Chapter 15 RQ
1. What are genes located on a sex chromosome called?
2. In fruit flies, what do we call the normal phenotype for a character?
3. Genes that tend to be inherited together are called what?
4. What is nondisjunction?
5. How do people have Down Syndrome?

1. Describe the contributions that Thomas
Hunt Morgan, Walter Sutton, and A.H.
Sturtevant made to current understanding of chromosomal inheritance.

• Morgan  selected Drosophila as organism
- proposed linkage (characteristics are inherited together)
• Sturtevant  Morgan’s student
- probability of crossing over between genes is directly proportional to the distance between them
• Sutton  noticed parallels of Mendel’s thoughts and the actual behavior of chromosomes 

2. Explain why Drosophilia melanogaster is a good experimental organism.
• It is easily cultured in the lab
• They are prolific breeders • They have a short generation time
• Their 4 pairs of chromosomes are easily differentiated and observed 

3. Define linkage and explain why linkage interferes with independent assortment.
• Linked genes do not independently assort • Linked genes  genes that are located on the same chromosome and that tend to be inherited together
- move together through meiosis and fertilization - F2 generation doesn’t show 9:3:3:1 ratio in the dihybrid 

4. Distinguish between parental and recombinant phenotypes.
Parental
Recombinant
• Progeny that have • Progeny whose the same phenotypes differ phenotype as one from either parent or the other of the

parents

5. Explain how crossing over can unlink genes. • During meiosis, exchange of parts between homologous chromosomes breaks linkages in parental chromosomes and forms recombinants with new allelic combinations 

6. Map the linear sequence of genes on a chromosome using the given recombinant frequencies from experimental crosses.
Loci
Recombinant
Frequency
Units b vg
17.0% 18.5* cn b

9.0%

9.0

cn vg

9.5%

9.5

Approximate Map

*higher because b & vg are relatively far apart and double crossovers occur between these loci and cancel each other out  leading to underestimation of map distance next slide

#6 continued…
1.
2.
3.

Establish distance between the genes farthest apart
Determine the frequency between the 3rd (cn) and the 1st
(b)
Consider possible placements

•

b-----------------------vg

•

cn---------b

•

cn----------b-----------vg

•

Determine recombinant frequency between 3 rd (cn) and
2nd (vg) to eliminate  b-------------cn---------------vg

 17

9
OR

b----------cn-----------vg

7. Explain what additional information cytological maps provide over crossover maps. • Cytological maps locate genes with respect to chromosomal features, such as stained bands that can be viewed with a microscope 

8. Distinguish between a heterogametic sex and a homogametic sex.
Heterogametic sex
• The sex that produces two kinds of gametes and determines the sex of the offspring
• Ex: human male
(sperm – X or Y)

Homogametic sex
• The sex that produces one kind of gamete
• Ex: human female
(egg – X only)


9. Describe sex determination in humans.
•
•
•
•

2 chromosomes determine sex – X and Y
Males carry X and Y options
Females carry X only
It is the gene SRY (sex determining region of Y) that triggers for complex events leading to the development of testes, etc.
• Default is the development of ovaries 

10. Describe the inheritance of a sexlinked gene such as color blindness.
• Refers to X-linked traits
• X is larger than Y, so there are more traits
• Fathers  pass on X to daughter - no sex-linked traits to son • Mothers  give to both type of offspring
- sex linked trait is a recessive allele
- females show trait only is homozygous 

11. Explain why a recessive sex-linked gene is always expressed in human males. • It is said to be ‘hemizygous’ - an organism having only one copy of a gene in a diploid organism
• Ex: son (XY)
- on X, the recessive trait for colorblindness - on Y, no dominant