The alleles IA and IB are dominant over i. A person who is homozygous recessive ii has type O blood. Homozygous dominant IAIA or heterozygous dominant IAi have type A blood, and homozygous dominant IBIB or heterozygous dominant IBi have type B blood. IAIB people have type AB blood, because the A and B alleles are codominant. Type A and type B parents can have a type AB child. Type A and a type B parent can also have a child with Type O blood, if they are both heterozygous (IBi,IAi). The table (''Bloodtype as Determined by Multiple Alleles'') shows how the different combinations of the blood group alleles can produce the four blood groups, A, AB, B, and O.
Bloodtype as Determined by Multiple Alleles IA IB i
IA IAIA
TYPE A
IAIB
TYPE AB
IAi
TYPE A
IB IA IB
TYPE AB
IB IB
TYPE B
IB i
TYPE B
i i IA
TYPE A
i IB
TYPE B
ii
TYPE O
Effects of Environment on Phenotype
Genes play an important part in influencing phenotype, but genes are not the only influence. Environmental conditions, such as temperature and availability of nutrients can affect phenotypes. For example, temperature affects coat color in Siamese cats.
Figure 7.15
The dark points, on this Siamese cat are caused by a gene that codes for a temperature-sensitive enzyme. The enzyme, which causes a darkening of the cats fur, is active only in the cooler parts of the body such as the tail, feet, ears, and area around the nose.
The pointed pattern is a form of partial albinism, which results from a mutation in an enzyme that is involved in melanin production. The mutated enzyme is heat-sensitive; it fails to work at normal body temperatures. However, it is active in cooler areas of the skin. This results in dark coloration in the coolest parts of the cat's body, such as the lower limbs and the face, as shown in Figure above. The cat’s face is cooled by the passage of air through the nose. Generally adult Siamese cats living in warm climates have lighter coats than those in cooler climates.
Height in humans is influenced by many genes, but is also influenced by nutrition. A person who eats a diet poor in nutrients will not grow as tall as they would have had they eaten a more nutritious diet. Scientists often study the effects of environment on phenotype by studying identical twins. Identical twins have the same genes, so phenotypic differences between twins often have an environmental cause.
Lesson Summary
Probability is the likelihood that a certain event will occur. It is expressed by comparing the number of events that actually occur to the total number of possible events. Probability can be expressed as a fraction, decimal, or ratio.
A Punnett square shows all the possible genotypes that can result from a given cross.
A testcross examines the genotype of an organism that shows the dominant phenotype for a given trait. The heterozygous organism is crossed with an organism that is homozygous recessive for the same trait.
A dihybrid cross-examines the inheritance of two traits at the same time.
A pedigree can help geneticists discover if a trait is sex-linked, if it is dominant or recessive, and if the person (or people) who have the trait are homozygous or heterozygous for that trait.
The Mendelian pattern of inheritance and expression does not apply to all traits. Codominant traits, incompletely dominant traits, and polygenic traits do not follow simple Mendelian patterns of inheritance. Their inheritance patterns are more complex.
An organism’s phenotype can be influenced by environmental conditions.
Review Questions
What does the probability equation help to determine?
How can probability be expressed?
Outline how Punnett squares are useful.
Identify the purpose of a testcross.
How do the Punnett squares for a monohybrid cross and a dihybrid cross differ?
Mendel carried out a dihybrid cross to examine the inheritance of the characteristics for seed color and seed shape. The dominant allele for yellow seed color is Y, and the recessive allele for green color is y. The dominant allele for round seeds is R, and the recessive allele for a wrinkled shape is r. The two plants that were crossed were F1 dihybrids RrYy. Identify the ratios of traits that Mendel observed in the F2 generation, and explain in terms of genotype what each number means. Create a Punnett square to help you answer the question.
Draw a pedigree that illustrates the passing of the dominant cleft chin trait through three generations. A person who has two recessive alleles does not have a cleft chin. Let us say that C is the dominant allele, c is the recessive allele.
A classmate tells you that a person can have type AO blood. Do you agree? Explain.
Mendelian inheritance does not apply to the inheritance of alleles that result in incomplete dominance and codominance. Explain why this is so.
Outline the relationship between environment and phenotype.
Further Reading / Supplemental Links
http://www.nbii.gov/portal/server.pt?open=512&objID=405&PageID=581&mode=2&in_hi_userid=2&cached=true
http://omia.angis.org.au/retrieve.shtml?pid=417
http://www.curiosityrats.com/genetics.html
http://www.macalester.edu/psychology/whathap/UBNRP/visionwebsite04/twotypes.html
http://www.nlm.nih.gov/
http://www.newton.dep.anl.gov/askasci/mole00/mole00087.htm
http://www.hhmi.org/biointeractive/vlabs/cardiology/content/dtg/pedigree/pedigree.html
http://www.ndsu.nodak.edu/instruct/mcclean/plsc431/mendel/mendel9.htm
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookgeninteract.html
Vocabulary
autosome
Any chromosome other than a sex chromosome.
carrier
A person who is heterozygous for a recessive allele of a trait.
codominance
Occurs when both traits appear in a heterozygous individual.
incomplete dominance
Occurs when the phenotype of the offspring is somewhere in between the phenotypes of both parents; a completely dominant allele does not occur.
