To successfully complete this assignment, first read the following exercise from the Laboratory Manual: Exercise 45 Principles of Heredity System. Student Discussion Assignment What is the difference

Introduction to the Language of Genetics

In humans all cells, except eggs and sperm, contain 46 chromosomes, that is, the diploid number. The diploid chromosomal number actually represents two complete (or nearly complete) sets of genetic instructions—one from the mother and the other from the father—or 23 pairs of homologous chromosomes.

Genes coding for the same traits on each pair of homologous chromosomes are called alleles. The alleles may be identical or different. For example, the pair of alleles coding for hairline shape on your forehead may specify either straight across or widow’s peak. When both alleles in a homologous chromosome pair have the same expression, the individual is homozygous for that trait. When the alleles differ in their expression, the individual is heterozygous for the given trait; and often only one of the alleles, called the dominant gene, exerts its effects. The allele with less potency, the recessive gene, is present but suppressed. Whereas dominant genes, or alleles, exert their effects in both homozygous and heterozygous conditions, as a rule recessive alleles must be present in double dose (homozygosity) to exert their influence.

An individual’s actual genetic makeup, that is, whether the person is homozygous or heterozygous for the various alleles, is called the genotype. The expression of the genotype, for example, the presence or absence of a widow’s peak (Figure 45.2) is referred to as a phenotype.

The complete story of heredity is much more complex than just outlined, and in actuality the expression of many traits (for example, eye color) is determined by the interaction of many allele pairs. However, our emphasis here will be to investigate only the less complex aspects of genetics. Dominant-Recessive Inheritance

One of the best ways to master the terminology and learn the principles of heredity is to work out the solutions to some genetic crosses in much the same way Gregor Mendel did in his classic experiments on pea plants.

To work out the various simple monohybrid (one pair of alleles) crosses in this exercise, you will be given the genotype of the parents. You will then determine the possible genotypes of their offspring by using a grid called the Punnett square, and you will record the percentages of both genotype and phenotype. To illustrate the procedure, an example of one of Mendel’s pea plant crosses is outlined next.

Alleles: T (determines tallness; dominant)

    t (determines dwarfism; recessive)

Genotypes of parents: TT (  ) × tt (  )

Phenotypes of parents: Tall × dwarf

To use the Punnett, or checkerboard, square, write the alleles (actually gametes) of one parent across the top and the gametes of the other parent down the left side. Then combine the gametes across and down to achieve all possible combinations (possible genotypes of their offspring), as follows:

Results: Genotypes 100% Tt (all heterozygous) Phenotypes 100% tall (because T, which determines tallness, is dominant, and all contain the T allele) Incomplete Dominance

The concepts of dominance and recessiveness are somewhat arbitrary and artificial in some instances because so-called dominant genes may be expressed differently in homozygous and heterozygous individuals. This produces a condition called incomplete dominance, or intermediate inheritance. In such cases, both alleles express themselves in the offspring. The crosses are worked out in the same manner as indicated previously, but heterozygous offspring exhibit a phenotype intermediate between that of the homozygous individuals. Some examples follow. Sex-Linked Inheritance

A cell’s chromosomes can be stained, photographed, and digitally rearranged to produce an image called a karyotype, which shows the complete human diploid chromosomal complement displayed in homologous pairs (Figure 45.1).

Of the 23 pairs of homologous chromosomes, 22 pairs are referred to as autosomes. The autosomes guide the expression of most body traits. The 23rd pair, the sex chromosomes, determine the sex of an individual, that is, whether an individual will be male or female. Normal females possess two sex chromosomes that look alike, the X chromosomes. Males possess two dissimilar sex chromosomes, referred to as X and Y. Possession of the Y chromosome determines maleness. The Y sex chromosome is only about a third the size of the X sex chromosome, and it lacks many of the genes that are found on the X.

Inherited traits determined by genes on the sex chromosomes are said to be sex-linked, and genes present only on the X sex chromosome are said to be X-linked. Some examples of X-linked genes include those that determine normal color vision (or, conversely, color blindness), and normal clotting ability (as opposed to hemophilia). The alleles that determine color blindness and hemophilia are recessive alleles. In females, both X chromosomes must carry the recessive alleles for a woman to express either of these conditions, and thus they tend to be infrequently seen. However, should a male receive an X-linked recessive allele for these conditions, he will exhibit the recessive phenotype because his Y chromosome does not contain alleles for that gene.

Figure 45.1 Karyotype (chromosomal complement) of human male.

Each pair of homologous chromosomes is numbered except the sex chromosomes, which are identified by their letters, X and Y.

The critical point to understand about X-linked inheritance is the absence of male to male (that is, father to son) transmission of sex-linked genes. The X of the father will pass to each of his daughters but to none of his sons. Males always inherit sex-linked conditions from their mothers via the X chromosome. Probability

Parceling out of chromosomes to gametes during meiosis and the combination of egg and sperm are random events. Hence, the possibility that certain genomes will arise and be expressed is based on the laws of probability. The randomness of gene recombination from each parent determines individual uniqueness and explains why siblings, however similar, never have totally corresponding traits (unless, of course, they are identical twins). The Punnett square method that you have been using to work out the genetics problems actually provides information on the probability of the appearance of certain genotypes considering all possible events. Probability (P) is defined as:

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If an event is certain to happen, its probability is 1. If it happens one out of every two times, its probability is ½; if one out of four times, its probability is ¼, and so on.

