12 Chapter 12

Learning Objectives

  1. Recognize the role of genes and genetics for inheritance patterns
  2. Apply principles of inheritance in a monohybrid cross with application of genotype and phenotype percentages
  3. Identify three patterns of inheritance in human disease

Mendel’s Experiments and Heredity

Johann Gregor Mendel (1822–1884) was a lifelong learner, teacher, scientist and man of faith. Supported by his monastery. in what is now the Czech Republic. In 1865, Mendel presented the results of his experiments with nearly 30,000 pea plants to the local Natural History Society. He demonstrated that traits are transmitted faithfully from parents to offspring independently of other traits and in dominant and recessive patterns.

Mendel’s work was accomplished using the garden pea to study inheritance. This species naturally self-fertilizes, preventing pollination from other plants. The result is “true-breeding” pea plants. When fertilization occurs between two true-breeding parents that differ in only one characteristic, the process is called a monohybrid cross, These are plants that always produce offspring that look like the parent. By experimenting with true-breeding pea plants, Mendel avoided the appearance of unexpected traits in offspring that might occur if the plants were not true breeding. The garden pea also grows to maturity within one season, meaning that several generations could be evaluated over a relatively short time. Finally, large quantities of garden peas could be cultivated simultaneously, allowing Mendel to conclude that his results did not come about simply by chance.

This illustration shows a monohybrid cross. In the upper P generation, one parent has a dominant yellow phenotype and the genotype upper Y upper Y, and the other parent has the recessive green phenotype and the genotype lower y lower y. Each parent produces one kind of gamete, resulting in an upper F subscript 1 baseline generation with a dominant yellow phenotype and the genotype upper Y lower y. Self-pollination of the upper F subscript 1 baseline generation results in an upper F subscript 2 baseline generation with a 3 to 1 ratio of yellow to green peas. One out of three of the yellow pea plants has a dominant genotype of upper Y upper Y, and 2 out of 3 have the heterozygous phenotype upper Y lower y. The homozygous recessive plant has the green phenotype and the genotype lower y lower y.

Pea plant with dominant yellow phenotype crossed with recessive green phenotype. This cross produces all heterozygotes with a yellow phenotype. A Punnett square analysis can be used to predict the genotypes of a cross of two heterozygotes.

Working with garden pea plants, Mendel found that crosses between parents differing in one trait produced F1 offspring expressing traits of one parent. Observable traits are referred to as dominant, and non-expressed traits are described as recessive. When the offspring in Mendel’s experiment were self-crossed, the F2 offspring exhibited the dominant trait (75%) and the recessive trait (25%) in a 3:1 ratio, confirming the recessive trait had been transmitted faithfully from the original P0 parent. By examining sample sizes, Mendel showed that his crosses behaved reproducibly according to the laws of probability, and that traits were inherited as independent events.

Gene variants which arise by mutation and exist at the same relative chromosomal locations are called alleles. Two alleles for a given gene in a diploid organism are expressed and interact to produce physical characteristics. The observable traits of an organism are referred to as phenotype. The organism’s underlying genetic makeup is referred to as  genotype. Mendel’s experiments demonstrate the difference between phenotype and genotype. When true-breeding plants in which one parent had yellow pods and one had green pods were cross-fertilized, all F1 offspring had yellow pods. Offspring were phenotypically identical to the parent with yellow pods. Allele donated by the parent with green pods was not simply lost, because it reappeared in some F2 offspring. This demonstrates F1 plants must have different genotypes from the parent with yellow pods.

The plants Mendel used in his experiments were homozygous for traits he was studying. Diploid organisms that are homozygous at a given gene have two identical alleles for that gene on their homologous chromosomes. Mendel’s parental pea plants always bred true because both gametes produced carried the same trait.When plants with contrasting traits were cross-fertilized, all offspring were heterozygous, meaning their genotype had different alleles for the gene being examined.

In all seven pea-plant characteristics, one of the two contrasting alleles was dominant, and the other was recessive. We now know this is due to genes on homologous chromosome pairs. For a gene that is expressed in a dominant and recessive pattern, homozygous dominant and heterozygous organisms will appear identical. This means they have different genotypes but the same phenotype. The recessive allele will only be observed in homozygous recessive individuals, so some refer to recessive alleles as masked.

Alleles do not always behave in dominant and recessive patterns. Incomplete dominance describes situations in which the heterozygote exhibits a phenotype that is intermediate between the homozygous phenotypes. Codominance describes the simultaneous expression of both of the alleles in the heterozygote. Although diploid organisms can only have two alleles for any given gene, it is common for more than two alleles of a gene to exist in a population. In humans, as in many animals and some plants, females have two X chromosomes and males have one X and one Y chromosome. Genes that are present on the X but not the Y chromosome are said to be X-linked, such that males only inherit one allele for the gene, and females inherit two. Finally, some alleles can be lethal. Recessive lethal alleles are only lethal in homozygotes, but dominant lethal alleles are fatal in heterozygotes as well.

