8.3 Mendelian Inheritance

Monohybrid Cross

A Punnett Square chart allows you to easily determine the expected ratios of possible genotypes in the offspring of two parents. You can see a simple example in Figure 8.3.1. This Punnett square is for a monohybrid cross when fertilization occurs between two parents that differ by only one character. In this case, both parents are yellow and are heterozygotes (Yy) for the gene controlling pea colour. Half of the gametes produced by each parent will have the Y allele (for yellow), and half will have the y allele (for green). That’s because the two alleles are on homologous chromosomes, which always separate and go to separate gametes during meiosis. The alleles in the gametes from each parent are written down the side and across the top of the Punnett square. Filling in the cells of the Punnett square gives the possible genotypes of their offspring. It also shows the most likely ratios of the genotypes, which in this case is 1 YY: 2 Yy: 1 yy (25% YY: 50% Yy: 25% yy). You can then determine the most likely ratios of the phenotypes, which is three yellow: 1 green (75% yellow: 25% green).

A Punnett square can also be used to show how the X and Y chromosomes are passed from parents to their children. Mothers pass only X chromosomes to their children. Fathers always pass their X chromosome to their daughters and their Y chromosome to their sons.

Probability Basics

The results of Mendel’s research can be explained in terms of probabilities, which are mathematical measures of likelihood. The probability of an event is calculated by the number of times the event occurs divided by the total number of opportunities for the event to occur. A probability of one (100 percent) for some event indicates that it is guaranteed to occur. In contrast, a probability of zero (0 percent) indicates that it is guaranteed not to occur, and a probability of 0.5 (50 percent) means it has an equal chance of occurring or not occurring.

Figure 8.3.1 shows a 25% chance of getting the genotype yy. That does not guarantee that there will be one yy pea plant out of every 4 offspring. Sample size plays a crucial role in observing these probabilities. In smaller sample sizes, the actual outcomes may deviate from the expected probabilities due to random chance. When flipping a coin, you have a 50% chance of getting heads and a 50% chance of tails. If you flip a coin 2 times, you aren’t guaranteed to get 1 heads and 1 tail. However, as the sample size increases, the observed ratios align more closely with the predicted probabilities. By flipping a coin 1000 times, you have a much better chance of observing the predicted ratios of 50% heads and 50% tails. Similarly, for genetics, there is a higher likelihood of observing the expected 25% yy genotype when considering a large number of offspring.

Dihybrid Cross

A dihybrid cross is when fertilization occurs between parents that differ by two characters. Consider the characteristics of seed colour and seed shape for two pea plants, one that has wrinkled, green seeds (rryy) and another that has round, yellow seeds (RRYY). Because each parent is homozygous, the law of segregation indicates that the gametes for the wrinkled–green plant are all Ry, and the gametes for the round–yellow plant are all RY. Therefore, the F1 offspring generation is all RrYy (Figure 8.10).

The gametes produced by the F1 individuals must have one allele from each of the two genes. For example, a gamete could get an R allele for the seed shape gene and either a Y or a y allele for the seed colour gene. It cannot get an R and an r allele; each gamete has only one allele per gene. The law of independent assortment states that a gamete into which an r allele is sorted would be equally likely to contain either a Y or a y allele. Thus, there are four equally likely gametes that can be formed when the RrYy heterozygote is self-crossed: RY, rY, Ry, and ry. Arranging these gametes along the top and left of a 4 × 4 Punnett square gives us 16 equally likely genotypic combinations. These genotypes show a phenotypic ratio of 9 round–yellow: 3 round–green: 3 wrinkled–yellow: 1 wrinkled–green. These are the offspring ratios we would expect, assuming we performed the crosses with a large enough sample size.

Mendelian Inheritance in Humans

Mendelian inheritance refers to the inheritance of traits controlled by a single gene with two alleles, one of which may be completely dominant to the other. The inheritance pattern of Mendelian traits depends on whether the traits are controlled by genes on autosomes or by genes on sex chromosomes.

Autosomal Traits

Autosomal traits are controlled by genes on one of the 22 pairs of human autosomes. Autosomes are all the chromosomes except the sex chromosomes (X and Y). Autosomes do not differ between males and females, so autosomal traits are inherited similarly, regardless of the parent’s sex or offspring.

Not many human autosomal traits are controlled by a single gene with two alleles, but they are a good starting point for understanding human heredity. Most forms of albinism in humans have a Mendelian inheritance pattern. Albinism is a condition that results from a lack of pigment melanin in the skin, hair, and eyes. It is usually controlled by a single autosomal gene with two alleles. The allele for normal pigmentation (let’s call it R) is dominant to the allele for albinism (r). Individuals with an RR or Rr genotype will not have albinism because the R allele dominates over the recessive r allele.

