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6.3: Chromosomes and Genes

Identical Twins, Identical Genes You probably can tell by their close resemblance that these two individuals are identical twins. Identical twins develop from the same fertilized egg, so they inherited copies of the same chromosomes and have all the same genes

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Identical Twins, Identical Genes

You probably can tell by their close resemblance that these two individuals are identical twins. Identical twins develop from the same fertilized egg, so they inherited copies of the same chromosomes and have all the same genes. Unless you have an identical twin, no one else in the world has exactly the same genes as you. What are genes? How are they related to chromosomes? And how do genes make you the person you are?

Figure \(\PageIndex{1}\): Identical twins

Chromosomes are coiled structures made of DNA and proteins. Chromosomes are encoded with genetic instructions for making proteins. These instructions are organized into units called genes . Most genes contain the instructions for a single protein. There may be hundreds or even thousands of genes on a single chromosome.

Human Chromosomes

Each species is characterized by a set number of chromosomes. The human number is 23. Human cells normally have two sets of chromosomes in each of their cells, one set inherited from each parent. There are 23 chromosomes in each set, for a total of 46 chromosomes per cell. Each chromosome in one set is matched by a chromosome of the same type in the other set, so there are actually 23 pairs of chromosomes per cell. Each pair consists of chromosomes of the same size and shape, and they also contain the same genes. The chromosomes in a pair are known as homologous chromosomes . As see in Figure \(\PageIndex{2}\), there are two types of chromosomes, autosomal and sex chromosomes. Read more about this in the genetics chapter.

Figure \(\PageIndex{2}\): All human cells (except gametes, which are sperm and egg cells) have the 23 pairs of chromosomes shown here. There are two types of chromosomes, autosomal (pairs 1-22) and sex (23rd pair). The chromosomes are numbered according to their size, the first chromosome pair being the longest and the twenty-second chromosome pair being the shortest. The 23rd pair is not always homologous. It can be XY or XX. The X chromosome is significantly longer than the Y chromosome. The stripes on the chromosomes represent genes.

Human Genes

Humans have an estimated 25,000 genes. This may sound like a lot, but it really isn’t. Far simpler species have almost as many genes as humans. However, human cells use splicing and other processes to make multiple proteins from the instructions encoded in a single gene. Only about 25 percent of the nitrogen base pairs of DNA in human chromosomes make up genes and their regulatory elements. Out of this 25 percent, only two percent code for genes. The functions of many of the other base pairs are still unclear. The majority of human genes have two or more possible versions, called alleles. Differences in alleles account for the considerable genetic variation among people. In fact, most human genetic variation is the result of differences in individual DNA base pairs within alleles.

Linkage

Genes that are located on the same chromosome are called linked genes . Linkage explains why certain characteristics are frequently inherited together. For example, genes for hair color and eye color are linked, so certain hair and eye colors tend to be inherited together, such as blonde hair with blue eyes and brown hair with brown eyes. Can you think of other human traits that seem to occur together? Do you think they might be controlled by linked genes? Genes located on the sex chromosomes are called sex-linked genes . Most sex-linked genes are on the X chromosome because the Y chromosome has relatively few genes. Strictly speaking, genes on the X chromosome are X-linked genes , but the term sex-linked is often used to refer to them. Figure \(\PageIndex{3}\) is called a linkage map. A linkage map shows the locations of specific genes on a chromosome. It shows the locations of a few of the genes on the human X chromosome, such as a blood group protein gene, Lethyosis (a skin disease gene), ocular albinism gene, and many more.

Figure \(\PageIndex{3}\): Linkage Map for the Human X Chromosome. This linkage map shows the locations of several genes on the X chromosome. Some of the genes code for normal proteins. Others code for abnormal proteins that lead to genetic disorders.

Chromosomes and genes

Figure \(\PageIndex{4}\): Graphic decomposition of a chromosome (found in the cell nucleus), to the bases pair of the DNA. Chromosomes are located in the nucleus of the cell. A duplicated chromosome has two chromatids, double-helix DNA is wrapped in histone proteins to form nucleosomes. This allows DNA to organize into chromosomes.

