LAB 6

GENETICS

  

Living organisms exhibit amazing diversity in appearance and activity.  Even members of a single species, which share common characteristics, differ considerably in the expression of those characteristics.  This variability is not haphazard; rather, individuals tend to express unique combinations of those traits observed in their parents.  These traits are determined by units of DNA called genes which are located in linear order on the chromosomes.  Genetics is the study of how genes are expressed to produce specific characteristics and how genes are transmitted from one generation to the next.

 

Genetic information is transferred from parents to offspring by sexual reproduction.  This process involves the formation of gametes, the egg and sperm, and their union in fertilization.  The cells of the human body which form gametes contain twenty‑three pairs of chromosomes.  Cells which contain pairs of chromosomes are termed diploid cells.  The members of each chromosome pair are similar in appearance.  They contain genes for the same traits and are referred to as homologues or homologous chromosomes.  The genes which occupy identical positions on the homologues are called alleles.

 

During meiosis the homologues separate producing gametes containing twenty-three single or unpaired chromosomes.  Since the egg and sperm contain one-half the chromosome number of diploid cells, they are said to be haploid cells.  The separation of the members of allelic pairs, which occurs during meiosis, is called segregation.  Due to segregation, the egg and sperm contain one allele for each trait.

 

The union of the sperm and egg re-establishes the diploid condition in a single cell, the zygote.  The zygote divides mitotically forming daughter cells which also undergo mitosis. Continued division and differentiation of the daughter cells results in the trillions of cells which comprise the human adult.

 

The diploid cells of the individual contain one pair of alleles for each characteristic.  If the two alleles for a particular trait are similar in their expression, (AA, aa) the individual is said to be a homozygote or homozygous for that trait.  If the members of an allelic pair are different in expression, (Aa) the individual is said to be a heterozygote or heterozygous for the trait.  The pair of alleles for a specific characteristic is termed the genotype and is represented by letter symbols.  The appearance of the individual, due to the expression of those alleles, is their phenotype.  The phenotype consists of a phrase describing the characteristic such as blue-eyed or brown-haired.

 

In many allelic pairs, one member masks or hides the expression of its alternative and is termed a dominant allele. The allele which is masked or hidden is called a recessive allele.  Capital letters are used to represent dominant alleles and small letters to represent recessive alleles.  Individuals that are homozygous (AA) or heterozygous (Aa) for a dominant allele express the dominant phenotype; whereas, only those individuals homozygous (aa) for a recessive allele express the recessive phenotype.

 

In some pairs of alleles, neither member is dominant, but the heterozygote exhibits a distinct phenotype different from that of either homozygote.  If the phenotype of the heterozygote is a combination of both alleles, the alleles are said to be co-dominant.  The alleles that produce the A and B blood types are co-dominant, resulting in the AB blood type in heterozygous individuals.  Incomplete dominance occurs when the phenotype of the heterozygote is intermediate between those of the two homozygotes.  In snapdragons, white flowers occur in one homozygote (r¹r¹) and red flowers in the other homozygote (r²r²).  Heterozygous plants (r¹r²) produce pink flowers.

 

The twenty-three pairs of chromosomes found in human diploid cells consist of two groups.  The twenty-two pair that do not determine sex are called autosomes.  The remaining pair of chromosomes are called allosomes. These allosome chromosomes determine gender and consist of a pair of X chromosomes in the female and an X and a Y chromosome in the male.  The genes located on the X chromosome are called sex-linked genes or alleles.  These alleles are represented twice in the diploid cells of the female (X^AX^a), but only once in those of the male (X^aY).  Hemophilia and red‑green color blindness result from sex-linked recessive alleles.  These traits occur much more frequently in males than females.  This is because the recessive allele is rare in the population, and the female must be homozygous (X^aX^a) to be affected.  A heterozygous female (X^AX^a) acts as a carrier for sex-linked recessive alleles.  Though not affected, she will pass the defective allele to about one-half of her sons (X^aY) who will be affected.

 

Question 1:         What are genes? 

 

 

Question 2:         Define genetics. 

 

 

Question 3:         What are diploid cells?

Question 4:         What is the diploid number of chromosomes in man? 

 

 

Question 5:         What are homologs? 

 

 

Question 6:         What are alleles? 

 

 

Question 7:         Which cells in the human body are haploid?  

 

 

Question 8:         What is meant by "segregation" of alleles? 

 

 

Question 9:         Explain the difference between a homozygote and a heterozygote. 

 

 

Question 10:       Distinguish between the genotype and the phenotype of an individual. 

 

 

Question 11:       What are dominant alleles?

 

 

Question 12:       What are recessive alleles? 

Question 13:       Explain how co-dominant and incompletely dominant alleles can be distinguished from dominant alleles. 

 

 

Question 14:       Explain the difference between co-dominant and incompletely dominant alleles. 

