Content Benchmark L.8.A.1
Students know heredity is the passage of genetic instructions from one generation to the next generation. E/S
Inside almost every cell of every living thing is the blueprint for building that organism. This blueprint contains information on all of the organism’s inherited characteristics. The blueprint isdeoxyribonucleic acid, or DNA. Much like a blueprint, DNA provides step by step instructions for building each part of the final product. The final productwould be an organism. DNA accomplishes this by providing the instructions to make all of an organism’s proteins. In humans for example, this blueprint gives the instructions for making a protein called melanin, which will determine how dark or light a person’s skin will be. In plants, DNA gives the instructions for proteins that influence traits like plant height and flower color. Where does the DNA and the information it holds come from? It is inherited from an organism’s parents through reproduction. During sexual reproduction, each parent donates half of its genetic material to the offspring. So, each parent gives half of the blueprint, and when they are put together, they form a complete blueprint from which the offspring can be made. Heredity is the reason organisms can look similar to their parents, yet also look unique. A thorough understanding of heredity requires at least a basic understanding of DNA, RNA, proteins synthesis, cell division, reproduction and genetics principles.
DNA Structure
Just a little less than a century ago, scientists were still trying to figure out what molecule held genetic information. In the early 1990s they knew cells were made of nucleic acids, proteins, lipids, and carbohydrates; but they did not know which of these was passed from parent to offspring. During this time, people thought DNA was too simple of a molecule to code for the variety of traits found in most organisms. Scientists believed proteins were more likely the genetic material because there were a greater variety of proteins known. Many experiments were done to find out which molecule contained the genetic material, but none definitively showed it was DNA until the 1950s. Alfred Hershey and Martha Chase proved that DNA, not protein was the genetic material in viruses. This experiment led most scientists to believe DNA was the genetic material for all life.
Soon after Hershey and Chase’s discovery, two scientists named James Watson and Francis Crick proposed the first accurate model of DNA’s structure. They used research done by other scientists such as Erwin Chargaff, Rosalind Franklin, and Maurice Wilkins to decipher the molecular structure of DNA. Watson and Crick’s model showed that DNA is a double helix (shown on the right side of Figure 1) and is composed of nucleotides (shown in the bottom left of Figure 1). Nucleotides are made of a sugar, phosphate, and a nitrogenous base (these components are also shown in Figure 1).The nitrogenous bases are adenine (A), thymine (T), cytosine(C), and guanine(G). Notice that if adenine is on one side, thymine is opposite and if cytosine is on one side, guanine is opposite. These are considered complimentary base pairs and they always pair together in DNA.
Figure 1. The Structure of DNA.
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For more information on DNA history, go to
For detailed information and an animation of DNA, go to
DNA, RNA, and Protein Synthesis
The nitrogenous bases of DNA – A, T, C, and G provide the basis for DNA to code for genetic characteristics. There can be hundreds to billions of nucleotides on just one side of the DNA strand, depending on the organism. The order of the nitrogenous bases on the nucleotides is called the base sequence. The base sequence is comprised of all the codes for each gene that an organism has. A gene is a specific nucleotide sequence on the DNA that codes, or contains the genetic instructions, for one protein. To discover how these genes are turned into proteins, we must take a closer look at RNA.
While DNA holds the genetic instructions for making proteins, it is ribonucleic acid, or RNA, that must read and translate them. Protein synthesis involves two parts, transcription and translation. It also involves 3 kinds of RNA- messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA). A summary of the process is pictured in Figure 2 and explained below it.
Figure 2. Protein Synthesis
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In transcription (#1 on Figure 2), mRNA transcribes, or copies down a gene from DNA. An enzyme called RNA polymerase opens the necessary gene in the DNA and begins adding complimentary nucleotides to “copy” the gene base sequence. RNA does not contain thymine; instead, it contains uracil (U). Therefore, as mRNA copies the gene from DNA, it pairs adenine with uracil, thymine with adenine, guanine with cytosine and cytosine with guanine. Once the gene base sequence has been copied, the mRNA leaves the nucleus to travel to the ribosome where the protein will be made.
Translation occurs in the cytoplasm (#2 on Figure 2). In translation, the mRNA first binds to a ribosome. Ribosomes are made of rRNA and provide the environment for protein synthesis. The mRNA molecule is read 3 bases at a time. These 3 base sequences are called codons. Each codon codes for a specific amino acid.Amino acids are the building blocks of proteins. The amino acids are brought to the ribosome one at a time by tRNA. Once all the codons are read and all the amino acids have bonded to form a protein, the mRNA and ribosome release the protein. The protein goes on to perform its function.
