Chapter 15 - Chromosomal Basis of Inheritance

Question 1

Chromosomal Theory of Inheritance

Answer 1

genes, segemnts of DNA coding for a product

are located at specific positions on chromosomes - gene locus

the behavior of chromosomes during meiosis accounts for inheritance patterns

Question 2

Why did Morgan choose Fruit Flies for Experiment

Answer 2

• short regeneration time allowing mass breeding
• easier to watch over multiple generations
• Only has 4 pairs of chromosomes

Question 3

Morgans Terms to describe Phenotypic expression

Answer 3

Wild Type: the phenotype for a trait most commonly observed in natural populations

Mutant phenotypes: any other trait alternative to the wild type as they are alleles assumed to have originated as changes or mutations in the wild type allele

Question 4

Morgans Experimental Conclusions from fruit flies

Answer 4

fly eye color was linked to its sex as mainly it was only white eyed traits showed up in males
- Provided support for the chromosome theory of inheritance

- His work also indicated that genes located on a sex chromosome exhibit unique inheritance patterns

Question 5

Chromosomal Basis of Sex

Answer 5

species sex are determined by the inheritance of sex chromosomes
- Biochemical, physiological and anatomical features associated with males and females are more complicated with many genes involved in their development

- Y chromosome is much smaller than the X chromosome and 
- only has short segments at either end which are homologous with regions on the X allowing them to pair in males

Question 6

Rules of Inheritance of X Linked Genes

Answer 6

• Fathers pass all X-linked alleles to their daughters and none to their sons
  
  • Mothers pass x linked alleles to all sons and possibly to daughters
  • If an X linked trait is recessive, a female will only express if she is homozygous for the allele however males will express if the mother carries it
    Hemizygous: describes males having 1 locus for x linked genes

Question 7

Examples of X Linked Disorders

Answer 7

Duchenne Muscular Dystrophy: an X linked disorder which is a progressive weakening of the muscle and loss of coordination

Hemophilia: an X linked recessive disorder defined as the absence of one or more of the proteins required for blood clotting

Question 8

X Inactivation

Answer 8

• females have two x linked chromosomes, but they do not produce twice as much proteins as 1 x gene is inactivated during early embryonic development

     - randomly and independently an X chromosome condenses into a barr body which is a compact object lying along the inside of the nuclear envelope
     - all mitotic descendents of that cell have the inactive X
  
     - They are reactivated in the ovaries which give rise to eggs allowing every female gamete to have an active X after meiosis

Question 9

Process of X Inactivation

Answer 9

• Inactivation of an x chromosome involved modification of DNA and histones, including the attachment of methyl groups to DNA nucleotides
  • Two regions, one on each X chromosome, associates briefly with each other early in embryonic development, and only one chromosomes XIST(X inactive specific transcript) becomes active, resulting in it forming the Barr Body
    • Multiple copies of the RNA product of this gene attach to the chromosome, covering it and assisting in X inactiviation

Question 10

Linked Genes

Answer 10

Two or more genes located near each other on the same chromosome tend to be inherited together in genetic crosses
- Deviate from Mendels Law of Independent Assortment as they are inherited together

Question 11

Genetic Recombination

Answer 11

the production of offspring with combinations of traits that differ from those found in either P generation parent

Question 12

Recombination of unlinked genes

Answer 12

• With unlinked genes, some offspring have the chance of having a combination of traits that don’t match those of either parent; nonparental types known as recombinant types
  • If 50% of offspring are recombinants, there is a 50% chance of recombination
    ○ This value and lower shows genes cannot be linked
• recombination is due to the random orientation of homologous chromosomes at metaphase 1, leading to independent assortment of the unlinked genes

Question 13

Recombination of Linked Genes; crossing over

Answer 13

• Recombination frequency of less than 50% indicates genes are on the same chromosome(<3% = linked)
  
  • Crossing over accounts for the recombination of linked genes by having two portions of non sister chromatids trade places
  • Two genes in close proximity on the same chromosome are more likely to be linked together
  • Chi-Square test analyzes the phenotypes of F1 testcross in order to see whether two genes are linked or unlinked.