Mendelian trait
A trait that is controlled by a single gene that has two alleles.
multiple alleles
When three or more alleles determine a trait, such as with the human ABO blood group.
probability
The likelihood that a certain event will occur.
pedigree
A chart which shows the inheritance of a trait over several generations.
polygenic traits
Traits that are affected by more than one gene.
Punnett square
A diagram that helps predict the probable inheritance of alleles in different crosses.
sex chromosome
A chromosome that determines the sex of an organism.
sex-linked trait
A trait whose allele is found on a sex chromosome.
testcross
A cross used to determine an unknown genotype.
Points to Consider
The next chapter is Molecular Genetics.
What do you think Molecular Genetics refers to?
How can DNA contain all the genetic information?
If DNA is in the nucleus, and proteins are made on ribosomes in the cytoplasm, how do you think this happens?
Chapter 8: Molecular Genetics
Lesson 8.1: DNA and RNA
Lesson Objectives
Discuss how the work of Griffith, Avery, Hershey, and Chase demonstrated that DNA is the genetic material.
Define transformation and explain that transformation is the change in genotype and phenotype due to the assimilation of the external DNA by a cell.
Discuss the findings of Chargaff. Describe the importanc
e of the finding that in DNA, the amount of adenine and thymine were about the same and that the amount of guanine and cytosine were about the same. This finding lead to the base pairing rules.
Explain Watson and Crick’s double helix model of DNA.
Describe how DNA is replicated.
Explain the importance of the fact that during DNA replication, each strand serves as a template to make a complementary DNA strand.
Describe the structure and function of RNA.
Discuss the role of the three types of RNA: mRNA, rRNA, and tRNA.
Introduction
What tells the first cell of an organism what to do? How does that first cell know to become two cells, then four cells, and so on? Does this cell have instructions? What are those instructions and what do they really do? What happens when those instructions don’t work properly? Are the “instructions” the genetic material? Though today it seems completely obvious that Deoxyribonucleic acid, or DNA, is the genetic material, this was not always known.
DNA and RNA
Practically everything a cell does, be it a liver cell, a skin cell, or a bone cell, it does because of proteins. It is your proteins that make a bone cell act like a bone cell, a liver cell act like a liver cell, or a skin cell act like a skin cell. In other words, it is the proteins that give an organism its traits. We know that it is your proteins that that make you tall or short, have light or dark skin, or have brown or blue eyes. But what tells those proteins how to act? It is the structure of the protein that determines what it does. And it is the order and type of amino acids that determine the structure of the protein. And that order and type of amino acids that make up the protein are determined by your DNA sequence.
The relatively large chromosomes that never leave the nucleus are made of DNA. And, as proteins are made on the ribosomes in the cytoplasm, how does the information encoded in the DNA get to the site of protein synthesis? That’s where RNA comes into this three-player act.
DNA → RNA → Protein
That’s known as the central dogma of molecular biology. It states that “DNA makes RNA makes protein.” This process starts with DNA. And first DNA had to be identified as the genetic material.
The Hereditary Material
For almost 100 years, scientists have known plenty about proteins. They have known that proteins of all different shapes, sizes, and functions exist. For this reason, many scientists believed that proteins were the heredity material. It wasn’t until 1928, when Frederick Griffith identified the process of transformation, that individuals started to question this concept. Griffith demonstrated that transformation occurs, but what was the material that caused the transforming process?
Griffith, Avery, Hershey and Chase
Griffith was studying Streptococcus pneumoniae, a bacterium that infects mammals. He used two strains, a virulent S (smooth) strain and a harmless R (rough) strain to demonstrate the transfer of genetic material. The S strain is surrounded by a polysaccharide capsule, which protects it from the host's immune system, resulting in the death of the host, while the R strain, which does not have the protective capsule, is defeated by the host's immune system. Hence, when mammalian cells are infected with the R strain bacteria, the host does not die (Figure below).
Griffith infected mice with heat-killed S strain bacteria. As expected, the heat-killed bacteria, as they were dead, had no effect on the mice (Figure 1). But then he tried something different. He mixed the remains of heat-killed S strain bacteria with live R strain bacteria and injected the mixture into mice. Remember, separately both of these bacteria are harmless to the mice. And yet the mice died (Figure below). Why? These mice had both live R and live S strain bacteria in their blood. How? Griffith concluded that the R strain had changed, or transformed, into the lethal S strain. Something, such as the "instructions” from the remains of the S strain, had to move into the R strain in order to turn the harmless R strain into the lethal S strain. This material that was transferred between strains had to be the heredity material. But the transforming material had yet to be identified. Transformation is now known as the change in genotype and phenotype due to the assimilation of external DNA (heredity material) by a cell.