Although middle school students are not responsible for protein synthesis, it is necessary background information for teachers to understand heredity.
For an animated simulation of protein synthesis and further explanation, go to
For information on how mutations affect the expression of DNA, see MS TIPS Benchmark L.8.A.2
Meiosis andGamete Formation
In eukaryotic organisms, DNA strands can be incredibly long due to the fact that it takes hundreds or thousands of nucleotides to code for one protein. For example, the DNA in just one human cell can be over 2 meters long from end-to-end! How does all of that DNA fit into a cell? The DNA coils tightly around itself and special proteins to form chromosomes. Human DNA has 46 chromosomes as shown in Figure 3, which is a human karyotype. A karyotype is a picture of an organisms chromosomes, lined up next to their homologues. Homologous chromosomes, or homologues, are chromosomes that are the same size, the same shape, and have the same genes. These homologous chromosomes may not have the same base sequences for the genes. For example, a gene that codes eye color would be located on the same spot in two homologous chromosomes;but one of the genes may code for blue eyes on one chromosome while the other codes for brown eyes on the other chromosome. These different forms of a gene are called alleles. Each parent donates one chromosome to the homologous pair. In order for this to be possible, each parent of any organism would need to produce a cell with half the total number of chromosomes for that organism. Or, in other words, a cell with only one homologue would be produced. This cell, used for sexual reproduction, would be called a gamete and is produced through the process of meiosis.
Figure 3. Human Karyotype and Chromosome Structure.
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Figure 4 depicts a simplified summary of meiosis. The figure shows 4 chromosomes, or 2 homologous pairs. In prophase 1, the chromosomes duplicate themselves, which is what gives them the X-shape. In metaphase 1, the homologues line up next to each other in the middle of the cell. Two events happen at this step that creates genetic variation among the gametes produced. The first is independent assortment. The homologues will line up and be separated randomly. In the figure, two chromosomes of the original four come from the mother and two come from the father. When the chromosomes are pulled to each side of the cell to create two new cells, (as seen in Anaphase 1 and Telophase 1), the daughter cells of the first cell division may end up with two chromosomes from the same parent or they may end up with one chromosome from each parent cell. The second event is crossing over. When homologues line up next to each other, parts of the chromosome may be swapped. This results in the daughter cells of the first division having different chromosomes than the parent cell. After two daughter cells are produced by the first division in meiosis, a second division occurs. In this division, each of the chromosomes are split in half. Notice the four daughter cells that result after Telophase 2 have half the number of chromosomes as the parent cells. These daughter cells would be considered gametes.
Figure 4. Meiosis Overview.
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Gamete formation is also wheremutations can happen, for more information, see MSTIPS Benchmark L.8.A.2
For meiosis animated simulations, go to
and,
For a meiosis tutorial, see
Genetics
Now that we have seen how gametes form, let’s take a look at how hereditary information is passed through these gametes. First, we will need some background information on genetics. Long before scientists knew that DNA was the genetic material, a monk named Gregor Mendel studied genetics in pea plants. His experiments led to the discovery of several important genetic principles. Mendel discovered that some alleles for genetic traits are dominant and some traits are recessive. Alleles are alternate forms of genes. When the dominant allele for a gene is present, it will mask the appearance of the recessive allele. For example, in his pea plants, Mendel discovered that green pea pods were dominant over yellow pea pods. The parent pea plants will each give one allele for pea pod color to their offspring. If one parent gave the allele for green pea pods and the other parent gave the allele for yellow pea pods, then the offspring would have green pea pods. This passing down of alleles is related to the previously discussed concept of meiosis. At the end of meiosis, each gamete contains one homologue of each chromosome for the given organism. This means the gamete also contains one allele for each trait on that chromosome. Each parent donates one allele to the offspring for each gene.
For more information on Gregor Mendel, go to
and,
Genetic traits are often symbolized by letters. Dominant alleles are often symbolized by capital letters, like ‘G’ for green pea pods. Recessive alleles are often symbolized by lower case letters, like ‘g’ for yellow pea pods. So the offspring from the previous example would have the genotype Gg and a phenotype of green pea pods. Genotype is the genetic makeup, while phenotype is the physical appearance of an organism. This genotype is called heterozygous, because there is one dominant and one recessive allele. Genotypes that have two of the same allele, such as GG or gg would be considered homozygous dominant and homozygous recessive, respectively.