 

 

Question 15:       What are autosomes? 

 

 

Question 16:       What are sex-linked genes? 

 

 

Question 17:       Why do sex-linked recessive traits appear more frequently in males than females?

 

 

Question 18:       How does a heterozygous female act as a carrier for sex-linked recessive traits?

 

 

 

LAB OBJECTIVE:

 

To gain an understanding of the basic principles of heredity and make application of those principles in the solution of selected genetics problems.

 

To gain practice in using the vocabulary associated with principles of heredity and genetics.

I.  Dominant and Recessive Traits in Humans

 

PROCEDURE:

 

Read the information describing each of the traits below.  Determine your phenotype and genotype, if possible, for each. Record them in the appropriate area of Table 1.  If your phenotype is dominant and one of your parents is recessive for that trait, then you must be heterozygous for the trait.  If you are unsure whether you are homozygous (AA) or heterozygous (Aa), represent your genotype with a capital letter followed by a dash (A_).

 

A.      Attached Ear Lobes:

The allele for free hanging ear lobes (E) is dominant to its recessive alternative (e), which produces adherent or attached ear lobes.

 

B.      Tongue Rolling: 

People with a dominant allele (R) have the ability to roll the tongue into a U‑shape when the tongue is extended from the mouth.  Those with the recessive genotype are unable to roll their tongue.

 

C.      Widow's Peak:

In most individual's, the hairline forms  a point in the center of the forehead.  This is termed a widow's peak.  This condition is due to a dominant allele (P).  Individuals homozygous for the recessive allele (p) do not express the trait.

 

D.      PTC Taster:

The ability to taste a specific chemical, phenylthio-carbomide, is due to a dominant allele (T).  People with the recessive genotype (tt) are unable to detect the chemical.

 

Obtain a piece of PTC paper from your lab instructor.  Place it on your tongue for about thirty seconds and determine your phenotype.  If you are uncertain as to whether you can taste the chemical, you are a non-taster.

 

E.      Pigmented Iris:

Eye color is determined by the amount of pigment present in the iris.  People who are homozygous for a recessive allele (bb) have blue eyes which lack pigment.  The dominant allele (B) causes pigment to be present in the iris.  Other genes determine the amount and type of pigment present.  Actual color may vary from light blue to dark brown.

F.      Mid-digital Hair:

The presence of hair on the middle joint of one or more fingers is due to a dominant allele (M).  People with the recessive genotype (mm) lack hair on the second joint of all fingers.

 

G.      Hitch Hiker's Thumb:

Hitch hiker's thumb (distal hyper-extensibility) is the ability to bend the thumb backward so that the terminal joint almost forms a 45^o angle with the joint below it.  This trait is due to a recessive allele (h) in the homozygous condition.  People with one or more dominant alleles (HH or Hh) will lack this ability.  Be sure to check both thumbs.

 

H.      Bent Little Finger:

A dominant allele (B) causes the terminal joint of the little finger to bend inward toward the ring finger.  In individuals with the recessive genotype, the last joint of the little finger is straight.

 

I.        Hair Color: 

1.       The allele for dark hair (brown or black) is dominant to the allele for blond hair (d). 

2.       The allele for red hair (r) is recessive to the allele for hair that is non-red (R).

 

J.       A-B-O  Blood Types:

 

The A-B-O blood types are differentiated according to the presence or absence of the A and B antigens on the red blood cells.  Type A contains the A antigen,

and type B contains the B antigen.  Type AB contains both antigens, while type O contains neither.

 

These antigens are determined by a group of three multiple alleles.  Multiple alleles      occur when there are more than two contrasting forms of a gene that

determine a particular trait.  The allele for the A antigen and the B antigen are co-dominant to the allele for no antigen or O type blood.  The genotypes and

phenotypes are as follows:

 

Blood type (phenotype)                Genotype

 

O                                  O

 

A                                   AA or AO

 

B                                   BB or BO

 

AB                                ^AB

 

Note the A and B phenotypes may be either homozygous or heterozygous.  If you have one of these phenotypes and either of your parents have the O

phenotype, you must be heterozygous for A or B type blood.

 

The blood types are detected by mixing drops of blood with sera containing the A antibody and the B antibody.  Clumping of the red blood cells with the A

antibody but not the B antibody indicates A type blood.  Clumping with the B antibody only indicates B type blood.  Clumping with both antibodies occurs with

AB type blood, and no clumping occurs with O type blood. 

 

Table 1 (Some Inherited Human Characteristics) 

                                                                              

 

 

Question 19:       How can an individual know if they are heterozygous  or homozygous for a dominant trait? 

 

 

Question 20:       What are multiple alleles?

