Dominant and Recessive Traits

Human Genetics

Children tend to look like their parents. Often a child may look more like one parent than the other. The study of how and why children look like their parents is called genetics. How we look is just a small part of our genetic heritage. Our genes influence the way our bodies work and thus, our health.

Dominant and Recessive Traits:

We get one half of our genes from each parent. Yet, we may look more like one parent than the other. Why? ______

A gene that governs a particular trait may come in alternative forms called alleles. Not all of the alleles that we inherit are shown or expressed. Some alleles, the dominant ones, cover the expression of the recessive alleles. The recessive alleles don’t disappear. They may show up in future generations when not paired with a dominant allele. When the same allele for a single gene is on both the paired homologous chromosomes the individual is homozygous for that trait. When the paired chromosomes carry different alleles for the same gene they are said to be heterozygous.

Physical Genetic Traits:

Hundreds of traits combine to determine appearance. A single gene determines some traits like tongue rolling. Others, like skin color, are determined by the combined effects of several genes. In the following procedure you will determine your phenotype, the detectable expression of a gene (what is shown), for several physical traits. Then you will determine your possible genotypes, or the alleles in your genetic makeup, for these traits.

Procedure:

Using Figure 1, have your lab partner help you determine your phenotype for each characteristic listed and record your phenotype in Table 1.
Next determine your possible genotypes for the traits listed and record in Table 1. Note the dominant alleles are represented by capital letters and recessive alleles by small letters. For example: “H” for hitchhiker’s thumb and “h” for normal thumb. If the recessive trait is expressed in the phenotype then it is always homozygous in the genotype “hh”. If the dominant allele is expressed it may be either homozygous “HH” or heterozygous “Hh”.

Table 1. Physical traits.
Dominant / Phenotype / Genotype
(there may be several) / Recessive
Example: (R) Tongue Roller / Roller / R R or R r / (r) Can’t roll tongue
(R) Tongue roller / (r) Can’t roll tongue
(W) Widow’s peak / (w) Straight hair line
(E) Unattached earlobes / (e) Attached ear lobes
(P) Ear points / (p) No ear points
(H) Straight thumb / (h) Hitchhiker’s thumb
(C) Cleft chin / (c) “Normal” chin
(R) Right thumb on top / (r) Left thumb on top
(D) Dimples / (d) No dimples
(F) Freckles / (f) No freckles
Dominant
Tongue roller / / Recessive:
Can’t roll tongue /
Dominant:
Widow’s Peak / / Recessive:
Straight Hairline /
Dominant:
Unattached earlobes / / Recessive:
Attached earlobes /
Dominant:
Has ear points / / Recessive:
No ear points /
Dominant:
Straight thumb / / Recessive:
Hitchhiker’s thumb /
Dominant:
“Normal” chin / / Recessive:
Cleft chin /
Dominant:
Right thumb on top / / Recessive:
Left thumb on top /
Dominant:
Has dimples / / Recessive:
No dimples /
Dominant:
Has freckles / / Recessive:
No Freckles /

Figure 1. Look at Yourself

Multiple Alleles: Human Blood Groups

Red blood cells have molecules calledantigens on their surface. An antigen is a molecule that causes the immune system to produce antibodies against it. When a foreign antigen enters the body the immune system will build antibodies against it. ABO antigens are attached to human red blood cells. The surface antigens on red blood cells are coded for by one gene that has three different alleles. This is an example of a multiple allelic system. The IA allele determines the A antigen, the IB allele determines the B antigen, and the “i” allele determines no antigens (type O).

An individual carries a matched pair of chromosomes and thus has two alleles for the ABO blood groups. Two alleles may be expressed at the same time. If an individual has IA and IB, they will have type AB blood. Since both alleles are expressed, this is an example of codominance. The possible genotypes and phenotypes are listed in Table 2. If red blood cells with foreign antigens on them enter the body the antibodies produced against them will cause the blood to clump (not clot—clotting is something quite different). A normal person never makes antibodies against his own antigens. If this occurred it would be disastrous! Imagine a person with A antigen making antibodies against A and clumping his own blood cells. Instead, a person with A would make antibodies against foreign antigens such as B (anti-B antibodies). This is why only a blood type that is compatible can be used during blood transfusions. For example, type A blood can only be given to persons with type A or type AB blood (See Table 2). The Rh system is completely separate, but works in much the same way. If you have the Rh antigen present on your red blood cells, you are Rh+. If it is absent, you are Rh-.

