2 - Linkage, Recombination & Deviations from Mendelian Ratios

Question 1

which 4 processes contribute to genetic variation?

Answer 1

1. mutation
2. independent assortment of chromosomes during meiosis I
3. crossing over between homologous chromosomes during meiosis I
4. random fertilisation of ova by sperm

Question 2

how does independent assortment + random fertilization promote genetic variation?

Answer 2

• The choice of which sperm fuses with which egg is random
• With 8,388,608 kinds of sperm and 8,388,608 kinds of eggs, the no. of possible chromosome combinations in the offspring from one couple is >70,000,000,000,000
• crossing over adds even more variation

Question 3

recombinant phenotype

Answer 3

where the combination of phenotypes differs from that found in either of the parents

Question 4

how do recombinant phenotypes occur?

Answer 4

through either independent assortment or crossing over

Question 5

what is crossing over?

Answer 5

the process of genetic recombination that gives rise to new combinations of linked genes

Question 6

crossing over explained

Answer 6

• Occurs during the pachytene phase of prophase I
• Begins with synapsis - pairing of homologous chromosomes
• The synaptonemal complex is a protein ‘zipper’ that holds homologous chromosomes together in the tetrad

Question 7

what is the result of crossing over?

Answer 7

recombinant chromosomes with new combinations of linked genes

Question 8

recombinant frequency

Answer 8

percentage of the progeny that inherit a combination of alleles that differs from either parent

Question 9

what are the results of a testcross when genes are on different chromosomes?

Answer 9

50% parental, 50% recombinant

Question 10

what are the results of a testcross when genes are on the same chromosomes?

Answer 10

> 50% parental

<50% recombinant

Question 11

who was the first to discover gene linkage?

Answer 11

T.H.Morgan using fruit flies

Question 12

testcross involving the linked genes black (b) and vestigial (vg)

Answer 12

Mutants are given a name that reflects their phenotype - usually abbreviated e.g. black (b) and vestigial (vg)
recombinant frequency = 17%

Question 13

what are the 2 possible arrangements of alleles in a double heterozygote?

Answer 13

coupling and repulsion

Question 14

coupling heterozygote testcross outcome

Answer 14

parentals = b+ vg+ and b vg 
recombinants = b vg+ and b+ vg

Question 15

repulsion heterozygote testcross outcome

Answer 15

parentals = b vg+ and b+ vg 
recombinants = b+ vg+ and b vg

Question 16

what do multiple crossovers lead to?

Answer 16

• an underestimate of the distance between 2 loci

- recombinant frequencies >50% are not possible

Question 17

3 point cross

Answer 17

allows the detection of double crossovers such that genes can be put into order

• largest numbers = parental types
• smallest numbers = double crossover

Question 18

who created chromosome maps?

Answer 18

A.H> Sturtevant (1913)

He saw that the recombination freq. between 2 genes should be related to their distance apart on the chromosome

Question 19

what are the 6 deviations from Mendelian ratios?

Answer 19

sex-linkage, 
incomplete dominance, 
co-dominance, 
pleiotropy, 
polygenic inheritance, 
epistasis

Question 20

sex linkage

Answer 20

occurs when certain traits are determined by genes on sex chromosomes

Question 21

occurs when certain traits are determined by genes on sex chromosomes

Answer 21

the sex chromosome found only in males.

cannot carry a gene

Question 22

incomplete dominance

Answer 22

Where a dominant allele does not completely mask the effect of a recessive allele at the same locus

Question 23

co-dominance

Answer 23

Where each allele affects the phenotype in separate, distinguishable ways

Question 24

incomplete dominance compared to codominance

Answer 24

• incomplete dominance = blending of traits and a ‘diluted’ phenotype
• co-dominance = no blending: traits appear together and remain distinct in the heterozygote

Question 25

Pleiotropy

Answer 25

Where a single gene has multiple effects on the phenotype

Question 26

polygenic inheritance

Answer 26

Where a single trait is determined by multiple genes

Question 27

Epistasis

Answer 27

When one gene masks or modifies the expression of another gene
(B is epistatic to A)
ratio of F2 in dihybrid cross = 9:3:4