BIO 2 GO! Sex- Linked Traits

Transcription

1 BIO 2 GO! Sex- Linked Traits 3323 Sex-linked traits are carried on the sex chromosomes. Because males have an X and a Y chromosome instead of 2 X chromosomes, they are more likely to inherit recessive traits. Upon successful completion of this unit, you should be able to do the following: 1. Explain how sex-linked traits are passed from parents to offspring. 2. Explain how the sex chromosomes, XX and XY, are involved with sex-linked traits. 3. Explain the genotype that creates females, and the genotype that creates males. 4. List several human maladies that are caused by sex-linked genes. 5. Define each of the following words: Trait sex-linked recessive genotype chromosome Phenotype chromosome pair dominant gene allele sex chromosome X chromosome Y chromosome XX XY

2 Sex-Linked Traits Sex-linked traits are easy to remember. Sex-linked traits are the traits that are carried on the sex chromosomes, either the X or the Y chromosome. This affects how they are passed from parent to offspring. Why is it that recessive genotypes are more often expressed in the male offspring than in the female? Answer: there are 23 pairs of chromosomes in each human cell. The 23rd pair is called the sex chromosome. Females have two X chromosomes. Males have an X chromosome and a Y chromosome. REMEMBER THIS!!! The male sex genotype is XY, the female sex genotype is XX. Question 1. If an organism has a genotype of XX, is it a male or female? There are many genes located on the 23rd chromosome pair. Traits that are produced by these genes are called sex-linked traits. Some genes that are on the X are missing from the Y because the Y chromosome is shorter than the X chromosome. Question 2. Why are some traits found on the X chromosome and not on the Y. Predict what effect this might have on the offspring. For example, the gene for normal color vision is on the X chromosome but it is not on the Y chromosome. Therefore, since males (XY) only have one X chromosome, they only receive one gene for normal vision or for colorblind vision. Therefore, only one gene is needed to determine a trait. As you recall, normally it takes two genes to control a trait one from the male and one from the female. In sex-linked traits, sometimes only one gene is present. When that is the case, that gene determines the trait.

3 For example, remember colorblindness is a recessive gene located on the X chromosome. Since all females are XX and all males are XY, then the mother contributes the X chromosome to the male offspring and the father contributes the Y chromosome to the offspring. Therefore, it only takes one recessive gene from the mother (the father contributes the Y gene) for the trait to be expressed. Colorblindness is more common in males because males only need to receive one recessive allele for colorblindness to inherit the trait. Females would need to receive two, since they have two X chromosomes (one from the mother and one from the father). REMEMBER THIS!!! If the mother has the trait, so will all of her sons. That is why it is called sex-linked. Question 3. If the mother is colorblind, will all of her daughter also have to be colorblind? The genotype for sex-linked traits is written with the symbols for the 23rd pair or sex chromosomes, either XX or XY. The gene that is sex-lined is written as a superscript to each chromosome. If the gene for normal color vision is represent by N, then the gene for colorblind vision is represented by n. A normal male s genotype would be X N Y A colorblind male s genotype would be X n Y. No letter is written by the Y chromosome because no gene is on the Y chromosome. Remember the Y chromosome is too short to carry all of the genes that are found on the X chromosome.

4 Since females have two X chromosomes, they have two genes for normal or colorblind vision. A normal female s genotype would be X N X N or X N X n. A colorblind female s genotype would be X n X n. REMEMBEER THIS!!! Because males (XY) only have to receive one recessive allele, and females (XX) have to receive two, colorblindness is more common in males than in females. Females (XX) have to inherit the recessive gene from both the mother and father. Males (XY) only need to inherit the recessive gene from the mother. Interesting Scientific Fact: Other sex-linked traits include hemophilia and muscular dystrophy. They are both recessive, sex-linked traits that occur more often in males than in females. Question 4. If the father is colorblind, what are the chances that his daughters will also be colorblind? Question 5. If the father is colorblind, what are the chances that his sons will also be colorblind?

5 Sex- Linked Traits TEST YOURSELF Matching 1. sex-linked trait 2. sex chromosome 3. X N X N or X N X n 4. X n X n 5. X N Y 6. X n Y 7. the female sex chromosomes 8. the male sex chromosomes a. XX b. XY c. the 23rd pair of human chromosomes d. genotype of a colorblind male e. genotype of a colorblind female f. a trait which is produced by a gene on the X chromosome g. genotype of a male with normal vision h. genotype of a female with normal vision True or False 1. The 23rd pair of chromosomes in a human cell are called sex chromosomes. 2. Females have two X chromosomes. 3. Males have two X chromosomes. 4. Females have two Y chromosomes. 5. Males have two Y chromosomes. 6. Females have one X and one Y chromosome. 7. Males have one X and one Y chromosome. 8. Sex-linked traits are produced by genes that are on the X chromosome but not on the Y chromosome. 9. Sex-linked traits are produced by genes that are on the Y chromosome but not on the X chromosome.

6 10. There are more genes on the X chromosome than on the Y chromosome because the X chromosome is longer. 11. The gene for normal vision or colorblind vision is on the X chromosome. 12. The gene for normal vision or colorblind vision is on the Y chromosome. 13. A normal male s genotype would be X N Y. 14. A normal male s genotype would be X n Y. 15. A colorblind male s genotype would be X N Y. 16. A colorblind male s genotype would be X n Y. 17. A normal female s genotype would be X N X N or X N X n. 18. A normal female s genotype would be X n X n. 19. A colorblind female s genotype would be X N X N or X N X n. 20. A colorblind female s genotype would be X n X n. Fill in the Blank sex chromosomes XX XY sex-linked trait colorblindness hemophilia muscular dystrophy males X N X N or X N X n X n X n X N Y X n Y females 1. Three examples of sex-linked traits are,, and. 2. A is a trait that is produced by genes on the X chromosome. 3. The 23rd pair of chromosomes, called the, are different in males and females. 4. The sex chromosomes in females are. 5. The sex chromosomes in males are. 6. If the gene for normal color vision is N, and the gene for colorblindness is n, then the genotype for a colorblind male is. 7. The genotype for a colorblind female is. 8. The genotype for a male with normal color vision is. 9. The genotype for a female with normal color vision is. 10. are more likely than to inherit colorblindness.

7 Answer the Following: 1. What sex chromosomes do females have? 2. What sex chromosomes do males have? 3. What are sex-linked traits? 4. What are three examples of sex-linked traits? 5. How are the genotypes of sex-linked traits written? Write the genotypes of a colorblind female. a colorblind male. a female with normal vision a male with normal vision. 6. Are males or females more likely to inherit sex-linked traits? Why?

8 Sex- Linked Traits Answer Sheet Question 1. If an organism has a genotype of XX, is it a male or female? Question 2. Why are some traits found on the X chromosome and not on the Y. Predict what effect this might have on the offspring. Question 3. If the mother is colorblind, will all of her daughter also have to be colorblind? Question 4. If the father is colorblind, what are the chances that his daughters will also be colorblind? Question 5. If the father is colorblind, what are the chances that his sons will also be colorblind?