Table 2. Human Blood Groups
Genotype / Phenotype / Antigen present / Antibody produced
IAIA / A / A / Anti-B
IAi / A / A / Anti-B
IBIB / B / B / Anti-A
IBi / B / B / Anti-A
IAIB / AB / A & B / None
ii / O / None / Anti-A & Anti-B
Rh+, Rh+ / Rh+ / Rh / None
Rh+, Rh- / Rh+ / Rh / None
Rh-, Rh- / Rh- / None / Anti-Rh (after exposure)

Blood type O is sometimes called the universal donor. Why? ______

TypeAB is sometimes called the universal recipient. Why? ______

Determining Your ABO Blood Type:

In this procedure you will use a serum with antibodies in it against a specific antigen.

Conduct a blood typing exercise as demonstrated by your instructor.

Table 3. Blood Antigen Test
Red Blood Cell Antigen / Antiserum Added / Results
A / Anti-A / clump
B / Anti-B / clump
AB / Anti-A & Anti-B / both clump
O / Neither Anti-A nor Anti-B / no clump
Rh / Anti-Rh / clump

What blood type are you? ______

What are your possible genotypes? ______

What type(s) of blood could you safely receive in a transfusion? ______

To which type(s) of blood could you safely donate in a transfusion? ______

Sex Determination in Humans: X and Y chromosomes

Human cells contain 23 pairs of chromosomes and 46 chromosomes in all. One chromosome in each pair comes from each parent. Twenty-two of the 23 pairs are matched and are called autosomes. The chromosomes in these 22 pairs are similar in size, shape, and the genes that they carry. The 23rd pair, XX or XY, determines the sex of the individual and are called sexchromosomes. Each egg contains one X chromosome. Each sperm contains either one X or one Y chromosome. If an “X” sperm fertilizes the “X” egg, the child is female (XX). See Figure 2. If a “Y” sperm fertilizes the “X” egg, the child is male (XY). Notice that the only thing that determines sex is the presence or absence of the Y chromosome. If you have a Y, you are male. If you lack the Y, you are female.

Sperm

Figure 2. Sex determination in humans.

Barr Bodies:

The X chromosome is fairly large and carries numerous genes necessary for life. The Y chromosome is tiny and has only a few genes. Early in the development of the female embryo one of the X chromosomes becomes inactive in each cell. This means that only one of the two X chromosomes is actually working. The inactive X becomes condensed and can be seen in certain cells as a Barr body. There is always only one active X chromosome in human cells. Thus, if the person is a normal female there should be one Barr body present and a normal male should not have any. People with extra X chromosomes will have more than one Barr body. For example an XXX female would have two Barr bodies. In the past Olympic athletes were required to have a Barr body test to determine their genetic sex before entering any events.

How many Barr bodies would you expect in a female with 4Xs (XXXX)? ______

How many in an XXY male? ______

Procedure:

Obtain a clean microscope slide.
Use the blunt end of a toothpick to scrape the inside of your cheek to obtain some epithelial cells.
Smear the cheek scraping onto the slide and add a drop of methylene blue.
Stir the dye and cells together and set aside for 2 minutes.
Add a coverslip, cover the slide with a piece of tissue and gently press straight down on the coverslip with your thumb to spread the cells.
Examine the slide with your microscope. Can you see any Barr bodies? The Barr body appears as a darkly stained structure against the nuclear envelope.
Exchange slides with someone of the opposite gender.
As an alternative, look at one of the prepared demonstration slides.

Which sex has Barr bodies? ______Why? ______

Draw a cell containing a Barr body in the space provided.

Sketch of a cheek cell with Barr body.

Sex-Linked Traits

Color blindness and hemophilia are examples of genetic disorders that are due to a change in a gene on the X chromosome. Females rarely show these recessive disorders because a gene on their second X chromosome may cover the effects of the changed gene. Males have these disorders more often because the Y chromosome does not have a gene that can cover the effect of the changed X gene. Color blindness is an X-linked, recessive trait. Therefore, the possible genotypes are

Females / Males
XCXC = normal vision
XCXc = normal vision (carrier)
XcXc = colorblind / XCY = normal vision
XcY = colorblind

Procedure:

Have your lab partner administer the color blindness test to you. Directions are included in the test books.
Write down the number you see so your partner does not see it. It is important that you do not discuss the numbers because colorblind people see a different number or figure than non-colorblind people.

Number of correct answers ______Are you colorblind? ______

What are your possible genotypes? ______

A female who is not colorblind may determine whether she is homozygous or a carrier (heterozygous) by knowing if any member of her family is colorblind. If her father is colorblind, what is her genotype? ______. If her mother is colorblind, what is her probable genotype? ______.

Why would more males be colorblind than females? ______

______

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