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Lab Exercise 15

Human Genetics

Objectives

- Become familiar with the human karyotype.

- Be able to recognize abnormal karyotypes.

- Be able to diagram a pedigree for a specific trait.

- Be able to interpret a pedigree for a specific trait.

Introduction

While Mendelian genetic principles apply to humans just

as they apply to all other organisms, as a society we

constrain ourselves from conducting breeding experiments

on human subjects in an effort to gather information on

the nature of inherited traits. Consequently we rely on

other sources of information to obtain this knowledge.

One source of information comes from study of the human

karyotype, the number and appearance of our chromo-

somes. Another source of information is from the study

and interpretation of pedigrees.

The number and appearance of the chromosomes as

they enter metaphase is called an organism’s karyotype.

Irregularities seen in an individual’s karyotype usually lead

to abnormalities which in most cases are quite detrimental

to the individual. The first exercise in the lab has you ex-

amine the human karyotype and then look at some of the

abnormalities which may occur.

When using pedigree analysis to study the nature of inheri-

tance of particular traits, families are investigated to deter-

mine which individuals within the family possess the trait

in question. Then using deductive reasoning, the nature

of the trait’s inheritance in inferred. The second exercise

in this lab asks you to determine your phenotype for some

inherited traits. The third exercise involves the construction

and interpretation of pedigrees.

Contents

Objectives 1

Introduction 1

Activity 15.1 Human Karyotype 2

Activity 15.2 Some Human Traits 2

Activity 15.3 Pedigree Analysis 4

Resutls Section 5


Activity 15.1Human Karyotype

An individual’s karyotype is the appearance and number

of the chromosomes in their cells. The normal human

karyotype consists of 22 pairs of chromosomes of various

sizes plus a pair of sex chromosomes for a total of 46

chromosomes. Examine the normal chromosomes of a

normal human karyotype in the Human Genetics simula-

tion on the BiologyOne DVD. Then examine and read

about some of the abnormal karyotypes that occur. These

abnormalities are the result of chromosomes failing to

separate properly during meiosis, events that are referred

to as nondisjunctions.

Examine the chromosomes displayed in the Results

Section. These are how they would appear through the

microscope when stained. Try cutting these out and ar-

ranging these to show the individual’s karyotype. Use the

chromosome size, location of the centromere and banding

patterns to pair chromosomes. Does this individual have a

normal karyotype? What is their sex?

You can also try constructing a karyotype using the il-

lustration of the chromosomes in the Results Section (this

will probably be easier than pairing with the images of the

actual chromosomes). What is this individual’s sex? Is

their kryotype normal?

Activity 15.2Pedigree Analysis

Since it is neither practical nor morally acceptable to breed

humans in an effort to determine the nature of genetic

inheritance of specific traits, scientists rely on another

method to infer modes of inheritance. This is the study of

family trees or pedigrees. By analyzing the pedigree, one

may be able to deduce how a gene for a specific trait is

inherited.

In a pedigree analysis, individuals are represented by

symbols. Open shapes usually represent individuals who

do not express the trait while filled shapes represent

individuals who express the trait being investigated. If

heterozygous individuals can be determined they are usu-

ally represented by partially filled shapes. Unions and the

offspring from unions are represented in the pedigree by

connecting lines as indicated in below.

Study the examples of a pedigrees shown in the Results

Section and then answer the questions concerning the

other pedigrees. Use the information in the Results Sec-

tion to construct a pedigree.

If you were able to collect information from your family for

some of the genetic traits listed in section 15.2, select one

of those traits and construct the pedigree for your family

for that trait.

Symbols used in Pedigree Charts

male without trait

male carrier

male with trait

female without trait

female carrier

female with trait

individual of unknown sex

male/femlae union

siblings

fraternal twins

identical twins


Activity 15.3Some Human Traits

Through pedigree analysis, a number of human character-

istics have been determined to be controlled by a single

gene and are easily demonstrated to follow Mendelian

laws of inheritance and the Hardy-Weinberg predictions.

Several of these traits will be explored in this exercise.

Use the following descriptions of traits to determine

your phenotype and possible genotype(s). Record your

observations in the Report Section. If you are able, also

determine the phenotypes of your relatives. Your instruc-

tor may also collect data from the class to determine the

population frequencies for these traits.

Tongue Rolling. The ability to roll one’s tongue lengthwise

is a genetic inherited trait. The ability to roll your tongue

(R) is dominant to the inability to roll your tongue (r).

Freckles. Freckles (F) are dominant over no freckles (f).

Widow’s Peak. If your hairline forms a point in the middle

of your forehead you have what is called a widow’s

peak. The allele for a widow’s peak (P) is dominant to no

widow’s peak.

Bent Phalanges. If you hold your hands with your palms

toward you and the last joints of your little fingers flare

away from each other, you have the trait known as bent

phalanges. Bent phalanges (B) is dominant over straight

phalanges (b).

