Mega Genetics Review: Mendelian and non-Mendelian Genetics

Summary

Highlights

• Exciting review of Mendelian and non-Mendelian genetics! 🎓
• Learn about different dominance types such as incomplete and codominance! 💡
• Explore genetic problems with multiple alleles and sex-linked traits! 💬
• Uncover the intricate dance of chromosomes in pedigree analysis! 🌀
• Engage with practical tips and reminders for genetic problem solving! 🚀

Key Takeaways

• Unmask the magic of Mendelian genetics and master those Punnett squares! 🧬
• Get the scoop on non-Mendelian traits like codominance and incomplete dominance! 🌈
• Discover the secrets of sex-linked traits and why X marks more than just a spot! 🕵️‍♀️
• Pedigrees are more fun when you solve the mystery of inheritance! 🕵️‍♂️
• Genetics can be tricky—embrace the complexity and stay curious! 🔍

Overview

Chapters

• 00:00 - 00:30: Introduction The chapter titled "Introduction" presents an overview of various educational resources available for biology review, including GIFs, study tips, and a mega review video titled "Stroll Through the Playlist." The video mentioned is noted as the longest in the creators' history, but the chapter clarifies that the current segment will not be as extensive. Instead, the focus will be on addressing areas not covered in depth previously, specifically providing a review on solving general genetic problems, such as Mendelian one-trait and two-trait crosses.
• 00:30 - 03:00: Review of Mendelian Traits The chapter 'Review of Mendelian Traits' discusses various aspects of genetic inheritance, particularly focusing on non-Mendelian traits. These include incomplete dominance, codominance, multiple alleles, and sex-linked traits. The chapter is designed as an interactive learning experience, suggesting the use of paper and pen for problem-solving and encouraging pauses to engage with exercises. Key genetic vocabulary is employed throughout, with the assumption that readers have a foundational understanding of these terms. The chapter concludes with an introduction to pedigrees, a tool used to trace traits through generations in a family.
• 03:00 - 04:00: Mendelian One-Trait Crosses The chapter 'Mendelian One-Trait Crosses' discusses the importance of understanding concepts over symbols in genetics, particularly in genetic problems involving incomplete dominance and codominance. It emphasizes that symbols used in textbooks can vary, for instance using superscripts or different letters, and suggests choosing letters with distinct uppercase and lowercase forms to avoid confusion.
• 04:00 - 06:00: Mendelian Two-Trait Crosses The chapter discusses Mendelian two-trait crosses, emphasizing the concept of probability in genetic problems. It highlights that while a Punnett square can predict the likelihood of an outcome, it does not guarantee it in every instance. The chapter also stresses the complexity of genetics, suggesting that it goes beyond the straightforward gene traits typically seen in standard Punnett square problems.
• 06:00 - 10:00: Non-Mendelian Inheritance This chapter discusses various forms of non-Mendelian inheritance, including polygenic traits where multiple genes control one trait, pleiotropy where one gene influences multiple traits, and epistasis where the expression of one gene is affected by another gene. It also touches on epigenetics, highlighting factors outside the DNA sequence that can impact gene expression.
• 10:00 - 11:30: Incomplete Dominance This chapter provides an explanation of incomplete dominance, using guinea pigs as a genetic example. It introduces the concept of Mendelian one-trait and monohybrid crosses, highlighting how a dominant allele leads to the presence of hair in guinea pigs. The chapter discusses different genotypes: homozygous dominant, heterozygous, and homozygous recessive, and challenges the reader to determine genotypes given specific genetic codes.
• 11:30 - 13:00: Codominance The chapter on 'Codominance' begins with an explanation of alleles using guinea pigs as an example. It describes that a guinea pig with at least one dominant allele (H) will have hair, while a hairless one is homozygous recessive (hh). The chapter includes an exercise to cross a hairless guinea pig (hh) with a heterozygous one (Hh) and calculate the phenotype and genotype ratios of their offspring.
• 13:00 - 15:00: Multiple Alleles The chapter delves into the concept of multiple alleles with a focus on genetic inheritance patterns using Punnett squares. It explains how to arrange a Punnett square and the significance of genotype ratios and phenotype ratios, highlighting that these ratios may not always align. The given example discusses a 1:1 genotype and phenotype ratio from a cross, showing that the offspring have either 'Hh' or 'hh' genotypes, which correspond to having hair or being hairless.
• 15:00 - 18:00: Sex-Linked Traits The chapter delves into the topic of sex-linked traits, using Mendelian genetic principles as a foundation. It introduces a fictional trait in cats, specifically an affinity for sinks, to explain dihybrid crosses and the influence of dominant alleles. The narrative uses a playful, fictional example to illustrate how traits can be passed on, despite the fact that the trait in question (love of sinks) is not likely a genetic one. This approach helps to elucidate the complexities of genetic inheritance in a lighthearted manner.
• 18:00 - 24:00: Pedigrees The chapter 'Pedigrees' discusses genetic inheritance with a focus on cat traits like sink-loving behavior and hair presence, illustrating how these traits are influenced by dominant and recessive alleles. A heterozygous cat with both traits (genotype HhSs) is crossed with another of the same genotype in a dihybrid cross, requiring a 16-square Punnett square to predict offspring outcomes.
• 24:00 - 25:00: Conclusion The conclusion chapter explains the concept of gamete combinations using a dihybrid cross. It discusses the use of the FOIL method to determine the gametes (HS, Hs, hS, hs) for the square and emphasizes that the gametes represent one allele of each type. The chapter concludes by instructing to complete the dihybrid cross and determine the phenotype ratio.

