BI111 Chapter Notes - Chapter 42: Wild Type, Chromosome, Meiosis

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23 Jan 2019

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Most fruit flies in nature have red eyes, so that phenotype is referred to as the wild type because it is the most common in the wild. The trait of white eyes in fruit flies is a sex-linked trait. It is located on a sex chromosome and is recessive, so its frequency in males and females differs. Because it is located on the x chromosome, it is more specifically called an x-linked gene. Humans have 22 pairs of autosomes (chromosomes other than sex chromosomes) plus two sex chromosomes for a total of 46 chromosomes. Offspring receive one version of each chromosome from each parent. If two genes are located very close to each other on the same chromosome, they are likely to be inherited together. Linked traits are usually inherited together because of the location of the genes that code for those traits.

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Sex-Linked Traits (traits located on the sex chromosomesâusually the X chromosome)

Follow the directions for the lab. Enter data you obtain for the following crosses and answer questions on this answer sheet. (NOTE: The traits used in this Module 3 Lab are sex-linked. Therefore, you CANNOT click the âignore sexâ button for these crosses. You need to see separate results for males and females to analyze data for sex-linked traits.)

Question: In this lab exercise, should you click the "ignore sex" button for the crosses?

1. a. Perform a monohybrid cross between a female fly with a tan body and a wild-type male fly. Show results for the F₁ offspring. (NOTE: You will use these F₁ flies in the next cross.)
ENTER DATA HERE:

Question: Are the phenotypes of the F₁ offspring what you would have predicted for this cross: why or why not?

Question: Each F₁ female received a tan allele from her mother and a wild allele from her father. Why do all of the F₁ females show the wild-type phenotype?

Question: Compare the number and type (X or Y) of sex chromosomes found in female and male flies.

Question: Compare the number of alleles present on the X and Y chromosomes.

Question: How can you tell that tan body is a sex-linked trait?

Question: How can you tell whether tan body is located on the X chromosome or the Y chromosome?

Question: If tan body was located on the Y-chromosome, what would be the phenotype of all the male F₁ offspring? Is this what you obtained for the cross above?

Question: If tan body was located on the Y-chromosome, could females even show the trait? (HINT: When analyzing the cross results, remember that Y-linked traits are passed down only from affected males to their sons. Females cannot display Y-linked traits since they do not have Y chromosomes. On the other hand, both males and females can inherit X-linked traits since they both have X chromosomes.)

PUNNETT SQUARE:

List the genotypes of the parent flies in this cross (HINT: You must use X and Y symbols):

List the gametes produced by each parent fly in this cross:

Show the Punnett Square for the above cross.

1. What is the chromosome theory of inheritance?

2. Understand how independent assortment can cause gametes to have four combinations of alleles when an individual is heterozygous for two genes on two different chromosomes.

3. What are linked characters and linked genes? Understand why linked genes will not independently assort.

4. How can crossing over affect linked genes?

5. What are recombinant gametes?

6. Define recombinant frequency. Be able to use the recombinant frequency to determine how close two genes are on a chromosome.

7. What is a linkage map?

8. What is the function of the SRY gene on the Y chromosome? What happens if you don't have this gene?

9. Know how sex is determined in mammals, birds, bees and ants, and reptiles.

10. How many autosomes and sex chromosomes do normal human haploid and diploid cells have?

11. What does it mean for a gene to be X-linked or Y-linked?

12. Be able to answer word problems and fill out a Punnet square involving an X-linked recessive trait.

13. Which sex is more likely to have an X-linked recessive disorder? Understand why.

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11) CIRCLE ALL CORRECT ANSWERS: Human males are hemizygous for

a) Sxl gene b) pseudoautosomal region of the Y c)SRY d) gene for testis determining factor e) Xist gene

12) The X inactivation center codes for

a) gonadal ridges to form ovaries b) gonadal ridges to form testes c) RNA d) testosterone e) protein enhancing transcription of sex linked genes

13) CIRCLE ALL CORRECT ANSWERS: The pseudoautosomal region is found on

a) autosomes in males b) autosomes in females c) X chromosome d) Y chromosome e) both the X and Y chromosome f) could occur in fruit flies

14) Comparing two monozygotic XX twins to two monozygotic XY twins

a) XX are more identical b) XY are more identical c) monozygotic twins do not occur

EXPLAIN YOUR ANSWER:

15) CIRCLE ALL CORRECT ANSWERS: Butterflies and Habrobracon differ in

a) which sex is homogametic b) which sex is monoploid c) one has sex chromosomes and the other does not d) one produces some gametes by mitosis, the other does not

BI111 Chapter Notes - Chapter 42: Wild Type, Chromosome, Meiosis

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