recombination and gene mapping

Question 1

Inheritance: the chromosome
theory

Answer 1

Drawing of chromosome
movement during mitosis by
Walther Flemming (1882)

Question 2

Sutton and Boveri (1902)

Answer 2

The chromosome
theory of inheritance states that genes are located
on chromosomes

Question 3

Chromosomes
segregate

Answer 3

independently
in meiosis

Question 4

meiosis

Answer 4

In a hypothetical diploid cell (with
N=2)
the chromosomes align in
metaphase of Meiosis I in one of
two equally possible ways.
This generates FOUR possible
gametes.

Question 5

Each progeny cell receives

Answer 5

a random
assortment of maternally- and paternally-derived
chromosomes

Question 6

for each homologous chromosome, one
is always_______ - and one is_________-derived

Answer 6

maternally
paternally

Question 7

Each progeny cell receives a random
assortment of maternally- and paternally-derived
chromosomes
Although: for each homologous chromosome, one
is always maternally- and one is paternally-derived
This gives rise to a large number of ___________

Answer 7

combinations
of possible gametes (2n where n = number of
chr)

Question 8

How can Mendel’s second law be
universally true?

Answer 8

● The chromosome theory of inheritance states that genes
are located on chromosomes
● Chromosomes segregate independently during meiosis
● Mendel’s second law states that all genes segregate
independently during meiosis, BUT humans have only 46
chromosomes and around 20,000 - 25,000 genes

Question 9

mendel´s second law It can’t be true in many cases.

Answer 9

It is true for genes on
different chromosomes, but not necessarily for genes
on the same chromosome.

Question 10

Genes on the same chromosome are

Answer 10

inherited together

Question 11

Genes on the same
chromosome cannot

Answer 11

segregate
independently (linkage) – as they
are physically connected

Question 12

Genes on the same
chromosome, and the
phenotypic traits they control,
are

Answer 12

inherited together

Question 13

Linkage mapping

Answer 13

can be used to map disease-causing
genes.
We don’t have to know what the gene is: we map
its effect on the disease phenotype relative to other
known loci(genes).

Question 14

‘Recombination’ allows

Answer 14

the fine
mapping of disease-causing genes

Question 15

Not all genes on the same chromosome are
necessarily always inherited together; only those that
are

Answer 15

close together

Question 16

The frequency that two gene alleles are inherited
together tells you how close together the genes are :

Answer 16

the more tightly ‘linked’, the closer they are together.
Due to the process of recombination

Question 17

Crossing over

Answer 17

‘Crossing-over’ can result in
homologous recombination (HR)
meiosis I,
Chromatids from homologous
chromosomes break and recombine
with the other chromatid.
new combinations of chromosomes
progeny will display non-parental
combination of alleles

Question 18

recombinant occurs between

Answer 18

two loci

Question 19

If recombination occurs between two loci, the offspring
can exhibit

Answer 19

non-parental genotypes and phenotypes,
even though the genes controlling the traits are linked (on
same chromosome)

Question 20

The probability of recombination occurring between two
loci is proportional

Answer 20

to the distance between the loci

Question 21

centi-Morgan
(cM), or map
unit, is a
measurement

Answer 21

of
genetic
distance

Question 22

Test cross for linkage between two genes

Answer 22

● Parental cross between inbred
strains containing either both
dominant, or both recessive traits.
● Test cross is F1 X homozygous
recessive.
● Inheritance of the dominant allele
is visible in the phenotype.
● Easy to see whether
recombination has occurred from
the phenotype.
● Quantify number of recombinant
offspring.

Question 23

Recombination frequency =

Answer 23

number of recombinants/total number of progeny x 100
* Measured in map units (mu) or cM (centimorgans)

Two genes are ‘linked’ when
recombination frequency < 50 %

Question 24

What is a multi-factorial disease?
polygenic -
environment -

Answer 24

polygenic – large number of genes influence pre-
disposition (no single causative gene; no mendelian
inheritance pattern)
environment - modifies genetic risk
lack of clear inheritance pattern, or sporadic
massive socio-economic importance, accelerating the
limits of genetic discovery

Question 25

most common diseases are multi-factorial E.g.

