Name:_______________________Due Date:___________

Probability, Genetics andPatterns of Inheritance

Lab and Data SheetBiol 2402 Lab

Jarzem, Ziser, 2001

I. Introduction to Probability

Think About It

1. If you toss a coin, what is the probability that it will come up heads? ________2. If you toss the coin again, what is the probability that it will come up heads again?

________3. Does the result of the first toss make any difference to the probability that it will

come up heads the second time? ________

Predicting Several Events at One Time

1. Toss the coin twice. What was the result of the first toss? ________

the second toss? ________

2. Toss two coins at the same time. What was the result? ______________3. Do you think that there is any difference in the probability that the coins

will come up two heads if you toss them one after another or at thesame time? Explain

As you have seen, if you toss a coin, there is a 50% chance that it will come up heads. If you toss it again,there is still a 50% chance that it will come up heads. However, if before you toss the coin, you decide thatyou would like to see it come up heads twice in a row (or two coins coming up heads if they are tossed atthe same time), you know that there is less than a 50% chance of this happening. In fact there is only a25% chance of getting two heads in a row; there are a total of four possibilities: head-head, head-tail, tail-head, and tail-tail. Only one of those fur is the one you wanted; and one in four is a 25% probability.

To calculate this mathematically, you multiply the two individual probabilities together:

50% x 50% = 25% or .5 x.5 = .25

4. What is the probability of getting three heads in a row? ___________________Test your hypothesis by actually tossing a coin 3 times,what were your results? ___________________

5. The Monroe’s are expecting their fifth child, Their first four childrenare girls. What is the probability that their fifth child will be a girl? ___________________

6. Fred Monroe and Sara Clifford are engaged. They plan to have fivechildren. What is the probability that all five children will be girls? ___________________

Figuring Probabilities Using Punnet Squares

A Punnet square is a way to represent visually the probabilities of two or more events happening at thesame time. You can represent the probability of the results of two coins being tossed with a Punnet squarein this way:

Coin ‘A’ may come up either:

‘A’ =Heads

‘A’ =Tails

‘B’ =Heads

A=headsB=heads

A=tailsB=heads

Coi

n ‘B

’ may

com

e up

eith

er:

‘B’=Tails

A=heads B=tails

A=tailsB=tails

As you can see, we get the same results as when we calculated probability mathematically. There are fourequally likely possibilities so each has a 25% chance of both coming up heads, 25% chance of bothcoming up tails, and a 50% chance of coming up one heads and one tails.

Define and distinguish between the following terms commonly used when discussing genetics:

DOMINANT

RECESSIVE

GENOTYPE

PHENOTYPE

ALLELE

COMPLETE DOMINANCE

INCOMPLETE DOMINANCE

HOMOZYGOUS

HETEROZYGOUS

II. Calculating Probabilities for Recessive Genes

Most genetic defects are caused by “recessive” genes. they got this name because they seemed to hide or“recede” in some generations, and come out, or “express” themselves, in other generations. Tounderstand what is actually happening, remember what a gene is – a code specifying how to make acertain protein. There are thousands of proteins that we must have made in our cells, at the proper time,and in the proper amounts, for the normal development and functioning of our bodies.

A mutation in DNA may result n its being an erroneous code – the protein that it no codes for is eitherdefective and nonfunctional, or perhaps it cannot be made at all. But remember, you have two DNA codesfor each protein – one found on one of the chromosomes in your father’s sperm and one found on thecorresponding chromosome in your mother’s ovum. If something is wrong with the DNA on one ofthese chromosomes, the proteins can still be made using instructions from the DNA on the otherchromosome. As long as you have one normal gene, and can make the normal protein, your health addevelopment will be normal.

For example, we each carry two genes that specify the code for how to make a cell membrane protein thatallows chloride ions to cross cell membranes. Without this protein mucus builds up in the respiratoryand digestive tracts – a life-threatening condition called cystic fibrosis.

Most of us have two good copies of this gene. About one in 20 Caucasians, however, has one functioninggene and one defective gene that cannot code for the protein. If one gene carries the correct code we canmake the protein and that means we have functional chloride channels. But for someone who has twodefective genes – and therefore no functional genes – there is no way to make the proper chloridechannels. Although everyone carries two genes for this protein, only one of the genes, randomly selected,will end up in each sperm or egg cell that we make. Which sperm meets a particular egg is also a randomevent. In that sense the genes inherited by a child are like thousands of coin tosses. Heads or tails? GeneA for chloride channel protein, or gene B which doesn’t properly code for the protein.

Punnet Squares and Genetics Problems:

If we know which genes the parents carry, a Punnet square can show the likelihood of a child being bornto them with cystic fibrosis (or any other genetic trait or disease that is controlled by a single gene pair).For example, suppose Jeremy carries one defective gene and one good gene, while his wife, Michele, hastwo good genes. If the good gene is represented by a ‘C’, and the defective gene, by a ‘c’ the PunnetSquare is drawn below:

Possible genefrom MomC C

C CC CC

Pos

sibl

e ge

nefr

om D

ad

c Cc Cc

The Punnet square lets us see a probability at a glance. In this case it is obvious that their child has 0%chance of having cystic fibrosis – No matter how the genes are distributed, this child would always get atleast one working gene.

