewhere between the two homozygous genotypes.

Mendel's Law of Segregation: during gamete formation, two copies of each hereditary factor segregate

Mendel's Law of Segregation, also known as Mendel's second law of inheritance, states that during the production of gametes, two copies of each hereditary factor segregate so that offspring acquire one factor from each parent. In other words, allele (alternative form of the gene) pairs segregate during the formation of gametes and recombine randomly during fertilization.

This law was formulated by Mendel after conducting experiments on pea plants for seven years. He noticed that certain factors were always being transferred down to the offspring in a stable way. These factors are now called genes, or the units of inheritance. Mendel found that there are alternative forms of factors—now called genes—that account for variations in inherited characteristics. For example, the gene for flower colour in pea plants exists in two forms, one for purple and the other for white. The alternative "forms" are now called alleles.

Mendel hypothesized that allele pairs separate randomly, or segregate, from each other during the production of the gametes in the seed plant (egg cell) and the pollen plant (sperm). Because allele pairs separate during gamete production, a sperm or egg carries only one allele for each inherited trait. When the gametes unite at fertilization, each contributes its allele, restoring the paired condition in the offspring. Mendel also found that each pair of alleles segregates independently of the other pairs of alleles during gamete formation.

The law of segregation is the only Mendelian law of inheritance without any exceptions. It is also known as the law of purity of gametes because a gamete carries only a recessive or a dominant allele, but not both. Mendel's Law of Segregation was a huge contribution to the world of science, and his work has stood the test of time, even as new discoveries have been made.

Mendel's Law of Independent Assortment: a pair of traits segregates independently of another pair during gamete formation

Mendel's Law of Independent Assortment, also known as Mendel's second law of inheritance, states that a pair of traits segregates independently of another pair during gamete formation. This means that the inheritance of one pair of genes is independent of the inheritance of another pair.

Mendel's Law of Independent Assortment can be explained through his experiments with pea plants. Mendel considered two traits, each having two alleles. He crossed wrinkled-green seeds and round-yellow seeds and observed that all the first-generation progeny (F1 progeny) were round-yellow. This meant that the dominant traits were the round shape and yellow colour.

Mendel then self-pollinated the F1 progeny and obtained four different traits: round-yellow, round-green, wrinkled-yellow, and wrinkled-green seeds in the ratio 9:3:3:1. This meant that the inheritance of the seed shape (round or wrinkled) was independent of the inheritance of the seed colour (yellow or green). The shape of the seeds did not impact their colour, and vice versa.

Mendel's Law of Independent Assortment can also be observed in a dihybrid cross, where the parents are hybrids for two different traits. In this case, the genotypes of the parent pea plants could be "R" (dominant allele for round seeds) and "r" (recessive allele for wrinkled seeds), and "G" (dominant allele for green pods) and "g" (recessive allele for yellow pods). The parents are hybrid for each trait (one dominant and one recessive allele for each trait).

When the genotypes of the parent pea plants are split, four possible gametes can be obtained: RG, Rg, rG, and rg. These gametes are then combined to form the genotypes of the offspring. The results from a dihybrid cross show that the different traits are inherited independently, as the phenotypes of the offspring exhibit a variety of combinations of dominant and recessive traits.

Mendel's Law of Independent Assortment demonstrates that the inheritance of one pair of traits is independent of the inheritance of another pair. This law, along with the Law of Dominance and the Law of Segregation, forms the basis of Mendel's Laws of Inheritance, which are essential to understanding the principles of genetic inheritance.

Non-Mendelian inheritance: many traits are inherited in a non-Mendelian fashion

Gregor Mendel's experiments with pea plants in the 1860s led to the discovery of three laws of inheritance: the Law of Dominance, the Law of Segregation, and the Law of Independent Assortment. Mendel's laws describe the inheritance of traits linked to single genes on chromosomes in the nucleus. However, not all traits follow these laws and are, therefore, inherited in a non-Mendelian fashion.

Non-Mendelian inheritance refers to any pattern in which traits do not segregate in accordance with Mendel's laws. In Mendelian inheritance, each parent contributes one of two possible alleles for a trait. Mendel's laws can be used to determine the distribution of phenotypes expected for the offspring. However, there are several situations in which the proportions of phenotypes observed do not match the predicted values.

One example of non-Mendelian inheritance is incomplete dominance, where one allele is not completely dominant over another. In such cases, the offspring may exhibit an intermediate phenotype. For instance, crossing a homozygous white-flowered plant with a homozygous red-flowered plant will result in pink-flowered offspring. Another example is co-dominance, where two alleles are simultaneously expressed in the offspring. A cross between a black chicken and a white chicken may result in offspring with both black and white feathers.

Some traits are polygenic, meaning they are controlled by multiple genes. For example, human skin colour is influenced by several different genes, resulting in a wide range of skin pigmentation. Additionally, environmental factors can also play a role in determining certain traits. For instance, the colour of hydrangea flowers can vary from blue to pink depending on the pH of the soil.

Non-Mendelian inheritance can also be observed in certain inherited diseases, such as colour blindness and haemophilia, which show specific non-Mendelian inheritance patterns. Furthermore, extranuclear inheritance, also known as cytoplasmic inheritance, is a form of non-Mendelian inheritance where the phenotype of a trait is determined exclusively by the maternal parent.

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