Many human diseases are genetically inherited. A healthy person in a family in which some members suffer from a recessive genetic disorder may want to know if he or she has the disease-causing gene and what risk exists of passing the disorder on to his or her offspring. Of course, doing a test cross in humans is unethical and impractical. Instead, geneticists use pedigree analysis to study the inheritance pattern of human genetic diseases. Autosomal dominant disorders are present in both male and female humans and appear in each generation of a pedigree. An example of this in humans is Huntington’s disease, in which the nervous system gradually wastes away. People who are heterozygous for the dominant Huntington allele (Hh) will inevitably develop the fatal disease. However, the onset of Huntington’s disease may not occur until age 40, at which point the afflicted persons may have already passed the allele to 50 percent of their offspring.

Recessive alleles can be “carried” and not expressed by individuals. Such heterozygous individuals are sometimes referred to as “carriers.” Further genetic studies in other plants and animals have shown that much more complexity exists, but that the fundamental principles of Mendelian genetics still hold true. Autosomal recessive disorders will show unaffected carriers and can skip generations in pedigrees.

This is a pedigree of a family that carries the recessive disorder alkaptonuria. In the second generation, an unaffected mother and an affected father have three children. One child has the disorder, so the genotype of the mother must be upper case A lower case a, and the genotype of the father is lower case a lower casea. One unaffected child goes on to have two children, one affected and one unaffected. Because her husband was not affected, she and her husband must both be heterozygous. The genotype of their unaffected child is unknown, and is designated upper A question mark. In the third generation, the other unaffected child had no offspring, and his genotype is therefore also unknown. The affected third-generation child goes on to have one child with the disorder. Her husband is unaffected and is labeled 3. The first generation father is affected and is labeled 1; The first generation mother is unaffected and is labeled 2 The Visual Connection question asks the genotype of the three numbered individuals.

Alkaptonuria is a recessive genetic disorder in which two amino acids, phenylalanine and tyrosine, are not properly metabolized. Affected individuals may have darkened skin and brown urine, and may suffer joint damage and other complications. In this pedigree, individuals with the disorder are indicated in blue and have the genotype aa. Unaffected individuals are indicated in yellow and have the genotype AA or Aa. Note that it is often possible to determine a person’s genotype from the genotype of their offspring. For example, if neither parent has the disorder but their child does, they must be heterozygous. Two individuals on the pedigree have an unaffected phenotype but unknown genotype. Because they do not have the disorder, they must have at least one normal allele, so their genotype gets the “A?” designation.

Because human males need to inherit only one recessive mutant X allele to be affected, X-linked disorders are disproportionately observed in males. Females must inherit recessive X-linked alleles from both of their parents in order to express the trait. When they inherit one recessive X-linked mutant allele and one dominant X-linked wild-type allele, they are carriers of the trait and are typically unaffected. Carrier females can manifest mild forms of the trait due to the inactivation of the dominant allele located on one of the X chromosomes. However, female carriers can contribute the trait to their male children, resulting in the male exhibiting the trait, or they can contribute the recessive allele to their female children, resulting in the children being carriers of the trait. Although some Y-linked recessive disorders exist, typically they are associated with infertility in males and are therefore not transmitted to subsequent generations. X-linked or sex linked disorders affect more males than females, as females can be unaffected carriers.

A diagram shows an unaffected male with a dominant allele and an unaffected carrier female with an x-linked recessive allele. Four figures of offspring are shown representing the various resulting genetic combinations: unaffected male offspring, unaffected female offspring, affected male offspring, and unaffected carrier female offspring.

The male offspring of a person who is a carrier of a recessive X-linked disorder will have a 50 percent chance of being affected. A female will not be affected, but she will have a 50 percent chance of being a carrier like the female parent.

Exercises

Reading Quiz

Key Takeaways

  1. DNA codes for proteins. Meiosis allows for alleles to be inherited from both parents. Some alleles exert effects when they are present (dominant), while others are not expressed (recessive).​
  2. Genotype refers to alleles and phenotype refers to appearance. A monohybrid cross allows analysis of heterozygote or homozygote offspring.​
  3. Pedigrees indicate whether a disease is autosomal dominant, recessive or x-linked.

 

Biology-2e. (2018). Houston, RX: website: OpenStax Book title: Biology 2e .

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Introductory Biology Copyright © 2023 by Mona Easterling is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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