Review

  • What are chromosomes and genes, and how are the two related?
  • Describe human chromosomes and genes.
  • Explain the difference between autosomes and sex chromosomes.
  • What are linked genes, and what does a linkage map show?
  • Explain why females are considered the default sex in humans.
  • True or False. Humans have 46 pairs of chromosomes.
  • True or False. Autosomes refer to any chromosome other than sex chromosomes.
  • True or False. The majority of human DNA does not encode for proteins.
  • Explain the relationship between genes and alleles.
  • Put the following in order of size, from smallest to largest: chromosome; gene; base pair
  • Sex-linked genes are usually found on which chromosome? Explain why these genes are called sex-linked.
  • Which of the following are considered homologous chromosomes? Chromosome 22 and the X chromosome The two copies of chromosome 22 that make up a pair All of the chromosomes in a skin cell and all of the chromosomes in a muscle cell Chromosomes 21 and 22
  • What is the one chromosome that is different between genetic males and females? Explain your answer.
  • Most males and females have two sex chromosomes. Explain why then, do only females have Barr bodies.

Explore More

Watch the video below to learn about sex chromosomal disorders, such as Turner syndrome.

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Related questions

01What Makes You...You?

This person has naturally red hair. Why is this hair red instead of some other color? And, in general, what causes specific traits to occur? There is a molecule in human beings and most other living things that is largely responsible for their traits. The molecule is large and has a spiral structure in eukaryotes. What molecule is it? With these hints, you probably know that the molecule is DNA.

Source: bio.libretexts.org ↗
02What Is the Human Genome?

The human genome refers to all the DNA of the human species. Human DNA consists of 3.3 billion base pairs and is divided into more than 20,000 genes onto 23 pairs of chromosomes. The human genome also includes noncoding sequences (e.g. intergenic region) of DNA, as shown in Figure \(\PageIndex{2}\).

Source: bio.libretexts.org ↗
03What Is Gene Expression?

Using a gene to make a protein is called gene expression . It includes the synthesis of the protein by the processes of transcription of DNA and translation of mRNA. It may also include further processing of the protein after synthesis. Gene expression is regulated to ensure that the correct proteins are made when and where they are needed. Regulation may occur at any point in the expression of a gene, from the start of the transcription phase of protein synthesis to the processing of a protein after synthesis occurs. The regulation of transcription is one of the most complicated parts of gene regulation in eukaryotic cells and is the focus of this concept.

Source: bio.libretexts.org ↗
04What Is the Genetic Code?

The genetic code consists of the sequence of nitrogen bases in a polynucleotide chain of DNA or RNA. The bases are adenine (A), cytosine (C), guanine (G), and thymine (T) (or uracil, U, in RNA). The four bases make up the “letters” of the genetic code. The letters are combined in groups of three to form code “words,” called codons . Each codon stands for (encodes) one amino acid unless it codes for a start or stop signal. There are 20 common amino acids in proteins. With four bases forming three-base codons, there are 64 possible codons. 61 codons are more than enough to code for the 20 amino acids, thus more than one codon codes for a single amino acid. Please find genetic codes in Table \(\PageIndex{1}\) or in appendix 1 .

Source: bio.libretexts.org ↗
05So how is it possible to know which genetic variants cause disease and which are passengers?

The way scientists look at disease variants is to compare the genetic makeup of a large number of people who have a specific disease with those who do not. This allows scientists to look for genetic variants that are more common in people with a disease compared to people without the disease. For example, if a particular genetic variant is present in 80 percent of patients with the disease but only 20 percent of the healthy population it suggests that this variant is increasing the risk of that disease. However, looking for a disease that is caused by variants in a single gene is the simplest example. There are many complex diseases where variants in many different genes might be involved. As well as the transcriptional and translational regulation of some enzyme production may vary due to the genetic variation in the enhancer and repressors of a gene. So, for this type of comparison to be effective very large groups of people need to be studied, usually in the tens of thousands, to find the variants that have subtle effects on disease risk. Researchers also try to pick individuals with similar phenotypes, in both the diseased and healthy groups, so that the disease genes are easier to identify and study.

Source: bio.libretexts.org ↗
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