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BIO-105 Lab 7 PRINCIPLES OF HEREDITY
The field of genetics is bristling with excitement! Complex, gene-splicing techniques have allowed researchers to isolate genes that code for specific proteins and then to use those genes to harvest large amounts of particular proteins and even to cure some dreaded human diseases. At present, growth hormone, insulin, and interferon produced by these genetic engineering techniques are available for clinical use, and the list is growing daily. Comprehending genetics involves a basic understanding and appreciation of how genes regulate our various traits (dimples, hair color, blood types, for example). The goal of this lab is to provide a “genetics sampler” or relatively simple introduction to heredity.
Objectives:
- To define some genetic terms: allele, dominant, recessive, genotype, phenotype, heterozygous, and homozygous.
- To gain practice working simple genetics problems, using a Punnett square.
- To observe selected human phenotypes and determine their genotypes.
Materials:
- PTC (phenylthiocarbamide) taste strips
- Sodium benzoate taste strips
- Chart for tabulation of class results of human phenotype/genotype determination.
- Blood typing supplies o Anti-A and Anti-B sera, slides, toothpicks, sterile lancet, wax pencils, alcohol swabs o Beaker containing 10% bleach solution o Disposable waste bags
Language of Genetics: In humans, all cells, except eggs and sperm, have 46 chromosomes, that is, the diploid number. This number is established when fertilization occurs and the egg and sperm fuse, combining the 23 chromosomes (or haploid number) each is carrying. The diploid chromosomal number is maintained through life in nearly all cells of the body by the precise process of mitosis. The diploid number actually represents two complete (or nearly complete) sets of genetic instructions –one from the egg and the other from the sperm-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 in their influence. For example, the numbers of gene pairs, or alleles, coding for hairline shape on your forehead may specify either a 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 typically only one of the alleles, called the dominant gene , will exert its effects.
- The alleles with less potency, the recessive gene, will be present but masked. 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 to exert their influence.
- An individual’s actual, genetic makeup, that is, whether he is homozygous or heterozygous for the various alleles, is called genotype.
- The physical expression of the genotype (for example, the presence of a widow’s peak or not, blue vs. brown eyes) is referred to as 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 will be to investigate 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 manner Gregor Mendel did in his classic experiments on pea plants. (Mendel, an Austrian monk of the mid-1800s, found evidence in these experiments that each gamete contribute just one allele to each pair on the zygote). To work out the various simple genetic crosses involving one pair of alleles (called a monohybrid cross ), 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 both genotype and phenotype percentages. To illustrate the procedure, an example of one of Mendel’s pea plant crosses is outlined next:
Alleles : T (determines tallness ; dominant) t (determines dwarfness ; recessive)
Genotypes of parents : TT (male) × tt (female) 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 as shown below:
T T T T
t t Tt Tt
Gametes J
t t Tt Tt
Results: Genotypes Æ 100% Tt (all heterozygous) PhenotypesÆ 100% tall (because T, which determines tallness, is dominant and all offspring contain the T allele)
b. Genotype of parents: Ww (males) × ww (female)
% of each genotype in the offspring: _____% WW ______% Ww ______ % ww ____ % of each phenotype in the offspring: _____% straight line ______% widow’s peak
B. Exploring probability Segregation (or parceling out) of chromosomes to daughter cells (gametes) during meiosis and the combination of egg and sperm are random or chance events. So 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 they are identical twins). The Punnett square method also provides information on the probability of the appearance of certain genotypes considering all possible events.
Probability ( P ) is defined as number of specific events or cases total number of events or cases
Gametes J
P =
If an event is certain to happen, its probability is 1. If it happens one out of every 2 times, its probability is ½, or 50%; if one out of 4 times, its probability is ¼, or 25%, and so on.
- Obtain two pennies and perform the following simple experiment to explore the laws of probability. a. Toss one penny into the air 10 times, and record the number of heads/tails observed ______ heads __________tails
Probability as fraction: ______/10 heads; _______/10 tails Probability as percentage: ______% heads; ________% tails
b. Now simultaneously toss two pennies into the air for 30 tosses, and record the results of each toss below. In each case, report the probability in the lowest fractional terms.
