19.1-.2

Mendel’s experiments with pea plants suggested that: (1) two “units” or copies exist for every gene; (2) alleles maintain their integrity in each generation (no blending); and (3) in the presence of the dominant allele, the recessive allele is hidden and makes no contribution to the phenotype. Therefore, 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. In the sections to follow, we consider some of the complexity. Incomplete Dominance Mendel’s results, that traits are inherited based on dominant and recessive alleles of a particular gene, contradicted the view at that time that offspring exhibited a blend of their parents’ traits. However, the heterozygote phenotype sometimes does appear to be intermediate between the two parents. Punnett square showing incomplete dominance In phenotypes that display incomplete dominance, the phenotype of the heterozygote is different from that of the dominant homozygote, and generally intermediate between the two homozygote phenotypes. (Figure by Melissa Hardy is used under a Creative Commons Attribution-NonCommercial license). For example, in the snapdragon, Antirrhinum majus, a cross between a homozygous parent with white flowers (CWCW) and a homozygous parent with red flowers (CRCR) will produce offspring with pink flowers (CRCW). (Note that different genotypic abbreviations are often used for Mendelian extensions to distinguish these patterns from simple dominance and recessiveness.) This pattern of inheritance is described as incomplete dominance. The allele for red flowers is incompletely dominant over the allele for white flowers. The results of a cross can still be predicted and diagrammed using a Punnett Square, just as with Mendelian dominant and recessive crosses. In this case, the genotypic ratio would be 1 CRCR:2 CRCW:1 CWCW, and the phenotypic ratio would be 1:2:1 for red:pink:white. Codominance Sometimes both alleles of a particular gene are expressed in a dominant fashion, meaning both alleles for the same characteristic are simultaneously expressed in the heterozygote. This is called codominance. Some flowering plants display this type of inheritance. For example, Camellia flowers can show patches of pink and white on the same flower. Two Camellia flowers show the difference between incomplete dominance and codominance. The heterozygote shows an intermediate phenotype if the trait displays incomplete dominance (left). If the trait displays codominance, then both phenotypes are simultaneously expressed in the heterozygote (right). (Left: Camellia by Fg2 is in the public domain. Right: Co-dominance in a Camellia cultivar by darwin cruz is used under a Creative Commons Attribution license). Multiple Alleles Individual humans, and all diploid individuals of other species, will have two copies of each gene (both copies might be the same, or there can be one copy each of two different alleles). However, at the population level, there are often more than two different alleles for a given gene. In this case, many combinations of two alleles are observed among the individuals of a population. Note that when many alleles exist for the same gene, the convention is to denote the most common phenotype or genotype among wild organisms of the species as the wild type (often abbreviated “+”); this is considered the standard or norm. Variants may be recessive or dominant to the wild-type allele. An example of multiple alleles is coat color in rabbits. Here, four alleles exist for the C gene. The wild-type phenotype (brown fur) is encoded by the C+ allele, the chinchilla phenotype (black-tipped white fur) is encoded by the cch allele, the Himalayan phenotype (black fur on the extremities and white fur elsewhere) is encoded by the ch allele, and the albino phenotype (white fur) is encoded by the c allele.In cases of multiple alleles, dominance hierarchies can exist. In this case, the wild-type allele is dominant over all the others, chinchilla is dominant over Himalayan and albino, and Himalayan is dominant over albino. Allelic series of coat colors Four different alleles exist for the rabbit coat color (C) gene. (Figure by OpenStax is used under a Creative Commons Attribution license). The complete dominance of a wild-type phenotype over all others often occurs as an effect of “dosage” of a specific gene product, such that the wild-type allele supplies the correct amount of gene product whereas the mutant alleles cannot. For the allelic series in rabbits, the wild-type allele may supply a given dosage of fur pigment, whereas the mutants supply a lesser dosage or none at all. Interestingly, the Himalayan phenotype is the result of an allele that produces a temperature-sensitive gene product that only produces pigment in the cooler extremities of the rabbit’s body. Another example of multiple alleles, as well as codominance, determines the ABO blood type in humans. The A and B phenotypes are based on the presence of the A and B antigens, respectively, which are present on the surface of red blood cells. There are three alleles: IA, IB, and i. Alleles IA and IB are codominant with respect to one another, and both are dominant to i. In humans, as well as in many other animals and some plants, the sex of an individual is determined by sex chromosomes. The sex chromosomes are one pair of non-homologous chromosomes. A sex chromosome can be defined as a chromosome that is present in a different copy number in the cells of a female compared to the cells of a male. The sex chromosomes in humans and many other species are named X chromosomes and Y chromosomes. An autosome is any chromosome that is present in the same number of copies in the cells of a female as in the cells of a male. Humans contain 22 pairs of autosomes (numbered 1 – 22) in addition to a pair of sex chromosomes. Human females generally have two copies of the X chromosome and 0 copies of the Y chromosome; while human males generally have one copy of the X chromosome and one copy of the Y chromosome. Although the Y chromosome contains a small region of similarity to the X chromosome so that they can pair during meiosis, the Y chromosome is much shorter and contains many fewer genes. When a gene is present on the X chromosome, but not on the Y chromosome, it is said to be X-linked. Until now, we have only considered inheritance patterns involving genes located on autosomes. Eye color in Drosophila was one of the first X-linked traits to be identified. Thomas Hunt Morgan mapped this trait to the X chromosome in 1910. Like humans, Drosophila males have an XY chromosome pair, and females are XX. In flies, the wild-type eye color is red ( based on the XR allele) and it is dominant to white eye color (based on the Xr allele). Because of the location of the eye-color gene, reciprocal crosses do not produce the same offspring ratios. Males are said to be hemizygous, because they have only one allele for any X-linked characteristic. Hemizygosity makes the descriptions of dominance and recessiveness irrelevant for XY males. Drosophila males lack a second allele copy on the Y chromosome; that is, their genotype can only be XRY or XrY. In contrast, females have two allele copies of this gene and can be XRXR, XRXr, or XrXr. Punnett squares showing reciprocal cross Reciprocal cross of red- and white-eyed Drosophila. Crossing a red-eyed female with a white-eyed male results in all red-eyed offspring. The reciprocal cross, a white-eyed female with a red-eyed male, results in white-eyed sons and red-eyed daughters. (Drosophila white by Melissa Hardy is used under a Creative Commons Attribution-NonCommercial license. Created with BioRender.com). In some groups of organisms with sex chromosomes, the sex with the non-homologous sex chromosomes is the female rather than the male. This is the case for all birds. In this case, sex-linked traits will be more likely to appear in the female, in which they are hemizygous. Human Sex-linked Disorders Discoveries in fruit fly genetics can be applied to human genetics. 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 will express the trait, as they will inherit their mother’s X chromosome with the recessive allele, and 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 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. Females must inherit recessive X-linked alleles from both of their parents in order to express the trait. Although some Y-linked disorders exist, they are rare. Moreover, they are often associated with infertility in males and are therefore not usually transmitted to subsequent generations.