1. CHROMOSOMES Chapter 11 Human X & Y chromosomes.
X chromosome carries more than 1,000 genes.
Y chromosome carries only a few hundred ( ).
Chapter 11

2. A. What Is a Chromosome?
A long, continuous strand of DNA, plus several types of associated proteins, and RNA.
Heterochromatin DNA is tightly wound and composed of many repetitive sequences. It makes up the telomeres, centromeres & helps maintain chromosomes’ structural integrity.
Euchromatin DNA is more loosely wound & is composed of many unique sequences. It encodes proteins.
To remain stable, a chromosome requires 3 basic parts:
telomeres
centromeres
origins of replication (sites where DNA replication begins)

3. Chromosome pairs are distinguished by: size, banding pattern & centromere position.
Karyotype is a chart of the metaphase chromosomes of a cell arranged by size and centromere position.
Centromere positions:
Metacentric - centromere is centrally located; divides chromosome into two arms of approximately equal length.
Submetacentric - centromere divides chromosome into a long and short arm.
Acrocentric - centromere is near one end.
Telocentric - centromere is located at tip.

4. B. Linked Genes Genes carried on the same chromosome.
The 7 traits that Mendel followed in his pea plants were found on different chromosomes (not linked). Had any of them been linked, he would have obtained remarkably different results in his dihybrid crosses.
Four chromosomes on left, A & B genes are NOT linked.
Two chromosomes on right, A & B genes are linked.
Linked genes violate Mendel’s law of independent assortment because they may not separate during crossing-over of meiosis I.

5. What types of gametes are expected from this individual?
¼ PL ¼ pl ¼ Pl ¼ pL
What types of gametes are expected from this individual?
½ PL ½ pl

6. If A & B genes are not linked, they assort independently into gametes during meiosis. Thus, an AaBb individual will produce equal numbers of AB, Ab, aB & ab gametes. A self cross would result in the expected 9:3:3:1 phenotypic ratio.
If A & B genes are linked, they tend to assort together into gametes during meiosis. Thus, an AaBb individual with linked genes (A & B alleles are linked, and a & b alleles are linked) tends to produce equal numbers of AB & ab gametes (parental types). A self cross would not give you the expected dihybrid 9:3:3:1 phenotypic ratio.

7. The further apart two linked genes are, the more likely they will separate during gamete formation.
Because there is more space between the 2 genes, the greater the likelihood they will be separated by crossing-over.
Note: genes located on opposite ends of the same chromosome behave as though they are not linked. The gametes would have all possible allele combinations.

8. Parental gametes retain the gene combinations from the parents.
Recombinant gametes result from the mixing of maternal & paternal alleles during crossing- over.
Parental allele combinations occur when crossing-over fails to separate parental alleles, so they are passed together into the gametes.
Recombinant allele combinations occur when crossing-over separates parental alleles, mixing maternal & paternal alleles into new combinations.
If genes are not linked, equal numbers of parental & recombinant allele combinations will occur in the gametes.
If genes are linked, gametes with parental allele combinations will occur more frequently than gametes with recombinant allele combinations.
Closely linked genes yield few recombinant chromosomes - will NOT obtain expected 9:3:3:1 phenotypic ratio.

9. Knowing allele arrangement is important in predicting trait transmission.
Ex. Two allele combinations are possible for a pea plant with genotype PpLl.
Alleles in coupling tend to be transmitted together.
Alleles in repulsion separate with each generation.

10. 1. Total chromosome number
C. Sex Determination
Mechanism by which an individual develops as a male or a female.
1. Total chromosome number
 is diploid
(develops from a fertilized ovum)
 is haploid
(develops from an unfertilized ovum)
Ex. bees

11. 2. X-O System (number of X chromosomes determines sex)
 is XX
 is XO
Ex. grasshoppers, crickets & roaches
3. X-Y System (presence of Y chromosome determines sex)
In X-O system, male determines sex of offspring because half his sperm contain X chromosome, while other half have no sex chromosome.
SRY gene produces a protein that switches on other genes that direct the embryo to develop male structures. It also activates a gene that encodes a protein that destroys rudimentary female structures.
 is XX
 is XY
Ex. all mammals
SRY gene

12. In X-Y system,  determines sex of offspring.
In X-Y system, male determines sex of offspring because half his sperm contain X chromosome, while other half contains Y chromosome. If a Y-bearing sperm fertilizes an oocyte, the resulting zygote is male (XY). If an X-bearing sperm fertilizes an oocyte, the resulting zygote is female (XX).