Question 14

Process of Chi Square

Answer 14

• If genes are unlinked, phenotypic ratio of offspring is expected to be 1:1:1:1 however if they are linked, it will be1:1:0:0

The observed data is compared to a set of expected data to predict whether genes are unlinked

Question 15

How are new variations in phenotypes available for natural selection

Answer 15

Physical behavior of chromosomes during meiosis

Differs through: 
- Recombination
    - Crossing over, independent assortment and random fertilization

Question 16

Distance between Genes

Answer 16

Map units: the distance between genes equivalent to 1% recombination frequency
- The observed recombination frequency in crosses involving two genes can have a max value of 50%
Any genes on the same chromosome but physically further apart are considered genetically unlinked due to the distance being so great, linkage cannot be observed in genetic crosses

Question 17

Nondisjunction

Answer 17

where the members of a pair of homologous chromosomes do not move apart properly during meiosis 1 or where sister chromatids fail to separate during meiosis 2.
-error in meiosis 1 results in all offspring having errors, error in meiosis 2 results in half having errors

Question 18

Anueploidy

Answer 18

zygote will have an abnormal number of a particular chromosome

types:
- Monosomic(2n-1)
- Trisomnic(2n+1)

Question 19

POlyploidy:

Answer 19

organisms possess more than two complete chromosome sets
- actually common in the plant kingdom as it assists in playing a role in plant evolution

Types of Polyploidy:
- Triploidy(3n):
- Tetraploidy(4n):

Question 20

Ways Polyploidy Occurs

Answer 20

• Tri: By fertilization of an abnormal diploid egg produced by nondisjunction of all its chromosomes
• Tetra: by the failure of a 2n zygote to divide after replicating its chromosomes leaving mitotic division to produce a 4n embryo

Question 21

Alterations of Chromosome Structure:

Answer 21

• Errors in meiosis or damaging agents can cause the breakage of chromosomes leading to 4 main structural changes:
  • Deletion: when a chromosomal fragment is lost and genes are missing
    • Duplication: Broken fragments can become reattached as an extra segment to a sister or nonsister chromatid
    • Inversion: Chromosomal fragments can also reattach to the original chromosome in the reverse orientation
  • Translocation: crossing over between non-homologous chromosomes
• often requires a reciprocal translocation

Question 22

Syndrome

Answer 22

set of traits characteristic of the type of aneuploidy

Question 23

Example of Trisomnic disease

Answer 23

Down Syndrome (Trisomy 21): the result of an extra chromosome 21 and characterized by developmental delays, heart and other defects which are treatable and non life-threatening.

Question 24

Disorders from Aneuploidy on Sex Chromosomes

Answer 24

Klinefelter Syndrome: when males develop and extra X chromosome leading to them having male sex organs that are smaller and produce little to no sperm

Trisomy X: when a female develops a third X chromosome however besides being at risk for learning disabilities, they are generally healthy and have no unusual physical features besides being slightly taller than average

Turner Syndrome: monosomy X where females lack the ability to reproduce because while they have sex organs, they do not mature

Question 25

Genomic Imprinting

Answer 25

he expression of an allele in offspring dependent on whether the allele is inherited from the male or female parent.
- Most are on autosomes
- Occurs during gamete formation and results in the silencing of a particular allele depending on the gene due to genes being imprinted differently in sperm and eggs

Question 26

Inheritance of Organelle Genes

Answer 26

Extranuclear/cytoplasmic Genes: genes located in mitochondira, chloroplasts plastids and in the cytoplasm

These organelles reproduce themselves and transmit their genes to daughter organelles however not via the process of mendelian inheritance

• passed on through maternal inheritance