The chromosomal theory of inheritance outlines how the movement and thereby transmission of chromosomes from parent to child, results in the patterns of inheritance described by Gregor Mendel. While the law of segregation stipulates the diploid set of each parent genome will separate into the haploid gametes, segregation does not ensure the chromosome will be identically inherited.
Having a complete set of genetic information is critical to organismal health and function. A regular human cell has 46 chromosomes, including 44 autosomes, which come in pairs, and two sex chromosomes, which specify gender (XX for female and XY for male).
The pairs of autosomes are called homologous chromosomes. One of each pair came from mom, and the other came from dad. Homologous chromosomes have all the same genes (arranged in the same order), but often exhibit slight differences in the DNA sequence of the genes.
Crossover is a unique meiotic event that occurs between homologous chromosomes aligned at the metaphase plate in meiosis I. Here, the diploid set of parental chromosomes may transpose homologous (regions coding for the same traits) sections of a given chromosome, contributing a new variation to the gene pool Crossover, which increases genetic diversity is considered beneficial.
In contrast, a single chromatid can be subject to anomalies, such as duplications, translocations, inversions, and deletions. These variations can be the result of incorrect distribution into gametes (the egg or sperm cells) during meiosis. The failure to segregate correctly is called nondisjunction. A gamete with too many or too few chromosomes, upon fertilization, may produce a zygote with too many (trisomy = three copies) or too few (monosomy = one copy) chromosomes. Most autosomal (non-sex chromosome) trisomies and monosomies are lethal. A few trisomies will result in viable offspring. However, these individuals exhibit severe genetic disorders.
Even within a chromosome, alternate versions of a trait may be coded for by the genetic content. Each trait is coded by the two distinct forms (alleles), one from each parent. Genetic conditions caused by a mutation in a single gene follow predictable patterns of inheritance within families. Inheritance patterns are described, depending on the type affected chromosome, and the relationship between genotype (genetic composition) and phenotype (physical expression).
Autosomal dominant conditions are expressed in individuals who have at least one copy of the mutant allele. Affected individuals, regardless of gender, have an equal probability of passing on the trait to their offspring. Affected individuals may retain one normal copy of the gene, providing each offspring a 50% chance of inheriting the normal allele.
Autosomal recessive conditions only manifest when an individual has two copies of the mutant allele. If an individual is heterozygous (contains one normal and one altered allele), they will be unaffected, but a carrier. Both genders are equally affected, and should two carriers mate; there's a 25% chance of the child being a homozygous mutant (affected) and a 50% chance of being a carrier.
Like the autosomal recessive trait, X-linked recessive traits do not typically manifest if there is a normal copy of the gene. However, because males have only one X-chromosome, any altered allele will be exhibited. Women, on the other hand, are rarely affected as they must possess two copies of the mutant allele. Because the gene is on the X-chromosome, there is no father to son transmission, but all daughters will become carriers. Both genders have a 50% chance of inherited the altered allele from the mother.
X-linked dominant transmission can result from an affected woman passing on the mutant allele (50% possibility if she is heterozygous) to her children or a father to his daughters (100% certainty). A single mutant allele will result in clinical manifestation.
Part 1: Prepare a Karyotype
1. Carefully cut out the individual chromosomes. Using the sample karyotype as a guide, arrange each chromosome on the blank karyotype. Note the size and banding patterns of each chromosome to correctly identify their placement in the karyotype
2. Count the chromosomes. The normal number is 46.
a. If there is extra chromosomal material there may be a trisomy, if the duplicated chromosome is a sex chromosome there are two possibilities: Klinefelter syndrome (a male with two X chromosomes) or an XYY male.
b. If there are 45 chromosomes, there may be monosomy, which is always lethal, except in the case of sex chromosomes. A single X chromosome accounts for the condition known as Turner syndrome (X).
3. Identify the gender of your patient. _____________________________
4. Identify the disorder ___________________________________________________________________________________________________________________________
5. Identify whether they will live ______________________________
Part 2: Personal Gene Inventory
Determine your phenotype and possible genotypes for each of the following characteristics.
For finger hair, even if you have only one hair on any of your mid-digits, you have finger hair.
1. Tongue Rolling: Attempt to roll your tongue into a “U” shape. Tongue rollers carry the dominant allele for the gene “R”.
2. Widow’s Peak: a dominant allele of the gene “W” causes the hairline to form a distinct point in the center of the forehead. Baldness will mask the expression of this gene.
3. Earlobe attachment: The inheritance of the dominant allele of the gene “E” results in the free, or unattached earlobe. Other genes affect the size and shape of the earlobe.
4. PTC Tasting: Phenylthiocarbamide (PTC) is a harmless chemical in small quantities. To individuals with the dominant allele of the gene “T”, it tastes undesirably bitter.
5. Hitchhiker’s Thumb: Some individuals can bend the last joint of their thumb backward at about a 45-degree angle. These individuals are homozygous recessive for the gene “H”. Note there is considerable variation in this trait.
6. Bent Little Finger: The dominant alleles of the gene “B” causes the terminal bone of the little finger to angle towards the fourth (ring) finger. Check for the characteristic by laying your hands flat on the table and relaxing them.
7. Mid-Digital Hair: The presence of hair on the middle segment of the fingers is caused by the dominant allele of the gene “M”.
8. Facial Dimples: the inheritance of cheek dimples is controlled by the dominant alleles of the gene “D”.
9. Hallux length: The length of the big toe (Hallux) is governed by the gene “Ha”, individuals whose hallux is shorter in comparison to the second toe possess the dominant allele.
10. Index finger length: Relax your hand on top of the table. Compare the length of your second finger (index finger) to that of your fourth (ring finger). The allele for the short index finger “SS” is sex influenced in its expression (like baldness) and is dominant in males and recessive in females. The allele for a long second finger is “SL”. See the table below to determine your possible genotypes:
|Gender||Phenotypes and Possible Genotypes|
|Short Second Finger||Long Second Finger|
|Male||SSSS or SSSL||SLSL|
|Female||SSSS||SLSL or SSSL|
11. Write your name on the board and identify your Dominant or Recessive form for the above-mentioned traits. Compile the class data and calculate the frequency of a given trait.
|Trait||Phenotype||Genotype||Class Occurance||Frequency of Expression|
|Bent Little finger|
|Index Finger Length|