Human Genetics - What will your childrenbe?

The purpose of this exercise is to give you some practiceworking with the major concepts of genetics using specific exampleof inherited traits present in humans. During this lab, we willobserve patterns of inheritance for three general types of traits:autosomal traits controlled by two alleles (hair color, Rh factor,PTC tasting), an autosomal trait controlled by multiple alleles(blood type), and a sex-linked trait (color-blindness). Before webegin this exercise, we should introduce or review some of theterminology used when discussing genetics.

All of an organism's traits, whether they are visible traitssuch as hair and eye color or non-visible traits such as bloodtype, are controlled by genes. A gene is a specific regionon a strand of DNA (deoxyribonucleic acid) that controls aspecific trait. The 46 long strands of DNA in human cells, eachcontaining many genes, are wound into chromosomes whichbecome visible during mitosis (remember Ex.7). Most human cells arediploid, meaning they contain two sets of geneticinformation. Diploid human cells contain 23 pairs of chromosomes(46 chromosomes total). A pair of these chromosomes are referred toas homologous chromosomes because both chromosomes havegenes that code for the same traits. One of the chromosomes in eachpair was inherited from the mother and the other from thefather.

While diploid cells have two copies of each gene, the copies maynot be identical. For example, each of our cells has two genes thatcontrol hair color, but one gene may code for light hair and onemay code for dark hair. The different versions of a gene are calledalleles. The different alleles of a gene are often notexpressed equally. For each trait, one allele is usuallydominant and the other is recessive. A dominantallele is expressed whenever it is present, even if only one copyis present.
A recessive allele must be present in two copies to beexpressed.

When we write out a genetics problem, we often use letters todenote the different alleles for a particular trait. An uppercaseletter denotes a dominant allele, and a lowercase letter denotes arecessive allele. As we will see later, other notations may also beused to indicate which alleles are present. If a person has twocopies of the same allele (AA or aa), they are consideredhomozygous for that trait. If they possess two differentalleles (Aa), they are considered heterozygous for thattrait. When we refer to the actual genetic makeup of an individual,the alleles that are present, this is called the genotype.If we refer to the outward appearance of the individual, or whattrait is expressed, then this is referred to as thephenotype of that individual.

Next, we will consider the specific traits that we will beworking with in lab today.
Autosomal traits controlled by two alleles: Genesfor these traits are carried on two homologous autosomal (non-sex)chromosomes. There are only two alleles or versions for each genepresent in the population.

Ability to taste PTC: PTC (phenylthiocarbimide) is a chemicalthat tastes bitter to some people and is tasteless to others. Theability to taste PTC is an inherited trait, and the ability totaste PTC is dominant (T) over the non-taster (t) condition.

Hair Color: In this case, we will consider only dark hair orlight hair. Dark hair (B) is dominant over light hair (b).

Rh factor: The Rh factor is an antigen found on red blood cells.It is named after the Rhesus monkey where it wasdiscovered. People who possess this antigen on their red bloodcells are considered Rh+ (dominant), while those without theantigen are Rh-(recessive). The Rh factor is of concern if an Rh-mother is carrying an Rh+ child, since the mother's body may formantibodies to the child's blood, resulting in numerous healthproblems.

Autosomal traits controlled by multiplealleles: Genes for these traits are also carried onhomologous autosomal chromosomes, but there are more than twoalleles (versions) of the gene present in the population. However,only two of these alleles can be
present in any one individual.

Blood type: A person's blood type or phenotype (A, B, AB, O) isdetermined by the presence of certain antigens on the surface oftheir red blood cells. Control for the inheritance of blood typesis based on three different alleles: IA, IB, and IO. Again, onlytwo of these alleles will be present in any one individual. The IAallele codes for the \"A\" antigen, the IB allele codes for the \"B\"antigen. The presence of the IO allele results in no antigen on thesurface of the red blood cell. A person's blood type (phenotype) isthe result of the combination of these alleles. The IA and IBalleles are co-dominant and are equally expressed if both arepresent. The IA and IB are both dominant to the IO allele. The onlyway for a person to have type O blood is to have both recessivealleles. Fill in the table at the right with the proper phenotype(blood type) for each possible genotype.

Sex chromosomes and sex-linked traits: Sex inhumans is determined by the presence or absence of certain wholechromosomes. Females have two X chromosomes and males have an X anda Y chromosome. Genes for sex-linked traits are usually carried onthe X chromosome and are absent on the homologous Y chromosome. Asa result, males only have a single copy of any gene located on theX chromosome and they will express whichever allele is present,even if it is normally a recessive allele.

