Chapter 15

Question 1

law of segregation

Answer 1

the two alleles for each gene separate during independent assortment

Question 2

law of independent assortment

Answer 2

alleles of genes on nonhomologous chromosomes assort independently during gamete formation

Question 3

chromosomal theory of inheritance

Answer 3

states that Mendelian chromosomes have specific loci

o Also, it is the chromosomes that undergo segregation and independent assortment

Question 4

wild type

Answer 4

red eyes (most common phenotype) found in drosophila (w+)
o	dominant trait

Question 5

mutant type

Answer 5

traits that are alternative to the wild type

o w- white

Question 6

X-Y system

Answer 6

sex of offspring depends on whether sperm cell contains X or Y chromosome

Question 7

X-0 system

Answer 7

females have two x chromosomes (XX), males have one sex chromosome (X0)
• Sex is determined based on whether sperm carries X chromosome or no chromosome at all
• Grasshoppers, cockroaches, and beetles

Question 8

Z-W system

Answer 8

sex chromosome present in egg, are either Z or W
•	Females- ZW
•	Males- ZZ
•	76 chromosomes
•	Birds, fishes, and insects

Question 9

Haplo-diploid system

Answer 9

females develop from fertilized eggs and are thus diploid, males develop from unfertilized eggs and are haploid (males have no fathers)
• Bees and ants

Question 10

SRY

Answer 10

“sex-determining region of Y”

o In absence of this region, gonads develop into ovaries

Question 11

sex-linked gene

Answer 11

gene located on either sex chromosome

Question 12

Y-linked gene

Answer 12

genes located on Y chromosome

• Passed down by father

Question 13

X-linked gene

Answer 13

genes found on the X chromosome

Question 14

hemizygous

Answer 14

when a male lacks a recessive allele from his mother

o One or more genes that have no allelic counterparts???

Question 15

Hemophelia

Answer 15

x-linked recessive disorder marked by absence in factors involved in blood clotting
o Bleeding can cause severe damage
o Results from mutation on X chromosome
Below… irrelevant?
o Almost all of one X chromosome is inactivated during female embryonic development
o Consequently, males and females have the same dose of X-linked genes

Question 16

Barr body

Answer 16

inactive X chromosome in a female
o Lies along the inside of a nuclear envelope
o Reactivated in ovaries to give rise to eggs so that each female gamete has an active X

Question 17

linked genes

Answer 17

genes located near each other on the same chromosome and which are inherited together

Question 18

genetic recombination

Answer 18

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

Question 19

parental types

Answer 19

when the offspring inherit a phenotype that matches either of the phenotypes of the parental (P) generation

Question 20

recombinant types/recombinants

Answer 20

offspring with new combinations of phenotype
o Results from random orientation of homologous chrom. at metaphase I of meiosis which leads to independent assortment of 2 unlinked genes

Question 21

crossing over

Answer 21

process that breaks the physical connection between specific alleles of genes on the same chromosome
o Accounts for recombination of linked genes

Question 22

genetic map

Answer 22

an ordered list of the genetic loci along a particular chromosome

Question 23

Sturtevant’s hypothesis

Answer 23

o Stated that the recombination frequency depends on the distance between genes on a chromosome
o Assumed crossing over was a random event
o Predicted that the farther two genes are, the higher the probability that a crossover will occur between them and therefore, the higher the recombination frequency

Question 24

recombination frequency

Answer 24

likelihood of recombination?
o The maximum value is 50% and occurs when two genes on a chrom. are so far apart crossover is basically guaranteed
• At this point, the genes are still considered genetically unlinked and assort independently (as if they were on different chromosomes).

Question 25

linkage map

Answer 25

genetic map based on recombination frequencies

Question 26

map units

Answer 26

how Sturtevant expressed the distance between genes

o 1 map unit was defined to equal a 1% recombination frequency

Question 27

cytogenic maps

Answer 27

locate genes with respect to chromosomal features, such as stained bands

Question 28

nondisjunction

Answer 28

o When members of a pair of homologous chromosomes do not separate properly during meiosis I
o Or when sister chromatids fail to separate during meiosis II
o Consequence: one gamete receives 2 copies of the same type of chromosome while another receives no copy
o If occurs during embryonic development, aneuploidy condition passed on by mitosis

Question 29

aneuploidy

Answer 29

when zygote has an abnormal number of particular chromosome

Question 30

monosomic (aneuploidy)

Answer 30

when fertilization results in a gamete with a missing copy of a chromosome
 Result: cell has 2n-1 chromosomes

Question 31

trisomic (aneuploidy)

Answer 31

when chromosome is present in three copies in zygote
 Result: cell has 2n+1 chromosomes
 Ex: down syndrome

Question 32

polyploidy

Answer 32

when organisms have more than 2 complete sets of chromosomes in all somatic cells
o Can result from failure of 2n zygote to divide after replicating its chromosomes

Question 33

deletion

Answer 33

when chromosomal fragment is lost

• Resulting chromosome is missing certain genes

Question 34

duplication

Answer 34

when 1 of 2 things happens:
• 1) When a detached fragment attaches to a nonsister chromatid of a homologous chromosome
• 2) When a chromosome is missing certain genes and the deleted fragment becomes attached as an extra segment to a sister chromatid.

Question 35

inversion

Answer 35

when a chromosomal fragment may reattach to the original chromosome but in the reverse orientation

Question 36

translocation

Answer 36

chromosomal breakage that causes a fragment to join to a nonhomologous chromosome

Question 37

Down syndrome

Answer 37

results from extra chromosome 21 (chrom. is present in 3 copies total)
o Each body cell has 47 chromosomes
o Results from nondisjunction during meiosis I

Question 38

Genomic imprinting

Answer 38

variation in phenotype depending on whether allele is inherited from male or female parent
o Occurs during gamete formation and is the result of silencing a particular allele of a set of genes
o Offspring expresses only one allele of an imprinted gene- the one inherited from either the female or the male parent
o In new generations old imprints are erased and chromosomes of developing gametes are newly imprinted according to the sex of the individual
o Sometimes consists of methyl groups that are added
o Methylation may silence the allele