Chapter 6: Linkage & Genetic Mapping (Exam 3)

Question 1

chromosomes and linkage

Answer 1

each chromosome has 100s-1000s of genes, these genes are linked together
chromosomes can be called linkage groups
number of linkage groups is number of types of chromosomes of species
(humans have 24 linkage groups)
genes that are far apart may independently assort

Question 2

phenotypes in linked genes

Answer 2

parental phenotypes occur at much higher rates than the intermediate phenotypes

Question 3

crossing over

Answer 3

produces recombinant (non parental) phenotypes
occurs during prophase 1
non sister chromatids of homologous chromosomes exchange DNA segments
more likely the farther apart two genes are

Question 4

frequency of crossing over

Answer 4

0-4 times per chromosome per meiosis
increases with maternal age

Question 5

Morgan’s three hypothesis (regarding x linked genes in drosophila)

Answer 5

1. genes for body color, eye color, wing length all located on x chromosome
2. due to crossing over, homologous x chromosomes exchange pieces of chromosomes
3. likelihood of crossing over depends on distance between genes

Question 6

genetic mapping

Answer 6

attempts to determine linear order of linked genes along chromosome

Question 7

uses of genetic mapping

Answer 7

understand complexity and genetic organization
improve understanding of evolutionary relationships
diagnose inherited diseases
predict likelihood couple will produce children with inherited diseases
provide info. for improving agriculturally important strains
estimate relative distances between linked genes

Question 8

syntony

Answer 8

the same genes on the same order in many species but in different places

Question 9

formula for map distance

Answer 9

number recombinant offspring / total number offspring x 100

Question 10

one map unit is equal to

Answer 10

1% recombination frequency

Question 11

testcross

Answer 11

heterozygous x homozygous recessive

Question 12

if distance between two genes is large:

Answer 12

likelihood of multiple crossovers increases
causes observed number of recombinant offspring to underestimate distance between genes

Question 13

how to determine likelihood of double crossover

Answer 13

product rule:
P (double crossover) = P (single crossover A) x P (single crossover B)

Question 14

Interference

Answer 14

presence of single crossover alters frequency of second crossover nearby

Question 15

positive interference

Answer 15

first crossover DECREASES probability of second crossover
most common

Question 16

negative interference

Answer 16

first crossover INCREASES probability second crossover
rare

Question 17

How to calculate interference (I)

Answer 17

I = 1 - C
C = observed # double crossovers / expected # double crossovers
I expressed as % which is how many crossovers did NOT occur

Question 18

mitotic recombination

Answer 18

does not involve homologous pairing –> crossing over less likely
does occur rarely
produces pair of recombinant chromosomes with new combination of alleles
if it occurs early in development, may result in patch of tissue different from the rest of the organism