Lab 3:  Karyotypes and Pedigrees

Objectives
 

Upon completion of this lab you should:

• Be able to identify human traits that are genetically controlled.
• Be able to determine the possible genotype(s) and phenotype of individuals with certain genetic traits.
• Be able to pair homologous chromosomes for a karyotype.
• Be able to determine the sex, number of chromosomes, and genetic condition (normal vs. abnormal) of the karyotype assigned.
• Be able to read pedigrees.
• Be able to determine the mode of inheritance for a trait using a pedigree.

Lab Report

A lab report is due at the beginning of your next lab period.   Answer the questions in this lab exercise.  Be complete; use sentences.

Part 1:  Mendelian Inheritance in Humans
 

There are a number of single gene traits that can be readily observed in humans. The genes for many of these traits have only 2 alleles, with each allele producing a distinct phenotype.  Work in pairs during this portion of the lab exercise.  Identify the following traits on yourself and your lab partner.  Enter your phenotypes, possible genotype(s) and possible gentoypes of your childern (assuming your partner is of the opposite sex) for each trait in the spaces provided.

PTC Taster: Place a piece of PTC (phenylthiocarbamide) paper on your tongue. If the paper has a bitter taste, you are a "taster" and would have the dominant allele (T). If the paper is bland, you are a non-taster and are homozygous recessive (tt) for this trait.
 

Your phenotype:
Your possible genotype(s):
Partner's phenotypes:
Partner's possible genotype(s):
Children's possible genotype(s)

 
Problem: Jack cannot taste PTC, but both his mother and father can taste PTC. Complete a Punnett Square to determine the expected phenotypic ratio among the offspring. In this family, what are the chances that a child will be able to taste PTC ? What are the chances that a child would not be able to taste PTC?

Sodium Benzoate Taster: Place a piece of sodium benzoate taste paper on your tongue. If the paper has a salty taste, you are a "taster" and would have the dominant allele (T). If the paper is not salty, you are a non-taster and are homozygous recessive (tt) for this trait.
 
 
 

Your phenotype:
Your possible genotype(s):
Partner's phenotypes:
Partner's possible genotype(s):
Children's possible genotype(s)

 

Widow's Peak: If you have a pointed hairline (forms a V), you have the dominant allele for widow's peak (W). Count Dracula is usually depicted with a widow's peak. If you have a straight hairline, you are homozygous recessive (ww) for this trait.\
 

Your phenotype:
Your possible genotype(s):
Partner's phenotypes:
Partner's possible genotype(s):
Children's possible genotype(s)

Attached Ear Lobes: If your ear lobes are even partially detached from the side of your face, you have the dominant allele for "detached ear lobes" (E). If your ear lobes are attached to the side of your face, you are homozygous recessive (ee) for this trait.
 
 

Your phenotype:
Your possible genotype(s):
Partner's phenotypes:
Partner's possible genotype(s):
Children's possible genotype(s)

 

Mid-Digital Hair: Each finger on your hands has 3 segments. If any hair is present on the middle segments, you have the dominant allele for mid-digital hair (D). If you do not have hair on any of the middle segments of your fingers, you are homozygous recessive (dd) for this trait.
 
 

Your phenotype:
Your possible genotype(s):
Partner's phenotypes:
Partner's possible genotype(s):
Children's possible genotype(s)

 

Blue-Gray Eye Color: If you have blue-gray eyes, there is little or no pigment present on the surface of your iris and you are homozygous recessive for this trait (ii). If your eyes are a color other than blue-gray, there is pigment present on the surface of your iris and you have the dominant allele for eye pigment (I). The exact eye color of a non-blue-gray-eyed individual is more complex than this (see the text) since eye color is controlled by other genes that code for combinations of various pigments such as brown, green, or hazel.
 