However, consider what happens if two individuals with the Rr genotype reproduce with each other. The outcome would be similar to the example shown in the Punnett square above for two hypothetical Yy individuals (Figure 8.3.1). Their possible offspring could be RR (normal pigmentation), Rr (normal pigmentation), or rr (albinism). This explains why a child with albinism (rr) can have two parents who do not have albinism. Both unaffected parents must be carriers of the recessive r allele, but they also have a dominant R allele that prevents them from having the condition themselves.

Some other human traits with a Mendelian inheritance pattern are Huntington’s disease and wet versus dry earwax. You may have heard about other human traits that were previously thought to be Mendelian, such as dimples, a widow’s peak hairline, a hitchhiker’s thumb, and the ability to roll your tongue. As science has progressed, it is now understood that these are not actually Mendelian traits. In fact, most human traits are controlled by multiple genes or have more than two alleles, which means they do not have a simple Mendelian inheritance pattern.

Sex-Linked Traits

Traits controlled by genes on the sex chromosomes are called sex-linked traits. Because of the small size of the Y chromosome, most sex-linked traits are controlled by genes on the X chromosome. These traits are called X-linked traits.

Because males have just one X chromosome, they have only one allele for any X-linked trait. Therefore, a recessive X-linked allele is always expressed in males. Because females have two X chromosomes, they have two alleles for any X-linked trait. Therefore, they must inherit two copies of the recessive allele to express an X-linked recessive trait. This explains why X-linked recessive traits are less common in females than males and why they show a different inheritance pattern than autosomal traits.

An example of a recessive X-linked trait is red-green colour blindness. People with this trait cannot distinguish between the colours red and green. More than one recessive gene on the X chromosome codes for this trait, which is fairly common in males but relatively rare in females. Figure 8.3.6 shows a simple pedigree for this trait. A female with one of the recessive alleles for the trait does not have the trait herself but can pass it on to her children. In this case, she is called a carrier of the trait. Half of any sons she has can be expected to have the trait because there is a 50 percent chance that they will inherit the X chromosome with the colour-blindness allele. Having only one X chromosome, the recessive allele will be expressed in the sons who inherit it. However, as long as the father is not affected, none of the woman’s daughters will have the trait. The daughters have a 50 percent chance of inheriting the X chromosome with the colour-blindness allele, but it won’t be expressed because it is recessive to the normal allele they inherit from their father.

Genetic Disorders

Some human genetic disorders follow Mendelian inheritance patterns. Table 8.3.1 lists several genetic disorders caused by mutations in just one gene. Some of the disorders are caused by mutations in autosomal genes, others by mutations in X-linked genes.

A dominant disorder is caused by a mutated dominant allele. Very few genetic disorders are controlled by dominant alleles. A dominant allele is expressed in every individual who inherits even one copy of it. If it causes a serious disorder, affected people may die young and fail to reproduce. Therefore, the mutant dominant allele will likely die out of a population.

A recessive disorder, such as sickle cell anemia or cystic fibrosis, is not expressed in people who inherit just one copy of it. These people are called carriers. They do not have the disorder themselves, but they carry the mutant allele, and their offspring can inherit it. Thus, the allele will likely pass on to the next generation rather than die out.

Queen Victoria carried hemophilia and passed the hemophilia allele to two of her daughters and one of her sons. When they married royalty in other European countries, they spread the allele across Europe, including the royal families of Spain, Germany, and Russia. Victoria’s son, Prince Leopold, also inherited the hemophilia allele from his mother, and he suffered from the disease. Understandably, hemophilia was once popularly called “the royal disease.”

Studying Inheritance Patterns

A pedigree shows how a trait is passed from generation to generation within a family. A pedigree can show, for example, whether a Mendelian trait is an autosomal or X-linked trait. It can also be used to infer the genotype of different family members. Pedigrees can also be used to track the inheritance of genetic disorders within families.

The trait represented by this chart is a hypothetical autosomal trait controlled by a dominant allele. At the top of the pedigree, you can see symbols representing a married couple. The husband has the trait (affected male), but the wife does not (unaffected female). The next row of the pedigree shows the couple’s children, as well as the spouses of three of the children. For example, the first child on the left is an affected male married to an unaffected female. The third row of the pedigree shows the next generation (the couple’s grandchildren at the top of the pedigree). One child in this generation — the affected female on the left — is the sibling of an unaffected male.

“5.13 Mendelian Inheritance” from Human Biology by Christine Miller is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

“Monohybrid Cross and the Punnett Square” from Principles of Biology by Catherine Creech is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

“8.2 Laws of Inheritance” from Biology and the Citizen by Colleen Jones is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

“5.15 Genetic Disorders” from Human Biology by Christine Miller is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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