Earlobes. If your earlobes hang below the point where

your ear attaches to your head, you have free earlobes. If

they do not hang down, you have attached earlobes. Free

earlobes (E) are dominant to attached earlobes (e).

Long Palmar Muscle. When you clench both fists, palms

up, and slowly rotate your hand, do you see two or three

tendons in your wrist. If you only see two tendons in both

wrists you do not have long palmar muscles and your

genotype is pp. If both wrists have three tendons, you are

probably homozygous for this trait (genotype PP). If only

one wrist has three tendons, you are probably heterozy-

gous (Pp).

Hitch-hiker’s Thumb. If you can bend the tip of your thumb

backwards so that the last joint is nearly a right angle to

the rest of your thumb, you have what is called a hitch-

hiker’s thumb. Hitch-hiker’s thumb is recessive (h) to a

non-hitch-hiker’s thumb (H).

Dimples. One or more facial dimples (D) is dominant over

no dimples (d).

Interlace Fingers. Clasp your hands together and look at

the position of your thumbs. If your left thumb is over the

right thumb, you have the dominant phenotype (II or Ii).

If your right thumb is over your left thumb you have the

recessive phenotype (ii).

Red-green Color Blindness. This is a sex-linked, recessive

trait. Since females have two copies of the X chromosome,

normal color vision females can be either CC or Cc while

color blind females will be cc. Normal males will have the

genotype CY and color blind males will be cY. At the end

of the Human Genetics simulation on the BiologyOne CD

is a color dot pattern you can use to test your color vision

but use the results of this test with caution since computer

color displays are somewhat variable.


Illustration of Some Human Traits

free earlobe

attached earlobe

straight hairline widow’s peak

bent phalangesstraight phalanges


Results SectionName _______________________Lab Exercise 15

Activity 15.1Human Karyotype

Cut out the photograph of chromosomes on the next page and arrange them in pairs in the spaces below.

Chromosome pairs are numbered from largest chromosomes to smallest with the sex chromosomes shown

last.

________

1

________

2

________

3

________

4

________

5

________

6

________

7

________

8

________

9

________

10

________

11

________

12

________

13

________

14

________

15

________

16

________

17

________

18

________

19

________

20

________

21

________

22

________

sex


Chromosomes


Cut out the illustration of chromosomes on the next page and arrange them in pairs in the spaces below.

Chromosome pairs are numbered from largest chromosomes to smallest with the sex chromosomes shown

last.

________

1

________

2

________

3

________

4

________

5

________

6

________

7

________

8

________

9

________

10

________

11

________

12

________

13

________

14

________

15

________

16

________

17

________

18

________

19

________

20

________

21

________

22

________

sex


Chromosomes


1 2 43

5 6 7 8 9 10 11

12 13 14

Activity 15.2Pedigree Analysis

Example of pedigree analysis: The following pedigree is for myopia (nearsightedness).

What is the nature of its inheritance?

Analysis:

1. The trait appear to be equally expressed in males and female so it is likely not sex linked.

2. The trait occurs in individual #1 but in none of his children and the trait does not occur in

parents 3 and 4 but does occur in their children. Conclusion: the allele for this trait is

probably recessive.

Test of conclusion: If the allele for myopia is recessive, individauls 1, 11, and 13 must be homozygous

for this trait. Others may be heterozygous or homozygous non-myoptic. Each child from

parents 1 and 2 would be heterozygous, inheriting the myoptic allele from their father. For

individual 11 to inherit the trait from parents 3 and 4, both parents must be heterozygous. They

would then be expected to have 1 out of 4 children with the trait (lucky here). Each normal

child would have a 2/3 chance of being heterozygous.

In the below space, diagram a family pedigree in which a couple has 5 children, 3 boys and 2 girls. One

of the girls and a boy are fraternal twins. The mother’s phenotype displays the trait in question. Two of

the boys (not the twin) and the twin girl display this phenotype as well. One of the boys, not the twin,

marries a woman who does not express this phenotype. They have two girls, neither of which has this

phenotype.

Diagram a Pedigree


Interpreting a Pedigree

21

3 764 5 9

9 10 11

The trait denoted in the following pedigree is known to be recessive.

Answer the below questions.

1. What is the possible genotype of individual #1? For individual #4?

2. Do you think this is trait is sex-linked?


If you collected family data for the human traits - construct a pedigree of your family below


Activity 15.3Some Human Traits

Tongue

Rolling

Freckles

Widow's

Peak

Bent

Phalanges

Earlobes

Long

Palmar Muscle

Hitch-hiker's

Thumb

Dimples

Interlace Fingers

Red-gree Color

Blindness

Your phenotype Father's phenotype Mother's phenotype Sibling phenotypes % of class

& genotype & genotype & genotype & genotypes with trait

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