sex determination- lecture 10

Question 1

what is sry

Answer 1

sex determining region on the y chromosome

Question 2

function of sry

Answer 2

sry is a transcription factor that is necessary for an individual to become biologically
male and is sufficient for male development. When an SRY transgene is
inserted into an XX embryo, even without the other Y chromosome genes, it
results in the embryo developing the male phenotype, including functional
male internal and external genitalia and gametes. This shows that SRY can
serve as a “switch” to initiate the typical male development pathway during
embryogenesis

Question 3

give one reason: disconnect in karyotype and actual sex of individual can be caused by

Answer 3

mutated or missing sry gene

Question 4

give another reason: disconnect in karyoype and actual sex of individual can be caused by

Answer 4

androgen insensitivity

Question 5

pseudo autosomal region

Answer 5

Question 6

turner syndrome

Answer 6

Turner syndrome, a condition that affects only females, results when one of the X chromosomes (sex chromosomes) is missing or partially missing. Turner syndrome can cause a variety of medical and developmental problems, including short height, failure of the ovaries to develop and heart defects

Their sex
chromosome genotype is denoted as “XO” to indicate that they have only
one X chromosome instead of 2 as a biological female should have

This indicates that at least one X chromosome is required for human
development, and two for fertility in females

Question 7

Klinefelter syndrome

Answer 7

Klinefelter syndrome (sometimes called Klinefelter’s, KS or XXY) is where boys and men are born with an extra X chromosome. Chromosomes are packages of genes found in every cell in the body. There are 2 types of chromosome, called the sex chromosomes, that determine the genetic sex of a baby.

This is the most common male sex chromosome disorder, and is characterized by traits such as small testes, reduced facial hair, and sterility.
Nonetheless, these individuals have many male sexual characteristics,
providing further support for the idea that a single Y chromosome, even with multiple X chromosomes, is sufficient for development of the male phenotype

have genotype XXY, XXXY, or XXYY

Question 8

poly x syndrome

Answer 8

is a genetic disorder caused by the presence of an extra X
chromosome in females

genotype XXX or XXXX

Poly-X syndrome does not lead to any particular physical features. Some individuals with the disorder are sterile, but this is not true of all Poly-X individuals. While little effect is
observed for physical features, the probability that a poly-X female will
exhibit intellectual disabilities increases with increasing number of X
chromosomes, underscoring the importance of gene dosage. Gene
expression and protein levels are finely tuned, so the more extra X’s an
individual has, the more negative consequences are observed, likely
because X-inactivation is not complete

Question 9

poly y syndrome

Answer 9

XYY syndrome, also known as Jacobs syndrome, is an aneuploid genetic condition in which a male has an extra Y chromosome. There are usually few symptoms (often
goes undiagnosed, because individuals who have it rarely exhibit any
significant observable anomalies)

These may include being taller than average, acne, and an increased risk of learning disabilities

Question 10

sex determination in other organisms

Answer 10

Question 11

sexual dimorphism

Answer 11

Sexual dimorphism is the systematic difference in form between individuals of different sex in the same species. For example, in some species, including many mammals, the male is larger than the female. In others, such as some spiders, the female is larger than the male.

Question 12

gynandromorph

Answer 12

Gynandromorphs are individuals that contains both male and female characteristics.

Gynandromorphs contain cells that possess different combinations of sex
chromosomes - some cells have sex chromosomes that make them female,
while others have sex chromosomes that make them male. This makes the
organism itself partially male and partially female. The mechanism by which
this occurs is best understood in butterflies, where it can be (though not
always) caused by spontaneous mitotic nondisjunction of sex chromosomes
during development

Question 13

mitotic nondisjunction

Answer 13

Question 14

mitotic nondisjunction of sex chromosomes

Answer 14

Question 15

mosaic gynandromorph

Answer 15

predominantly display the phenotype associated
with one sex, with patches of cells having characteristics of the other. The
difference between bilateral and mosaic phenotypes is due to differences in
the timing of sex chromosome nondisjunction (with very early non-disjunction
leading to bilateral gynandromorphs)

When this nondisjunction happens later in development, a mosaic gynandromorph can be produced. If we start with an initial ZW individual,
most of the cells would develop the female phenotype, but if the W chromosome is “lost” by a cell later in development, it (and its descendent
cells) will display the male phenotype instead. The earlier nondisjunction
happens, the more cells will be affected and the greater the extent of the
mosaicism.

