Study Guide

Genetics - Common Mistakes

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Common Mistakes

Mendelian Genetics

  • A capital letter in a genotype represents a dominant allele; a lower case letter represents a recessive allele. It doesn't really matter what letter you use, just try to pick one that looks different in upper & lower case if you're writing them out by hand
  • Capital letters always go first! So when you're writing out a heterozygous genotype, remember its Rr, and not rR
  • Seriously, you can ace any problem about Mendelian genetics with a Punnett square. Always write out your crosses in a grid design; you are less likely to make mistakes. And don't forget: practice makes perfect!
  • Genetic crosses only give you the probability that a particular genotype or phenotype will occur (e.g. ¼ wrinkled to ¾ round peas). You only tend to see the expected ratios if you look at a large enough sample, which is one reason why Mendel chose peas. If you look at humans, just because there's a 50% chance that your child will be a boy and 50% chance that they will be a girl, it doesn't mean that if you have two children you'll get one of each!
  • All those numbers! As you might have noticed, you can express the relative numbers of offspring from a cross as a fraction (½) and a ratio (1:1). You can also express them as a percentage (50%), or even a decimal if you really want to (0.5). It doesn't matter which you use, just make sure everything adds up in the end…
  • A good way to remember what the first and second laws are about and not mix them up: the first law is about one locus (principle of segregation - alleles at one locus go their own way into separate gametes); the second law is about two or more loci (principle of independent assortment—genes for different traits assort independently of one another in the production of gametes).

Genetic Linkage

If you are analyzing a cross involving two genes, if the ratio of F2 phenotypes from a self-fertilized F1 cross don't closely match the expected 9:3:3:1 ratio, you might be dealing with linked genes!

Modification of Dominance Relationships

  • Don't get confused between codominance and incomplete dominance! In codominance, both alleles are expressed: the offspring of a bird with blue feathers and a bird with yellow feathers has both blue and yellow feathers. In incomplete dominance, the heterozygote shows a phenotype that is intermediate between the parents: the offspring of a bird with blue feathers and a bird with yellow feathers has green feathers.
  • When writing out incomplete dominance crosses, you can use different letters for the different alleles – this just helps to remind you that you're not dealing with a standard dominant/recessive relationship
  • Just because you can only have two alleles at every gene locus (one on each of your homologous chromosomes) doesn't mean that every gene only has two alleles in total. As we've seen, the ABO blood system has three, and some genes in the fruit fly Drosophila have over 30!

Lethal Alleles

  • If you don't see a 3:1 phenotypic ratio in the F2, you might be dealing with a lethal allele
  • Recessive or dominant? The best way to think about it is to look at the number of alleles you need to cause death. If its only one, then it's a dominant lethal allele; if it's two, then it's a recessive lethal allele.

Gene Interactions

  • How can you make sure you don't mix up epistasis and pleiotropy? Epistasis always involves multiple genes but one phenotype – while pleiotropy is about one gene affecting multiple phenotypes.
  • How about penetrance and expressivity? Penetrance refers to whether the expected phenotype is expressed for a particular genotype, while expressivity refers to the particular degree to which a phenotype is expressed (when it is expressed). They are of course tightly related: there is no possibility of variation of expression (expressivity) if the phenotype is not expressed (penetrance)!

Sex Determination and Inheritance of Sex-Linked Traits

  • Sex linkage of genes really messes with your phenotypic and genotypic ratios, because there's nothing to mask the recessive phenotype in affected males
  • If a trait is sex-linked, a father won't be able to pass it on to his sons

Quantitative Genetics

A common misconception is to assume that heritability estimates the proportion of a phenotype that is due to genetic effects: for example, it does not help to answer how much of a person's height is determined by her genes. Heritability is actually a population parameter, and measures instead how much of the variation for a given phenotype across many individuals is due to variation at the genetic level. So heritability could help you understand how much of the variation in height in the USA might be explained by genetic factors. Think of the chocolate cookies you make for the bake sale - no cookie looks the same, right? Do 43% ingredients and 57% baking methods define each cookie's look? Doesn't make much sense, right? The best approach is to ask whether the difference between cookies is mostly due to variation in ingredient composition or differences in how the cookies were baked.

Extranuclear Genetics

The transmission of both X-linked and mitochondrial genes are associated with sex. However, X-linked traits tend to be expressed mostly in males, but transmitted by fathers to daughters, and by mothers to both sons and daughters. Mitochondrial traits however, are only transmitted by mothers to both sons and daughters.

The Chi-Squared Test

  • Think carefully about the ratios you might be expecting to see in a cross: some of the most common for monohybrid crosses are 3:1, 1:2:1, 2:1 and 1:1, and for dihybrid crosses 9:3:3:1, 9:3:4 and 1:1:1:1 (depending on what type of cross you're carrying out). If you don't know what sort of cross you're looking at, or what the expected ratio is, then you won't be able to work out what your expected values should be and you'll end up proving your null hypothesis wrong!
  • Break it down: one step at a time is the best way to go with chi-squared – that way, you're less likely to make a mistake when juggling all those numbers

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