Mega Genetics Review: Mendelian and non-Mendelian Genetics Transcription

• 00:00 - 00:30 So we’ve made resources for reviewing in biology: from our GIF review, to our study tips video, to our mega biology review video called stroll through the playlist. Our Stroll Through the Playlist was actually our longest video in Amoeba Sisters history. This one won’t be quite that long. But it will address an area that the Stroll Through the Playlist couldn’t go into very much---a review of how to do general genetic problems. By that, I mean this video will review: Mendelian one-trait and two-trait crosses.
• 00:30 - 01:00 Then non-Mendelian traits like incomplete dominance, codominance, multiple alleles, and sex-linked traits. We’ll end with pedigrees. Before starting, I’ve got five things to mention first. Number 1, a sheet of paper will be very useful for this. You might want to pull one out now with a pen or pencil. Just like our stroll through the playlist, when you see Gus pop up, you can pause the video and work out a problem to check how you’re doing. Number 2, we’ll use some genetic vocabulary here, and we are making the assumption that
• 01:00 - 01:30 you’ve already seen these words before. Number 3, with genetic problems, the symbols used in textbooks---especially with incomplete dominance and codominance---can vary. Some use superscript; some use different letters. It’s not the symbol you need to focus on but rather the concept. But just to note: if you’re choosing the letters to write out, you might want to pick letters that have different looking capital and lowercase letters unless you go to a lot of effort to make them look different.
• 01:30 - 02:00 Number 4, remember that when you’re doing a genetic problem, you are determining a probability. But just because a Punnett square tells you that 1 out of 4 guinea pig offspring will be hairless, doesn’t mean that there will always be one hairless guinea pig every time you have four offspring. It can show it’s possible, but it’s still a probability. Number 5, and this is the big one, please realize the topic of genetics is complex. Far more complex than simple gene traits you tend to see in Punnett square problems.
• 02:00 - 02:30 Traits can be polygenic, which means MANY genes control ONE trait. Some traits can be pleiotropic which means ONE gene controls MANY different traits. We mention epistasis in one of our videos, which means that a gene’s expression, whether it’s expressed or not, can be impacted by the expression of another gene. And we released a video about epigenetics: this involves factors at work - that are not part of the DNA sequence -but yet can influence gene expression. Okay, with those five things said, let’s get started!
• 02:30 - 03:00 First, with understanding a Mendelian one-trait cross, which includes monohybrid crosses. Let’s assume here we have these three guinea pigs. We’re using the letter H to represent an allele, and we’re assuming that the presence of a dominant allele means the guinea pig will have hair. This guinea pig is homozygous dominant, this one is heterozygous, and this one is homozygous recessive. Knowing that, can you complete the genotypes?. [PAUSE]
• 03:00 - 03:30 HH, Hh, and hh. Remember it only takes one dominant allele and it would have hair. Also remember there are two alleles per guinea pig here because each guinea pig gets one allele from each of their parents. Please cross a hairless guinea pig with a heterozygous guinea pig and give the phenotype and genotype ratios of the offspring. [PAUSE] Notice that I had to figure out the hairless guinea pig’s genotype would be hh. I remembered it only takes one dominant allele in this example and it would have hair, so
• 03:30 - 04:00 it can’t have a capital H. I arrange the Punnett square. It doesn’t matter which side I put the parents on. You will notice the genotype ratio of the offspring here is 2 Hh: 2 hh. This can be reduced to 1:1: ratio. The phenotype ratio is 2 with hair: 2 hairless which can be reduced to 1:1. Genotype ratios and phenotype ratios don’t necessarily match although they did in this
• 04:00 - 04:30 case. On to a Mendelian two-trait cross, which includes dihybrid crosses. Moving on to a different animal: cats. With our Mendelian two-trait and dihybrid cross video, we mentioned a fictional trait of the love for sinks. Because our cat Moo, and many cats we’ve been told, seem to have an affection for sinks. Again, probably not a genetic trait, but for this example, let’s use this fictional trait. We will assume here that the presence of a dominant S allele leads to the trait of a
