Part I: Predicting Genetic OutcomesA DNA fingerprint consists of segments of DNA that are processed...

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Genetic Outcomes Mechanisms of Genetics

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 in the form of a code, the information carried in DNA can be copied and transferred to offspring. Not only does each species have its own unique code, but so does each individual within that species. Those unique codes 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. He was able to predict the outcomes of plants with dominant traits vs. recessive traits. Mendelian crosses are used to predict the possible genetic and phenotypic outcomes of offspring. That is what you will be doing in this activity, but you do not have to grow the plants!! The DNA code, 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. Each chromosome will carry the gene for a specific trait, but 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. It is this unique combination of alleles in each individual that creates species diversity.

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Part I: Predicting Genetic Outcomes, continued Below you will see two chromosomes from the offspring of true-breeding parents. 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 eye color. If the allele for brown eye color is represented by B, the true-breeding genotype must be BB (one from each chromosome), and the phenotype will always be brown eyes. On the other hand, if the allele for blue eye color is represented by b, the true-breeding genotype has to be bb, and the phenotype will always be blue eyes. 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 eye color. It is this specific location on the chromosome that specifies the gene for eye color. You can see, however, that each allele of the gene expresses a different trait, or phenotype. The offspring has inherited the blue allele b from the father and the brown allele B from the mother. Genetics tells us this particular offspring will have brown eyes. How can you predict that outcome?


Mother Father

Go to Part I of your Student Journal.

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Part I: Predicting Genetic Outcomes, continued Eye 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 eye color, which means the individual carries two of the same allele, one allele for each parent's sex chromosome. Therefore, the genotype of one parent is BB (homozygous for brown eyes), and the genotype of the other parent is bb (homozygous for blue eyes). 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 eyes. Our cross between a parent with homozygous dominant eye color BB and a parent with homozygous recessive eye 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 eyes.


BB x bb b b

B Bb Bb B Bb Bb

This box represents the cross of the mother (BB) with the father (bb). Both parents are homozygous.

Each B represents one allele found on one maternal chromosome.

Each b represents one allele found on one paternal chromosome.

The resulting possibilities represent the F1 generation as they are offspring of the parents. Each Bb represents the genotype of each of the four possible offspring produced. In this case, all offspring are heterozygous.

Answer the Eye Color Example questions in Part I in your Student Journal.

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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.

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 eyes, 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 eyes and 1 with blue eyes, so the phenotypic ratio would be 3:1. Hair Color Trait Examples


Hair Color Trait

Phenotype Allele

Purple P (dominant)

White p (recessive)

Complete Part I in your Student Journal.

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Part II: 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 group, a five-carbon sugar, and a nitrogenous base. There are four different nitrogenous 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.



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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 a specific base sequence. When the DNA is cut, there are many fragments of different sizes.

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.



Part II: DNA Fingerprinting, continued The process of making a DNA fingerprint is called gel electrophoresis.

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Part II: 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.


Complete Part II in your Student Journal.

Example B. Look at the example to the right. Is it likely the suspect committed the crime?

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Part III: 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.


Complete Part III and the Reflection and Conclusions sections in your Student Journal.

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Complete Part II in your Student Journal.

Genetic Disorder Chromosome Abnormality Symptoms

Down's Syndrome Extra chromosome on # 21 (trisomy 21)

Possible heart defects, prone to respiratory disease, short stature Usually some degree of mental retardation

Cri du chat Deletion of a small portion of chromosome # 5

Severe mental retardation Small head with unusual facial features A cry that sounds like a distressed cat

Turner's Syndrome Missing one of the sex chromosomes (monosomy) # 23 XO

Genetically female, but do not reach sexual maturity, sterile Normal intelligence

Patau Syndrome

Extra chromosome on # 13 (trisomy 13)

Serious eye, brain, circulatory defects Cleft palate Many do not survive past a few months

Edward's syndrome Extra chromosome on # 18 (trisomy 18)

Rare disorder, majority of people with the syndrome die as a fetus Infants who survive have serious defects Short life expectancy

Part III: Karyotyping, continued

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Part I: Predicting Genetic OutcomesA DNA fingerprint consists of segments of DNA that are processed...

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Transcript of Part I: Predicting Genetic OutcomesA DNA fingerprint consists of segments of DNA that are processed...