Download Mendelian Laws of Inheritance

SCI 102 Spring 2010
CONCEPTS IN HUMAN GENETICS
MENDELIAN TRAITS, PEDIGREE AND KARYOTYPE ANALYSES
THEORY
Humans, in common with other multicellular organisms, are diploid; that is, they have
homologous chromosomes bearing genes for the same traits. The chromosomal location of a
gene is called its locus. Two genes at homologous loci are referred to as a gene pair and, if
these genes are in different forms, they are called alleles. The phenotype is the observable
result of the genotype. However, please remember that not all traits are inherited in a
Mendelian fashion, and moreover, environmental factors can modify phenotype. The
inheritance of most human traits is complex, involving many genes or interactions between
genes. As an example, hair color is determined by at least four genes, each one coding for the
production of melanin, a brown pigment. Because the effects of these genes are cumulative,
hair color can range from blond (little melanin) to very dark brown (much melanin). A
number of human traits, however, are fairly simple and follow a Mendelian pattern of
inheritance. For instance, human earlobes can be either attached or free. This trait is
determined by a single gene consisting of two alleles, F and f. An individual whose genotype
is FF or Ff has free earlobes. This is the dominant condition. The presence of one or two F
alleles results in the dominant phenotype, free earlobes. The allele F is said to be dominant
over f, its allelic partner. The recessive phenotype, attached earlobes, occurs only when the
genotype is ff. When both alleles for a given trait are identical, the condition is called
homozygous. When both the dominant and recessive alleles are present in a single person, the
individual is heterozygous for that trait.
Human somatic cells normally contain 23 pairs of chromosomes. Of these 22 pairs are
autosomes, and 1 pair is sex chromosomes, XX in females and XY in males. Errors occurring
during meiosis or mitosis can lead to chromosomal aberrations in which whole
chromosomes or large parts of chromosomes are missing or added. If a gamete with a
chromosomal aberration participates in fertilization, the resulting zygote will have an
abnormal DNA content. Chromosome abnormalities are very common, such that they occur at
least 7.5 % of all conceptions. However most are spontaneously miscarried and the average
live birth frequency is 0.6 %. Down syndrome is an example of one such abnormality. It is
caused by inheriting three copies of chromosome 21 (trisomy 21), instead of the normal two.
Chromosomes are determined by examining them directly at the metaphase stage of mitosis.
A photograph of the condensed chromosomes is taken and enlarged. Later the chromosomes
are cut out from the photograph and sorted to form a karyotype. Karyotype is an arrangement
of chromosome pairs by size and morphology that allows determination of the chromosome
number and any other abnormalities. The autosomes are arranged into seven groups, A
through G, and the sex chromosomes are located separately. Karyotyping can even be done
prior to birth (prenatal testing) to determine if the fetus has any chromosomal abnormality.
In this session you will learn to identify a phenotype that has a Mendelian inheritance, to
assess genotypes using a pedigree, to understand traits with multiple alleles, to determine
blood type and to prepare a karyotype.
PROCEDURE
A. Some readily observable human traits
1. Read the human traits given below. For each trait examine your phenotype and fill in the
table.
2. List your possible genotype(s) for each trait.
3. If possible, examine your parents’ phenotypes and attempt to determine your genotype.
You may also make use of your siblings phenotypes for more clues. Fill in the table.
4. In your lab reports construct pedigrees (family trees) showing the inheritance of each
separate trait in your family.
5. Examine twenty unrelated persons for each trait and try to calculate (estimate) the probable
frequency of each allele in your sample of gene pool. Present your results as a table.
Traits we will investigate are:
1. Mid-digital hair: Examine the middle joint of your fingers for the presence of hair, the
dominant condition (MM, Mm). Complete absence of hair is due to the homozygous recessive
condition (mm). You may need a hand lens to determine your phenotype. Even the slightest
amount of hair indicates the dominant condition.
2. Tongue rolling: The ability to roll one’s tongue is believed to be due to a dominant allele
(T). The homozygous recessive condition (tt) results in inability to roll one’s tongue.
3. Widow’s peak: Widow’s peak describes a distinct downward point in the frontal hairline.
It is due to the dominant allele, W, and the recessive allele, w, in homozygous state results in a
continuous hairline. (If baldness is affecting the hairline, consider the person’s hairline prior
to the onset of balding using photographs or memory).
4. Earlobe attachment: Most individuals have free earlobes (FF, Ff), due to a dominant
allele. Homozygous recessives (ff) individuals have earlobes attached directly to the head.
5. Hitchhiker’s thumb: Considerable variation exists in this trait. We will consider those
individuals who cannot extend their thumbs backward to approximately 450 to be carrying the
dominant allele, H. Homozygous recessive persons (hh) can bend their thumbs at least 450, if
not farther.
6. Relative finger length: An interesting sex-influenced (not sex-linked) trait relates to the
relative lengths of the index and ring fingers. In males, the allele for a short index finger (S) is
dominant, and in females, it is recessive. In rare cases each hand may be different. If the
length of one or both index fingers is greater than or equal to the length of the ring finger,
males will have recessive whereas females dominant genotype.
