Characteristics and Traits Inquire: Characteristics and Traits Overview 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 chromosome and one Y chromosome. Genes that are present on the X chromosome, 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. Big Question: What are other ways to explain variance in genetic characteristics and traits? Watch: Robbery at Big Petunia s Cafe There are many ways that the traits you see can be coded in our DNA. When Gregor Mendel was studying genetics, he wanted to find out how he could predict the genotypes, or the specific genetic code, of a plant based on the phenotypes, or the characteristics, expressed. Let s use the following scenario to help understand how genotypes and phenotypes can be different. There s been a robbery at Big Petunia s Cafe, the hot hangout for all of the local plant life. Upon investigation of the crime scene, Sheriff Sage has discovered that a pea plant leaf was left near the register and now he s taking it in for DNA testing. He will look at the alleles, which are gene variations that arise by mutation and exist at the same relative locations on two chromosomes. Sheriff Sage will look at whether or not the criminal is homozygous dominant, which is when the suspect has two dominant alleles (represented by two capital letters). The suspect could also be homozygous dominant, which is when the suspect has one dominant allele and one recessive allele (represented by one capital letter and one lower case letter). Lastly, the suspect could be homozygous recessive, which is when the suspect has two recessive alleles (represented by two lower case letters). Meanwhile, Deputy Dehlia has wrangled up the pea plant suspects and has collected all of their DNA information. Here s what she knows about the suspects; you might want to pause the video here to take notes. Copyright TEL Library 2018 Page 1
Blossom has white flowers. Her husband, Ash, has violet flowers. Their four children all have violet flowers. Poppy has the dominant phenotype of violet flowers. Her husband, Alder, has the recessive phenotype of white flowers. Poppy and Alder have two children: one has white flowers and one has violet flowers. Aspen has white flowers and his wife, Ivy, has white flowers. They have no children. Although this problem is focused on monohybrid crosses, where the offspring of an organism only expresses one trait from a parent, other types of genetics can be seen through incomplete dominance, codominance, and x-linked traits. Sheriff Sage reports that the genotype for the criminal is heterozygous dominant. Who committed the crime? Poppy, Alder, Blossom, Ash, Aspen, or Ivy? Read through the lesson to find the answer! Read: Characteristics and Traits Overview Physical characteristics are expressed through the genes that are carried on chromosomes. Many people used to think that genetics were a blend of the parent s genes; for example, if your mother has blue eyes and your father has green eyes, then your eyes would be a blue/green color. However, when Gregor Mendel was conducting his work on genetics on pea plants, he found that it was more likely that some genes were passed on and others were repressed. Therefore, you were actually more likely to have green eyes like your father, rather than a blend from both parents. In the example of Mendel s pea plant, the genetic makeup of peas consists of two homologous (same) chromosomes, one from each parent. Each pair of homologous chromosomes has the same order of genes which represent the same characteristics, such as a violet flower. Gene variations that arise by mutation and exist at the same relative locations on homologous chromosomes are called alleles. Mendel examined the inheritance of genes with just two allele forms (violet or white flower, wrinkled or round pea), but it is common to encounter more than two alleles for any given gene in a natural population, such as hair and eye color. Phenotypes and Genotypes Two alleles for a given gene in a diploid organism are expressed and interact to produce physical characteristics such as eye color. The observable traits expressed by an organism are referred to as its phenotype (think pheno = what you see). An organism s underlying genetic makeup is called its genotype (think geno = genes). Diploid organisms that are homozygous at a given gene have two identical alleles for that gene on their homologous chromosomes. For example, a pea plant s genotype might be two alleles for white flowers, and the phenotype means that the plant actually shows white flowers. Diploid organisms having two different alleles for a given gene on the homologous chromosome are heterozygous for the trait. For example, a pea plant that has one allele for a violet flower and one allele for a white flower (those are the genotypes) actually has violet flowers (phenotype). Copyright TEL Library 2018 Page 2
