Mendelian Genetics Open Stax

University:
Kentucky State University
Course:
BIO 307 | Genetics
Academic year:

2025
Views:

142

Pages:

75
Author:

Patrick Farrand

Biology 520 Mendelian Genetics – Chapter 8 Textbook link A. Gregor Mendel’s early experiments with peas Mendel’s technique – section 8.1 Mendel’s seven traits – fig. 8.4 Article source: Terms to know: Homozygous dominant Genotype Homozygous recessive Phenotype Heterozygous in humans What about more complex traits like height? One of the largest human genetic studies so far has just been completed. Writing in Nature, Yengo et al.1 describe how they have used genetic data from almost 5.4 million people to identify DNA variants associated with differences in height between individuals. . . . The researchers found 12,111 genetic variants associated with human height, which clustered into small genomic segments that cover 21% of the genome. This confirms earlier predictions9 that a large proportion of the human is determined by many genes • genes for production of the brown pigment (melanin) • genes that regulate production of the brown pigment • genes for assembly of the pigment into pigment granules • genes for modification of the pigment (producing red pigment) All of these genes work together to produce the final hair color. Some of these same genes also influence eye color and skin color (hence the common association of light hair, light skin, and blue eyes, vs dark hair, dark skin, and brown eyes). In many animals, there are various alleles of the genes that regulate pigment production, resulting in different patterns of dark and light-colored hair. In humans, there are few, if any, such alleles that create color patterns; most of the alleles of these genes affect the overall quantity of pigment, resulting in darker or lighter hair. Therefore, among the many shades of brown, there are contributions from multiple alleles of multiple genes. In general, however, we can make two statements relating the genotype to phenotype: 🡪 The more pigment the hair cells produce, the darker the hair color. 🡪 Darker hair-color alleles tend to be dominant over lighter alleles. http://www.hhmi.org/biointeractive/biology-skin-color Two key compounds: Folate Vitamin D Vitamin D Folate is a water-soluble B vitamin that is naturally present in some foods, added to others, and available as a dietary supplement. Folate, formerly known as folacin, is the generic term for both naturally occurring food folate and folic acid, the fully oxidized monoglutamate form of the vitamin that is used in dietary supplements and fortified foods. Folic acid consists of a p-aminobenzoic molecule linked to a pteridine ring and one molecule of glutamic acid. Folate functions as a coenzyme or cosubstrate in single-carbon transfers in the synthesis of nucleic acids (DNA and RNA) and metabolism of amino acids Folate Fur color in Siamese cats can be explained as a interplay between genetics and environment – warmer temperature inactivate the enzyme produces colored pigment. Lemon juice denatures the catechol enzyme in apples. The Science Teacher, Jan 2016 Eye color is a complex polygenic trait – at least 15 different genes are involved (American Biology Teacher, May 2014). One of these genes (PAH) has 67 known alleles Animals with white fur are not usually true albinos, which lack all pigment and have pink eyes. Rather they have a trait called “Leucism”. Deer and other mammals can also be “piebald”, which means they have patches of white fur mixed with another color. (http://blog.nature.org/science/2016/02/03/white-deer-understanding-a-common-animal-of-uncommon-color/) Musician Woody Guthrie had Huntington’s disease Although a dominant disease, the phenotype varies: males show symptoms in adolescence if the disease is inherited from the father, but in middle-age if inherited from the mother. This is an epigenetic effect. Huntington's disease is caused by a genetic defect on chromosome #4. The defect causes a part of DNA, called a CAG repeat, to occur many more times than it is supposed to. Normally, this section of DNA is repeated 10 to 35 times. But in persons with Huntington's disease, it is repeated 36 to 120 times. As the gene is passed on from one generation to the next, the number of repeats - called a CAG repeat expansion - tend to get larger. The larger the number of repeats, the greater your chance of developing symptoms at an earlier age. Therefore, as the disease is passed along in families, it becomes evident at younger and younger ages. Using pedigrees Mitochondrial DNA codes for 13 proteins. An additional 1000 