II.  Segregation in Monohybrid Crosses

 

Monohybrid crosses are those in which the parents differ with respect to a single pair of alleles.  The Punnett square commonly used to determine the different genotypes and phenotypes which may result from monohybrid crosses.  Consider a man and woman, both heterozygous (Aa) for a dominant trait. Each can produce two different types of gametes ( A  or  a ).  If a Punnett square with four divisions is constructed and the possible gametes of the parents are placed at the top and to the left of each division, the proportion of genotypes and phenotypes obtained during fertilization can be determined as follows:

 

Genotype of parents:                    (female) Aa         X               Aa (male)

 

Meiosis (segregation)

 

Types of gametes:                      

STUDENTS, WORKING IN PAIRS, WILL ATTEMPT TO REPRODUCE THE ABOVE RATIOS BY THE FOLLOWING SIMPLE EXPERIMENT:

 

PROCEDURE:

 

A.      Each pair should obtain a paper cup containing an equal number of blue and green toothpicks.  The blue toothpick will represent the dominant allele (E) for free ear lobes, and the green toothpick will represent the recessive alternative (e) for attached ear lobes.

 

B.      Without looking into the cup, each student will draw one toothpick from the cup.  Record the alleles represented by the colored toothpicks in Table 2 under the columns labeled first and second gametes.  Record the genotype (EE, Ee, or ee) and phenotype

(free or attached) resulting from the combination of the two gametes.

C.      Return the toothpicks to the cup.  Mix them and select two more toothpicks.  Record the results in Table 2.

 

D.      Repeat this procedure until Table 2 is complete.

 

E.      Tabulate the genotypes and phenotypes obtained and record the numbers in Table 3.

 

F.      Determine the genotypic and phenotypic ratios by dividing the number in the smallest group into each group.  Record the genotypic and phenotypic ratio in Table 3.  The instructor may summarize the results from all groups on the board. 

 

Question 21:       What are monohybrid crosses? 

 

Question 22:       What are the expected genotypic and phenotypic ratios from a monohybrid cross involving individuals that are heterozygous for a dominant allele?

 

                                                     

                                                                          Remember:  Blue = E    Green = e

                                                      

 

 

 

III.  Independent Assortment in Dihybrid Crosses

 

Matings in which two pair of alleles are considered are termed dihybrid crosses.  During meiosis, the members of one pair of alleles segregate independently of those of the other pair. Thus, the gametes contain random combinations of representatives from both pairs.  This random segregation of members of different allelic pairs is called independent assortment.  Consider a man and a woman both heterozygous (AaBb) for two dominant traits. Each can produce four different types of gametes ( AB ,  Ab ,  aB , and  ab ).  The possible genotypes and phenotypes of their children can be determined as follows:

 

Genotypes of Parents:                 (female) AaBb     X     AaBb (male)

 

Meiosis (segregation)

 

Types of Gametes:                       AB  Ab  aB  or  ab      AB  Ab  aB  or  ab

 

Fertilization:                                                 Male Gametes

 

                                                        AB           Ab          aB          ab

 

                                        AB          AABB     AABb     AaBB     AaBb

 

Female         Ab        AABb      AAbb     AaBb     Aabb

                   Gametes

           aB           AaBB      AaBb     aaBB     aaBb

 

                                        ab           AaBb      Aabb      aaBb     aabb

 

 

Genotypic ratio:                            1 AABB: 2 AABb: 1 AAbb: 2 AaBB: 4 AaBb: 2 Aabb:

1 aaBB: 2 aaBb: 1 aabb                               

Phenotypic ratio:                          9 dominant for both traits: 3 dominant for

                           first trait and recessive for second trait:

3 recessive for first trait and dominant for

second trait: 1 recessive for both traits

Students, working in pairs, will test the principle of independent assortment by performing the following experiment: 

 

PROCEDURE:

 

A.      Each pair of students should obtain two paper cups containing colored toothpicks.  One cup will contain an equal number of blue and green toothpicks. These toothpicks will represent the same alleles (E and e) as in II.  The second cup will contain an equal number of red and yellow toothpicks.  The red toothpick will represent the dominant allele (T) for tasting PTC, and the yellow ones will represent the recessive alternative (t) for nontasters.

 

B.      Without looking at the contents, draw one toothpick from each cup.  Record the combination of alleles (ET, Et, eT or et), represented by the toothpicks, in the first column of Table 4.  Return the tooth‑picks to the appropriate cups and mix them.  Repeat the process and record the combination of alleles in the second column of Table 4.

 

C.      Determine the genotype and phenotype, resulting from the combination of the two gametes.  Record the information in Table 4.

 

D.      Repeat B and C until Table 4 is complete.

 

E.      Tabulate the number of each genotype and phenotype in Table 5.

 

F.      Calculate the genotypic and phenotypic ratios, as in II, and record them in Table 5.  The instructor will summarize the data for all groups on the board.

Table 4 (Dihybrid Cross)

 

First Gamete	Second Gamete	Geontype of          Offspring	Phenotype of          Offspring
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.