Dominant and Recessive Alleles In all seven pea-plant characteristics, one of the two contrasting alleles was dominant, and the other was recessive. Mendel called the dominant allele the expressed unit factor; the recessive allele was referred to as the latent unit factor. The recessive allele will only be observed in homozygous recessive individuals, meaning that the genotype of the organism has to have two recessive alleles to be shown as a phenotype. For the purposes of this lesson, we will abbreviate genes using the first letter of the gene s corresponding dominant trait. For example, violet is the dominant trait for a pea plant s flower color, so the flower-color gene would be abbreviated as V. Furthermore, we will use uppercase and lowercase letters to represent dominant and recessive alleles, respectively. Therefore, we would refer to the genotype of a homozygous dominant pea plant with violet flowers as VV, a homozygous recessive pea plant with white flowers as vv, and a heterozygous pea plant with violet flowers as Vv. The Punnett Square Approach for a Monohybrid Cross When fertilization occurs between two true-breeding parents that differ in only one characteristic, the process is called a monohybrid cross, and the resulting offspring are monohybrids. A Punnett square, devised by the British geneticist Reginald Punnett, applies the rules of probability to predict the possible outcomes of a genetic cross or mating and their expected frequencies. Try drawing a Punnett square (explained in the previous lesson titled: Mendel s Experiments and Punnett Squares) with the following characteristics: Vv and VV. What are the phenotypes for the resulting offspring? If you answered all violet flowers, you would be correct! The Test Cross Distinguishes the Dominant Phenotype Beyond predicting the offspring of a cross between known homozygous or heterozygous parents, Mendel also developed a way to determine whether an organism that expressed a dominant trait was a heterozygote or a homozygote. This method is called the test cross. In a test cross, the dominant-expressing organism is crossed with an organism that is homozygous recessive for the same characteristic. If the dominant-expressing organism is a homozygote, then all F1 offspring will be heterozygotes expressing the dominant trait. Alternatively, if the dominant expressing organism is a heterozygote, the F1 offspring will exhibit a 1:1 ratio of heterozygotes and recessive homozygotes. You can try this out by doing a Punnett square in reverse. Place the following offspring into the four boxes of a Punnett square: Vv, vv, vv, Vv. Can you determine from the offspring the genotype of the parents? If you guessed that one parent is Vv and the other is vv, then you would be correct! Mendel didn t know that pea plants with violet flowers had the allele for a recessive white flower until he did a test cross with a recessive plant. Reflect Poll: How Did I Get Blue Eyes? Did you know the gene for blue eyes began as a mutation? After time, the gene became a recessive gene. Do you know anyone that has blue eyes and their parents are both brown-eyed? Yes No Copyright TEL Library 2018 Page 3
Expand: Alternatives to Dominance and Recessiveness Overview According to Mendel s experiments with pea plants, recessive alleles can be carried and not expressed by individuals. Such heterozygous individuals are sometimes referred to as carriers, like our Vv plant example in the previous section. 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. Incomplete Dominance The heterozygote phenotype occasionally appears to be intermediate between the two parents. For example, a cross between a homozygous parent with white flowers and a homozygous parent with red flowers will produce offspring with pink flowers. This pattern of inheritance is described as incomplete dominance, denoting the expression of two contrasting alleles such that the individual displays an intermediate phenotype. The allele for red flowers is incompletely dominant over the allele for white flowers. Codominance A variation on incomplete dominance is codominance, in which both alleles for the same characteristic are simultaneously expressed in the heterozygote. An example of codominance is the MN blood groups of humans. The M and N alleles are expressed in the form of an M or N antigen present on the surface of red blood cells. Homozygotes express either the M or the N allele, and heterozygotes express both alleles equally. In a self-cross between heterozygotes