Lecture 8: Sex Determination and Sex Chromosomes

Question 1

Define Sex Chromosomes

Answer 1

Chromosomes whose presence, absence, or quantity play a role in determining the sex of an organism

Question 2

Contrast primary and secondary sexual differentiation.

Answer 2

Primary Sexual Differentiation:
differentiation between two sexes only as it pertains to gonads(ovaries and testes; where gametes are produced)

Secondary Sexual Differentiation:
Morphological differences between the sexes do not involve gonads (e.g: female mammals have more breast tissue)

Question 3

3. What is the heterogametic sex and what is Haldane’s rule?

Answer 3

Heterogametic Sex: The sex which does not produce identical gametes with respect to sex chromosomes (XO,XY,ZW)

Haldanes Rule: When you cross two diverged species and one of the offspring is sterile than the sterile one is the heterogametic sex.

Question 4

What is Haldane’s Rule?

Answer 4

When two closely related species are crossed and only one sex of the hybrids is missing or sterile, the sterile sex is the heterogametic(non-identical gametes)

Question 5

Define intersex and contrast the meaning of this word with the meaning of hermaphroditic.

give examples

Answer 5

Intersex:
reserved for individuals of a gonochoric (only female OR male reproductive organs) species with an intermediate sexual phenotype

the intermediate sexual phenotype is typically characterized by:
being sterile
ovotestes(gonads with both ovaries and testicular aspects)

e.g: in some mole species all the females have ovotestes but do not produce functional sperm

Hermaphroditic: Produce both eggs and sperm and self-fertilize

Question 6

dioecious/gonochoric

Answer 6

refers to an individual containing only male or female reproductive organs

Question 7

Monoecious

Answer 7

refers to individuals containing both male and female reproductive organs

can also refer to species lacking distinct sexes which is common in plants

Question 8

Does Chlamydomonas have sexual differentiation? Define isogamous in your answer.

Answer 8

Yes,
two types of cells produce + and - gametes under stress
They combine to form the zygote
the gametes are morphologically the same in size and shape making them isogamous

when no longer stressed zygote undergoes meiosis and produces 4 haploid sells that asexually reproducir

Question 9

You are performing a mutant screen on maize. You notice that one of the plants in the screen is only capable of forming female flowers. What can you infer about the mutation that is causing this?

Answer 9

begins with an ambiguous structure and sexual differentiation occurs from the spontaneous abortion of the male or female genes. If only female plants are produced there is a mutation in male sex determining genes that leads to the spontaneous abortion of female genes.

Question 10

Compare and contrast sex determination in C. elegans and Drosophila.

Answer 10

C. Elegans:
most individuals are hermaphroditic(produce eggs and sperm)

Males are XO ( do not receive Y chromosome due to XO sex determination(meiotic nondisjunctions)

Drosophila:
Y chromosome is not involved in sex determination in females

Females are XX or XXY
Males are XY
Sterile males are XO

for both the ratio 2:1 autosome to sex chromosomes determines sex.

Question 11

Compare and contrast sex determination in humans and Drosophila. What is the sex in both
species of XXY and X individuals?

Answer 11

Drosophila:
XXY–> females
X–> males

Humans:
XXY–> Males (kinefelter syndrom)
X–> Females (turner syndrome)

Question 12

How do we explain the phenotype of a bilateral gynandromorph in genetic terms?

Answer 12

females were heterozygous for two X-LINKED genetic variants
m=x linked recessive mutant for mini wings
w= x linked mutatation for white eyes

one cell lost its x in the first mitotic division (results in one side have male phenotype(no mutations)

other cells retained two xx’s(female phenotype w mutations on the other side)

based on the orientation of the spindle in the first mitotic division the line seprating the phenotypes can be along any axis

fly is mosaic of the two sexes

Question 13

Why does the white throated sparrow effectively have four sexes? Why might this mating system
be evolutionarily transient?

Answer 13

There is white and tan morph and they mate via negative assortive mating making effectively 4 different sexes.

due to increased cost of finding a mate this is evolutionary transient

Question 14

In temperature-dependent sex determination, why must there be a genotype by environment interaction when comparing the different species mentioned in lecture?

Answer 14

aromatase: gene encodes enzyme that converts adnrogens to estrogens. At different temperatures aromatase activity increases or decreases
due to different species having different charactertic patterns of temperature-dependent sex determination

Question 14

How is sex determined in Hymenoptera. What is one genetic consequence of this sex
determination system?

Answer 14

rely on haplodiploidy females are diploid and males derived from unfertilized eggs are haploid

results in sisters being more closely related than diploid diploid organisms(humans)

Question 15

Contrast sex determination in humans and platypodes. Explain how platypodes are an exception
to the law of independent assortment.

Answer 15

unlike humans platyplodes do not abide by independent assortment as chromosomes are segregated into xxxxx and yyyyy gametes in male meiosis

Question 16

For each of the following human karyotypes, give the sex and a common phenotype associated with the karyotype: X,XXX,XYY,XXY, Y

Answer 16

X: One X but Female ( Turner Syndrome)
smaller ovaries, smaller stature
XXX: triplo X in Females
typically phenotypically normal
XYY: Jacob syndrome
anti-social behavior, learning disabililty
Males
XXY: Klinefelter syndrome 2X’s however still Male
often have some degree of female sexual development
(gyno, rounded hips)
Y:
WOULD NOT Develop as the X chromosome contains may vital genes where as the Y chromosome only contains SRY gene

Question 17

What is a Barr body? How does the logic of dosage compensation in humans contribute to higher tolerance of aneuploidy of the sex chromosomes than in the autosomes?

Answer 17

A Barr body is a condensed cell resulting from the inactivation of the X chromosome. So no matter how many X chromosomes an individual has only one X is activated

Dosage compensation: mechanism to form bar bodies

contributes to higher tolerance of aneuploidy of sex chromosomes as it ensures males and females have the same gene dosage

Without gene dosage females would be developing double the genes due to two activated X activation

Sex chromosomes are more tolerant than autosomes to aneuploidy as autosomes have no mechanism to compensate for aneuploidy which leads to more fatal developmental issues

Question 18

Explain how the mechanism of X inactivation in mammals leads to females being a mosaic of their two X chromosomes in the context of calico cats

Answer 18

Female Cats: carry two xx each carrying fur color allele

Since fur color is X-linked

The different colors of calico female cats highlights the different x chromosomes that were inactivated early in development

Question 19

Suppose you find a male calico cat. What is its likely karyotype and why? What human condition is this analogous to?

Answer 19

Likely XXY as this would allow for x-inactivation without loss of important hereditary material on the X chromosome.

This would be analogous Klinefelter where one of the X chromosomes are randomly inactivated allowing males to have two X chromosomes

Question 20

How do we know that the SRY gene specifies maleness? What type of gene is SRY?

Answer 20

SRY gene is a trancription factor that intiates testes/male development located on Y chromosome
There are XY individuals who are phenotypically female and lack the SRY gene
Injecting xx mice embryos with mouse SRY –> male

Question 21

What is the likely reason for the secondary male-to-female ratio in humans not being 1:1?

Answer 21

Primary Sex Ratio:
The ratio of males and females
conceived (difficult to measure)

Secondary Sex Ratio:
The ratio of males and females
born
Female embryos:

There is Sperm that carries X chromosomes and there is sperm that carries Y chromosome, it is hypothesized that the sperm carrying Y travels faster

Female embryos just die faster