• 04:30 - 05:00 cat loving sinks. Only two recessive “s” alleles would result in the non-sink loving trait. So, if we have a cat that is heterozygous for both the traits of having hair and loving sinks, what would that cat’s genotype be? [PAUSE] HhSs. Let’s cross it with another cat with that same genotype. HhSs. Now a dihybrid calls for a 16 square box here.
• 05:00 - 05:30 What would be the gamete combinations that we put on top and side of this square? [PAUSE] Recall the FOIL method---the gametes along the side would be HS, Hs, hS, and hs. Same for the other side. Remember it doesn’t matter which parent you put on which side. Notice how the gametes have one of each allele type. Meaning, you wouldn’t have a HH or a SS in a gamete; you get one of each. Go ahead and fill that dihybrid cross in and give us the phenotype ratio.
• 05:30 - 06:00 [PAUSE] Here we are. Notice we put the H letters here first and then the S letters, similar to how the genotype was written for the parent cats. That’s a 9:3:3:1 phenotype ratio, which if you cross two organisms that are heterozygous for both traits in a dihybrid cross, you will find that phenotype ratio to occur. But for a two-trait example that doesn’t have two heterozygote parents, you can check
• 06:00 - 06:30 that out on our full content video. Ok, we’re leaving Mendelian genetics now. Mendelian genetics followed an inheritance pattern where having a dominant allele meant the dominant trait was expressed. But in non-Mendelian inheritance, we’ll see that’s not always how it works. Take incomplete dominance. Keep in mind before starting there should be clues that a problem is involving incomplete dominance or some other non-Mendelian trait. Incomplete dominant traits tend to have an intermediate phenotype, almost an in-between
• 06:30 - 07:00 phenotype. The snapdragon example is a popular one. Here is a red snapdragon with genotype RR. A white snapdragon with genotype rr. But the Rr genotype leads to a pink phenotype. In incomplete dominance, one allele is not completely dominant over the other.. What would be the genotype and phenotype ratios of the offspring from two pink snapdragons crossed? [PAUSE] 1 RR: 2 Rr: 1 rr would be the genotype ratio.
• 07:00 - 07:30 1 red: 2 pink: 1 white would be the phenotype ratio. This is different from codominance, in codominance, both traits are expressed fully. So for codominance, we like using different letters entirely for this reason. In a certain type of chicken, genotype BB results in black chickens, WW results in white chickens, and BW results in black and white speckled chickens!
• 07:30 - 08:00 What would be the genotype and phenotype ratios of offspring from one black chicken and one black and white speckled chicken? [PAUSE] 2 BB: 2 BW, reduced to 1:1 would be the genotype ratio. And as for the phenotype ratio? 2 black chickens: 2 black and white speckled chickens reduced to 1:1. Now it’s time for a genetic problem with multiple alleles. Blood types are a great example of this. If we have these four blood types: A, B, AB, and O… can you write the genotypes?
• 08:00 - 08:30 And just a hint, it’s common to write the alleles as exponents on the letter I, although it doesn’t have to be written that way. [PAUSE] Here they are! Now, if there is one parent that is heterozygous type B and another that is heterozygous type A, what is the percent chance that the baby from these two parents will be type O? [PAUSE] So after working this out with the correct genotypes around the Punnett square,
• 08:30 - 09:00 it is a 25% chance that the baby will be type O. Keep in mind that blood types can also be positive or negative, which is related to Rh factor, which our video does not go into. Next, sex-linked traits on sex chromosomes. In these Punnett square problems, you are usually told it is a sex-linked trait and then also given information about whether an individual is male or female. In these problems, you are working on Punnett squares that involve alleles on sex chromosomes.
• 09:00 - 09:30 But as we mention in our old video, please know that individuals can have more or fewer sex chromosomes than what might be written in a Punnett square. Let’s consider the recessive, sex-linked disorder hemophilia. If I tell you it’s sex-linked recessive, and we use the letter “h,” how would you write the genotype for a male that has this disorder? [PAUSE] You’d write XhY. Notice that it’s only placed on the X chromosome.
