Sex Determination in Animals

Question 1

What does homogametic mean?

Answer 1

Organisms possessing two identical sex chromosomes
(e.g. XX in female humans or ZZ in male birds)

Question 2

What does heterogametic mean?

Answer 2

Organisms possessing two different sex chromosomes
(e.g. XY in male humans and ZW in female birds)

Question 3

What is the Lygaeus System?

Answer 3

Females: two X- chromosomes, homogametic (XX)
Male: single X chromosome and Y chromosome (XY), heterogametic

Most common, eg humans, rats, some reptiles- green iguana

Question 4

What are some Human Abnormalities?

Answer 4

Normal Humans
XX- female
XY- male
Males’ Y chromosomes have the sex-determining region Y (SRY) gene which is Holandric (Y-linked) and codes for testis-determining factor (maleness)

Human Abnormalities
XX- male (SRY gene located on the top homologous region of the X chromosome)
XY- female (the SRY gene was mutated or missing on the y chromosome)

Question 5

What is the Protenor System?

Answer 5

Females: two X- chromosomes, homogametic (XX)
Male: single X chromosome, heterogametic (XO)
O is not a chromosome

eg. insects like Hemiptera, Orthoptera (grasshopper)

Question 6

What is the Birds ZW-ZZ System?

Answer 6

Females: one Z- chromosome and one W chromosome, heterogametic (ZW)
Male: two Z chromosomes, homogametic (ZZ)

eg. Birds, moths, some isopods, some amphibians and some fishes

Question 7

What is the ZO-ZZ system?

Answer 7

Females: one Z- chromosome, heterogametic (ZO)
Male: two Z chromosomes, homogametic (ZZ)
O is not chromosome

eg. Butterflies, caddisflies (moths)

Question 8

How does the environment determine the sex of an organism?
Hint: Marine Mollusk- common slipper limpet

Answer 8

• Lives in stacks- one on top of the other
• First to settle on a solid unoccupied substrate develops into a female
• Those that settle on top become the males and serve as mates for females
• After a while the males turn into females and attract more male larvae on top of them
• This is referred to as sequential hermaphroditism- each animal can be both male and female, although not at the same time

Question 9

How does the environment determine the sex of an organism?
Hint: Temperature

Answer 9

For some reptiles temperature during embryonic development determines sexual phenotype
- Turtles: warm temperatures produce females; cool temperatures produce males
- Alligators: warm temperatures produce males (>34 degrees C); cool temperatures produce females (<30 degrees C)

Question 10

If temperature effects the sexual phenotype of some reptiles, how would climate change effect these reptiles?

Answer 10

If there is a climate change then more of one sex would be produced over the other

Question 11

How does Chromosome Balance Theory of Sex Determination (Genetic balance) determine the sex of an organism?
Hint: Drosophila melanogaster (fruit fly)

Answer 11

Sex is determined by the balance between sex chromosomes and autosomal complement of chromosomes… How?

Lets say that ‘A’ represents a haploid set of autosomal chromosomes
X/A ratio = 1 or higher = female
X/A ratio = 0.5 or lower = male
X/A = >0.5-<1.0 = intersex

Female genome- AAXX
Male genome- AAXY *(Y chromosome is involves in male fertility but not in the determination of sex)

Question 12

What is the evidence for the Genetic Balance Theory in Drosophila melanogaster?

Answer 12

Bridges (1922) observed that Drosophila occasionally produces a triploid due to non- disjunction

Abnormal Normal
AAXY x AAXX AAXY x AAXX
/ \ / / \ /
AX AY AAXX AX AY AX
AAAXXX AAAXXY AAXX AAXY
X/A=1 X/A=0.66 X/A=1 X/A=1

AAXY x AAAXXX
/ \ / \
AX AY AAX AXX
AAAXX AAXXX AAAXY AAXXY
X/A=0.6 =1.5 =0.33 =1
intersex superfemale supermale female
(infertile (fertile male)
male)

Question 13

What is a Gynandromorph?

Answer 13

an abnormal individual, especially an insect, having some male and some female characteristics

Question 14

What are the characteristics of Intersex (Gynandromorphs) in Drosophila melanogaster?

Answer 14

• Have male and female organs
• Highly influenced by the environment (temperature)
  - high T- more female
  - low T- more male
• Modifying factors also influence - more male or female

Question 15

What is the Transformer Gene in Drosophila melanogaster?

Answer 15

Superimposed on the genetic balance system is another level of control of the determination of sex
-transformer gene (autosomal gene)

This gene in the homozygous recessive state (tra.tra) converts a diploid female into a sterile male
tra.tra
AAXX ————–> Sterile Male
Tra.___
AAXX ————–> Female

Question 16

What is Haplo-diploidy sex determination?
Hint: Bees

Answer 16

a sex-determination system in which males develop from unfertilized eggs and are haploid, and females develop from fertilized eggs and are diploid
Autosomes are involved- the number of autosomal complements determine sex

eg. Hymenopteran insects such as bees, wasps, ants
In Bees- eggs ———————-> haploid males (A)
l Parthenogenesis
l (arrhenotoky)
————————–> diploid females (AA)
/ \ (quality of food)
Queen Female worker
*The queen controls sex ratio

Question 17

What is Haplo-diploidy complimentary gene method sex determination?
Hint: Wasps

Answer 17

In Wasps- eggs ———————-> haploid males (A)
l Parthenogenesis
l (arrhenotoky)
————————–> diploid male worker
l Homozygous (infertile-no meiosis)
l gene ‘S’
————————–> diploid female worker
Heterozygous gene ‘S’

Question 18

What is Haplo-diploidy complimentary gene method sex determination?
Hint: Alleles in Wasps

Answer 18

The ‘S’ gene has 9 multiple alleles
Homozygous states
S^aS^a, S^bS^b, S^cS^c, S^dS^d…. S^iS^i =2N worker male

Heterozygous states
S^aS^b, S^aS^c, S^aS^d etc. =2N worker females

*There are more heterozygous combinations than homozygous and hence more worker females than worker males

Question 19

What is the genic sex determination in Fungi?

Answer 19

Lower plants such as Chlamydomonas and fungi
- one or a few autosomal genes control sex.

e.g. Chlamydomonas - ‘A’ = + mating type
= - mating type
e.g. Basidiomycete fungi – two genes, A & B
AB – mating type-1
Ab – mating type-2
aB - mating type-3
ab – mating type-4
*males and females look similar