13. D. Inheritance of Sex-Linked Traits
Most sex-linked traits are carried on the X chromosome (X-linked) & are recessive.
Ex. colorblindness, hemophilia
more common in 
 cannot be a carrier ( is hemizygous)
 inherits condition from his mother, NOT his father
Hemophilia (free bleeders disease) - afflicted individuals lack a protein clotting factor that is essential in blood clotting.
Hemizygous - have only 1 set of X chromosomes, so they either have the trait or not; cannot be a carrier.

14. Hemophilia: recessive X-linked trait
Genotype Phenotype
XHXH non-carrier 
XHXh carrier 
XhXh  with hemophilia
XHY normal 
XhY  with hemophilia

15. What is the probability that a carrier  and a normal  will have a son with hemophilia?
Punnett square showing possible offspring of a woman carrier (XHXh) and a normal man (XHY).
What parental genotypes would give rise to a daughter with hemophilia?
Father would have to have hemophilia & mother would have to be a carrier or have hemophilia as well. Hemophilia is usually so life threatening that the individual dies before they reach reproductive age.
What is the probability that a non-carrier  and a hemophiliac  will have a son with hemophilia?
ZERO

16. Pedigree of several royal families with hemophilia
Pedigree of several royal families with hemophilia. The mutant allele apparently arose in Queen Victoria, who was either a carrier or produced an oocyte that mutated to carry the allele.

17. E. X Inactivation
Female mammals have 2 alleles for every gene on the X chromosome, while males have only 1.
This inequality is balanced by “turning off” one X chromosome in each cell of a 3 week old  embryo.
some cells turn off paternal X
some cells turn off maternal X

18. How many Barr bodies would cells of a male possess?
Inactivated X appears as a dark-staining structure called a Barr body.
Because the inactivation occurs early in development, females are genetic mosaics for any heterozygous genes on the X chromosome (some cells express one allele, and other cells express the other).
Normal males would not have Barr bodies in their cells.
If a cell from a male possessed a Barr body, what would that tell you about the male’s genotype?
He would be an XXY male.
How many Barr bodies would cells of a male possess?

19. X inactivation is responsible for the appearance of calico cats.
Coat color in calicos is an X-linked trait.
Each orange patch is made of cells descended from a cell in which the X chromosome carrying the coat color allele for black was inactivated.
Each black patch is made of cells descended from a cell in which the X chromosome carrying the orange allele was turned off.
Calicos are almost always female because both orange & black alleles are required produce the distinctive patches.
[Only way to have a calico male is if he is XXY.]
The earlier X inactivation occurs, the larger the patches.

20. F. Chromosome Abnormalities
1. Polyploidy - extra full sets of chromosomes.
animal polyploids spontaneously abort or die shortly after birth
plant polyploids are relatively common (wheat, lilies)
Euploid cell (“true set”) has the normal chromosome number for the species.
Polyploidy can be the result of an error in meiosis that produced a gamete with a diploid set of chromosomes. When this gamete is fertilized, it will possess 3 sets of chromosomes (triploidy).

21. 2. Aneuploidy - an extra (trisomy) or missing (monosomy) chromosome.
Aneuploidy is usually due to a meiotic error called nondisjunction.
Nondisjunction:
A chromosome pair fails to separate, either at the first or second meiotic division. The result is a sperm or oocyte with either two copies of a particular chromosome or none at all, rather than the normal one copy. If one of these abnormal gametes in a human is fertilized, the resulting zygote has either 45 or 47 chromosomes instead of the normal 46.
Most aneuploids spontaneously abort in humans.

22. trisomy 21 (Down syndrome)
Autosomal aneuploids
trisomy 13
trisomy 18
trisomy 21 (Down syndrome)
Sex chromosome aneuploids
Turner syndrome XO 
Triplo-X XXX 
Klinefelter syndrome XXY 
Jacobs syndrome XYY 
Trisomy 21 is the most common autosomal anueploid.

23. 3. Deletion - part of a chromosome is missing.
4. Duplication - part of a chromosome is present twice.
5. Inversion - part of a chromosome is reversed.

24. 6. Translocation - nonhomologous chromosomes exchange parts (reciprocal translocation) or combine (Robertsonian translocation).
Certain viruses, drugs, and radiation can cause translocations, but we often do not know how they arise.