Question 16

types of sex determination

Answer 16

chromosomal, genic, environmental

Question 17

sex chromosome evolution

Answer 17

Question 18

dosage compensation

Answer 18

Question 19

Barr body

Answer 19

A Barr body or X-chromatin is an inactive X chromosome

Question 20

X-chromosome inactivation

Answer 20

X-chromosome inactivation is the transcriptional silencing of one X chromosome in female mammalian cells that equalizes dosage of gene products from the X chromosome between XX females and XY males [1–3]. X-chromosome inactivation in the embryo proper occurs early in development

Question 21

lyon hypothesis

Answer 21

Lyon’s hypothesis states that the phenotypic effect of the X chromosome is the same in the mammalian female which has two X chromosomes as it is in the male which has only one X chromosome. One out of two X chromosomes in females is inactivated early in embryonic development.

Question 22

random x inactivation

Answer 22

Question 23

escaping x inactivation

Answer 23

some genes on an inactivated x chromosomes escape and are transcriptionally active

Question 24

why do y chromosomes get smaller

Answer 24

y chromosomes never meet another y chromosome, no recombination

Question 25

how many protein coding genes does the x chromosome have?

the y chromosome?

Answer 25

x= 800
y= 75

Question 26

Sex-linked traits

Answer 26

(either X-linked and Y-linked traits) in humans show unique inheritance patterns that can be detected in
pedigree analysis

Question 27

what happens if the sry gene is absent or nonfunctional

Answer 27

. If the SRY gene is absent (or
nonfunctional), the embryo “defaults” to develop female reproductive traits.
The Mullerian duct will give rise to the female reproductive organs – fallopian tubes, uterus, cervix, and upper vagina, while the Wolffian duct will
degenerate

Question 28

what happens if the sry gene is present and functional

Answer 28

If SRY is present and functional, a male-specifying transcription factor is made. Even though the SRY transcription factor is only produced at a low
level for a few days, it initiates a cascade of developmental events that leads to male sexual differentiation. SRY causes the bipotential gonad to
develop into the testis, which secretes (1) anti-Mullerian hormone, causing the Mullerian ducts to degenerate, and (2) the androgen hormone testosterone, which promotes male characteristics. Testosterone (and other androgens) cause the Wolffian duct to differentiate into the epididymis, vas
deferens, seminal vesicles, and ejaculatory duct, and for other male reproductive organs like the prostate and penis to form.

Question 29

give two reasons why an individual can possess a Y chromosome and still be anatomically
female

Answer 29

One possibility is that SRY is either deleted or mutated so that it no
longer functions. In the absence of a functional SRY transcription factor, the male-development “switch” is not activated, and the individual will follow the path to female sex organ differentiation.

If the mutation is not a
complete loss-of-function in SRY, it is possible for intermediate or variable phenotypes to emerge; for example, an individual may possess some male
and some female sex characteristics (an intersex individual). Mutations on
SRY are responsible for 10-15% of cases in which a genetically XY individual develops as female.

Alternatively, mutations in any one of a number of genes that function
after SRY activation can also prevent activation of the male differentiation
pathway. Some XY individuals have a mutation in the autosomal androgen
receptor that responds to testosterone (i.e., androgen insensitivity
syndrome)

Question 30

androgen insensitivity
syndrome

Answer 30

These individuals may produce a high level of testosterone, but
their cells cannot respond to the hormone, and so instead they develop as
female.

Question 31

how do the sex chromosomes pair during meiosis

Answer 31

During meiosis I, homologous chromosomes pair so that they can be
properly segregated into gametes. Though the X and Y chromosomes are
not very similar, it is nonetheless necessary for them to pair up during meiosis.
This ensures they too can segregate properly such that each gamete
receives either an X or a Y chromosome.

The X and Y chromosomes are not very similar in
size or gene content, making it difficult to understand how they could be recognized as “homologous.” However, both the X and Y do contain
specific regions that have similar sequences and can therefore be
recognized as homologous. Both contain sequences known as
pseudoautosomal regions, or PARs. These are specific regions at the end of both the X and Y chromosomes that have sequences that are similar
enough to pair up and cross over during meiosis. In other words, the PARs are homologous, and therefore facilitate X-Y chromosome pairing during meiosis in males. Because at least one cross-over event is required at pairing, the PAR has the highest recombination rate (number of cross-overs
per unit of DNA) in the genome

Question 32

pseudoautosomal regions, or PARs

Answer 32

These are specific regions at the end of
both the X and Y chromosomes that have sequences that are similar enough to pair up and cross over during meiosis. In other words, the PARs are homologous, and therefore facilitate X-Y chromosome pairing during meiosis in males.

Question 33

how can Mistakes in X-Y chromosome pairing can still happen despite the
existence of PARs (sex chromosome non-disjunction)

Answer 33

Unlike most
autosomal aneuploidies, sex chromosome abnormalities are relatively well
tolerated in humans. Because the Y chromosome contains few genes, having an “extra” copy of it does not lead to the extreme imbalances in
gene dosage that are observed when extra autosomes (especially large ones with lots of genes) are present. Additionally, cells that have more than
one X chromosome “turn off” all but one of them, in a process known as X inactivation, so that any “extra” X chromosomes don’t cause major dosage
imbalances.