Answer 25

Type 2 diabetes, Type 1 diabetes, hypertension, asthma, rheumatoid arthritis, Crohn’s disease

Question 26

Genes influencing risk of multifactorial diseases can be identified by

Answer 26

GWAS Genome Wide Association Studies – identify gene variants with individually small effects on disease risk. SNPs (single nucleotide polymorphisms) are genotyped across the whole genome. * Test whether a particular SNP is more common in people with the disease or controls : “genetic association” * Very large numbers needed!

Question 27

Manhattan plot

Answer 27

Used to illustrate the association of SNPs across the genome with disease

Question 28

Sutton and Boveri proposed that genes are located on: a) Ribosomes b) Chromosomes c) Centrioles d) Nuclear pores

Answer 28

b

Question 29

Independent assortment occurs because: a) Sister chromatids separate in meiosis II b) Crossing-over always happens c) Homologous chromosomes align randomly in meiosis I d) Mitosis produces identical daughter cells

Answer 29

c

Question 30

The number of gamete combinations produced by independent assortment alone is given by: a) n² b) 2n c) 2ⁿ d) 4ⁿ

Answer 30

c

Question 31

Humans produce approximately 8.4 million gamete combinations because: a) They have 46 chromosomes b) Crossing-over increases variation c) n = 23 d) Linked genes undergo recombination

Answer 31

c

Question 32

Independent assortment does NOT apply when: a) Genes are on different chromosomes b) Genes are far apart on the same chromosome c) Genes are on the same chromosome and close together d) Recombination frequency is 50%

Answer 32

c

Question 33

What Is Recombinant Frequency?

Answer 33

👉 Recombinants = offspring whose allele combinations are different from the parents This happens because of crossing over during meiosis. 👉 Recombination frequency = The percentage of offspring that are recombinants. Formula: RF = number of recombinants /total offspring × 100

Question 34

Genetic linkage occurs when two genes: a) Are on different chromosomes b) Are far apart on the same chromosome c) Are close together on the same chromosome d) Undergo independent assortment

Answer 34

c

Question 35

Linked genes typically show: a) 50% recombination b) More recombinants than parentals c) More parentals than recombinants d) No recombination ever

Answer 35

c

Question 36

A linkage group refers to: a) All genes influencing the same trait b) All genes on one chromosome c) All alleles that assort independently d) All genes in the nucleus

Answer 36

b

Question 37

Linked genes violate Mendel’s second law because: a) They are recessive b) They do not undergo meiosis c) They are inherited together d) They randomly mutate

Answer 37

c

Question 38

The recombination frequency (RF) of linked genes must be: a) Exactly 50% b) Less than 50% c) More than 50% d) Always 0%

Answer 38

b

Question 39

Crossing over occurs between: a) Sister chromatids b) Homologous chromatids c) Non-chromosomal DNA d) Mitochondrial genomes

Answer 39

b

Question 40

Recombinant gametes arise because: a) Chromosomes duplicate b) Chromosomes assort independently c) Homologous chromosomes exchange DNA d) DNA replication makes mistakes

Answer 40

c

Question 41

If two genes show a recombination frequency of 1%, they are: a) 1 base pair apart b) 1 map unit (cM) apart c) 10 cM apart d) Unlinked

Answer 41

b

Question 42

Two genes far apart on the same chromosome usually show: a) High recombination frequency b) No recombination c) Low recombination frequency d) 0% recombination

Answer 42

a Since they are far apart, there are more places where crossing over can happen between them. So the probability of a crossover between them is high. This = high recombination frequency (RF) Usually close to 50%. If two genes are close, the chance that a crossover happens exactly between them is small. So they stay together → low RF.

