7.2: Chromosomes, Genes and DNA

Learning Objectives

1. Describe the basic structure and function of genes and chromosomes.
2. Differentiate between genotypes and phenotypes.
3. Explain how dominant and recessive alleles influence inherited traits.
4. Explain the role of sex chromosomes in determining biological sex.
5. Identify key characteristics of polygenic inheritance and incomplete dominance.

Genes and Chromosomes

Now, let’s look more closely at nature. Nature refers to the contribution of genetics to one’s development. The basic building block of the nature perspective is the gene. Genes are recipes for making proteins, which influence the structure and functions of cells. Genes are located on the chromosomes, and according to the Human Genome Project (NIH, 2015), there are an estimated 20,500 genes for humans.

Figure \(\PageIndex{1}\): DNA’s location in the cell. Image by Radio89 is licensed under CC BY-SA 3.0.

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 when a cell’s nucleus makes an exact copy of all the chromosomes and splits into two new cells.

However, the cells used in sexual reproduction, called the gametes (sperm or ova), are formed in 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. Thus, each sperm and egg possesses only 23 chromosomes and combine to produce the normal 46.

Table \(\PageIndex{1}\) - Mitosis & Meiosis ⁶

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 genes between the chromosomes (crossing over) Step 2. Chromosomes make a duplicate Step 3. First cell division Step 4. Second cell division

Figure \(\PageIndex{2}\): Mitosis and Meiosis. Image by Community College Consortium for Bioscience Credentials is licensed under CC BY 3.0.

Given the amount of genes present and the unpredictability of the meiosis process, the likelihood of having genetically identical offspring (and not twins) is one in trillions (Gould & Keeton, 1997).

Of the 23 pairs of chromosomes created at conception, 22 pairs are similar in length. These are called autosomes. The remaining pair, or 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.⁸

Here is an image (called a karyogram) of the 23 pairs of chromosomes. Notice the differences between the sex chromosomes in females (XX) and males (XY).

Figure \(\PageIndex{3}\): The 23 pairs of chromosomes. Image by Nami-ja is in the public domain.

Genotypes and Phenotypes & Patterns of Inheritance

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 or not 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 the characteristic the gene influences. If we receive a different version of the gene from each parent, that is referred to as heterozygous.

In the homozygous situation, we will display that characteristic. In the heterozygous condition, 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 of a gene. Geneticists refer to different versions of a gene as alleles. Some dominant traits include having facial dimples, curly hair, normal vision, and dark hair. Some recessive traits include red hair, nearsightedness, and straight hair.

Most characteristics do not result from a single gene; they are polygenic, meaning they result from 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, which is called incomplete dominance. ¹⁰

References, Contributors and Attributions

6. Lifespan Development: A Psychological Perspective (page 34) by Martha Lally and Suzanne Valentine-French is licensed under CC BY-NC-SA 3.0 (content modified: image made into table)

8. Lifespan Development: A Psychological Perspective (page 34-35) by Martha Lally and Suzanne Valentine-French is licensed under CC BY-NC-SA 3.0

10. Lifespan Development: A Psychological Perspective (page 35) by Martha Lally and Suzanne Valentine-French is licensed under CC BY-NC-SA 3.0