B. Multiple alleles
The major blood groups in humans are determined by multiple alleles. There are three
possible alleles, any one of which can occupy the locus. In this ABO blood group system, a
single gene can exist in any of three allelic forms: IA, IB, or i. Alleles IA and IB are
codominant, while allele i is recessive. Four blood groups are possible from combinations of
these alleles.
The alleles A and B code for the synthesis of antigen A and antigen B, glycoproteins on the
surface of erythrocytes. These antigens will cause agglutination if exposed to the
complementary antibodies. If one or both antigens are not present in the cells, the plasma
contains the antibody or antibodies for the missing antigen(s). Thus a person with type A
blood could not safely receive blood from type B or AB individuals, because the donor
erythrocytes would be agglutinated by the anti-B antibodies in the plasma.
¾ What are the possible genotypes, phenotypes in a population?
¾ Which blood type can theoretically give blood to any other type and therefore is called
the universal donor? Explain why this is so.
¾ Which blood type can theoretically receive blood from any other type and therefore is
called the universal recipient? Explain why this is so.
A standard blood typing procedure also includes a test for the Rh factor, another red blood
cell surface antigen. For example, a person with A+ blood has both the A and Rh antigens on
the surface of his or her erythrocytes. Rh- individuals normally do not have anti-Rh antibodies
unless they have been exposed to Rh+ blood. This can happen during blood transfusion or
during the birth of an Rh+ child.
C. Determining your blood type
1. Obtain a clean glass slide. Using a marker divide the slide in thirds by drawing two lines
perpendicular to the long axis of the slide. Label the upper left-hand corner of the first box
with an A, similarly label the middle box with a B and label the last box with Rh.
2. Now, each student should have his or her finger pricked by an assistant. Please be careful
not to contaminate anything with your blood, as blood may contain pathogens.
3. Drip or touch the center of each box to deposit one drop of blood in the middle of each box.
4. The assistant then quickly drips one drop of anti-A serum (contains anti-A antibodies)
next to the drop of blood in the A box, one drop of anti-B serum next to the drop of blood in
the B box, and one drop of anti-Rh (or anti-D) serum next to the drop in the Rh box.
5. Mix together the two drops in each box with a toothpick, using an unused end for each box.
If the antigen is present, the erythrocytes will agglutinate in the appropriate box. If it is not
present, the solution will remain cloudy. Record the subject’s blood type.
If you have trouble deciding whether a result is positive or negative, check the slide with the
microscope.
6. Write the subject’s blood type on a sheet of paper, which will be passed around to all of the
groups. It is not necessary to include the person’s name. In a table, record the total number of
tested students in the lab for each blood type (eg. 3 A+, 6 B+, …). Calculate the percent of
each blood type and record it in your table. Is Rh+, or Rh-, more common?
D. Preparation of a human karyotype
You will be provided with photographs of chromosomes from cultured human white blood
cells from three patients. Note that since these cells are undergoing mitosis, their
chromosomes are duplicated. Each of the 46 chromosomes consists of two chromatids, held
together by a centromere that has not yet divided. From such a preparation it will not be
difficult to assign each of the chromosomes to one of the seven groups. These groups are
distinguishable by the length of their chromosomes and the position of the centromere, which
can be either more or less in the middle of the chromosome (median), somewhat off from the
middle (submedian), far off from the middle (distal), or nearly terminal (acrocentric). Each
group contains 2 to 8 pairs of chromosomes. The chromosome groups are distinguished as
follows:
Group
A
B
C
D
E
Chromosome #
1-3
4-5
6-12, X
13-15
16-18
Length
Long
Long
Medium
Medium
Short
F
G
19-20
21-22, Y
Shorter
Very short
Centromere position
Median
Distal
Submedian
Acrocentric
16 - Median
17, 18 - Submedian
Median
Acrocentric
You will be given one karyotype prepared from a normal individual and an abnormal
chromosome spread (having a chromosomal abnormality);
1. Examine a prepared karyotype of a normal individual understand better how the
chromosomes are sorted.
2. Prepare a karyotype from the abnormal spread. Cut out the individual chromosomes one by
one from the picture. Sort out the chromosomes, assigning each one of them to a certain group
based on their size and centromere position. Put homologous chromosomes side by side and
place them on the karyotype analysis form provided with the centromere on the dotted line.
Do not glue them in place until you are sure of their correct positions.
3. Observe the karyotype for any abnormalities, describe it and try to identify the resultant
disorder.
4. Indicate for which patient you have prepared a karyotype.
5. In your lab reports, outline briefly how a chromosome preparation, like the one you were
given today, is prepared. Just give the basic steps of the procedure, mentioning only the
essential components of solutions. To be able to answer this question, you will have to do a
research.
Metaphase chromosomes in three individuals. Sex chromosomes are shown in outline
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Individual A:
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Individual B:
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Individual C: