Principles of Genetics Biology 204 Marilyn M. Shannon, M.A.

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1 Principles of Genetics Biology 204 Marilyn M. Shannon, M.A. Introduction Nature versus nurture is a topic often informally discussed. Are world-class musicians that good because they inherited the right genes, or is it only because of excellent training and diligent practice? In most cases it s a combination of genetic inheritance ( nature ) together with lifestyle practices ( nurture ). But not always. Blue eyes or brown eyes, naturally curly hair or straight hair have little to do with lifestyle and everything to do with inheritance. Actually, many characteristics that are inherited are simply too complex to easily categorize. For example, you may have a higher or lower risk than average for a particular disease, but so many genes affect that risk that only generalities may be made. The term polygenic is used to refer to traits that are determined by many different genes. Many things about your body are determined in a complex way by polygenic inheritance your height, your body build, your skin color, even your tendency to put on weight or not. Again, lifestyle choices diet, exercise, sleep, stress management, and so forth-- also impact many complexly inherited characteristics. On the other hand, certain normal and abnormal traits (characteristics) are inherited in a simple enough way that we can use them to learn basic principles of genetics. Objectives Our goal is to gain understanding of (1) inheritance of male or female sex; (2) dominant versus recessive gene inheritance; (3) co-dominance; (4) multiple genes for the same trait; and (5) sex-linked traits. Suggested reading: Genetics, pages Review The nucleus of human cells houses the genetic material, DNA, which contains the genetic code. DNA strands are composed of a specific sequence of subunits called nucleotides which code for all the amino acids of all the proteins that any cell could make. The chromosomes are the DNA molecules wrapped compactly around protein molecules (Fig. 3.14, p. 54). Human Chromosomes Ordinary human cells have 46 chromosomes, half (23) inherited from the mother and half from the father. However, the 23 chromosomes that a person received from the mother at fertilization are not identical to the 23 that he or she received from the father. They do contain genes that code for the same traits, but the specific details of that trait may vary for example, brown eyes versus blue eyes, dimples versus no dimples, and so forth. For convenience, the first 22 chromosomes are numbered from 1 to 22 and the similar pairs from each parent are called homologous chromosomes. 1

2 Chromosomes 1-22 are known as autologous chromosomes. The last pair are the sex chromosomes (Fig , p. 579). If they are both similar, they are a pair of the X chromosomes, and the embryo will develop as a female. If one is an X chromosome and the other is a very small chromosome called Y, the embryo will develop as a male. 2 Meiosis It s necessary for the male and female gametes, the sperm and ovum, to be haploid cells; that is, with only one set of 23 chromosomes. Why? When sperm meets ovum at fertilization, the newly conceived individual, called a zygote, receives 23 chromosomes from the mother and 23 chromosomes from the father, and now has the diploid number of chromosomes 23 pairs (46 total chromosomes). To become haploid cells with only 23 chromosome, the original diploid cells, spermatogonium and and oogonium, undergo a special cell division called meiosis to generate the sperm and ovum. Each sperm and each ovum ends up with only 23 single chromosomes. These chromosomes are randomly assorted from their own male and female parents genetic material. The process will be taught in lecture. Mitosis Do not confuse meiosis with mitosis, which is the ordinary cell division that occurs during development and throughout life. During mitosis, cells which already have 23 pairs of chromosomes, the diploid number of the human species, divide in such a way that the two resulting cells are also diploid. (The exceptions, of course, are the male and female gametes.) Determination of Sex When the sex chromosomes are both X (XX), the offspring will develop as a female. If the sex chromosomes are XY, the offspring develops as a male (Fig , p. 580). Worksheet Item 1: Make a Punnett square to determine the probability of a couple having boy versus girl children. Dominant Versus Recessive Trait Inheritance The genotype is the record of the genes that the person has for a particular trait. The phenotype is the physical expression of the genotype that which you can observe. Different genotypes can give the same phenotype due to the masking effect of one of a gene pair over another. As a simplified example, you may inherit a gene for blue eyes from one parent and a gene for brown eyes on the homologous (similar) chromosome of the other parent. These different possibilities for the same trait are called alleles, and they actually are found at the same location (locus) on the pair of homologous chromosomes.