Figure 8.1
The transformation experiments of Griffith. The rough (R) strain has no effect on the mouse, whereas the smooth (S) strain is harmful to the mouse. Heat-killed S strain also has no effect on the mouse, but the mixture of heat-killed S strain and the R strain is harmful to the mouse.
Over the next decade, scientists, led by Oswald Avery, tried to identify the material involved in transformation. Avery, together with his colleagues Maclyn McCarty and Colin MacLeod, removed various organic compounds from bacteria and tested the remaining compounds for the ability to cause transformation. If the remaining material did not cause transformation, than that material could not be the heredity material. Avery treated S strain bacteria with protease enzymes, which remove proteins from cells, and then mixed the remainder with R strain bacteria. The R strain bacteria transformed, meaning that proteins did not carry the genes for causing the disease. Then the remnants of the S strain bacteria were treated with deoxyribonuclease, an enzyme which degrades DNA. After this treatment, the R strain bacteria no longer transformed. This indicated that DNA was the heredity material. The year was 1944.
However, this finding was not widely accepted, partly because so little was known about DNA. It was still thought that proteins were better candidates to be the heredity material. The structure of DNA was still unknown, and many scientists were not convinced that genes from bacteria and more complex organisms could be similar.
In 1952, Alfred Hershey and Martha Chase put this skepticism to rest. They conclusively demonstrated that DNA is the genetic material. Hershey and Chase used the T2 bacteriophage, a virus that infects bacteria, to prove this point. A virus is essentially DNA (or RNA) surrounded by a protein coat (Figure below). To reproduce, a virus must infect a cell and use that host cell’s machinery to make more viruses. The T2 bacteriophage can quickly turn an Escherichia coli (E. coli) bacteria into a T2 producing system. But to do that, the genetic material from T2, which could only be protein or DNA, must be transferred to the bacteria. Which one was it?
Figure 8.2
Structural overview of T2 phage. A 2-dimensional representation is on the left, and a 3-dimensional representation is on the right. The phage is essentially nucleic acid surrounded by a protein coat.
Hershey and Chase performed a series of classic experiments, taking advantage of the fact that T2 is essentially just DNA and protein. In the experiments, T2 phages with either radioactive 32P-labeled DNA or radioactive 35S-labeled protein were used to infect bacteria. Either the radioactive proteins or radioactive DNA would be transferred to the bacteria. Identifying which one is transferred would identify the genetic material. In both experiments, bacteria were separated from the phage coats by blending, followed by centrifugation. Only the radioactively labeled DNA was found inside the bacteria, whereas the radioactive proteins stayed in the solution (Figure below). These experiments demonstrated that DNA is the genetic material and that protein does not transmit genetic information.
Figure 8.3
The Hershey and Chase experiment. T2 virus with either radioactive DNA (upper section) or radioactive protein (lower section) were used to infect bacteria. A blender was used to remove the phage from the bacteria followed by centrifugation. The radioactive DNA was found inside the bacteria (upper section), demonstrating that DNA is the genetic material.
Chargaff’s Rules
It was known that DNA is composed of nucleotides, each of which contains a nitrogen-containing base, a five-carbon sugar (deoxyribose), and a phosphate group. In these nucleotides, there is one of the four possible bases: adenine (A), guanine (G), cytosine (C), or thymine (T) (Figure below).
Figure 8.4
Chemical structure of the four nitrogenous bases in DNA.
Erwin Chargaff proposed two main rules that h
ave been appropriately named Chargaff's rules. In 1947 he showed that the composition of DNA varied from one species to another. This molecular diversity added evidence that DNA could be the genetic material. Chargaff also determined that in DNA, the amount of one base always approximately equals the amount of a particular second base. For example, the number of guanines equals the number of cytosines, and the number of adenines equals the number of thymines. Human DNA is 30.9% A and 29.4% T, 19.9% G and 19.8% C. This finding, together with that of the DNA structure, led to the base-pairing rules of DNA.
The Double Helix
In the early 1950s, Rosalind Franklin started working on understanding the structure of DNA fibers. Franklin, together with Maurice Wilkins, used her expertise in x-ray diffraction photographic techniques to analyze the structure of DNA. In February 1953, Francis Crick and James D. Watson of the Cavendish Laboratory in Cambridge University had started to build a model of DNA. Watson and Crick indirectly obtained Franklin's DNA X-ray diffraction data demonstrating crucial information into the DNA structure. Francis Crick and James Watson (Figure below) then published their double helical model of DNA in Nature on April 25th, 1953.
Figure 8.5
James Watson (left) and Francis Crick (right).
DNA has the shape of a double helix, just like a spiral staircase (Figure below). There are two sides, called the sugar-phosphate backbone, because they are made from alternating phosphate groups and deoxyribose sugars. The “steps” of the double helix are made from the base pairs formed between the nitrogenous bases. The DNA double helix is held together by hydrogen bonds between the bases attached to the two strands.
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