Episode 18 recAPs all things Mendelian and Non-Mendelian in the genetics world.
Episode 18 recAPs all things Mendelian and Non-Mendelian in the genetics world. It's important to remember the Law of Independent Assortment and Law of Segregation when reviewing pea experiments (1:30). Do you know what Pater and Filius mean in Latin? (2:00) The classic monohybrid flower example is broken down with a Punnett square walk through and vocabulary reminders (2:30). Always go back and reread the genetics problem when you think you’ve solved it! (4:00) Melanie concludes the episode by providing heredity examples of Non-Mendelian genetics (5:00). Don’t forget that mitochondria and chloroplasts have their own genes! (5:55)
The Question of the Day asks (6:40) “Explain how sexual reproduction increases genetic variation.”
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Hi and welcome to the APsolute Recap: Biology Edition. Today’s episode will recap Genetics
Lets Zoom out:
Unit 5 - Heredity
Topic - 5.3 (Mendelian) and 5.4 (Non-Mendelian) Genetics
Big Idea: Evolution and Information Storage and Transmission
Once upon a time, an Augustinian Monk by the name of Gregor Mendel grew some peas. He noticed several patterns in the plants he was producing and lived happily ever after. The End. Actually, like most scientists (and Big G was really a farmer), his work and observations weren't appreciated for decades. Gregor Mendel is now regarded as the father of genetics. This episode will first focus on the patterns of heredity that follow Mendel’s rules and then dive into those that don’t.
Lets Zoom in:
Two of Mendel’s major observations are highly rooted in meiosis and the manner in which homologous chromosomes separate. The first is the law of independent assortment which states that alleles of genes located on non-homologous chromosomes will separate during meiosis I without influencing each other. For example, so long as two genes are on different chromosomes - there won’t be a greater chance of inheriting them together, they are not linked. The second is the law of segregation which states that the two alleles for each gene will separate during meiosis, as diploid cells become haploid.
Time for a quick Latin lesson. Pater and filius or father and son. And ding ding ding - now it makes sense why genetics uses the P, F1 and F2 designations. Hetero means other and homo means same. So heterozygous has two different alleles while homozygous has two of the same. We will come back to these later.
Each of Mendel’s laws is displayed when completing Punnett squares. Lets review the classic monohybrid (or one trait) example. In flower expression, a purple phenotype is dominant to white. Consider a cross between two heterozygous purple flowers and interpret the genotypes and phenotypes of the offspring. When we set up our Punnett square (the diagram with four boxes) the parental alleles are independently separated into each of the boxes. This models the formation of gametes during meiosis. Just like the distributive property in math, It does not matter if Mom’s alleles are written across the top of the Punnett Square or down the side - you will get the same answer. Each box has a restored diploid number, representing a fertilized zygote when alleles from each parent join as homologous chromosomes. By convention, dominant alleles are written first if present - so a heterozygous genotype would be capital P then lowercase p.
OK, lets interpret the results of a cross between two heterozygous purple flowers. We have one homozygous dominant, two heterozygous and one homozygous recessive. Phenotypically, a 3:1 ratio of purple to white. This first puzzled Mendel as well. How can a new phenotype result that wasn’t present with the parents? It's important to note that the probability expressed in each recombination in gametes is unique. If you flipped a coin and it landed on heads the first time, it still has a 50% chance of landing on heads the second time.
Whether using Punnett squares or pedigrees - most of solving genetics problems is to look for patterns, explain and interpret results. Best strategies are to always make a key to reference. And when you think you’ve solved it - go back and reread the question to make sure you interpret your results properly. Here are a few other patterns of inheritance to practice
Dihybrid cross: Following the independent inheritance of two traits in a 16 box Punnett square. A heterozygous cross will produce a 9:3:3:1 phenotypic ratio. Sex-linked recessive: Genetic problems with the 23rd chromosme. Because males only inherit one X chromosome, they cannot be carriers, and are disproportionately affected. These disorders seem to skip generations in pedigrees and are often passed from unaffected moms to sons. Hemophilia is sex linked disorder that was common in the European royal family.
There are also several patterns of inheritance that seem to defy Mendel’s laws. Genes that are adjacent and close to one another on the same chromosome may appear to be genetically linked and would even travel together during crossing over. The probability that genetically linked genes will separate is calculated and expressed in map units. Other patterns include multiple alleles and codominance, like with ABO blood typing, Incomplete dominance where the heterozygote expresses a new phenotype, and epistasis, where the phenotypic expression of one gene affects the expression of another, like with the fur color of labradors.
There are also some traits that result from non-nuclear inheritance. These include traits determined by chloroplasts and mitochondrial DNA (remember - they have unique prokaryote traits as referenced by the endosymbiosis theory). In animal cells, mitochondria are only transmitted through egg not sperm. In plant cells, both mitochondria and chloroplasts are only inherited through ovules, not pollen. All the mitochondria you have only came from your mom. Pretty cool right? Go thank your mom for all the ATP power.
To recap….
Genetics, love them or hate them - they make us who we are! The pattern of inheritance of alleles follows certain laws of Mendel and probability. When Mendel’s laws are broken, look for other explanations through pedigrees. When in doubt, Punnett square and work it out.
Today’s Question of the day is about variation.
Question: Explain how sexual reproduction increases genetic variation?