Having dimples is an example of a phenotype which is

Lab #5 – At Home

Human Genetics

Pre-lab:

1. Explain what is meant when a gene is referred to as “dominant” or “recessive”:

2. Define genotype:

3. Define phenotype:

Purpose:

1. Understand how genetics relates to the diversity of the human population visible around us.

2. Find out the genotype of a real person.

3. Practice determining the results of genetic crosses.

Background:

Each living thing that reproduces sexually has two sets of chromosomes. One set of chromosomes is inherited from each parent. Each set of chromosomes carries a full set of genes, where each gene is the "code" for a particular type of protein. Having two sets means everyone has two copies of each gene.

Proteins have many roles in an organism and sometimes a single gene is critical in making a certain obvious trait (something observable, like dimples). You can think of each gene as a location and you can put specific information (allele) in each location. The different alleles for a particular gene may produce proteins with identical functions or very different functions. Each type of protein that an organism makes is a product of the alleles that it has. Often the resulting trait is a blend of the effects of the proteins from two alleles, but sometimes one allele (a dominant allele) produces a protein with an effect so powerful that it can completely cover up the effect of a protein produced by a weaker allele (a recessive allele). (2)

Humans are sexually reproducing organism and are different from each other in appearance and in their biochemical make up. Even so, if you compare any two people you will find that 99.9% of their DNA is the same. (3) Human genetic traits are difficult to study because of our relatively long life span and the limited number of human offspring produced. In addition, the number of chromosome pairs (23) increases the possible number of genetic combinations which in turn compounds the difficulty of studying human genetics. In spite of these difficulties humans have some genes that exhibit monohybrid inheritance. For these genes it is possible to get a good idea of your own genotype.

Procedure:

In this exercise you will observe either your own or someone else’s phenotype (part 1) and determine part of your/their genotype (part 2). Then you will figure out one possible genotype of a hypothetical child -a combination of your genes and the genes that are listed for that trait (part 3).

You will Submit the Report Page to Canvas.

Part 1. Determine your phenotype

Read the descriptions of the traits listed below and use a best guess to determine whether you have the dominant or recessive phenotype for each trait. Record the word that describes your phenotype in the spaces provided in the Table on the Report Pages at the end of the lab.

Example:

Trait

Your phenotype

Your possible genotype

Dimples A_

No Dimples aa

No Dimples

Part 2. Deduce your possible genotype(s)

Each of the alleles of a given gene are symbolized by an assigned letter. Normally the dominant form is symbolized by an upper case letter (e.g. E), and the recessive form is symbolized by a lower case of the same letter (e.g. e). If you express the dominant trait (phenotype) use only a single capital letter (e.g. E_ ) for your genotype since you don't know whether you are homozygous or heterozygous for that trait. If you express the recessive phenotype for a given trait you must be homozygous recessive, represented by two lower case letters (e.g. ee). Note: Many of the traits in this lab, while presented as under the control of a single gene, are hypothesized by some scientists to be controlled by several genes. For the purposes of this exercise we will assume the more simple, single gene inheritance.

Part 3. Predict Potential Offspring Phenotypes

The genotype of a potential mate is given in the Report Pages table. Use that information to predict one possible phenotype of an offspring. Record your possible genotypes for each trait in the table on the Report Pages at the end of the lab.

Example (answers are in red):

Trait

Your phenotype

Your possible genotype

Mate’s genotype

One possible phenotype of offspring

A

Dimples A_

No Dimples aa

No dimples

aa

aa

No Dimples

Human Genetic Traits

Dimples (A)

Dimples are small, natural indentations on the cheeks on one or both sides. Some people are born with dimples that disappear when they are adults; others develop dimples later in childhood. Dimples are highly heritable, meaning that people who have dimples tend to have children with dimples—but not always. Because their inheritance is not completely predictable, dimples are considered an “irregular” dominant trait. Having dimples is probably controlled mainly by one gene but also influenced by other genes.

Cleft Chin (B)

A cleft chin looks like a dimple or indentation in the middle of a person’s chin. This trait is due to a single gene with a cleft chin dominant and a smooth chin recessive.

Tongue Rolling (C)

The ability to roll the tongue upward from the sides (so it looks like an "O" or "U" from the front) is a dominant trait. It is probably the result of several genes with an environmental influence, though in genetics labs it is usually treated as a one-gene trait. It probably means that tongue rollers have more flexible muscle or connective tissue associated with the tongue. With the non-roller recessive trait (cc) a person only produce a slight downward curve of the tongue.

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Widow's Peak (D)

The widow's peak is a distinct downward point on the hairline at the top center of the forehead. This is a dominant trait. If you have a straight hairline, you have recessive alleles for this trait.

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Free or Attached Ear Lobe (E)

Look for where the lower earlobe attaches to the head - does it hang freely, more like a "U" (unattached), or curve directly into its attachment on the face, more like a "J" (attached)? Free earlobes are dominant over attached earlobes and are found in the majority of people.

Freckles (F)

Freckles are small, concentrated spots of the skin pigment melanin. This trait is probably due to a single gene called MC1R. The presence of freckles is dominant while the absence of freckles is recessive. Alleles of MC1R control freckle number; other genes and the environment influence freckle size, color, and pattern. For example, sun exposure can temporarily cause more freckles to appear.

