Part I: Predicting Genetic Outcomes

Transcription

1 Part I: Predicting Genetic Outcomes Deoxyribonucleic acid (DNA) is found in every cell of living organisms, and all of the cells in each organism contain the exact same copy of that organism s DNA. Because the genetic information is encoded as a sequence of nucleotides, the information carried in DNA can be copied and transferred to offspring. Not only does each species have its own unique nucleotide sequence, but so does each individual within that species. Those unique sequences are what determine the varying traits that are observed from one individual organism to the next. In other words, the DNA is the organism s blueprint, and each organism s blueprint is slightly different from the next. Gregor Mendel, considered the father of genetics, studied pea plants by crossing them and noting the resulting offspring. Through his studies with pea plants, Mendel developed three principles or laws of inheritance: 1. The Law of Segregation: During reproduction, alleles are randomly separated into gametes during the process of meiosis. 2. The Law of Independent Assortment: Genes located on the same chromosome will be inherited independently of one another. This means that even if a plant has a long stem, it doesn t mean that it will also have to have purple flowers. 3. The Law of Dominance: A dominant allele will completely mask a recessive allele. A homozygous dominant and heterozygous genotype will produce the same phenotype. Mendel was able to predict the outcomes of crossing plants with dominant traits vs. recessive traits. Mendelian crosses are used to predict the possible genetic and phenotypic outcomes of offspring. That s what you ll do, but you don t have to grow the plants! The DNA sequence, or the information contained in the gene, can be used to create proteins that specify a particular trait, such as eye color. The information carried on the DNA is the genotype. The way that information is expressed, such as blue or brown eyes, is the phenotype. Remember that an organism s genes are carried on the chromosomes in the nucleus of the cell. During meiosis, gametes, or reproductive cells, are formed. When an egg and sperm from two individuals unite, the offspring inherits one set of chromosomes from the mother s egg and one set of chromosomes from the father s sperm. Therefore, this creates a unique set of chromosomes for each offspring. Please continue to the next page. 1

2 Part I: Predicting Genetic Outcomes, continued The maternal and paternal chromosomes in a chromosome pair will carry the same gene for a specific trait, but often in a slightly different version. A different version of the same gene is called an allele. The combination of alleles, one from the father and one from the mother, results in a unique genetic combination. This unique combination of alleles in each individual helps create diversity within species. Below you will see two chromosomes from the offspring of true-breeding parent guinea pigs. We call the parents the P Generation and their offspring the F1 Generation. True-breeding means that each parent has two matching alleles for a trait, in this case fur color. If the allele for brown fur color is represented by B, the true-breeding genotype must be BB (one from each chromosome), and the phenotype will always be brown fur. On the other hand, if the allele for black fur color is represented by b, the true-breeding genotype has to be bb, and the phenotype will always be black fur. Let s walk through this step by step. In our diagram below, the F1 offspring has inherited one chromosome from the father (shown in blue) and one from the mother (shown in pink). On these two chromosomes, you can see the specific locus for the gene for fur color. It is this specific location on the chromosome that contains the gene for fur color. You can see, however, that each allele of the gene expresses a different trait, or phenotype. The offspring has inherited the black allele b from the father and the brown allele B from the mother. Genetics tells us this particular offspring will have brown fur. How can you predict that outcome? Black allele Go to Part I of your Student Journal. 2

3 Part I: Predicting Genetic Outcomes, continued Fur Color Trait Example Each genotype for a trait may have dominant and/or recessive alleles. The dominant allele is represented by an upper-case letter (B), while the recessive allele is represented by a lower-case letter(b). In our example, both parents are homozygous for fur color, which means that each carries two of the same allele, one allele from each parent. Therefore, the genotype of one parent is BB (homozygous for brown fur), and the genotype of the other parent is bb (homozygous for black fur). Homozygous individuals always express the phenotype that matches their alleles. If the parents had a genotype that was heterozygous, that would mean that they would carry one of each allele for that trait, or Bb. Heterozygous individuals express the phenotype of the dominant allele, in this case brown fur. Our cross between a parent with homozygous dominant fur color BB and a parent with homozygous recessive fur color bb is shown in the Punnett Square below. You can see that all of the possible combinations result in a dominant B and a recessive b, which means they are all heterozygous. The dominant trait will be expressed in this phenotype, so all of the offspring will have brown fur. Answer the Fur Color Example questions in Part I in your Student Journal. 3