Question 43

The maximum recombination frequency possible is: a) 25% b) 40% c) 50% d) 100%

Answer 43

c

Question 44

use the question on slides What are recombinant phenotypes in this cross? a) The two most common phenotypes b) The phenotypes showing dominant traits only c) The two least common phenotypes d) All phenotypes with recessive traits

Answer 44

c

Question 45

use the question on slides Which offspring represent the parental phenotypes? a) Red, white and black, yellow b) Red, yellow and black, white c) Red, white only d) Black, yellow only

Answer 45

b

Question 46

use the question on slides How many recombinant offspring were produced? a) 58 b) 64 c) 122 d) 805

Answer 46

c

Question 47

use the question on slides What is the recombination frequency? a) 8% b) 12% c) 15.2% d) 22%

Answer 47

c

Question 48

use the question on slides If recombination frequency is 15.2%, the genetic distance between the genes is: a) 1.52 cM b) 15.2 cM c) 152 cM d) 0.152 cM

Answer 48

b

Question 49

Multifactorial disorders are influenced by: a) One gene only b) Many genes and environment c) Only environment d) Only sex chromosomes

Answer 49

b

Question 50

Which of the following is a multifactorial disorder? a) Cystic fibrosis b) Type 2 diabetes c) Red-green colour blindness d) Huntington’s disease

Answer 50

b

Question 51

Why can’t we use Mendelian ratios for multifactorial traits? a) Because they are all recessive b) Because multiple genes and environment affect the trait c) Because the dominant allele is lethal d) Because chromosomes don’t segregate

Answer 51

b

Question 52

Multifactorial disorders usually show: a) Clear, predictable inheritance in families b) Sporadic or unclear inheritance patterns c) Only maternal inheritance d) Only paternal inheritance

Answer 52

b

Question 53

Studying multifactorial disorders often requires: a) A single test cross b) Large population studies c) Punnett squares only d) No genetic analysis

Answer 53

b

Question 54

1. The FTO gene was identified using: a) Test crosses in plants b) GWAS in humans c) Family pedigrees d) Chromosome staining

Answer 54

b

Question 55

Carrying two copies of the FTO risk allele causes: a) Dominant monogenic disease b) On average 2.3 kg higher body weight c) Immediate diabetes d) No measurable effect

Answer 55

b

Question 56

How many people were included in the study? a) 3,000 b) 30,000 c) 300 d) 300,000

Answer 56

b

Question 57

What does genotyping allow researchers to do? a) Change the FTO gene sequence b) Identify homozygotes and heterozygotes for the SNP c) Make people heavier d) Ignore environmental effects

Answer 57

b

Question 58

Why is the FTO gene important in genetics? a) It proves all obesity is genetic b) It shows that single SNPs can affect complex traits c) It replaces lifestyle as a factor d) It’s the only gene affecting BMI

Answer 58

b

Question 59

Crossing-over occurs during: a) Meiosis I, prophase b) Meiosis II, anaphase c) Fertilization d) DNA replication

Answer 59

a

Question 60

If a test cross between a double heterozygote (RrYy) and double recessive (rryy) produces mostly parental phenotypes, this indicates: a) Genes are far apart b) Genes are linked c) Genes are unlinked d) Independent assortment is occurring

Answer 60

b

Question 61

Recombination frequency is calculated as: a) Total offspring ÷ recombinants × 100 b) Recombinant offspring ÷ total offspring × 100 c) Parental offspring ÷ recombinant offspring × 100 d) Total offspring ÷ parental offspring × 100

Answer 61

b

Question 62

In the same tomato cross, which phenotypes are recombinant? a) RY and ry b) Ry and rY c) All phenotypes d) Only RY

Answer 62

b

Question 63

Genes separated by 50 cM or more: a) Are tightly linked b) Show recombination frequently and behave as if unlinked c) Are lethal when combined d) Cannot be mapped

Answer 63

b

Question 64

SNPs are: a) Structural proteins b) Single nucleotide polymorphisms c) Entire genes d) Only found on sex chromosomes

Answer 64

b

Question 65

In a Manhattan plot: a) The x-axis shows chromosomes, the y-axis shows strength of SNP association b) The x-axis shows allele frequency, the y-axis shows recombination c) The x-axis shows phenotypes, the y-axis shows genotypes d) The x-axis shows genes, the y-axis shows environmental factors

Answer 65

a

Question 66

Large sample sizes are required in GWAS because: a) Each SNP has a very small effect size b) To perform test crosses c) Only to detect Mendelian disorders d) Recombination frequency is very high

Answer 66

a