Polygenic Inheritance or Quantitative Inheritance

POLYGENIC INHERITANCE OR QUANTITATIVE INHERITANCE Mendel mainly looked at traits with clear choices, like flowers being either purple or white. But if you look closely, you'll see that some traits aren't so simple; they fall along a range. Take human height, for example it's not just tall or short but can be various heights. These kinds of traits are influenced by many genes and are called polygenic traits. The way these traits are inherited is called polygenic or quantitative inheritance. In quantitative inheritance, dominant alleles add up their effects. Each dominant allele contributes to part of a working protein, and you see the complete trait when all dominant alleles are there. The genes responsible for quantitative inheritance are called polygenes. Scientists like H. Nilsson-Ehle (1908) and East (1910) showed how genes controlling traits along a spectrum separate and mix. For instance, they studied the color of kernels in wheat and the length of the corolla in tobacco. a) wheat kernel color, the Swedish geneticist H. Nilsson-Ehle (1908) conducted an experiment. He crossed a type of wheat with red kernels with another type with white kernels. The grains of the first generation (F₁) were all uniformly red but fell somewhere in between the red and white of the parent generation. When these F₁ plants were crossbred with each other, the second generation (F2) showed five different appearances, with a ratio of 1:46:4:1. i. The extreme red 1/16 (as red as to the parent of F:) ii. Deep red (Dark red) - 4/16 iii. Intermediate red-6/16 (similar to F1) iv. Light red-4/16 v. White-1/16 (as white as to the parent of F1) Nilsson-Ehle discovered that the color of wheat kernels is controlled by two sets of genes, AA and BB. Genes A and B are responsible for the red color, and they are dominant over their recessive counterparts. Each pair of genes follows Mendelian segregation. When plants have heterozygous combinations for both gene pairs (AaBb), they separate into 15 plants with red kernels and one with white kernels. Interestingly, not all red kernels look the same; they vary in the intensity of redness. The level of redness was found to match the number of dominant alleles present. b) Skin Color in Humans The color of our skin is determined by the presence of a pigment called melanin. The amount of melanin that develops in a person is influenced by three (sometimes two) pairs of genes. These genes are found at three different places in our genetic code, and each dominant gene plays a role in producing a specific amount of melanin. The impact of all these genes adds up, and the amount of melanin produced is directly linked to the number of dominant genes present. Later studies, following Davenport's research, suggest that as many as six genes could be involved in deciding the skin color of human beings. The combined effect of these genes is additive, meaning the overall skin color is influenced by the total number of dominant genes, as shown in the table. White aabbcc (Very light) abc AaBbCc Intermediate (Mulatto) Marriage between mulattoes Negro / Black AABBCC (Very dark) ↓ ABC Parents Gametes F, generation Gametes ABC 1 ABC AbC abC ABC Abc aBc abc ABC ABC AABBCC Very dark AaBBCC Dark AABBCC Dark AaBbCC AABBCC Dark AABBCC Fairly dark Fairly dark Fairly dark AaBBCc Inter-mediate AaBbCc AABBCC Dark aaB BCC Fairly dark AaBbCC Fairly dark aaBbCC Inter-mediate AaBB Cc Fairly dark Inter-mediate AaBbCc aa BBCC Inter-mediate Fairly light aaBbCc AbC AABBCC Dark AaBbCC Fairly dark AAbbCC Fairly dark AabbCC Inter-mediate AABb Cc Fairly dark AAbbCc Inter-mediate AaBbCc Inter-mediate AabbCc Fairly light abc Aa Bb CC Fairly dark aa Bb CC Inter-mediate Aabb CC Inter-mediate aabbCC Fairly light AaBbCc Inter-mediate Aabb Cc Fairly light aaBbCc Fairly light aabb Cc Light ABC AABBCC Dark AaBBCe Fairly dark AABb Cc Fairly dark AaBbCc Inter-mediate AABBCC Fairly dark AABbec Inter-mediate AaBBcc Inter mediate Aa Bbee Fairly light Abc AABBCC Dark AaBbCc Inter-mediate AAbbCc Inter-mediate Aabb Cc Fairly light AABbc Inter-mediate AAbbce Fairly light AaB bcc Fairly light Aabbcc Light aBe AaBBCC Fairly dark aaBBCc intermediate AaBbCc Inter-mediate aa Bb Cc Fairly light Inter-mediate AaBB Fairly light AaBbce aaB Bcc Fairly light aa Bbce Light abe AaBbCc Inter-mediate aa Bb Ce Fairly light Aabb Cc Fairly light aabbCc Light AaBbce Fairly light Aabbee Light aaBbee Light aabbcc Very light Phenotypes: 1 (Very dark): 6(Dark): 15(Fairly dark): 20(Intermediate): 15(Fairly light): 6(Light): 1(Verylight) Fig.: Results of polygenic inheritance of skin colour in man When a very light-skinned person and a person of African descent, known as a mulatto, have offspring (F, generation), the result is a skin color that falls in between. In the next generation (F2), the colored offspring display various shades, and their distribution follows a ratio of 1:6:15:20:15:6:1. Relative Frequency in F 20 Very light Light Fairly light Intermediate Dark Fairly dark Very dark Skin Pigmentation The different skin colors in a population can be shown either with a histogram or a bell-shaped normal distribution curve. By examining the histogram, we can understand that in polygenic inheritance, the very light and very dark skin tones are uncommon, while the ones in between are more common. There are other examples of quantitative traits, such as the length of the cob in maize, human intelligence, milk and meat production, human height, and the size, shape, and number of seeds and fruits in plants. For polygenic traits: (a) The number of different appearances (phenotypes) is calculated as 2n + 1. (b) The number of different genetic combinations (genotypes) is 3", where n represents pairs of polygenes. Chromosomal Theory of Inheritance Mendel began studying peas in 1856 and shared his findings in 1865. Unfortunately, his work went unnoticed and unappreciated until 1900, and there are a few reasons for this: (a) Communication was challenging back then, and Mendel's work couldn't be widely spread. It was published in the "Proceedings of Brunn Natural Science Society," which had limited circulation. (b) Mendel's idea of stable, separate units for different traits didn't align with the views of his contemporaries like Charles Darwin and A.R. Wallace. They didn't see it as an explanation for the continuous variations observed in nature. (c) Mendel introduced a new and unfamiliar approach, using mathematics and statistics to explain biological phenomena. Many biologists at that time found this method to be entirely new and unacceptable. (d) Mendel faced a challenge because he couldn't offer any tangible evidence for the existence of these factors or what substance they were composed of. (e) At the time Mendel conducted his research, crucial discoveries like chromosomes, mitosis, and meiosis hadn't been made yet. Mendel passed away in 1884, long before his groundbreaking work gained acknowledgment. In 1900, three scientists-de Vries from Holland, Carl Correns from Germany, and Von Tschermak from Austria-independently rediscovered the principles of heredity that Mendel had already outlined. Note: 1. de Vries found out the paper of Mendel and got it published in Flora in 1901. 2. Bateson, Punnett and other subsequent workers found Mendel's work to be universal application including animals.