 

Your phenotype:
Your possible genotype(s):
Partner's phenotypes:
Partner's possible genotype(s):
Children's possible genotype(s)

 

Incomplete Dominance in Humans - Long Palmar Muscle: Clench your left hand tightly and feel your wrist with the right hand; do the same for the right hand. If you have 3 tendons in a wrist you have a long palmar muscle. If you have a long palmar muscle in both wrists, you are homozygous recessive (pp). If you do not have a long palmar muscle in both wrists, you are homozygous dominant (PP). If you lack a long palmar muscle in only one arm, you are heterozygous (Pp) for this trait. This trait would be an example of incomplete dominance.
 
 

Your phenotype:
Your possible genotype(s):
Partner's phenotypes:
Partner's possible genotype(s):
Children's possible genotype(s)

Sex-Linked Traits - Red-Green Color Blindness:

The ability to distinguish all colors is determined by a number of genes. A defect at one of these genes may remove the ability to perceive certain colors (e.g., red-green color blindness). We will only be concerned here with red-green color blindness.

Since red-green color blindness is a sex-linked trait, the allele is located on the X chromosome. If you see colors normally, you carry the dominant allele for this trait (XBXB or XBXb for females or XBY for males). If you are red-green color blind, you carry the recessive allele. Since this is a sex-linked trait, the only possible genotype for color blind males is XbY and the only possible genotype for color blind females is XbXb.

 
 

Your phenotype:
Your possible genotype(s):
Partner's phenotypes:
Partner's possible genotype(s):
Children's possible genotype(s)


Questions

 
• For a female to be red-green color blind her mother's genotype may be either   _______or ________  but the her father's genotype must be  ________.  Why?
• If a male is color blind, but his mother is not color blind, then her genotype must be  _______. Why?  Is it possible for the mother's genotype to be different if the male's father is color blind?

Part 2:  Karyotypes

Typing of human chromosomes is carried out to detect hereditary defects.  The resulting chromosome preparation is called a karyotype.  The chromosomes in a karyotype are chromatid pairs, since mitosis is interrupted at metaphase with the addition of colchicine (this drug prevents subsequent mitotic steps).  Certain abnormalities, such as an extra chromosome, can be identified in karyotypes.  In some cases, genetic counseling centers use karyotyping to advise couples whether or not to have children.  Karyotypes can be made while the fetus is still in the uterus.  The process of removing amniotic fluid containing fetal cells is called amniocentesis.

In this lab exercise, you will have the opportunity to analyze diagrams depicting actual karyotypes.  By pairing homologous chromosomes, you will determine the sex of the individual and his/her chromosomal complement (karyotype).  You will then reference the table below to determine the name of the chromosomal abnormality.

Procedure

1. Obtain a copy of an unknown karyotype and a blank karyotype form from your instructor.
2. Write down the number of your unknown karyotype in the top right corner of the blank karyotype form.
3. Count the total number of chromosomes in the preparation and double check your count. This will establish whether you have a karyotype with the normal number of chromosomes or a variant karyotype, such as Edward's syndrome.
4. Cut out the chromosomes individually, leaving a small border of white paper around each chromosome. Be careful not to lose any!
5. Arrange the chromosomes as homologous pairs on the blank karyotype form on the following page, with the largest chromosomes in the "1" position and the smallest at the end. You can recognize homologous chromosomes by similarity of centromere position and banding patterns.
6. Do not permanently attach the chromosomes to the form until you are sure of the placement of ALL chromosomes!
7. After you are sure of the placement of the chromosomes, tape them in place on the blank karyotype form.
8. Answer the questions below based on your karyotype.

Questions

1. What is the sex of the karyotyped individual?
2. How many chromosomes does this individual possess?
3. What (if any) chromosomal abnormality does this individual demonstrate? Is there a name (syndrome) associated with this chromosomal abnormality?
4. In what sex(es) can this chromosomal abnormality occur?

Many studies of human inheritance involve the tracing of a conspicuous trait, such as hair color, in individual families.  Data on the expression of the trait in each member of a family are recorded.  The relationship of the individuals, their sex, and transmission of a trait over several generations are illustrated in a diagram called a pedigree.

A pedigree, to be of value for analysis, must include several generations and preferably be from a family with many children.  In the pedigree diagram, a Roman numeral indicates each generation.  The members of that generation, whether related or not, are indicated by Arabic numbers.  Females are represented by circles and males by squares.  Individuals demonstrating the trait are shaded black.