Someone who carries a defective gene, but who is not affected by a geneticdisease is called a carrier. What is the probability that Jeremy and Michele’schild would be a carrier? _______________

Sure enough, Jeremy and Michele’s son is a carrier for cystic fibrosis. He marries Jennifer, who is also acarrier. Make a Punnet square showing the probability that Jason and Jennifer’s child will have cysticfibrosis:

What are your odds?

As you answer the following questions, be sure to show your calculations (when necessary) used to arriveat your answer. Also, use either fractions or decimals to show probability, but be consistent

1. If you are Caucasian,what is the probability that you carry the cystic fibrosis gene? ________________

2. What is the probability that your (Caucasian) husband/wife is (will be)a carrier? ________________

3. What is the probability that both you and the person you decide to marryand have children with will be carriers? ________________

4. If both parents are carriers, what is the probability that a child born to themwill actually have cystic fibrosis? ________________

5. What is the probability that both you and the person you decide to marryand have children with will be carriers and your child will actuallyhave cystic fibrosis? ________________

6. In fact, cystic fibrosis occurs I about 1 in 2000 births to Caucasian couples.Does this agree, more or less, with your calculations? ________________

III. Other Patterns of Inheritance

1. Genetic Defects Caused by Dominant Genes:Sometimes only one copy of a mutated gene can cause serious health or developmental problems. Thepresence of the defective protein in some way causes something to go wrong in cells. For example, someof the proteins that our cells make act to “turn on” or “turn off” genes. A defective protein of this typemade by only one gene on one chromosome (even if the other gene was normal) could cause harm to thebody.

Huntington’s chorea is a genetic disease caused by a dominant mutation. Make a Punnet square showingthe probability of inheriting Huntington’s chorea if one parent has the disease (assume this parent has onedefective and one normal gene)

2. Incomplete Dominance:Sometimes both genes must be normal for complete health. One normal and one abnormal gene causessome problems, and two defective genes cause even more serious problems.

An example of this type of gene is the one that causes sickle-cell anemia. The DNA code for one of thehemoglobin chains specifies an amino acid with a hydrophobic region at a place on the amino acid chainthat normally has a different amino acid, one with a hydrophilic region. If both copies of the DNA forhemoglobin are defective and all of the hemoglobin made has the wrong amino acid, the hemoglobins willstick together when oxygen levels are low. The hemoglobins form stiff rods, deforming the RBC’s. Suchdeformed cells can damage tissues and organs and be extremely painful. This person has sickle cellanemia.

With one copy of the defective gene, there is enough normal hemoglobin so that there are usually nodeformed cells produced. Occasionally, under extreme stress, some abnormal cells form, causing mildsymptoms. This person is a carrier of the sickle-cell gene and is said to have sickle cell trait.

Now, imagine two people with sickle cell trait marry and have children. Make a Punnet square that showsthe probability that each of their children will have sickle cell anemia, sickle cell trait, or be completelyhealthy.

Probability that the child will have sickle-cell anemia?_______________

Probability that the child will have sickle-cell trait? _______________

Probability that the child will be completely healthy? _______________

3. Sex-Linked Traits:

There is one major exception to the rule that a person has two copies of each gene. For genes found onthe X-chromosome, men have only one copy while women do have two copies. Men are male becausethey have an X and a Y for their 23rd pair of chromosomes while women have two X’s.\

The Y chromosome is a small chromosome that carries only genes that are necessary for “maleness” (nowisecracks from the nonmale portion of the class!!). It is logical that there are no basic genes needed byall human beings on this chromosome because then all women would be in big trouble, geneticallyspeaking (no wisecracks from the nonfemale portion of the class!!)

On the other hand, X chromosomes are large chromosomes that have many genes that have nothing to dowith gender and are needed by both men and women. That works because everyone has at least one X.And as we have seen, most of the time, you only need one gene that works in order to have the necessaryprotein.

When a woman has a defective gene on one of her X chromosomes, she can use the corresponding geneon the other X chromosome to make the protein, develop normally, and to be healthy, just as with any otherrecessive gene. Only if the genes are defective on both of her X chromosomes, will she have the geneticdefect. However, any time a man has a defective gene on his X, he has no “backup” to code for thenormal protein. If he has a defective gene on his one and only X chromosome, he has the problem itcauses.

