3.2: Phenotype Analysis
Background Information
Most cells making up the human body contain a nucleus. Inside of the nucleus are chromosomes that are responsible for transmitting hereditary information from cell to cell in cell division and from parent to offspring in reproduction. Chromosomes are thread-like structures that consist of tightly coiled DNA (deoxyribonucleic acid), the genetic material, and associated proteins. DNA essentially serves as the blueprints for life. DNA is organized into genes. Genes (i.e., "packets of DNA") control the structure and function of all cells, as well as large-scale processes such as development and reproduction. Most genes encode for a specific protein. Proteins play a variety of different essential roles in the body, including acting as enzymes, allowing communication between cells, immune functions, regulation of molecules into and out of cells, contraction of muscles and many more.
Humans have 23 pairs of chromosomes (46 total chromosomes) in the nucleus of all somatic cells (non-sex cells). However, the sex cells (sperm cells and eggs) contain only one copy of each chromosome. Human reproduction requires a sperm cell to fuse with an egg during fertilization to create the zygote. The zygote contains 23 pairs of chromosomes. Twenty-two pairs of the chromosomes are autosomes, containing genes that are unrelated to sex. The 23rd pair of chromosomes are the sex chromosomes, X and Y. Females have two X chromosomes and males have an X and a Y chromosome. A single gene on the Y chromosome, SRY (sex-determining region Y), triggers an embryo to develop male physical characteristics.
Chromosomes that are the same length and contain the same genes are called homologous chromosomes. Humans have 22 pairs of homologous chromosomes, receiving one of each pair from the mother and one of each pair from the father. Each chromosome contains thousands of individual genes. There are thought to be about 20,000 genes in human DNA. Interestingly, the number of genes in human DNA is not appreciably different from the number of genes in chimpanzees or mice.
Some human traits are determined by a single gene, or mostly determined by a single gene. Different versions of a gene allow for variations in a population. The different versions of a gene are called alleles. For example, earlobe shape is controlled by a single gene. One allele for earlobe shape causes individuals to have free earlobes and another allele causes people to have attached earlobes. Humans have two alleles for each gene (receiving one allele from our mom and one allele from our dad). If both alleles are the same version then this is referred to as homozygous. When an individual has two different alleles for the same gene this is referred to as heterozygous. The alleles that an individual has are referred to as the genotype. But, the outward expression of the alleles (what we see) is the phenotype. The phenotype that is expressed in heterozygous individuals is the dominant phenotype and is caused by the dominant allele. The recessive allele is the phenotype that is not seen in the phenotype of heterozygous individuals. The dominant allele is generally represented by a capital letter, while the recessive allele is generally represented by a lowercase letter. The alleles for earlobe shape are E and e. The dominant allele, E, determines free earlobes. Homozygous dominant (EE) and heterozygous (Ee) individuals have the free earlobe phenotype. But, homozygous recessive (ee) individuals have the recessive attached earlobe phenotype.
Most traits are determined by more than one gene. For example, skin color and height are determined by many genes. A few human phenotypes however, are determined by a single gene. We will explore some of these single gene traits in the laboratory.
Materials needed
- A group of students to observe phenotypes
Exercise 1: Phenotype Analysis
- Use a lab partner to help you determine your phenotype for the traits listed.
- Complete the table below. Use two alleles per trait for the genotype. If you exhibit the dominant phenotype, use a dash to represent the second allele.
Example :
B_ genotype for the phenotype of brown eyes (dash indicates second allele could be B or b which means a genotype of BB or Bb)
bb genotype for the phenotype of blue eyes
Interlocking fingers
Interlock fingers. Observe which thumb is on top (right or left). The tendency to place the left thumb on top is due to a dominant allele (I) and the genotype is I- (either II or I-). The right thumb on top is determined by the ii genotype.
Ear lobes
The dominant allele (E) results in the phenotype of free earlobes. The recessive allele (e) is for attached earlobes.
Widow’s peak
Widow’s peak occurs when the hairline forms a distinct point in the center of the forehead. Widow’s peak is controlled by a dominant allele (W).
Tongue curling
A dominant allele (T) gives the individual the ability to curl the tongue in a U-shape.
Hitch hiker’s thumb
A person homozygous recessive for this trait (hh) can bend the last (distal) thumb joint back to about a 90 degree angle. Those with the H allele cannot.
PTC tasting
If you can taste the bitterness of PTC, you have the dominant allele (P). Place the PTC paper on your tongue for a few seconds. If you cannot taste anything, you do not possess the dominant allele.
Straight hair
A person with the dominant allele (H) has straight hair. Homozygous recessive individuals have curly hair.
Handedness
If you are right-handed then you have the dominant allele (R). The recessive allele (r) encodes left-handedness.
Mid-digital hair
A person with the dominant allele (H) has hair on the middle joint of any finger. People with no hair on the middle finger joint are homozygous recessive.
Number of students in class_________
| Trait and alleles | My phenotype is: (dominant or nondominant) | My genotype is (X_ or xx) | # in class with dominant phenotype | % of class with dominant phenotype |
|---|---|---|---|---|
| Interlocking fingers (I or i) | ||||
| Earlobes (E or e) | ||||
| Widow’s peak (W or w) | ||||
| Tongue curling (T or t) | ||||
| Hitchhiker’s thumb (H or h) | ||||
| PTC tasting (P or p) | ||||
| Straight hair (H or h) | ||||
| Handedness (R or r) | ||||
| Mid digital hair (h or h) |