4 Part I: Predicting Genetic Outcomes, continued The results of Punnett Square analyses are expressed as ratios for both genotypes and phenotypes. In our first example on the previous page, all 4 of the offspring genotypes are Bb, with no other combinations, so the genotypic ratio would be 4:0. The only phenotype expressed in the offspring will be brown fur, with no other choices, so the phenotypic ratio would be 1:0 (or 100% brown). If the results of another cross were BB = 1, Bb = 2, and bb = 1, the genotypic ratio would be 1:2:1. The expression of the phenotype would be 3 offspring with brown fur and 1 with black fur, so the phenotypic ratio would be 3:1. Hair Color Trait Examples Scenario 1 A man has a genotype of Pp. P will be dominant for purple and p will be recessive for white. This means his hair color is purple. A woman also has a genotype of Pp, and her hair color is also purple. The pair decides to have offspring. The genotype cross is Pp x Pp. This means it is a cross between two parents who are heterozygous for hair color. Scenario 2 A man has a genotype of Pp, which means his hair color is purple. A woman has a genotype of pp, which means her hair color is white. They also decide to have offspring. This is a cross between a parent who is heterozygous for hair color with a parent who is homozygous recessive. In your Student Journal, determine what the hair color of the offspring in the F1 Generation will be for each scenario. Complete Part I in your Student Journal. 4

5 Part II: Non-Mendalian Genetic Outcomes There have been advances in our current knowledge of the mechanisms of genetic inheritance. Now we know that not all genetic outcomes follow Mendel s laws for inheritance. Below we explain four different instances where Mendel s laws do not apply. A. Incomplete Dominance Mendel s law of dominance stated that that the recessive allele will always be masked by dominant alleles. However, in nature, incomplete dominance can be seen in several instances All where neither allele is considered dominant. offspring With Mendel s experiments, the offspring would R R pink always looks like one of the two parents. However, in incomplete dominance, since R RR RR neither allele is considered dominant, the phenotypic trait will actually be a blending of the two parents. An example of this can be R RR RR seen with the cross-pollination of a red snapdragon flower (RR) and a white snapdragon flower (R R ). Since neither flower color is considered dominant, a blending of the two phenotypes take place to produce a new pink flower phenotype (RR ). B. Co-Dominance In certain species of plants and animals, two homozygous dominant parents will cross to produce a F1 generation that fully expresses both parental phenotypes. This can be seen in a certain variety of chickens. The alleles for both black feathers (BB) and white feathers (WW) are dominant. If a heterozygous offspring is produced (BW), the chicken would be black and white speckled, not grey. Please continue to the next page. B B W BW BW W BW BW 5

6 Part II: Non-Mendalian Genetic Outcomes (cont.) C. Multiple Alleles With Mendalian genetics, genes could only have two alleles, a dominant or a recessive. However, in nature it has been shown that genes could have more than two alleles. Blood types are an example of a gene having more than two alleles. There is a dominant allele for A blood type (I A ), a dominant allele for B blood type (I B ), and a recessive allele for O blood type (i). The three different alleles give four different possible phenotypic blood types: A, B, AB, or O. Phenotype A B AB O Genotypes I A I A I A i I B I B I B i I A I B ii I A I B I A I B I B i i I A i ii i D. Sex-Linked Traits The X and Y chromosome are designated the human sex chromosomes. Females have two X chromosomes (XX) while males have an X and Y chromosome (XY). Some traits and diseases are carried on these sex chromosomes, especially the X chromosome. Since males only have one X chromosome, they are more likely to express a sex-linked trait since they can not be heterozygous. Hemophilia and color-blindness are two examples of sex-linked traits that are primarily seen in mainly males. For a female to inherit these diseases, she must receive a recessive X chromosome from each parent. Below is an example of a sex-linked Punnett square showing the inheritance of colorblindness which is a recessive trait. The mother is normal but heterozygous (X C X c ) and the father is normal (X C Y) The two will have a 25% chance of X C having a child who is color blind. Y Notice that it only occurs in the X C X C X C X C male, the female that receives the Y recessive allele is masked by the X c X C X c X c dominant normal allele. She would Y be considered a carrier and could E. Polygenic Traits pass this trait off to her offspring. Scientists now understand that not all traits are produced by just one gene on one chromosome. Some of our traits are influenced by multiple genes on multiple chromosomes. This is one reason there is variety in certain traits like eye color, hair color, and skin color. Complete Part II in your Student Journal 6