Your first task in any pedigree analysis is to determine the mode of transmission of the trait in question.  That is, does it appear to be transmitted as a dominant or recessive trait, does it appear to be autosomal or sex-linked, etc.  Of course, none of these may apply, since the trait may be polygenic. In the latter case, little definitive evidence can be drawn from the pedigree.

The analysis of family history is an important tool for genetic counselors.  It has been estimated that about 1% of all children born have a chromosomal abnormality with moderate to severe phenotypic effects.  An additional 1 to 2% bear the results of single gene defects.  By looking at several generations in a family, transmission of a particular trait may be studied and some predictions made about the next generation.

1.  Red Hair Pedigree

The pedigree for red hair inheritance in three generations illustrates the use of a pedigree.

In generation I, two individuals (I-2 & I-3) have red hair. No individuals in generation II show the trait, but it reappears in generation III in individuals III-2 and III-3.  The reappearance of the trait in generation III among children whose parents' hair is not red indicates that red hair is a recessive characteristic.  If it was caused by a dominant allele, at least one of the parents would have been carrying it, and therefore would have had red hair.  This is not the case.  With a recessive allele, both parents could be carrying it (as heterozygotes) without showing the trait.  .

It is not always possible to tell if a recessive gene is sex-linked or autosomal. However, this pedigree does give us enough information to determine that the red hair recessive gene is autosomal.  Individual III-3 is an affected female.  If the allele was X-linked, this female would have received it from both her parents.  This is not possible because her father (individual II-3) would have had red hair (males only have one allele of X-linked genes because they have a Y chromosome instead of a second X chromosome).  The gene cannot be Y-linked because females have the trait.

Since we have determined that the red-hair gene is autosomal recessive, we can write the genotypes entirely or in part for the individuals in the pedigree. Each individual showing the trait is homozygous recessive (rr: I-2, I-3, III-2, and III-3).  All individuals in generation II are heterozygotes because none of them have red hair, but each of them has a homozygous recessive parent (Rr: II-1, II-2, II-3, and II-4).  All other individuals must have at least one dominant allele, because they are not red-haired, but they could be either homozygotes or heterozygotes (R_: I-1, I-4, III-1, III-4, and III-5).
 

 

 

 

2.  Trait A Pedigree

Use the Pedigree for Trait A to determine the genetic basis of this trait.

 
 

Questions

1.  Does a dominant or recessive allele produce the trait?  Explain.
2.  Is it autosomal or sex-linked?  Explain.
3.  What are the genotypes of all the individuals in the pedigree?  (Write them on the pedigree.)
4.  What is the genotype of individual IV-2?  Explain.
5.  What is the genotype of individual IV-6?  Explain.
6.  What is the genotype of individual I-1?  Explain

 

3.  Alkaptonuria Pedigree

This inherited defect is characterized by darkening of cartilage in the ears, melanin spots in the eyes, proneness to arthritis, and darkening of urine upon exposure to air. It is caused by a metabolic error that results in failure to produce functional homogentisic acid oxidase (originally called alkapton oxidase).

Use the Pedigree for Alkaptonuria to determine the genetic basis of this trait.
 

Questions

1.  Does a dominant or recessive allele produce the trait?  Explain.
2.  Based on information from this pedigree alone, is it autosomal or sex-linked?  Explain.
3.  Assume that alkaptonuria is caused by an autosomal allele.  Of which individuals are we certain of the genotype?  List them and explain.
4.  Assume that alkaptonuria is caused by an X-linked allele.  Of which individuals are we certain of the genotype?  List them and explain.
5.  Assume that the allele causing alkaptonuria is autosomal and rare.  What is the probability that III-2 is a heterozygote?  Explain.
6.  Assume that the allele causing alkaptonuria is autosomal and rare.  What is the probability that II-4 is a heterozygote?  Explain.
7.  Assume that the allele causing alkaptonuria is autosomal and rare.  What is the probability that IV-2 is a heterozygote?  Explain.