Mendelian Inheritance in Humans

BIOL209: General Genetics Laboratory

Mendelian Inheritance in Humans


Students are expected to read all pages before coming to the lab to complete the experiments.

Print this entire lab packet and bring it to the laboratory.


After completing this laboratory assignment, students will be able to:

1. Observe some single-gene genetic traits in humans.

2. Solve problems involving Punnett squares and pedigrees

Modern genetics began with the work of Gregor Mendel and his study of pea plants. Mendel observed patterns in the number of each phenotype (physical appearance) of offspring from different parents. Mendel described genetic concepts that are used today to explain simple heredity (what traits are passed from one generation to the next) and variation (the differences between parents and their offspring). Through his experiments, Mendel determined that there can be multiple forms of the same gene. These alternate forms of genes, which are called alleles, code for slightly different expressions of a genetic trait. He recognized that some alleles exhibited dominant tendencies and others, recessive tendencies. When a dominant allele combined with a recessive allele, the dominant allele was expressed in the phenotype. Following the system that Mendel used in his experiments, a dominant allele is represented with a capital letter, and a recessive allele is represented with a lowercase letter.

For decades, teachers and professors have used a few classic human traits to demonstrate the concepts of simple autosomal inheritance. In this activity, you will observe the phenotypes of people in your class. You will apply your results, along with research findings, to predict whether each trait follows patterns of simple autosomal inheritance.

Mendelian Laws apply to the simple autosomal inheritance that Gregor Mendel studied, but these laws do not always apply to more-complex modes of genetic inheritance.

1. The Law of Segregation: Inherited traits are determined by genes, with two copies of each gene. Parental genes are randomly separated and segregated into each sex cell (gamete) with one copy of each gene per gamete. Offspring inherit one copy of each gene from each parent.

2. The Law of Independent Assortment: The inheritance of one trait is not dependent on the inheritance of another.

It turns out that some genes, and therefore some traits, are linked. Therefore, they are not assorted independently. Genes in close proximity to one another on the same chromosome are, in fact, more likely to be inherited together.

3. The Law of Dominance: If the inherited genes have multiple alleles, the dominant allele will determine the phenotype of the organism.

In fact, some alleles are codominant, meaning that multiple alleles contribute to the phenotype of the individual. Traits may have more than two alleles for the same gene. Furthermore, multiple genes may contribute to the phenotype of one trait.


This lab is designed to demonstrate genetics, or the study of how heritable characteristics are passed from generation to generation. Genetic traits are determined by genes, or small segments of DNA carried on chromosomes that determine physical characteristics. This exercise demonstrates how different alleles of the same gene segregate and reassort (separate and come together) in a population.

Part I: Observing Mendelian Inheritance in Human Traits

1. Students will work in groups of two to complete the assignments.

2. Every person will submit an individual lab report for this lab.

3. Mark your appropriate phenotype for each trait on the chart.

4. Determine your corresponding genotype for each trait; if dominant, use the heterozygous genotype.

Human Genetic Traits

A. Facial Dimples: Dimples, or indentations, at the corner of the mouth are a dominant trait.

B. Bent little finger: A dominant allele causes the last joint of the little finger to dramatically bend inward toward the 4th finger. Lay both hands flat on a table relax your muscles, and note whether you have a bent or straight little finger.

C. Eye Color: Brown pigmented irises are dominant. The absence of brown pigment results in blue eyes, which is recessive. Hazel or green eye color is the result of a second gene that produces a yellow pigment. Hazel eyes have both brown and yellow iris pigment, while green eyes have both the recessive blue iris and dominant yellow pigment. For our purposes, assess only the presence of absence of brown pigment.

D. Free Earlobes: Free earlobes are dominant over attached earlobes. Read the introduction to Chapter 9 for an explanation of how this trait is produced during fetal development.

E. Mid-digital Hair: The presence of hair on the middle joint of the finger is a dominant trait. Hair may not be present on all of your fingers, but if you have hair on even one finger, you are dominant.

F. Hand clasping: When the hands are clasped (without thinking about it!), most people place their left thumb on top of their right.