• 09:30 - 10:00 Generally sex-linked traits will be found on the X chromosome, although there are some exceptions. If a male was XHY, this individual would not have the disease since the disease is a recessive sex-linked disorder. Which of these female genotypes would have the disease hemophilia? [PAUSE] Only the female genotype with XhXh. Remember, the heterozygous genotype XHXh still has a dominant allele, represented by the
• 10:00 - 10:30 capital H, which means this individual does not have this disorder. If a male with hemophilia and a female who is homozygous dominant decide to have a child together, what percent chance is it that their child would have hemophilia? [PAUSE] It is a 0% chance. Also, do you notice how the male children receive their X chromosome from the female?
• 10:30 - 11:00 They receive their Y chromosome from the male. All right, that’s a lot of genetic problems. Now, our last topic, pedigrees. Pedigrees can be used to track a trait of interest, and they use many of the concepts we’ve been reviewing. A reminder that the shaded shape in a pedigree is generally the trait of interest. Some people will also do a half-shading to represent the heterozygous genotype, as we mention in our pedigree video, however, this isn’t always done and we’re going to assume half shadings have not been done in our examples.
• 11:00 - 11:30 Take a look at this pedigree. What shape is supposed to represent females? [PAUSE] That’s right, the circles. The males would be represented as squares. So in this pedigree, assume you are told the shaded shapes represent individuals that have an autosomal recessive trait. Because it is autosomal, the trait is not on a sex chromosome. In our pedigree video, our trait of interest was tracking attached earlobes, but as we mentioned in that video, this trait may be more complex than a single gene trait.
• 11:30 - 12:00 We’re going to use the letter “e” here so any of the shaded shapes must have the genotype ee. So, if given this pedigree, can you determine the genotypes of the rest of the individuals? Just a reminder, with pedigrees, it’s often ideal to fill out the genotypes of the shaded shapes first before determining the others. And in this case, you know the shaded shapes will all be ee. So now try and fill out the rest of the genotypes!
• 12:00 - 12:30 [PAUSE] Here are all the genotypes! A few things to point out. Notice in generation I, individual I, the individual must be Ee. That’s because there is a ee offspring and so if the individual was EE, that would not be possible. This is the same situation with individual 1 in generation 2. Notice in generation 3, individual 2, this male must be Ee.
• 12:30 - 13:00 This male cannot be EE, because it would not be possible to receive a dominant allele from both parents. All offspring receive one allele from each parent. Notice in generation II, individual 4, this male could be EE or Ee. We don’t know. Even if this individual had 10 children that did not have the trait, you still would not know the genotype for sure. The only way you would know for sure is if they had a child with the genotype ee.
• 13:00 - 13:30 Because a child with the ee genotype would reveal that both of these parents would have to be the heterozygous genotype. But there is not a child represented by a shaded shape here. What if I didn’t tell you this trait was autosomal recessive? Could you show why this pedigree is likely NOT tracking a sex-linked recessive trait? So if this was tracking a sex-linked recessive trait, the shaded shapes would then represent genotypes
• 13:30 - 14:00 that are sex-linked recessive. Filling out the genotypes of shaded shapes first. Here they are. So can you determine why this is NOT likely? [PAUSE] So take a look at parents 1 and 2 in generation II. We know that Generation II, individual 1 is a male. The individual must have genotype XEY, because if the genotype was XeY, the shape would be
• 14:00 - 14:30 shaded. Notice a Punnett square with parents 1 and 2 of generation II shows it is not possible to have those offspring genotypes in generation III from the pedigree. This specific pedigree is not likely to be tracking a sex-linked recessive trait. So that was a lot to review! What if you’re still stuck? Check out the full content videos, which are each under 10 minutes, in our genetic series. Practice. Practice a lot. And, finally, connect this to why it matters. We have some links to check out with more fascinating real-life examples in our video
• 14:30 - 15:00 details so you can discover why gaining an understanding of genetics is such a worthwhile endeavor. Well, that’s it for the Amoeba Sisters, and we remind you to stay curious!