expressing a codominant trait, the three possible offspring genotypes are phenotypically distinct. X-Linked Traits In humans, as well as in many other animals and some plants, the sex of the individual is determined by sex chromosomes. When a gene being examined is present on the X chromosome, but not on the Y chromosome, it is said to be X-linked. When a female parent is homozygous for a recessive X-linked trait, she will pass the trait on to 100 percent of her offspring. Her male offspring are, therefore, destined to express the trait, as they will inherit their father's Y chromosome. In humans, the alleles for certain conditions (some forms of color blindness, hemophilia, and muscular dystrophy) are X-linked. Females who are heterozygous for these diseases are said to be carriers and may not exhibit any phenotypic effects. These females will pass the disease on to half of their sons and will pass carrier status to half of their daughters; therefore, recessive X-linked traits appear more frequently in males than females. Who Robbed Big Petunia s Cafe? In the watch section, you learned about a scenario involving the difference between phenotype and genotype. If you haven t seen this video, take a minute to go back and watch it! The criminal was heterozygous dominant, which means that they have violet flowers. That narrows down the suspects to Ash and Poppy. However, we need to determine which suspect would have the genotype Vv, carrying the recessive white flower gene, but not showing it. You may notice that all of Ash s children have the dominant violet flower phenotype and his wife is recessive, meaning that Ash must be homozygous Copyright TEL Library 2018 Page 4
dominant (VV). Try a Punnett square to check this work. That leaves us with Poppy. Notice that one of her children has the white flower phenotype, which they can only inherit if Poppy were heterozygous dominant! Poppy is the criminal! Lesson Toolbox Additional Resources and Readings An Amoeba Sisters video covering Punnett squares and sex-linked traits Link to resource: https://www.youtube.com/watch?v=h2xufrhwg3e&t=305s An Amoeba Sisters video explaining incomplete dominance and codominance Link to resource: https://www.youtube.com/watch?v=yjhgfbw55l0 A video helping you practice with Punnett squares Link to resource: https://www.youtube.com/watch?v=keioqp1mjre Lesson Glossary alleles : gene variations that arise by mutation and exist at the same relative locations on homologous chromosomes codominance : in a heterozygote, complete and simultaneous expression of both alleles for the same characteristic genotype : underlying genetic makeup, consisting of both physically visible and non-expressed alleles of an organism heterozygous : having two different alleles for a given gene on the homologous chromosome homozygous : having two identical alleles for a given gene on the homologous chromosome incomplete dominance : in a heterozygote, expression of two contrasting alleles such that the individual displays an intermediate phenotype monohybrid : result of a cross between two true-breeding parents that express different traits for only one characteristic phenotype : observable traits expressed by an organism Punnett square : visual representation of a cross between two individuals in which the gametes of each individual are denoted along the top and side of a grid, respectively, and the possible zygotic genotypes are recombined at each box in the grid test cross : cross between a dominant expressing individual with an unknown genotype and a homozygous recessive individual; the offspring phenotypes indicate whether the unknown parent is heterozygous or homozygous for the dominant trait X-linked : gene present on the X, but not the Y, chromosome Copyright TEL Library 2018 Page 5
Check Your Knowledge 1. The observable traits expressed by an organism are described as its. a. phenotype b. genotype c. alleles d. zygote 2. A recessive trait will be observed in individuals that are for that trait. a. heterozygous b. homozygous or heterozygous c. homozygous d. diploids 3. If black and white true-breeding mice are mated and the result is all gray offspring, what inheritance pattern would this be indicative of? a. dominance b. codominance c. multiple alleles d. incomplete dominance Answer Key: 1. A 2. C 3. D Citations Lesson Content: Authored and curated by Jill Carson for The TEL Library. CC BY NC SA 4.0 Adapted Content: Title: 12.2 Characteristics and Traits Phenotypes and Genotypes; The Punnett Square Approach for a Monohybrid Cross. OpenStax CNX. License: CC BY 4.0. Link to resource: https://cnx.org/contents/jvcgr5sl@8.18:d6qqvhfc@5/characteristics-and-traits Copyright TEL Library 2018 Page 6