#head/head(HH) _________Probability as fraction__________ Probability as % _______
head/tail (HT) _________ Probability as fraction__________ Probability as % _______
tail/tail (TT) _________Probability as fraction__________ Probability as % _______
Does the first toss have any influence on the second? ___________________ Does the third have any influence on the fourth? _______________________
c. Do a Punnett Square using head/tail ( HT) for one coin and HT for the alleles of the other.
How closely do your coin-tossing results correlate with the percentage obtained from the Punnett square results? _________________________________________
- Gender in humans is determined by the presence of certain chromosomes , commonly called the sex chromosomes because they determine the primary sexual characteristics: If there are 2 identical chromosomes, called XX Æ female If there are 2 different chromosomes, called Xy Æ male
a. Determine the probability of having a boy or girl offspring for each conception. Parental genotypes: Xy × XX
Probability of HH : __________ Probability of HT ___________ Probability of TT : __________
Probability of males _________ Probability of females ________
b. 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. But the probability of a tossed penny coming up heads four times in a row is: ½ × ½ × ½ × ½ = 1/16.
Dad wants a boy’s baseball team! What are the chances of having nine sons in a row?
C. 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. We will investigate some of those here, and try to determine the genotype based on your phenotype.
For each of the characteristics described here, determine (as best you can) both your own phenotype and genotype, and record this information in Table 1.
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 ).
Double-jointed thumb: A dominant gene determines a condition of loose ligaments that allows one to throw the thumb out of joint. The homozygous recessive condition determines tight joints. Use J for the dominant allele and j for the recessive allele.
Bent little finger: Examine your little finger on each hand. If its terminal phalanx angles toward the ring finger, you are dominant for this trait. If one or both terminal digits are essentially straight, you are homozygous recessive ( ll ) for the trait. Use L for the dominant allele and 1 for the recessive allele.
Middle digital finger hair: Critically examine the dorsum of the middle segment (phalanx) of fingers 3 and 4. If no hair is obvious, you are recessive ( hh) for this condition. If hair is seen, you have the dominant gene ( H ) for this trait (which, however, is determined by multigene inheritance).
Freckles: Freckles are the result of a dominant gene. Use F as the dominant allele and f as the recessive allele.
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: Inheritance of the ABO blood is based on the existence of three alleles designated as IA, IB , and i. Both I A^ and I B^ are dominant over i, but neither is dominant over the other. Thus the possession of IA^ and IB^ will yield type AB blood, whereas the possession of the IA^ and i alleles will yield type A blood, and so on. There are four ABO blood groups or phenotypes, A, B, AB, and 0, and their correlation to genotype is indicated in the next table:
ABO blood group (phenotype) Genotype A IAIA^ or IAi B IBIB^ or IBi AB IAIB O ii
If you have previously typed your blood, record your phenotype and genotype in Table 1.
If not, type your blood following the instructions with the blood typing kit; then enter your results in Table 1.
IMPORTANT: dispose of any blood-soiled supplies by placing the glassware in the bleach- containing beaker and all other items in the red bag.
Table 1. Record of Human Phenotypes and Genotypes Your Information Class Data Characteristics Phenotypes Genotype Dominant phenotype
Recessive phenotype Interlocking fingers ( I, i) PTC taste ( P, p ) Sodium benzoate taste ( S, s ) Dimples ( D, d ) Tongue rolling ( T, t ) Attached earlobes ( E, e ) Widow’s peak ( W, w ) Double-jointed thumb ( J , j ) Bent little finger ( L, l ) Middigital hair ( H, h ) Freckles ( F, f ) Blaze ( B, b ) ABO blood type ( IA, IB, i ) Sex ( XX or Xy )
Please answer these questions:
- Of these 13 traits (excluding gender), how many dominant phenotypes do you have? ___________ How many recessive phenotypes?______________.
- Once class data have been tabulated, examine the results. a. Is there a single trait that is expressed in an identical manner by all members of the class?
b. Of the 13 traits, which trait (phenotype) is shared by most individuals in the class?
- Why is it easy to determine the genotype of a person with type AB or type O blood?
- Because all human beings have 23 pairs of chromosomes and each pair separates into separate gametes during the process of meiosis, the number of possible combination at segregation is more than 8 million! On the basis of this information, what would you guess are the chances of any two individuals in the class having identical phenotypes for all 14 traits investigated?
