2.4: Genes and Chromosomes
- Page ID
- 233816
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Now, let’s look more closely at just nature. Nature refers to the contribution of genetics to one’s development.
- Genes - the basic building block of the nature perspective and are the recipes for making proteins
- Proteins - influence the structure and functions of cells.
- Chromosomes - a package of DNA containing part or all of the genetic material of an organism. an estimated 20,500 genes for humans
Figure 2.4 – DNA’s location within a cell.
Image Source: Eukaryote DNA. Wikimedia Commons. This file is licensed under the Creative Commons Attribution-Share Alike 3.0 license.
Human Chromosomes
Chromosomes are string-like structures located inside the nucleus of animal and plant cells. Scientists gave this name to chromosomes because the structures become strongly stained when colorful dyes are applied to them.
Normal human cells contain 46 chromosomes (or 23 pairs; one from each parent) in the nucleus of the cells. After conception, most cells of the body are created by a process called mitosis. Mitosis is defined as the cell’s nucleus making an exact copy of all the chromosomes and splitting into two new cells.
However, the cells used in sexual reproduction, called the gametes (sperm or ova), are formed in a process called meiosis. In meiosis, the gamete’s chromosomes duplicate, and then divide twice resulting in four cells containing only half the genetic material of the original gamete.
Fig. 2.4.1. Types of Cell Division: Mitosis and Meiosis
| Type of Cell Division | Explanation | Steps |
| Mitosis |
All cells, EXCEPT those used in sexual reproduction, are created by mitosis. |
Step 1. Chromosomes make a duplicate copy Step 2. Two identical cells are created |
| Meiosis |
Cells used in sexual reproduction are created by meiosis. |
Step 1. Exchange of gene between the chromosomes (crossing over) Step 2. Chromosomes make a duplicate Step 3. First cell division Step 4. Second cell division |
The diagram shows the replication of parent cells replication, and then how the cells replicate via mitosis and meiosis.
Fig. 2.4.2. Diagram showing parent cells, replication, then how the cells replicate via mitosis, then meiosis.
Image Source: Community College Consortium for Bioscience Credentials. Mitosis v. Meiosis. Wikmedia.com. This file is licensed under the Creative Commons Attribution 3.0 license.
Two Types of Chromosomes
Chromosomes in humans can be divided into two types:
- autosomes - body chromosomes
- allosomes - sex chromosomes
Human cells have 23 pairs of chromosomes (22 pairs of autosomes and one pair of sex chromosomes), a total of 46 per cell. Given the number of genes present and the unpredictability of the meiosis process, the likelihood of having offspring that are genetically identical (and not twins) is one in trillions
Of the 23 pairs of chromosomes created at conception, 22 pairs are similar in length. These are called autosomes (body chromosomes). The remaining pair, are called allosomes (sex chromosomes), may differ in length. If a child receives the combination of XY, the child will be genetically male. If the child receives the combination XX, the child will be genetically female.
Below is an image of what the 23 pairs of chromosomes look like. Notice the differences between the sex chromosomes in female (XX) and male (XY)
Fig.2.3.3. Chromosome Chart showing 22 autosomes which vary in length, the longest being the first chromosome and gradually shortening. The 23rd chromosome is represented by an example of XX female chromosome of identical lengths, and the XY male chromosome with the Y chromosome being approximately half the height of the X chromosome.
Image Source: Inside the Cell. CML Support Group. Open Access. July 2021.
Genotypes and Phenotypes & Patterns on Inheritance
The word genotype refers to the total of all the genes a person inherits. The word phenotype refers to the features that are expressed. Look in the mirror. What do you see, your genotype or your phenotype? What determines whether genes are expressed?
Because genes are inherited in pairs on the chromosomes, we may receive either the same version of a gene from our mother and father, that is, be homozygous for that characteristic the gene influences. In this situation we will display that characteristic (eye or hair color for example).. If we receive a different version of the gene from each parent, that is referred to as heterozygous. It is in the heterozygous condition that it becomes clear that not all genes are created equal. Some genes are dominant, meaning they express themselves in the phenotype even when paired with a different version of the gene, while their silent partner is called recessive. Recessive genes express themselves only when paired with a similar version gene. Geneticists refer to different versions of a gene as alleles.
Ex: dominant (alleles)
- curly hair, dark hair
- normal vision
- ability to roll the tongue
- right-handedness
- freckles
Ex: recessive (alleles)
- red hair, straight hair
- nearsightedness
- Type O blood
- blue eyes
- left-handedness
Most characteristics are not the result of a single gene; they are polygenic, meaning they are the result of several genes. In addition, the dominant and recessive patterns described above are usually not that simple either. Sometimes the dominant gene does not completely suppress the recessive gene; this is called incomplete dominance.
Sources
- Lally, Martha a and Suzanne Valentine-French. Lifespan Development: A Psychological Perspective (page 34-35) Licensed under CC BY-NC-SA 3.0.