When figuring the probability of separate events occurring together (or consecutively), the probability of each event must be multiplied together to get the final probability figure. For example, the probability of a penny coming up “heads” in each toss is ½ (because it has two sides—heads and tails). But the probability of a tossed penny coming up heads four times in a row is ½ × ½ × ½ × ½ × 1∕16. Genetic Determination of Selected Human Characteristics

Most human traits are determined by multiple alleles or the interaction of several gene pairs. However, a few visible human traits or phenotypes can be traced to a single gene pair. It is some of these that will be investigated here in our “genetics sampler.”

Activity 5 Using Phenotype to Determine Genotype

For each of the characteristics described here, determine (as best you can) both your own phenotype and genotype, and record this information on the Activity 5 chart. Since it is difficult to know if you are homozygous or heterozygous for a nondetrimental trait when you exhibit its dominant expression, you are to record your genotype as A—(or B—, and so on, depending on the letter used to indicate the alleles) in such cases. However, if you exhibit the recessive trait, you are homozygous for the recessive allele and should record it accordingly as aa (bb, cc, and so on). When you have completed your observations, also record your data in the chart on the board for tabulation of class results.

Activity 5: Record of Human Genotypes/Phenotypes

Characteristic Phenotype Genotype

PTC taste (P,p)    

Sodium benzoate taste (S,s)    

Sex (X,Y)    

Dimples (D,d)    

Widow’s peak (W,w)    

Proximal finger hair (H,h)    

Freckles (F,f)    

Blaze (B,b)    

ABO blood type (IA,IB,i)    

PTC taste: Obtain a PTC taste strip. PTC, or phenylthiocarbamide, is a harmless chemical that some people can taste and others find tasteless. Chew the strip. If it tastes slightly bitter, you are a “taster” and possess the dominant gene (P) for this trait. If you cannot taste anything, you are a nontaster and are homozygous recessive (pp) for the trait. Approximately 70% of the people in the United States are tasters.

Sodium benzoate taste: Obtain a sodium benzoate taste strip and chew it. A different pair of alleles (from that determining PTC taste) determines the ability to taste sodium benzoate. If you can taste it, you have at least one of the dominant alleles (S). If not, you are homozygous recessive (ss) for the trait. Also record whether sodium benzoate tastes salty, bitter, or sweet to you (if a taster). Even though PTC and sodium benzoate taste are inherited independently, they interact to determine a person’s taste sensations. Individuals who find PTC bitter and sodium benzoate salty tend to like sauerkraut, buttermilk, spinach, and other slightly bitter or salty foods.

Sex: The genotype XX usually determines the female phenotype, whereas XY usually determines the male phenotype.

Dimpled cheeks: The presence of dimples in one or both cheeks is due to a dominant gene (D). Absence of dimples indicates the homozygous recessive condition (dd).

Widow’s peak: A distinct downward V-shaped hairline at the middle of the forehead is referred to as a “widow’s peak”. It is determined by a dominant allele (W), whereas the straight or continuous forehead hairline is determined by the homozygous recessive condition (ww) (Figure 45.2).

Figure 45.2 Selected examples of human phenotypes.

Proximal finger hair: Critically examine the dorsum of the proximal phalanx of fingers 3 and 4. If no hair is obvious, you are recessive (hh) for this condition. If hair is visible, you have the dominant gene (H) for this trait (which, however, is determined by multigene inheritance) (Figure 45.2).

Freckles: Freckles are the result of a dominant gene. Use F as the dominant allele and f as the recessive allele (Figure 45.2).

Blaze: A lock of hair different in color from the rest of scalp hair is called a blaze; it is determined by a dominant gene. Use B for the dominant gene and b for the recessive gene.

Blood type: Some genes exhibit more than two allele forms, leading to a phenomenon called multiple-allele inheritance. Inheritance of the ABO blood type is based on the existence of three alleles designated as I A, I B, and i. Both I A and I B are dominant over i, but neither is dominant over the other. The I A and I B alleles are codominant. Thus the possession of I A and I B will yield type AB blood, whereas the possession of the I A and i alleles will yield type A blood, and so on (as explained in Exercise 29). The four ABO blood groups, or phenotypes, are A, B, AB, and O.   Their correlation to genotype is indicated in Table  45.1 .

If you have previously typed your blood, record your phenotype and genotype in the Activity 5 chart. If not, type your blood following your instructor’s instructions (see Exercise 29, pp. 436–438), and then enter your results in the table.

Dispose of any blood-soiled supplies by placing the glassware in the bleach-containing beaker and all other items in the autoclave bag.

Once class data have been tabulated, scrutinize the results. Is there a single trait that is expressed in an identical manner by all members of the class?

Table 45.1 Blood Groups

ABO blood group (phenotype) Genotype

A I A I A or I A i

B I B I B or I B i

AB I A I B

O iiHemoglobin Phenotype Identification Using Agarose Gel Electrophoresis

Agarose gel electrophoresis separates molecules based on size and charge. In the appropriate buffer with an alkaline pH, hemoglobin molecules will move toward the anode of the apparatus at different speeds, based on the number of negative charges on the molecules.

Sickle cell anemia and sickle cell trait are discussed in Activity 2 of this exercise. The beta chains of hemoglobin S (HbS) contain a base substitution where a valine replaces glutamic acid. As a result of the substitution, HbS has fewer negative charges than the predominant form of adult hemoglobin (HbA) and can be separated from HbA using agarose gel electrophoresis.