9.3: Chromosomes and Genes
- Page ID
- 92610
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)You probably can tell by their very close resemblance that these two individuals are identical twins. Identical twins develop from the same fertilized egg, so they inherit the same chromosomes and have all the same genes. However, even identical twins have some differences from the moment of conception due to small variations in nutrients and other in-utero factors, as well as diverse environmental factors throughout a lifetime.
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?
Chromosomes are coiled structures made of DNA and proteins. Chromosomes contain genetic instructions for making proteins. These instructions are organized into units called genes. Most genes contain the instructions for a single protein. There are thousands of genes on each chromosome, with the exception of the Y chromosome, which contains a few hundred.
Watch this video about chromosomes from Learn.Genetics
Human Chromosomes
Each species normally has a fixed number of chromosomes. Humans have 23 pairs of chromosomes in every cell, except for gametes (egg and sperm cells), with one chromosome from each pair inherited from each parent. These individual chromosomes in each pair are known as homologous chromosomes and are the same size and shape, and they contain the same genes.
Most humans have 46 chromosomes in each cell. As seen in Figure \(\PageIndex{2}\), there are two types of chromosomes, autosomal and sex chromosomes. Autosomal chromosomes are pairs 1-22. The sex chromosomes are pair 23, and generally XX or XY. The XY chromosomes are the exception to the statement above about homologous chromosomes.
Human Genes
Humans have an estimated 25,000 genes. While this may sound like a lot, simpler species have far more genes than humans do. However, human cells use different processes to make multiple proteins from the instructions encoded in a single gene. The region of DNA that encodes a protein (provides a blueprint via its nucleotide sequence) is known as the coding region.
Less than 1% of human DNA encodes proteins. Approximately 24% of our DNA encodes for proteins that regulate protein production. The functions of much of the rest of human DNA are still unclear but are undergoing extensive study.
The majority of genes have two or more possible variations, called alleles. These alleles differ due to variation in individual DNA base pairs within a coding region. The considerable genetic variation among people results from the different allele combinations each individual possesses.
Linkage
Genes that are located on the same chromosome are called linked genes. Alleles of these genes tend to segregate together during meiosis unless separated by crossing over. Crossing over occurs when two homologous chromosomes exchange genetic material during meiosis I (Figure \(\PageIndex{3}\).
The closer together two genes are to one another on a particular chromosome, the less likely their alleles are to be separated by crossing over and the more likely it is to be inherited together. Remember that each individual chromosome is inherited from either the egg or the sperm. Linkage explains why certain alleles from certain parents are frequently inherited together.
Figure \(\PageIndex{3}\): Linked genes are inherited together as their alleles are located close to each other and are less likely to be separated by crossing over. CK-12 Foundation CC-BY-NC 4.0
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.
The diagram in Figure \(\PageIndex{4}\) is called a linkage map. A linkage map shows the locations of specific genes on a chromosome. It shows the locations of a few genes on the human X chromosome, including a blood group protein gene, a gene for lethyosis (a skin disease), an ocular albinism gene, and many others.
Figure \(\PageIndex{4}\): 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 proteins that are associated with genetic disorders. Which pair of genes would you expect to have a lower frequency of crossing over: the genes that code for hemophilia A and G6PD deficiency, or the genes that code for Protan and Xm? Sam McCabe CC-BY-NC 3.0
Scientists have used genetic linkage to discover the location in the human genome of many genes that cause disease. They locate disease genes by tracking the inheritance of traits across generations within families and by creating linkage maps that measure recombination among groups of genetic “markers.”
The two BRCA genes, which can lead to breast and ovarian cancers, were among the first genes discovered through genetic mapping. Women with family histories of these cancers can now be screened to determine whether one or both of these genes carry a mutation. If so, they can opt to have their breasts and ovaries surgically removed. This reduces their risk of developing cancer later in life.
The actress Angelia Jolie brought this to the public’s attention when she opted for surgery in 2014 and again in 2015 after doctors found she carried a mutated BRCA1 gene.
Zedalis, J., & Eggebrecht, J. (2018, March 8). 13.1 chromosomal theory and genetic linkages. Biology for AP®. https://openstax.org/books/biology-a...netic-linkages CC-BY
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.
How is DNA packaged into the cell? Each chromosome consists of one continuous thread-like molecule of DNA coiled tightly around proteins, and contains a portion of the 6,400,000,000 base pairs (DNA building blocks) that make up your DNA. The way DNA is packaged into chromatin influences how protein production is controlled.
Chromosomal Disorders
The number, size, shape, and banding patterns of chromosomes make them easily identifiable in a karyogram and allow for the assessment of many chromosomal abnormalities. Although the cell cycle, mitosis, and meiosis are highly regulated to prevent errors, the processes are not perfect. One example is the failure of homologous chromosomes or sister chromatids to separate evenly (nondisjunction) during meiosis I or meiosis II. This results in gametes with too many (polyploidy) or too few (monosomy) chromosomes. These types of disorders in autosomal chromosomes (collectively known as aneuploidy) are typically lethal to the embryo, although a few trisomic genotypes are viable (e.g., Down syndrome).
Examples of chromosomal disorders are Down Syndrome (three copies of chromosome 21) and Turner Syndrome (only one X chromosome in women instead of the normal two). Geneticists can also identify large deletions, excess copies (additions), inversions of segments of DNA, or insertions of DNA from pieces of chromosomes where they do not normally belong (translocations).
These can result in symptomatic phenotypic effects. Diagnostic testing, such as karyotyping, can detect many of these chromosomal disorders in individuals well before birth, resulting in medical, ethical, and civic issues, such as the right to privacy and the ability to be covered by insurance (in the United States).
Watch the video below to learn about sex chromosomal disorders, such as Turner syndrome.
Attributions
- Gene map by Sam McCabe for CK-12 licensed CC BY-NC 3.0
- Chromosome by KES47, licensed CC BY 3.0 via Wikimedia Commons