Hitchhiker's Thumb (G)

When present, the thumb, held up, has almost a right angle bend on the outside and the pad is almost parallel to the ceiling. The ability to bend the thumb backward (at least 45o) is caused by a dominant allele. The proper term for this is distal hyperextensibility. People with dominant alleles have more flexible ligaments and thus looser joints.

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Finger Mid-digital Hair (H)

Some people have hair on the back of the middle segment of some fingers. Individuals with hair on the middle segment of at least one finger are considered to have the dominant trait mid-digital hair, while other people have no mid-digital hair (recessive). Lack of hair, however, can also be influenced by your environment - for example if you have a job where the hairs may be worn away.

Bent Little Finger (I)

In some people the fifth “little” finger bends towards the fourth finger near the tip. Bent fingers are believed to be dominant, while straight is recessive. There is limited evidence to support that the trait is controlled by one gene with two alleles.

Hair on Back of Hand (J)

Some people have hair on the back of their hands (dominant) while others have no hair (recessive).

Long Palmar Muscle (K)

Most people lack a forearm muscle (dominant) called the palmaris longus, which sends a tendon through the wrist into the hand. Approximately 14% of the population has this muscle (recessive). Make a tight fist, and tilt your hand toward you. If you see one obvious tendon down the middle, and can feel a tendon to either side of this middle tendon, the muscle is present. If you see (or feel) only two tendons, this muscle is absent. (Alternately, tightly press your opposing thumb and pinkie together to observe the tendons.)

Interlocking Fingers (L)

Fold your hands together by interlocking your fingers. If the left thumb is on top, you have the dominant allele. If the right thumb is on top you are homozygous recessive. This trait is not from a single gene but it acts like a dominant - recessive pair of alleles.

Morton’s Toe (M)

In some people, the big toe is longer than the second toe (recessive), while other people have the big toe shorter than the second toe (dominant). Whether the big toe is longer or shorter than the second toe is influenced by genetics, but it may be determined by more than one gene, or by a combination of genetics and the environment.

Handedness (N)

Recent discoveries make right-handedness dominant over no pre-determination – people with two recessives are about 50% left-handed. Handedness appears to be influenced by genetics, environment and chance.

Darwin Tubercle (O)

A Darwin tubercle is a projection on the upper portion (helix) of the ear resulting from a thickening of the cartilage. The tubercle may be inherited as a dominant, although some individuals may have the tubercle only on one ear and the size of the tubercle can vary. Absence of the tubercle may be recessive.

Inter-eye Distance (P)

Close-set eyes are incompletely dominant to eyes set far apart. Medium-set eyes are heterozygous.

Eye Color (Q)

Presence of pigment is usually dominant over absence of pigment in eye, hair and skin color. The actual color is a result of the interaction of several genes i.e. a polygenic trait. You can use two gene pairs to determine eye color. (This model is simplified since more than two genes are involved in eye pigmentation but using just two of them works pretty well.) One gene codes for pigment deposited in front of the iris; the other gene codes for pigment deposited behind the iris. Brown eyes are the result of black or brown pigment in the iris. Blue eyes have no pigment. Note that hazel or green eye color is the result of a second gene which produces a yellow pigment. Hazel eyes have brown iris pigment while green eyes have the recessive blue iris. (The alleles for Iris color are Q and q, and the alleles for the color behind the iris are Z and z.)

Phenotype

Genotype

Dark Brown

QQZZ and QQZz

Brown w/green flakes

QqZZ

Brown

QqZz and QQzz

hazel

Qqzz

green

qqZZ

Dark blue

qqZz

Pale Blue

qqzz

Hair Whorl (R)

When viewed from above and behind the head, many people's hair whorls in either a clockwise (dominant) or counterclockwise (recessive) direction. While this trait seems to be controlled by a single gene, there may be other genetic influences at play.

Report Page Name

Trait

Your phenotype

Your possible genotype

Mate’s genotype

One possible phenotype of offspring

A

Dimples A_

No Dimples aa

aa

B

Cleft Chin B_

No Cleft bb

BB

C

Tongue Roller C_

No Tongue Rolling cc

CC

D

Widow’s Peak D_

Straight Hairline dd

DD

E

Free Ear Lobe E_

Attached Ear Lobe ee

EE

F

Freckles F_

No freckles ff

FF

G

Hitchhiker’s Thumb G_

Normal Thumb gg

GG

H

Finger Mid-digital Hair H_

No Mid-digital Hair hh

hh

I

Bent Little Finger I_

Straight Little Finger ii

ii

J

Hair on Back of Hand J_

No Hair on Back of Hand jj

JJ

K

No Long Palmar Muscle K_

Long Palmar Muscle kk

KK

L

Interlock Left on top L_

Interlock Right on top ll

ll

M

Big Toe Shorter M_

Big Toe Longer mm

MM

N

Right Handed N_ or nn

Left Handed nn

NN

O

Darwin Tubercle O_

No Darwin Tubercle oo

OO

P

Close set eyes PP

Medium set eyes Pp

Wide set eyes pp

pp

Q, Z

Phenotype

Genotype

Dark Brown

QQZZ and QQZz

Brown w/green flakes

QqZZ

Brown

QqZz and QQzz

hazel

Qqzz

green

qqZZ

Dark blue

qqZz

Pale Blue

qqzz

Qqzz

R

Clockwise Whorl R_

Counterclockwise Whorl rr

RR