Color-blindness: The gene that controls color vision is locatedon the X chromosome and has two alleles: normal (X) and color-blind(X°). The normal condition is dominant over the color-blindcondition. Females that are phenotypically normal, but heterozygousfor color-blindness (X X°) are often referred to as carriers

Now that you have become familiar with the basic terms ofgenetics and the specific traits that we will observe today, youare ready to put this knowledge to use. To do this, we will play a\"family\" game. During the course of the lab, each of you willdetermine your genetic makeup, find a mate, and determine thecombination of traits that would be possible in your offspring.Here is how it works:

Step 1: Names - These will be used to determineyour mates. Men will draw a last name from the beaker marked malesand women will draw a last name from the beaker marked female. Ifwe do not have equal numbers of men and women, some people may havemultiple mates.

Step 2: Determine sex & sex-linked traits -Men will draw only ONE X chromosome andONE Y chromosome. Women will drawTWO X chromosomes. The X chromosome will have anallele for normal vision or color blindness.

Step 3: Determine autosomal traits - In orderto find out your traits, you will draw TWOchromosomes with alleles for each of the following traits: haircolor, ability to taste PTC, Rh factor, and blood type.

Step 4: Take a social break and find your mate.When you find someone with the same last name, have a seat besideyour new mate.

Step 5: Once you have become acquainted withyour mate;-) arrange your alleles in front of you in the ordershown below for Jane Smith.

Genotype XX

Bb
IA IO Rh+Rh- TT

Phenotype
Female with normal vision Dark hair
Blood type A
Rh+
PTC Taster

Step 6: Fill in the table below with theinformation for you and your mate. If needed, review theinformation on pages 1-2

MY RESULTS

my phenotypes: SEX/VISION = xBxb (female normal vision),BLOOD TYPE=IAIA (A), +/- BLOOD TYPE = Rh-Rh- (-), PTC TASTING = Tt(taster), HAIR COLOR = BB (dark)

Mates phenotypes: SEX/VISION = xbY (colorblind male),BLOOD TYPE= IAIO (A), +/- BLOOD TYPE = Rh- Rh- (-), PTC TASTING =tt (non taster), HAIR COLOR= Bb (dark)

Mates Phenotypes:

Step 7: You are now ready to determine all thepossible gametes that each of you could contribute to youroffspring. Remember, gametes are formed by meiosis, which isreduction division. Each gamete will only get one allele from eachpair of homologous chromosomes.

The number of possible gametes is equal to 2 (because we aredealing with 2 alleles) raised to the ?n?

power, where ?n? equals the number of pairs of heterozygousalleles. For example, looking at Jane Smith=s genotype we can seethat she is heterozygous for three traits: hair color (Bb), bloodtype (IAIO), and Rh factor (+-). According to our formula, shecould have (23) = 8 different gametes. How many possible gametescould you have? _____ How many possible gametes could your matehave?_____

Step 8: Each person should now determines theirown gametes and writes them below. The method illustrated belowusing Jane Smith=s genotype will allow you to systematically findall possible gametes for you and your mate. Each column representsone possible gamete. Jane could have eight different gametes (23 =8), so there should be eight columns shown below when we arefinished. You may have more or less than eight columnsdepending on the number of heterozygous pairs of alleles that youhave. Choose one trait that Jane (or you) is heterozygousfor (ex., Rh factor) and give the possible gametes. (Hint: thereshould be 2, one for each allele).

Use the space below to write out all of the possible gametecombinations for you and your partner.

The total number of different looking children that you and yourmate could produce can be determined by multiplying your number ofpossible gametes times your mate's number of possible gametes.Considering only these traits, how many different children couldyou and your mate produce?_________

Look at your gamete possibilities and genotypes and those ofyour mate to answer the following questions. Use simple Punnettsquares to determine the possible outcomes of the crosses whereneeded.

1. Will any of your children be color-blind?

2. What are the possible hair colors of your children?

3. Will any of your children be non-tasters?

4. What are the possible blood types of your children?

5. Will any of your children be blood type O-?

6. Will any of your children be AB+?

7. Could you possibly produce a child who would have dark hair,could taste PTC, and have AB- blood?

8. Would you bet on the sex of your next child? Why or whynot?

9. If you were a betting person, are there any traits,concerning your child, on which you would bet? Explain.

10. Today we can determine the genetic makeup of individuals anddetermine whether they carry traits that are potentially harmfulsuch as Huntington=s disease. Knowing this, would you participatein a genetic typing for certain various characteristics before youget married? Why or why not?