BIO 2 GO! Phenotypes / Dominant and Recessive Traits and

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1 BIO 2 GO! Phenotypes / Dominant and Recessive Traits and The phenotype of an organism is what the organism looks like. The phenotype is determined by the action of dominant and recessive genes. Upon successful completion of this unit, you should be able to do the following: 1. Describe the phenotype of an organism. 2. Describe and explain the interaction of dominant and recessive traits.

2 Phenotypes / Dominant and Recessive Traits and Phenotype Phenotype is easy. The phenotype of an organism tells what the organism looks like, its Physical appearance. The phenotype is the expression of inherited traits or characteristics. If the offspring of a plant has red flowers, then we can say that the offspring s phenotype is red flowers. If a horse has a black color, then we can say the horse s phenotype is black coat. Phenotype is easy. REMEMBER THIS!!! Phenotype is the expression of inherited characteristics. Question 1. What would be the phenotype of blue-green algae? As you recall, an organism s characteristics, also called traits, such as eye color, weight, hair color, are passed from parents to offspring through sexual reproduction. Genetics is the study of the inheritance of traits from parent to offspring. These traits are passed on by means of the DNA in the nucleus of the sperm and egg cell. This DNA contains the genes that will produce the traits in the offspring. Since each parent contributes one gene for each trait, the offspring then inherits 1 from each parent for a total of 2. There are two genes for every trait within an organism. Adults have 2 sets of genes for each trait. These two genes for a particular trait or characteristics are called alleles. The two alleles may code for the same expression of a trait or they may code for two different expressions of a trait. Somehow, the two alleles combine to produce one trait. For example, if one organism has an allele for brown hair from one parent, and an allele for brown hair from the other parent, the offspring will have brown hair. The term phenotype is used to describe how the two alleles are expressed. The easiest way to determine the phenotype of an organism is to look at it. If the organism has brown hair, then that is the phenotype. The phenotype is what the organism looks like.

3 Question 2. Explain how an organism s phenotype is determined. Dominant and Recessive Genes The reason the plant with two different alleles will be tall is that the allele for tallness is dominant over the allele for shortness. The allele for short plants is called the recessive allele. Therefore, if an offspring has two tall alleles, its phenotype will be tall. If it has two alleles for shortness, its phenotype will be short. If it has one allele for tallness and one allele for shortness, its phenotype will be tall. REMEMBER THIS!!! A dominant gene is always expressed. A recessive gene is never expressed unless it is homozygous. When both a dominant gene and a recessive gene are present, the dominant gene is expressed and the recessive gene appears to be hidden. Question 3. If R is dominant for rough and r is recessive for smooth. Predict the phenotype for the following offspring possibilities. RR Rr rr Another trait in pea plants is the color of the seeds. Yellow is the dominant trait. Green is the recessive trait. If an offspring has one allele for yellow seed and one allele for green seed, its phenotype will be yellow seed. If the offspring has two alleles for yellow seed, its phenotype will be yellow seed. If the offspring has two alleles for green seed, its phenotype will be green seed.

4 Question 4. Using the above paragraph, explain why an offspring with one allele for yellow and one allele for green has a yellow phenotype. Question 5. From the above example, if the phenotype for the seed is yellow, what is the color of the seed? Summary Traits often come in two forms, a dominant form and a recessive form. The dominant allele hides the recessive allele in individuals that have two different alleles for a trait. If the offspring inherits a dominant trait from both parents, that trait will be exhibited as the phenotype. If the offspring inherits 1 dominant trait from either parent and 1 recessive trait from the other parent, then it will express the dominant trait only. In this case, the recessive trait remains hidden. In the case where the offspring inherits recessive traits from both parents, the recessive trait is exhibited. In this case, there is no dominant gene to hide the recessive gene so the recessive gene is the phenotype. Here is another example: Assume that X is the dominant allele for brown color in rats, and x is the recessive allele for gray color in rats. Then, a XX rat would have a brown phenotype, because the X is dominant. If a rat had an Xx genotype, then its phenotype would be brown, because the X is dominant. If a rat had an xx genotype, then its phenotype would be gray, because there is no dominant gene to hide the effect of the recessive gene.

5 Matching Phenotypes / Dominant and Recessive Traits TEST YOUSELF 1. genetics 2. alleles 3. tall 4. short 5. phenotype 6. green seeds 7. yellow seeds True or False a. the dominant phenotype for seed color b. the recessive phenotype for seed color c. the dominant phenotype for height d. the recessive phenotype for height e. the study of inheritance of traits f. the outward expression of a trait g. genes that code for different phenotypes for one trait 1. Genetics is the study of cells. 2. Genetics is the study of inheritance of traits. 3. Traits are passed on through DNA in the egg cell only. 4. Traits are passed on through DNA in the egg and sperm cell. 5. Genes are found in the DNA. 6. Traits are produced by genes. 7. Genes are produced by traits. 8. There are two genes for every trait within an organism. 9. There is one gene for every trait within an organism. 10. Alleles are genes that code for different forms of one particular trait. 11. A pea plant that inherits two alleles for tallness will be tall. 12. A pea plant that inherits two alleles for tallness will be short. 13. A pea plant that inherits one allele for tallness and one allele for shortness will be tall. 14. A pea plant that inherits one allele for tallness and one allele for shortness will be short. 15. A pea plant that inherits two alleles for shortness will be tall. 16. A pea plant that inherits two alleles for shortness will be short. 17. The phenotype describes the way two alleles are expressed in an organism. 18. The dominant allele for seed color in pea plants is yellow. 19. The dominant allele for seed color in pea plants is green. 20. The recessive allele for seed color in pea plants is yellow.

6 21. The recessive allele for seed color in pea plants is green. 22. A pea plant that has two dominant alleles for seed color will have a green phenotype. 23. A pea plant that has two recessive alleles for seed color will have a green phenotype. 24. A pea plant that has one dominant allele and one recessive allele for seed color will have a green phenotype. 25. The dominant allele hides the recessive allele in individuals that have two different alleles for a trait. Fill in the Blank - Some words may be used more than once. genetics alleles phenotype dominant recessive yellow tall short green allele 1. A pea plant has two sets of for each trait, one from each parent. 2. When combined, the alleles produce one, or outward expression of a trait. 3. Alleles are described as or. 4. A dominant will hide the recessive. 5. If a pea plant has two dominant alleles for height, its phenotype will be. 6. If a pea plant has one dominant and one recessive allele for height, its phenotype will be. 7. If a pea plant has two recessive alleles for height, its phenotype will be. 8. If a pea plant has two dominant alleles for seed color, its phenotype will be. 9. If a pea plant has one dominant and one recessive allele for seed color, its phenotype will be. 10. If a pea plant has two recessive alleles for seed color, its phenotype will be. 11. is the study of inheritance from parent to offspring.

7 Answer the Following: 1. What is an allele? 2. How many alleles does an individual have for each trait? 3. How is the phenotype determined when an individual has two dominant alleles for a trait? 4. How is the phenotype determined when an individual has two recessive alleles for a trait? 5. How is the phenotype determined when an individual has one dominant and one recessive allele for a trait?

8 Phenotypes / Dominant and Recessive Traits Short Answer Questions Question 1. What would be the phenotype of blue-green algae? Question 2. Explain how an organism s phenotype is determined. Question 3. If R is dominant for rough and r is recessive for smooth. Predict the phenotype for the following offspring possibilities. RR Rr rr Question 4. Using the above paragraph, explain why an offspring with one allele for yellow and one allele for green has a yellow phenotype. Question 5. From the above example, if the phenotype for the seed is yellow, what is the color of the seed?