3 If the person has an identical pair of genes for the same trait; for example, for brown eyes, he or she is said to be homozygous for the brown-eye trait. If he or she has both genes at that locus for blue eyes, he or she is said to be homozygous for the blue-eye trait. If he or she has inherited one gene for blue eyes and one gene for brown eyes, he or she is heterozygous for eye color. Often the presence of one allele masks the presence of another. If so, the one that is expressed and masks the other is said to be dominant, and the masked trait is recessive. The recessive trait will only be expressed in the phenotype if the person is homozygous for the recessive trait that is, he or she has inherited copies of the recessive trait on both homologous chromosomes. Several genetic errors that cause physical problems or diseases are recessive traits. Albinism is an example of a recessive trait. It is caused by the inherited inability to make the pigment melanin in the skin, eyes, and hair. This inability to make melanin is due to an error in the gene that codes for the enzyme which makes melanin. Without the correct genetic information, the enzyme is nonfunctional. Recall that genes are the segments of DNA that code for one protein, and that all enzymes are proteins (Fig.3.22, p. 60). If a person has the normal allele for normal melanin, it is dominant over the allele for albinism. Why? One gene for making the required enzyme is enough for cells that produce melanin. To have the albinism phenotype, the individual must inherit two recessive genes for albinism, one from each parent (Fig , p. 581). By convention the dominant allele is given a capital letter, such as A. The recessive allele is given the lower-case letter, a. The heterozygous person, who does not have the albino phenotype (appearance), is called a carrier. He or she would have the genotype Aa. He or she carries one recessive gene, which is not expressed. Worksheet Item 2: Try several Punnet squares with parents with different genotypes to find out the probability of their offspring having the trait, carrying the trait, or neither having nor carrying the trait. AA x AA AA x aa AA x Aa Aa x Aa Aa x aa aa x aa Phenylketonuria and cystic fibrosis are two rare, serious, but treatable genetic disorders that are inherited in the same way that albinism is, as recessive traits. Phenylketonuria is the result of an inheritable genetic mutation that makes a particular liver enzyme nonfunctional. Cystic fibrosis is the result of a genetic mutation in a channel protein that normally transports chloride ion out of cells. (See Clinical Impact: Cystic Fibrosis, p. 51) Again, since genes code for proteins, the genetic errors result in poor- or nonfunctioning proteins. But one normal gene is enough for a person to be be free of the disease, though a carrier. 3

4 4 Worksheet Item 3: For phenylketonuria, make a Punnett square that shows how two parents without phenylkeonuria can have a child that has the disease, and the probability that such parents will have a child with the disease. Use P for the normal allele, and p for the abnormal allele. For cystic fibrosis, make a Punnett square showing one parent who does not have the disease but is a carrier, and one parent that is a normal non-carrier. What is the probability that they will have children who are non-carriers, carriers, or who have the disease themselves? Use C for the normal gene and c for the gene that carries this genetic defect. Co-dominance In some cases, both alleles are expressed in the phenotype; that is, one allele does not dominate the other. The A and B of blood types are a good example. A and B stand for particular antigens on the surface of red blood cells. If a person inherits the gene for A from one parent and the gene for B from the other, his red blood cells will have both A and B antigens, and he will have AB blood (Fig , p. 309). Worksheet Item 4: Make a Punnett square with two parents with AB blood. What is the probability that their children will have A, B, or AB blood? Multiple Alleles At one locus, a person can only have two alleles, one inherited from each parent on the homologous chromosomes. But in the human population, there may be more than two possible alleles for that trait. ABO blood grouping is an example of three possible alleles for a locus. Moreover, even though A and B are co-dominant, both A and B are each dominant over O. Worksheet Item 5: What is the phenotype for the following? AA, AB, BB, AO, BO, OO Make a Punnett square for two parents, one AO and the other BO. What are the phenotypes that are possible in their children? What is the probability of each phenotype?

5 Sex-linked Inheritance You may be aware that red-green color-blindness is more common in males than in females. This is true because it is a sex-linked trait. Like most sex-linked traits, it is recessive and is on the X chromosome. The allele for normal vision, C, dominates the allele for color-blindness, c. Therefore a female, with two X chromosomes, must inherit alleles for color-blindness from both parents. In the male, the small Y chromosome does not have a matching locus for the allele. Thus, in the male, if c, the recessive color-blindness allele, is carried on the X chromosome, it will be expressed in the phenotype of color-blindness, since there is no other X chromosome to mask it. Worksheet Item 6: Using a Punnett square, try the following matings. Find the probability of children of each sex having color-blindness, being carriers, or being noncarriers with normal vision. 5 XCXC x XCY XCXC x XCY XCXC x XCY XCXC x XcY XCXC x XCY XCXC x XcY Classic hemophilia is also a sex-linked, recessive trait (Fig , p. 581). Are there any genes on the Y chromosome? Yes, the genes that express male development are on the Y chromosome. Polygenic Inheritance As stated in the introduction, many traits involve the expression of several different genes on different chromosomes. Skin color (Fig , p. 582) is one example. Just for Fun Some common genetic traits are thought to be simply inherited as dominant or recessive though like so many other traits, be aware that these may actually be polygenic.. Refer to the photographs of these traits provided in lab. Dominant Dimples Widow s peak Straight thumb Unattached ear lobes Recessive no dimples straight hairline curved thumb attached ear lobes See if your classmates demonstrate these traits.