1. An examples of genes found on the X chromosome are ones that code for proteins needed in the conesof the retina. if these proteins cannot be made, a form of color-blindness results. If a woman has the genefor normal color vision on one of her X chromosomes and a defective corresponding gene on her other Xchromosome, and if she is married to a man with normal color vision, What is the probability that her sonor daughter will be color-blind? Show your word below:

Probability son will be color bind? _______________

Probability daughter will be color blind?_______________

2. If a woman is color blind, is it certain that both her mother and her father were color bind? Show yourwork:

Probability that mother was color blind?______________

Probability that father was color blind?______________

IV. Distribution of Genes in the Human Population

So far, we have only discussed patterns of inheritance where there are only two possible genes for a trait –one gene which is dominant and another which is recessive. You either have one or both dominant genes –and therefore the dominant trait, or you have two recessive genes and show the ‘recessive’ characteristic.In a few cases, such as sickle-cell anemia, there is an ‘in-between’ state caused by having one dominantand one recessive gene. Many (most) times the genetic possibilities are not so simple.

1. Traits determined by many gene pairs:Observe the many different shades of skin color in humans. Clearly , there is not simple a “very-dark-brown gene” and a “sort-of-peachy-pink gene” if there were there would only be two different possibleskin colors; or in the case of incomplete dominance, three colors at most. In fact, many different genepairs combine to determine skin color.

Determining eye color requires genes that specify which pigments are made, genes that specify how muchpigment, and genes that specify the pattern by which the pigment is deposited in the iris. Surprisingly, thislast one is the most important – the patterns causes the iris to act like a prism, reflecting subtle shades thatcan include variations like hazel, aquamarine, or even violet.

2. More than two genes in the population (Multiple Alleles):Even when a single gene pair determines a trait, that doesn’t necessarily mean that only tow genes exist forthat trait. ABO blood types are a prime example. We each carry only two genes (one from mom and onefrom dad, for this characteristic of our blood cells, but there are three different genes in the humanpopulation that can actually affect this trait. One gene codes for the ‘A’ protein on the surface of RBC’s,another codes for the ‘B’ protein, and a third gene (‘o’) doesn’t code for either one. So each of us cancarry any combination of two of the three possible types of genes. Remember, as with cystic fibrosis,make a protein equals dominant, doesn’t make a protein equals recessive.

Genes Producing the ABO Blood Groups

1st gene 2nd geneprotein(s) on

RBC’s Blood TypeA A AA o A Type A

A B A & B Type ABB B BB o B

Type B

o o Neither Type O

1. If mom has type AB blood and dad has type B blood what is the possibility that their child will havetype A blood? Show your work

V. Which Genes Do You Carry?

Most of the examples above involve genetic defects of some kind, but many normal traits and functionsshow variation form person to person because some normal trait s are dominant (show up when the personhas the gene on either one or both chromosomes) or recessive (show up only when the genes are presenton both chromosomes).

the Phenotype of a person is the actual appearance or function that a person has as a result of the genes onhis or her chromosomes. For example , the phenotype of a person might be “has dimples” or “does nothave dimples”. The genotype of a person is a description of the actual genes the person carries for thattrait. Dominant genes are usually represented by a capital letter, recessives by lower-case letters. Agenotype for dimples might be written “DD” or “Dd” to indicated the two possible gene combinations(genotypes) that would produce this dominant phenotype. Since, for example, dimples are due to adominant gene, to have dimples you need only one of these genes, D. The other gene might also be a D orit could be a ‘no-dimple gene’, d. So if you have the ‘dimples’ phenotype, you could have either the DDor the Dd genotype; if, however, you have the ‘no dimples’ phenotype, the only possible genotype is dd.

Various genetic traits are described in your lab manual. Read the descriptions and the testing proceduresfor each and record your phenotype, and as much of your genotype as you can determine. If you knowwhether your parents or siblings have the dominant genotype this might help you to determine your owngenotype. Record this additional information (if available) in the table below along with your own data:

Genetic Traits

Characteristic(and genes)

Your PhenotypeFamily Members’Phenotypes

(if known)

YourGenotype

Tongue Rolling (T, t)

Free Earlobes (E,e)

Interlocking Fingers (I,i)

PTC Taster (P,p)

Sodium Benzoate Taste (S,s)

Gender (X,Y)

Dimples (D,d)

Widow’s Peak (W,w)

Bent Little Finger (L,l)

Double-Jointed Thumb (J,j)

Middigittal Hair (H,h)

Freckles (F,f)

Blaze (B,b)

ABO Bloodtype (A,B,o)

Not Color Blind (C,c)

Additional Genetics Problems (show your work)

1. If one parent is heterozygous dominant for the ability to taste PTC and the other is homozygousrecessive, what are the chances that their offspring will be able to taste PTC?

2. If you have attached earlobes yet both your parents have free earlobes, what can you deduce about thegenotypes involved? What are the chances of your parents producing an offspring with free earlobe(assuming they are still young enough to have kids)

3. In humans, hemophilia, the bleeders disease in which blood clotting time is much greater than normal, isa sex linked recessive trait. A man whose father was a hemophiliac but who is not one himself, marries awoman with no record of hemophilia in her heritage. What is the chance of hemophilia showing up intheir children?

4. A woman whose father was a hemophiliac but who does not have hemophilia, herself, marries a normalman (and NO, a normal man is NOT an oxymoron!!). What is the chance of hemophilia in her daughter?In her son?