Lecture 4 Linked genes and sex determination

Question 1

Chromosome theory of inheritance

Answer 1

● Mendelian genes have specific loci (sites) along chromosomes,
and it is the chromosomes that undergo segregation and
independent assortment
● Morgan Hunt’s experiments with fruit flies (Drosophila melanogaster)

Question 2

Sex-Linked Characteristics: genes on the Sex Chromosomes

Answer 2

.Morgan first observed and noted
-Wild type (most common) phenotypes that were common in the fly populations
.Traits alternative to the wild type are called mutant phenotypes

Question 3

Morgan Hunt’s experiments

Answer 3

slide 5-9

Question 4

Sex-linked genes

Answer 4

-X and Y chromosomes pair during meiosis, even though they are not
homologous (the genes located on each are different)
-Sexual reproduction: alternates between haploid and diploid states
-Most organisms have two sexual phenotypes, male and fem

Question 5

Chromosomal Sex-Determination Systems

Answer 5

• Sex chromosomes vs autosomes
  It’s the genes on the sex chromosomes that determine sex phenotypes

Question 6

XX-XO System - insects

Answer 6

● In some insects - Males are X0 and females are XX
● In other insects (fruit fly, for example) - Males are XY (heterogametic) and females are XX
(homogametic)
○ The Y chromosome does not determine maleness
● Rather, it is the ratio between the X chromosomes and the number of sets of
autosomes (X/A)
○ If X/A = 0.5, the fly becomes a male
○ If X/A = 1.0, the fly becomes a female

slide 12

Question 7

XX-XY system – mammals - Some plants, some insects, reptiles

Answer 7

● Humans have 46 chromosomes
○ 44 autosomes
○ 2 sex chromosomes
● Males contain one X and one Y chromosome - heterogametic● Females have two X chromosomes - homogametic
● The Y chromosome determines maleness

slide 13

Question 8

ZZ-ZW system – birds, snakes, butterflies, amphibians , fishes

Answer 8

● Sex chromosomes:
○ Z and W (distinguish them from the X and Y chromosomes of mammals)
● Males - two Z chromosomes (homogametic)
● Females - one Z and one W chromosome (heterogametic)

slide 14

Question 9

Haplo-diploid system - bees and ants

Answer 9

● No sex chromosomes
● Females - fertilized eggs - 2n (diploid)
● Males - unfertilized eggs - n (haploid)

slide 15

Question 10

Difference between X and y chromosomes

Answer 10

slide 16

Question 11

Pseudoautosomal regions SRY gene determines maleness

Answer 11

slide 17

Question 12

Errors in Meiosis

Answer 12

slide 18-19

Question 13

In humans - X-linked color blindness

Answer 13

slide 20

Question 14

Dosage Compensation

Answer 14

● Female mammals inherit two X chromosomes
● Twice the number inherited by males
● Do females produces double the number of proteins?

slide 26

Question 15

The Lyon Hypothesis

Answer 15

1940’s two Canadian scientists noticed a dark
staining mass in the nuclei of cat brain cells
● in female but not males.
● cats and humans
● They thought the spot was a tightly condensed X
chromosome.
● During embryonic development one X chromosome
becomes inactive (Barr Body).
● Found only in female somatic cells.

Question 16

Selection of inactivated X

Answer 16

slide 28

Question 17

X inactivation and the tortoiseshell cat

Answer 17

slide 29

Question 18

Mechanism of Random X Inactivation

Answer 18

Requires two steps
1. Assessment of how many X chromosomes are present.
2. one X chromosome is selected to become the active X
chromosome and all others are inactivated.

For inactivation to occur, each X chromosome must have a gene
named Xist (for X-inactive specific transcript)
■ Xist gene - only expressed on the inactive X chromosome
■ Does not encode a protein
■ codes for a long RNA, which coats the inactive X chromosome
■ Other proteins bind/promote chromosomal compaction into a Barr body

Question 19

Genetic Recombination and Linkage

Answer 19

● The number of genes in a cell > the number of chromosomes
● Genes located near each other on the same chromosome tend
to be inherited together in genetic crosses;
● genes inherited together = linked genes
How does linkage affects inheritance?
● Experiment fruit flies: are genes for body color and wing size linked?
○ body color b+
(gray) and b (black),
○ wing size are vg+ (normal) and vg (vestigial).

slide 33

Question 20

Chromosomal basis for recombination of linked genes

Answer 20

slide 36-39

Question 21

Calculating the recombination frequencies

Answer 21

Linkage mapping by Sturtevant
● A genetic map based on recombination frequencies
● use crossover frequency to position loci along a chromosome
● distances between genes in map units (1 map unit = 1%
recombination frequency)
slide 34

ex: slide 40