G. Widow’s Peak: A distinctive downward point of the frontal hairline is a dominant trait known as a widow’s peak. If you have a straight hairline, you are recessive for this trait.

H. Tongue Rolling: The ability to roll the tongue upward from the sides is a dominant trait. For some reason, people who exhibit this trait seem to think it is a desirable thing to do. As far as anyone knows, tongue rolling has no obvious anatomical or physiological advantage or disadvantage.

I. Chin cleft: A prominent cleft in the chin is inherited as the dominant phenotype. The cleft is due to the bond structure which underlies the Y-shaped fissure of the chin. Females appear to be less conspicuously affected than males.

J. Hitchhiker’s Thumb: The ability to bend the thumb backward at least 45 is a dominant trait. The proper term for this is distal hyperextensibility.

K. Handedness: Right-handedness is an autosomal dominant trait.

L. Finger number: The number of fingers on the hand is controlled by genes. Having more than 5 digits is dominant.

M. PTC Tasting: The ability to taste the chemical phenylthiocarbamide, or PTC, is a dominant trait. Place a piece of PTC paper on the back of your tongue. If you can detect this chemical, it will have a bitter taste. If the paper does not taste nasty to you, then you are recessive for this trait.

N. Relative Length of the Big Toe: If your big toe is shorter than your second toe, you are dominant for this trait.

O. Palmaris Longus Muscle: The presence of this muscle is a dominant genetic trait. If you have this muscle you will have three wrist tendons. To determine this, clench your fist tightly and flex your hand toward you. If you can see or feel three tendons in your wrist, you have the long palmar muscle and are dominant. If you have only two tendons, then you are recessive for this trait.

Data Sheet

Class Data
Traits Symbols Dominant Phenotype Your phenotype Your genotype Dom Rec
1. Facial dimples D, d Dimples d
2. Bent little finger B, b Bent little finger b
3. Eye Color E, e Brown eyes E
4. Free ear lobe F. f Free ear lobe F
5. Mid-digital hair H, h Presence of hair h
6. Hand clasping L, l Left on top L
7. Widow’s peak W, w Widow’s peak w
8. Tongue Rolling T, t Tongue roll T
9. Chin cleft C, c Cleft in chin c
10. Hitchhiker’s Thumb H, h Straight


11. Handedness R, r Right R
12. Finger number S, s Six-digits s
13. PTC Tasting P, p Tasting P
14. Big Toe Length G, g Longer Big toe g
15. Palmaris Longus Muscle M, m Three tendons m


1. Did you have mostly dominant or recessive traits?


2. Compare your findings with other students.

a. For which trait were most students dominant?

b. For which traits were most students recessive?

Part II: Mendelian Genetics: Monohybrid Crosses

A monohybrid cross is the genetic transmission of a single trait. For this exercise, you will “mate” (exchange gametes) with the person listed and determine the genotypes and phenotypes of the offspring that could result from each mating, or “cross”.

1. Complete a Punnett Square using your genotype and the given genotype. Calculate the probability of all the different possible genotypes and phenotypes of your offspring in percentages.

a. Widow’s Peak: Yours_____Ww___________ & your partners: ww

b. Tongue Rolling: Yours_____TT___________ & your partners Tt

c. Hand Clasping: Yours______LL__________ & your partners LL

2. Two people who are heterozygous for tasting the chemical PTC marry. List the genotypes possible for their children regarding the tasting of PTC. Show your work.

Part 2: Dihybrid Crosses

1. Alfred is heterozygous for tongue rolling and has five fingers. Alfreda, his wife, cannot roll her tongue and is heterozygous for polydactyly.

a. What is Alfred’s genotype?

b. What is Alfreda’s genotype?

c. What are Alfred’s possible sperm with regard to these two traits?

d. What are Alfreda’s possible eggs with regard to these two traits?

e. What is the probability that their first child will not roll its tongue and will have the normal number of fingers?

Part 3: Pedigrees

1. Determine the mode of inheritance.

2. Label the generations, genotypes and correct symbols (if applicable)

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