KARYOTYPES AND INHERITANCE OF CHROMOSOMES
1992 Woodrow Wilson Biology Institute
Too often when teaching heredity, teachers do not emphasize that whole
chromosomes (linkage groups) are inherited from parents. Students often think
they inherit separate genes. Mendel's Law of Independent Assortment is only
applicable if genes are on separate chromosomes. If genes are linked on the
same chromosome, the probability of inheriting both those genes increases.
Students are also taught that they have half of each parent's chromosomes,
twenty-five percent of each grandparent's chromosomes, etc. This exercise
will visually present the mode of inheritance of chromosomes through three
generations, demonstrating the percent of chromosome inheritance from a
grandparent as being a matter of chance.
Grades 10 - 12: Biology or Genetics classes
To visually demonstrate the concept that chromosomes (linkage groups) are
inherited, and not individual genes.
To familiarize students with the procedure of locating gene loci.
To understand the concept of homologous chromosomes and crossing-over.
To increase understanding of the use of karyotypes and their analysis.
To demonstrate the following genetic concepts: dominance, recessive-ness,
homozygous, heterozygous, genotype, phenotype, complete and domi-nance,
polygenic inheritance, sex-linkage, probability, gene loci, karyo-typing, and
Mendel's laws of Segregation and Independent Assortment.
To understand the relationship between genes on a chromosome and the
Time Allotment: 2 to 3 class periods. Some activities may be completed at
- Scotch tape/paste/glue
- colored pencils
- pens - 2 colors per student
- 1 coin per student
- blank paper for Karyotype sheet - 1 per 2 students
- Chromosome sheet - 2 per student
- Human Genome sheet -1 per student
- Case study sheet - 1 per group
This hands-on activity is highly adaptable. It can be used as an
introduction to many genetic concepts, or as a reinforcement to concepts
already introduced in
class. I recommend terms and concepts such as dominant, recessive, homozygous,
heterozygous, genotype, phenotype, complete and incomplete dominance, sex
linkage, linkage groups, crossing-over, polygenic inheritance, homologous
chromosomes, Punnett squares, karyotypes, and Mendel's Laws of Segregation and
Independent Assortment should first be introduced to insure the smooth
continuance of this model. This activity is an excellent application and it
reinforces all the terminology and concepts listed.
I have included the reference numbers from McKusick's Mendelian
Inheritance in Man. These will be helpful if further studies include
research of the etiology of each condition.
Answers to the application questions are included on the student worksheet.
INHERITANCE OF CHROMOSOMES
- You should have two copies of the chromosome sheet to represent your
genetic makeup. One sheet represents the chromosomes of your mother's egg and
the other sheet represents the chromosomes from your father's sperm. You
should use one pencil color for your mother and another color for your father.
(Remember males have an X chromosome from their mother and a Y chromosome from
their father. Females have two X chromosomes, one from each parent.) Place
the correctly colored dot under each of the metaphase replicated chromosome
- Flip a coin (heads = the dominant trait, tails = the recessive trait) to
determine the gene carried on each of your parent's chromosomes. Use the
Human Genotype sheet to determine the traits obtained. After each flip of
the coin, write the allele(s) at the top of each of your parents' chromosomes.
After you have completed both parents, your traits will be represented from a
combination of both sheets.
- Determine the locus of each gene listed on the Human Genome sheet.
Indicate its position by coloring in the locus using the correct color
previously assigned each parent. Some genes are hypothetical to illustrate a
concept, so you may place them anywhere on the chromosome.
- Find a partner to marry. Be sure this person has used two different
colors for the parents. You will have one child with this person, sharing
your chromosomes to create the next generation. Each of you will have to
toss a coin to determine which of your parental chro-mosomes will be passed
down in your gamete (e.g., heads will be the chromosome you obtained from
mother and tails will be your father's chromosome). You may only pass on one
chromosome � your mother's or your father's. Toss your coin 23 times, making
a mark by the one which was determined by the toss. Cut out these chromosomes,
keeping the colored dot at the bottom and the genotype at the top. Paste or
tape the chromosome pairs together on the karyotype sheet, starting with the
first chromosome pair and ending with two X's or the X and Y.
- Determine the entire genotype of your child using the Human Genome sheet.
List these in order starting with the first chromosome set.
- Determine the entire phenotype of your child, describing all traits
starting from the first set of chromosomes.
- Count the number of chromosomes in your child which were inherited from
his maternal grandmother, his maternal grandfather, his paternal grandmother,
and his paternal grandfather. Determine the total percentage of chromosomes
inherited from each grandparent.
- What is the significance of using a coin in this exercise? (It represents
a 50% chance.)
- Give one example to illustrate the difference between genotype and
phenotype. What other factor(s) will affect phenotypic expression? (Answers
- Give an example to illustrate the difference in phenotype between complete
and incomplete dominance. (Answers will vary.)
- What is the difference in inheritance when two genes are on different
chromosomes vs when they are on the same chromosome? (They will be inherited
together if they are on the same chromosome, unless crossing over occurs.)
- Explain how it might be possible that a person could be genetically
unrelated to one of his/her biological grandparents. What assumption is being
made here? (Through random chance, none of the grandparent's chromosomes
ended in one of the gametes producing the person. The assumption here is that
no crossing-over occurs on any of the chro-mosomes.)