or so are active in the mitochondria but coded by nuclear DNA (Nature, Jan 16, 2014) American Biology Teacher, Nov 2013 Sickle-cell: simple gene with many phenotypic effects and an interesting geographic connection See “evolution connection” in section 8.3 Sickle-cell anemia https://bitescis.org/lesson-plan/f rom-gene-to-disease-sickle-cell -anemia/ Gene therapy options for sickle cell disease (Dec 2019 - https://www.nature.com/articles/d41586-019-03698-8) (more details in chapter 16) Life Cycle of the Malaria Parasite 1 - A female Anopheles mosquito carrying malaria-causing parasites feeds on a human and injects the parasites in the form of sporozoites into the bloodstream. The sporozoites travel to the liver and invade liver cells., (2) grow, divide, and produce tens of thousands of haploid forms, called merozoites, per liver cell. Some malaria parasite species also produce hypnozoites in the liver that remain dormant for extended periods, causing relapses weeks or months later. (3) The merozoites exit the liver cells and re-enter the bloodstream, beginning a cycle of invasion of red blood cells, known as asexual replication. In the red blood cells they develop into mature schizonts, which rupture, releasing newly formed merozoites that then reinvade other red blood cells. 4 - Some of the merozoite-infected blood cells leave the cycle of asexual replication. Instead of replicating, the merozoites in these cells develop into sexual forms of the parasite, called male and female gametocytes. When a mosquito bites an infected human, it ingests the gametocytes. (5) gametes are fertilized and develop in the mosquito 6 - Growth and division of each oocyst produces thousands of active haploid forms called sporozoites. After 8-15 days*, the oocyst bursts, releasing sporozoites into the body cavity of the mosquito, from which they travel to and invade the mosquito salivary glands. The cycle of human infection re-starts when the mosquito takes a blood meal, injecting the sporozoites from its salivary glands into the human https://www.niaid.nih.gov/topics/Malaria/Pages/lifecycle.aspx bloodstream. Other Inheritance patterns Sex determination systems X-linked trait: white eyes in fruit flies Fig 8.16 X-linked traits Hemophilia Nature, Dec 2014 Color-blindness is also X-linked. Genetic testing of the remains indicate that 1 of the 3 daughters was a carrier 🡪 American Biology Teacher, Nov 2013 Incomplete dominance – fig 8.12 Both genes are expressed if both are present Cholesterol commercial http://www.youtube.com/watch?v=kBfWybm0218 Blood types – multiple alleles and co-dominance See fig 8.13 Three alleles: ● ● ● A (dominant) B (dominant) o (recessive) Blood Type & Rh How Many Have It Frequency O Rh Positive 1 person in 3 37.4% O Rh Negative 1 person in 15 6.6% A Rh Positive 1 person in 3 35.7% A Rh Negative 1 person in 16 6.3% B Rh Positive 1 person in 12 8.5% B Rh Negative 1 person in 67 1.5% AB Rh Positive 1 person in 29 3.4% AB Rh Negative 1 person in 167 .6% http://bloodcenter.stanford.edu/about_blood/blood_types.html More here: http://anthro.palomar.edu/vary/vary_3.htm Two traits at once: Independent assortment See fig 8-10 Normal situation – independent assortment Fig. 8-10 Actual results Two-factor (Dihybrid) cross https://www.youtube.com/watch?v=qIGXTJLrLf8&ab_channel=AmoebaSisters Independent assortment: another example https://learn.genetics.utah.edu/content/evolution/allele/ Crossing over (fig 8.17) Gene linkage and independent assortment + gene maps Fig 8.17 A gene map Genome organization: gene vs. locus vs. allele The address numbers here are analogous to a gene locus. 173, 245, and 125 would be homozygous, while 211 and 267 are heterozygous. 321 does not represent a pair of alleles. Fruit fly chromosome map Epistasis – the presence of one gene affects another Gene pathways Epigenetics refers to inherited changes to the structure of the chromosomes or associated proteins, rather than the genetic sequence. One common example is methylation – methyl groups (-CH3) are often added to DNA http://www.pbs.org/wgbh/nova/genes/mice.html Nature, Oct 21, 2010 Mitochondial DNA (Nature, Sept 24, 2015) Nature, Jan 17, 2019 Summary of inheritance patterns you should know: •Autosomal vs. X-linked •Dominant vs. recessive •Incomplete or Co-dominance •Multiple alleles (ABO blood types) •Polygenic inheritance (height, skin color, hair color) •Linked genes •Pleiotropy (one gene influence multiple traits – sickle cell disease) •Heterozygote superiority (sickle cell) •Dihybrid crosses (2 traits at once)

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