7 Part III: Pedigrees Pedigrees are useful because they are a graphic representation of the traits that are inherited within a family. The graphics show which phenotypes each offspring inherited. Scientists can use pedigrees to understand where certain trait were inherited and even to help determine the genotypes of parents. The table below shows a key to the accompanying pedigree. Review the pedigree and then answer the questions in your student journal. =Normal Male =Normal Female =Affected Male =Affected Female =Marriage =Siblings Complete Part III in your Student Journal. 7

8 Part IV: DNA Fingerprinting In Part I, you explored how to predict genetic outcomes using Punnett Squares. Now, you will investigate a method for matching genetic outcomes with possible sources for that outcome. What is DNA Fingerprinting? A DNA fingerprint is not an actual fingerprint like you might imagine. A DNA fingerprint consists of segments of DNA that are processed and can be used for comparison against the DNA of another organism. DNA fingerprinting can be used to determine paternity, in criminal investigations and in finding evolutionary relationships. Here s how it works: The structure of any organism s DNA is the same. All DNA nucleotides have a phosphate, a fivesided sugar and a nitrogen base. There are four different nitrogen bases. The only difference in the DNA of every living thing is the number and order of the base pairs. Although there are millions of base pairs in humans, scientists only isolate, process and compare the repeating sequences found in the genome. Please continue to the next page. 8

9 Part IV: DNA Fingerprinting, continued The process of making a DNA fingerprint is called gel electrophoresis. Below are the steps of the procedure: 1. A sample of DNA is collected. The sample can come from hair, saliva, blood, or semen. 2. Restriction enzymes cut the DNA into smaller pieces at specific base sequences. When the DNA is cut, there are many fragments of different sizes. Because the sequences are slightly different among individuals, the enzymes make their cuts at different places, producing different sized fragments in different individuals. 3. The fragments are put into a gel made of agarose. 4. An electric current pulls the fragments across the gel. The pieces of DNA are sorted by size. Smaller fragments move farther across the gel than the larger ones. 5. The gel is blotted with nylon, and the DNA is transferred onto the nylon. 6. Radioactive probes are washed onto the nylon. A film is placed on the nylon and X-rays will be used to produce an image. Please continue to the next page. 9

10 Part IV: DNA Fingerprinting, continued Once the DNA fingerprint is made, comparisons can be made. Let s practice! Example A. The sample shows blood collected at a criminal investigation. DNA from three suspects will be compared to the crime scene blood at each numbered probe. Start with: # 1. The blood has a dark band. Suspects 1 and 3 also have that dark band. # 2. The blood has a thin band. All suspects have the band, too. # 3. The blood has a dark band. Suspects 1 and 3 have the dark band. # 4. The blood has a thin band. Suspects 2 and 3 have these bands as well. Suspect 3 is the closest match because he shares the most bands in common. Example B. Look at the example to the right. Is it likely the suspect committed the crime? Complete Part IV in your Student Journal. 10

11 Part V: Karyotyping A karyotype is a map of an organism s chromosomes. Chromosomes are extracted from an organism s cells and then stained. The chromosomes are then arranged and numbered by size, from largest to smallest. Once the chromosomes are organized by size, they are further paired by using banding characteristics and location of the centromere. During chromosomal analysis, this arrangement helps scientists quickly identify chromosomal alterations that could possibly indicate a genetic abnormality. A karyotype is shown below. Notice that there are 23 pairs of chromosomes in a normal karyotype. The first 22 pairs are called autosomes. The 23rd pair of chromosomes indicates the sex of the person. Sex chromosomes XX = female Sex chromosomes XY = male The example shown here is a normal karyotype. There are 23 pairs of chromosomes. There are no chromosomes missing, and there are no extra chromosomes on any pair. Analyze the karyotypes in your Student Journal. Use the data table to determine which genetic abnormality exists. Continue to the next page. 11

12 Part V: Karyotyping, continued Complete Part V in your Student Journal. 12