- What is the difference between crossing-over in sister chromatids and
crossing-over in homologous chromosomes? (There would be no difference in
sister chromatids as they are identical. There would be no new genetic
- If two genes linked on the same chromosome have a 50% cross-over rate,
what could you summarize about their inheritance? What would you infer about
their positions on the chromosome? (It would be the same as their being on
separate chromosomes. They probably have loci which are far apart.)
- What are your assumptions of the probability of inheritance between the
two genes for colorblindness and hemophilia vs the genes for Marfans and
Familial Hypercholesterolemia? (The genes for colorblindness and hemophilia
would be inherited together more frequently due to being linked on the same
- Which example illustrates polygenic inheritance? (Answers will vary.)
- Develop a model using chromosomes 13,14,16, and 18 which could illustrate
how a male could grow to a height of five feet ten inches. (Example could
be: AaBbCcDd = 5'4", each active gene adds 3", therefore
Karyotypes and Inheritance of Chromosomes
|1 (11700)||Rh blood type|
|85% Rh+ phenotype|
|Fragile, hyperflex skin|
|3 ** (with 6)||Acne (2 Locus model)|
N=active allele for acne
|NNNN = severe|
NNnn = mild
Nnnn = very mild
nnnn = none
|4 (143100)||Huntington's disease|
|mid-life neurologic decline|
|6 **(with 3)||Acne|
|6 (222100)||Diabetes mellitus, insulin dependent|
|7 (219700)||Cystic Fibrosis
|1/20 Caucasian carriers|
|9||ABO blood group|
IA, IB, i
|10 **||Short/long index finger
male - dominant
|11 (141900)||Sickle Cell hemoglobin|
|HbAHbS=sickle cell trait|
P = normal
p = PKU
Karyotypes and Inheritance of Chromosomes
(see 14, 16, 18)
A = active
B = active
b = inactive
|15 (272800)||Tay Sachs|
T = normal
t = Tay Sachs
|death usually within 2 years|
C = active
c = inactive
M = Marfans
m = normal
|20,000 affected in USA|
15% new mutation
N = normal
D = active
d = inactive
|18 (137589)||Tourette Syndrome
T = Normal
t = Tourette
|19 (143890)||Familial Hypercholesterolemia|
F = affected
f = normal
|300-500 Cholesterol levels|
|X (303700)||Xcb, XN = Colorblindness / Normal||Xq28|
|8% Caucasian males|
|X (306700)||Xh, XN = Hemophilia / Normal||Xq28|
|X (310200)||Xdmd, XN = Duchene
Muscular Dystrophy / Normal
|Testis determining factor|
KARYOTYPES AND INHERITANCE OF CHROMOSOMES
Dr. John Q. Frothingham III was a very respected, wealthy man from a
socially prominent family. He was the head of a major genetics research
laboratory in the East. He felt an intense duty to continue his family name
and genetic background. At the age of 55, he died in a plane crash. He had
stipulated that his estate would go to any males who would carry on his family
name and gene pool. His daughter Alice is married with a son and a daughter.
His son Gerald is 34 and still single. Dr. Frothingham's second wife
Christine is 12 weeks pregnant and her amniocentesis procedure confirmed that
her child is male.
Should Alice's son receive the entire estate? Should Reginald receive his
share of the estate? Should Alice or her daughter receive any of the estate?
Does Dr. Frothingham have a son with his second wife? Does the fetus have
a right to a part of the estate?
If you were a close personal friend of Dr. Frothingham, a colleague at
work, and named executor of his will, what would you suggest to follow Dr.
Frothingham's intentions as he had meant for them to be carried out?
Karyotyping and Inheritance of Chromosomes
The haploid human genome contains 23 chromosomes, containing a total of
about 50,000 to 100,000 genes. Each chromosome has a few hundred to several
thousand genes, depending upon its length and the size of the genes.
Chromosomes are arranged on a karyotype form by size, position of centro-mere,
and banding patterns.
The largest chromosomes are placed first and sequentially become smaller,
except for the X and Y chromosomes. Chromosomes which are metacentric have
the centromere (which binds the two replicated chromosomes together) in the
middle, submetacentric chromosomes have the centromere off-center, and
acrocentric chromosomes have the centromere close to one end. The shorter
arms of the chromosomes are called the p- petite arms and are positioned on
the top in a karyotype. The longer arms are the q arms. The locus (location
on a chromosome) for each gene is represented by the chro-mosome number,
which arm it occurs on, the section number, and the gene position. In the
illustration, the cystic fibrosis gene is indicated as 7q31. This reading
indicates its position is on the seventh chromosome, the q or longer arm,
section 3, gene position 1.
|Special thanks for original ideas to:||Gordon
Lawrence Central High School
McKusick, V.A. Mendelian Inheritance in Man . (9th ed.).
Baltimore: The Johns Hopkins University Press, 1990.
O'Brien, S.J. (Ed.). Genetic Maps Locus Maps of Complex Genomes
(5th ed.). Book 5 Human Maps. Cold Spring Harbor, NY: Cold Spring Harbor
Offner, S. "A Plain English Map of the Human Chromosomes."
The American Biology Teacher Feb (1992) 87-91.