4.2: Genes and Chromosomes - the nuts and bolts

Let’s look more closely at just nature/heredity. 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, and proteins influence the structure and functions of cells. Genes are located on the chromosomes and there are an estimated 20,500 genes for humans, according to the Human Genome Project (NIH, 2015).

Figure \(\PageIndex{1}\): DNA’s location in the cell, within its nucleus, and therein within the chromosome.[1]

Normal human cells contain 46 chromosomes (or 23 pairs; one set 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. Thus, each sperm and egg/ovum possesses only 23 chromosomes. In conception those two fuse to make a cell with 46 chromosomes.

Table \(\PageIndex{1}\): - Mitosis & Meiosis[2] Contrasting Meiosis and Mitosis

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

Figure \(\PageIndex{2}\): Mitosis in the frame above represents cells multiplying and making copies of themselves. In the frame below Meiosis involves two steps whereby chromosomal strands divide, cross over, reattach, and randomly half get assigned to one cell, and half to another in a process of gamete production.[3]

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 (Gould & Keeton, 1997).

Of the 23 pairs of chromosomes created at conception, 22 pairs are similar in length and the genes on each of the chromosomes correspond with the genes on the other in the pair. These 22 pairs are called autosomes. The remaining pair, or sex chromosomes, could be X or Y, and X and Y chromosomes are not the same 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.[4]

So while all children receive 22 autosomes and one sex chromosome from each of their parents, the particular sex chromosome received by boys will be the Y chromosome from dad and an X from mom. Girls will receive an X from both parents.

Here is an image (called a karyogram) of what the 23 pairs of chromosomes look like. Notice the differences between the sex chromosomes in female (XX) and male (XY). As is clear from the karyotype, the Y chromosome is much shorter than the X chromosome.

Figure \(\PageIndex{3}\): A female Karyotype or picture of the 23 pairs of human chromosomes, and one y chromosome represented additionally at the end. [5]

Genotypes, Phenotypes & Patterns of Inheritance

The word genotype refers to the sum total of all the genes a person inherits. The word phenotype refers to the features that are actually 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 (alleles) from our mother and father, that is, be homozygous for that characteristic the gene influences. If we receive a different version or allele of the gene from each parent, that is referred to as being heterozygous.

In the homozygous situation we will display that characteristic. For example, if you receive genes for attached earlobes from each parent, of course you will have attached earlobes. Similarly, if you receive genes for unattached earlobes from each parent, you will have unattached earlobes.

It is in the heterozygous condition that it becomes clear that not all genes are created equal. If you get an attached earlobe allele from one parent and an unattached allele from the other parent, you are heterozygous on the earlobe attachment gene. In this case, you will have unattached earlobes. This indicates that the unattached earlobe allele is dominant over the attached one. The attached earlobe allele is recessive.

Dominant/Recessive principle

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. Some dominant traits include having unattached earlobes, facial dimples, curly hair, normal vision, and dark hair. Some recessive traits include red hair, being nearsighted, and straight hair.

Polygenic inherittance

Most characteristics are not the result of a single gene; they are polygenic, meaning they are the result of several genes. For example, skin color, the likelihood of developing schizophrenia, height, and many other human traits are the result of multiple genes working together.

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.[6]

Attributions:

Child Growth and Development by Jennifer Paris, Antoinette Ricardo, and Dawn Rymond, 2019, is licensed under CC BY 4.0

[1] Image by Radio89 is licensed under CC BY-SA 3.0 (original image has been modified)

[2] 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)

[3] Image by Community College Consortium for Bioscience Credentials is licensed under CC BY 3.0

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

[5] Image by Nami-ja is in the public domain

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