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In the previous section, we explored the idea that all organisms likely descend from a common ancestor. Heritable information, primarily in the form of DNA and RNA, is what makes this continuity of life possible. We also discussed how genetic information is passed on to the next generation through the formation of gametes during meiosis. But how do these processes inform our understanding of inheritance?
In the mid-19th century, a man named Gregor Mendel sought to understand how phenotypic traits were passed on to offspring. Many of his experiments were done using pea plants that he carefully crossbred in order to investigate how traits such as flower color and pea shape were inherited. The alleles, or specific versions of a trait, are often expressed as letters. For example, an allele that codes for a yellow pea may be expressed as a capital Y and an allele for a green pea may be expressed as a lowercase y. Through his observations, without even the knowledge of what DNA was, he understood that information was passed from parent to offspring and the resulting phenotypes displayed by the offspring were a combination of the traits from the parents. Through his experimentation, he developed three main laws of inheritance – segregation, independent assortment, and dominance.
Law of Segregation
This law states that each organism has two alleles for any given trait and that one is inherited from each parent. During the production of gametes, these alleles are separated and only one of the alleles will be passed on to any given offspring with the second allele coming from the other parent. For example, if a parent is heterozygous Yy and has an allele for a yellow pea (Y) and a green pea (y), the offspring will inherit either the Y or the y but not both.
Law of Independent Assortment
This law states that different traits are inherited independently of one another. For instance, in a pea plant, the trait for flower color is inherited independently from the trait for pea shape.
Law of Dominance
This law states that if an organism has two alleles for a trait and these alleles are different, one of these alleles may mask the expression of the other allele. The dominant allele is the allele that is expressed phenotypically. The recessive allele is the allele that is not expressed phenotypically. For example, imagine in a pea plant the allele for a yellow pea is dominant (Y) and the allele for a green pea is recessive (y). If the genotype for a pea plant is Yy, in other words it has one allele for yellow peas and another allele for green peas, the pea plant will produce yellow peas.
Punnett Squares
Punnett squares are a tool that can help us understand how the alleles for single-gene traits segregate into gametes and then are passed on from parents to offspring. Organisms that are heterozygous are those with two different alleles for a gene. Organisms that are homozygous are those with two of the same allele for a gene. Sometimes, one of these alleles is dominant over the other. This means that if the dominant allele is present, the dominant phenotype will be expressed even if the individual is heterozygous. Using Punnett squares, we can determine the likelihood of an offspring to have specific allele combinations and specific phenotypes expressed. By dividing the alleles from each parent, a Punnett square can show all the possible genotypes that an offspring may have as well as the relative ratios of these offspring.
Figure 5.06: Example of a Punnett Square cross between two heterozygous parents. Y is the dominant allele for a yellow pea and y is the recessive allele for a green pea.
In the Punnett square above, the allele for a yellow pea (Y) is dominant over the allele for a green pea (y). As you can see, even though both parents express yellow phenotypes, since they are heterozygous, they are still able to produce a green pea. This example is called a monohybrid test cross because it only shows the genotypes for one trait and both parents are heterozygous. Dihybrid crosses occur when both parents are heterozygous for two traits. Punnett squares can be expanded to show this as well but will result in 16 potential offspring genotypes.
Laws of Probability
While Punnett squares are useful, we can also use mathematical calculations to find the same probabilities. The probability equations are:
If A and B are mutually exclusive, then:
P (A or B) = P (A) + P (B)
If A and B are independent, then:
P (A and B) = P (A) x P (B)
Where P stands for the probability of something occurring.
Using the example above, where parent Yy is crossed with parent Yy, let’s try calculating the probability of the offspring having a recessive phenotype. There is a ½ chance of getting y from the mother and a ½ chance of getting y from the father. Looking at the equations shown below, since these are independent from each other, the probability of the offspring being yy is ½ x ½ which is equal to ¼. This is the same likelihood we found when using the Punnett square. Now let’s try calculating the probability of the offspring having a dominant allele. This can occur if the genotype is YY, Yy, or yY. The probability of getting any one of these specifically is ¼. However, to find the probability that one of these will happen, since these events are mutually exclusive, we can add these probabilities together. ¼ + ¼ + ¼ = ¾ which is the likelihood of the offspring having a dominant phenotype. While these calculations may seem simple in this example, we can then use these equations to do more complex crosses with multiple traits involved.
Patterns of Inheritance
Not all patterns of inheritance are simple. What we described above is a monohybrid inheritance pattern, when a trait is controlled by one gene with two alleles. In these situations, the dominant trait follows what is called an autosomal dominant inheritance pattern and the recessive trait follows what is called an autosomal recessive inheritance pattern. One level more complicated than this is a dihybrid inheritance pattern. This occurs when a trait is controlled by two genes, each of which has two alleles. Both of these patterns of inheritance follow Mendel’s Laws of Inheritance and exhibit typical phenotypic ratios. However, not all inheritance patterns follow these laws. In the next section, we will discuss some examples of deviations from Mendel’s model of inheritance and how these influence patterns of inheritance.
