18. Linkage and sex determination

Question 1

How does gene linkage interact with mendelian inheritance?

Answer 1

Gene linkagee contradicts mendelian inheritance: law of independent assortment denied

Question 2

Explain the discovery of gene linkage

Answer 2

Crossing between different phenotypes -> both phenotypes observed in offspring

Question 3

How are linked and non-linked genes written?

Answer 3

The letters on one chromosome or two

Question 4

Explain why there are four possible genotypes in gametes despite genes being linked?

Answer 4

Although genes are linked, recombination can occur -> four different phenotypes in offspring

Question 5

Explain the mechanism of breaking gene linkage

Answer 5

• Crossing over (2 gene linked chromosomes, 2 recombinant)
• Crossing over is regulated: if genes close - higher chance of crossing over
• Double crossing over possible - undoes the effect of crossing over

Question 6

Explain double crossing over

Answer 6

Question 7

How can a map of genes be created?

Answer 7

• Probability of crossing over can be used to determine the gene map (if close - higher probability, if far - lower probability) → percent of recombinant offspring can be used to determine the distance between genes on a chromosome
• Distance can be determined from 9% → 1% recombination frequency = 1 map unit in centimorgan (cM)

Question 8

What is the proof of double crossing over in gene mapping?

Answer 8

The differences in recombination frequencies:

Question 9

What is sex linkage?

Answer 9

When a trait is linked to sex: white eyes can only be inherited by males, females - red eyes although have the mutate white eye allele w- → the phenotype of white eyes only shows when mutant w- and Y chromosome come together

Question 10

What is hemizygous?

Answer 10

When only one allele of a certain gene is present (one allele is missing) → males are hemizygous because of XY sex chromosome combination - one allele present, not in pairs

Question 11

What are the examples of sex linkage in humans?

Answer 11

• hemophilia: mother is the crarrier → pass on to male offspring (X from mother, Y from father)
• daltonism

Question 12

Autosomes vs heterosomes

Answer 12

autosomes - don’t determine the sex of an organism (22 pairs of autosomes in humans)

heterosomes - determine the sex of an organism (2 same or different heterosomes in humans)

Question 13

Which sex is the heterogametic sex?

Answer 13

• Humans: males - two different gametes chromosomes - hetero
• Chicken: females heterogametic
• Drosophila: males heterogametic

Question 14

How is the sex determined in Drosophila vs humans when different heterosomes are present?

Answer 14

• in humans: if Y present → male
• in Drosophila: depends on the ration of X and Y chromosomes
• in C. elegans: most are hermaphrodites, males rare - develop due to a spontaneous non-disjunction during meiosis → the sex is determined by the dosage of X chromosomes in C. elegans (XX - hermaphrodite, X0 - male)

Question 15

What is the possible molecular mechanisms in sex determination? Dosage compensation

Answer 15

• Humans: in X chromosome - many genes for inactivation of another X chromosome’s genes (double dose of same genes in females than in males) ← dosage compensation (random inactivation)
• Worms: downregulation by half of both X in hermaphrodites
• Drosophila: upregulation of X chromosome (?)

Question 16

Explain how genes in X chromosomes are inactivated in human females? Example?

Answer 16

• High condensation of one X chromosome (not apparent in males in micrographs)
• Tortoiseshell cats: different X chromosome is inactivated in each cell → heterozygous female cats fur in patches

Question 17

What are the non-genetic systems of sex determination in organisms?

Answer 17

Environment:

• temperature dependent sex determination in reptiles
• many animals can bias the sex

Question 18

Lecture topic summary

Answer 18