When the genotype of parents is known, Punnett Squares can be used to determine the possible genotypes of the offspring. For example, the allele for being tall (T) in pea plants is dominant over the allele for being short, so if we breed a heterozygous plant (Tt) with a homozygous recessive plant (tt). The possible offspring genotypes are shown in Figure 5. Punnett squares can also help us determine the probability that offspring will turn out a certain way. Figure 5 shows that there is a 50% chance that an offspring of these parent plants would be tall and a 50% chance that it would be short. Punnett Squares shows the possible gamete combinations that would be made by parents during meiosis. So, in this example, for the Tt parent, meiosis would produce a gamete with the T allele in it 50% of the time and a gamete with t in it 50% of the time. The tt parent would only produce gametes with t in them. This Punnett square is an example of a monohybrid cross, which mean it only contains one inherited trait. Punnett squares can be much larger when they are used for dihybrid or trihybrid crosses.
Figure 5. Punnett Square for Tt and tt Pea Plant Cross
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For step by step instructions on making Punnett squares, go to
For an online animated tutorial of making Punnett square and how they relate to breeding and probability, go to
For a Punnett square calculator (shows you the Punnett square if you type the genotypes), go to
For more information on selective breeding, see MSTIPS Benchmark L.8.A.3.
Now let’s take a look at a human trait that is often passed down through generations. The trait is thumb straightness. In human, as discussed in previous sections, there are 46 chromosomes. When meiosis occurs and gametes are formed, the resulting cells have 23 chromosomes, which means one homologue and one allele for each gene. In humans, the allele for hitchhikers thumb (h) is recessive, while the allele for a straight thumb (H)is dominant. These thumb types are pictured in Figure 6. Each person has two alleles for this trait. A genotype of HH or Hh would result in a straight thumb, while hh would result in hitchhikers thumb.
Hitchhiker's Thumb / Regular ThumbFigure 6. Hitchhiker Thumb Compared to a Straight Thumb
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Now let’s take two parents, a male with hitchhikers thumb and a female with a homozygous straight thumb (hh and HH respectively). When their gametes are formed through meiosis, the male will produce gametes with the h allele. The female will produce gametes with the H allele. When the gametes fuse together to form a zygote during fertilization, the zygote would receive the genotype Hh and therefore have a straight thumb. Let’s say the offspring of this child was a female who has children with a male who has the genotype Hh. Both parents will have straight thumbs, but it will be possible for them to have a hitchhiker thumbed child. Each parent will form some gametes with the H allele and some with the h allele. As demonstrated in the Punnett Square in Figure 7, there is a 25% chance that they will have a child with hitchhiker’s thumb and a 75% chance they will have a child with straight thumbs. This example illustrates how a recessive trait can be passed through generations and stay hidden or unseen in some individuals. These individuals carry the allele for the recessive trait, but do not express it.
H / HH / HH / Hh
h / Hh / Hh
Figure 7.Punnett Square of a heterozygous cross for hitchhikers thumb
Heredity is often not as simple as monohybrid crosses. Most human characteristics are polygenic, which means they are controlled by many genes. Eye color, for example, is controlled by at least three genes and there may be more. Many human traits are also complex characters, which means the environment plays a role in the phenotype. Skin color, for example, can be influenced not only by several genes, but by the amount of sunlight a person’s skin receives. Some traits are incompletely dominant. This means that in heterozygous individuals, the phenotype is somewhere in between the phenotypes of the homozygous individuals. For example, if a curly haired Caucasian and a straight hair Caucasian have children, the child will have wavy hair. Some traits are controlled by multiple alleles, such as the ABO blood types in humans. The three alleles for blood typing areA (IA), B (IB), and O (i). The IAand IB alleles are also codominant, which means that if a person has both alleles, they are both expressed as the phenotype and that person would have AB blood. The type O (i) allele is recessive to A (IA) and B (IB)allele. In order to have type O blood, an individual must inherit a recessive allele (i) from each parent. If an individual inherits an A allele (IA) from one parent and an O allele (i) from another parent, then the individual will have type A blood (IA i) because the A allele is dominant to the O allele.
For more information on blood typing and inheritance see
Some traits are sex-linked, which means they are found on the sex chromosomes. These traits, such as colorblindness, are usually located on the X chromosome and are more prevalent in men. For colorblindness, women would only be colorblind if the colorblind allele were on both X chromosomes; but in men, the allele only needs to be on their one X chromosome. Some traits are sex-influenced, which means males and females will show different phenotypes when they have the same genotype. Pattern baldness is an example of a sex-influenced trait, as it is dominant in males but recessive in females. Many genetic traits found in organisms, especially in humans, are not controlled by two alleles where one allele is dominant and one is recessive; but that kind of trait is the simplest way to explain heredity.