Question 34

in Drosophila melanogaster (fruit fly) how is sex determined

Answer 34

sex is also determined by an individual’s chromosome content; but in flies, it is not
the presence or absence of a Y that matters, instead it is the ratio between
X chromosomes and autosomes. Though most Drosophila males do contain a Y chromosome, sex is determined by the ratio of X chromosomes to
haploid sets of autosomes (e.g., each X chromosome counts as 1, and each
haploid set of autosomes also counts as 1). In a “normal” diploid female,
there are 2 X chromosomes and 2 sets of autosomes (one homologous pair
per chromosome) for an X:A ratio of 1:1. Most often, diploid males have one
X, one Y, and 2 sets of autosomes, for an X:A ratio of 1:2, or 0.5. Variations on
these ratios are possible because fruit flies can survive as triploid, producing
other ratios that can lead to intersex, meta-female, and meta-male
phenotypes

Question 35

in C. elegans (nematode worm) how is sex detemined?

Answer 35

number of X chromosomes that determines an individual’s sex. XX
individuals (the most common sex chromosome genotype) are hermaphrodites (individuals producing both eggs and sperm), and XO individuals (with only a single X chromosome) are male. There are no Y
chromosomes (and no females)

Question 36

how is sex determined in birds and butterflies

Answer 36

In these organisms, the sex chromosomes are called Z and W to differentiate them from X and Y
because their sex determination is like the mammalian system, but backwards. Females are heterogametic – they have two different sex chromosomes, one Z, and one W. Males are homogametic and have the sex chromosome genotype ZZ. In contrast, in mammals, males (XY) are
heterogametic, and females (XX) homogametic.

Question 37

heterogametic

Answer 37

have two different sex
chromosomes

Question 38

homogametic

Answer 38

have the
sex chromosome genotype

Question 39

haplo-diplo sex determination

Answer 39

it is used in some insect groups (e.g.,
bees, ants and wasps). In these insects, females are diploid, and produced by “standard” sexual reproduction (sperm + egg à offspring), while males
are haploid, and produced by parthenogenesis

Question 40

Parthenogenesis

Answer 40

form of asexual reproduction in which an unfertilized egg develops into a new
individual

Question 41

Bilateral gynandromorphs

Answer 41

are those that are half male and half
female, with the right half of the body displaying male characteristics and
the left half of the body displaying female characteristics (or vice versa).

Question 42

why is gynandromorphy unlikely to occur in humans or other
mammals

Answer 42

because of the mechanisms that control the development of
mammalian sex characteristics (e.g., the role of circulating hormones)

Question 43

Genic sex determination

Answer 43

is similar to chromosomal sex determination in
that genetic factors – genes – are responsible for determining an individual’s biological sex. However, the difference is that those genes are not located
on observable sex chromosomes. In cases where many genes are involved, the sex-determining genes can be scattered around the genome. The
specific combination of alleles that an individual inherits at these loci will
determine their biological sex, rather than a combination of sex chromosomes. This mode of sex determination occurs in many species of
fish.

Question 44

Environmental sex determination

Answer 44

describes systems in which an individual’s environment, not their genotype, determines their sex. There are
a number of different environmental factors that influence sex determination in different species.

For example, in turtles, the temperature at which eggs are incubated determines what sex they will develop into. At low
temperatures, sexual development tends to take the “male” path, while at higher temperatures, more eggs develop into females. The effect of
temperature can vary between species. For example, in alligators, the extreme temperatures (hot or cold) result in males, whereas intermediate
temperatures lead to females.

In contrast, slipper limpets use location to determine sex. These small marine creatures form piles on the ocean floor – the first slipper limpet larvae
that settles in a particular location will develop into a female, and secrete chemical signals that attract other larvae to her location. Other slipper
limpets will then join the pile, and develop into males due to their location on
“top” of the pile. The males and original female become reproductive
partners. Over time, though, the males switch sex, becoming female and start to secrete attractive chemical signals themselves. This causes additional larvae to join in and develop into males that become reproductive partners
with the females below

Another type of environmental sex determination uses social environment as the “deciding factor” in determining sex. For example, in clownfish, sex is based on social hierarchy, and can change when an
individual’s social circumstances change. All clownfish are born male, and only the dominant individual in any group will be female. The second-incommand male is usually the largest and most dominant of the males, and only he will mate with the female. If the female dies or is removed, this large,
dominant male will switch sex, becoming female, and the largest, most dominant males of the remaining members of the group will become her
breeding partner

Question 45

Y chromosome degeneration

Answer 45

we start with the assumption that sex determination was
originally not chromosomal (and may not even have been genetic at all;
perhaps it was environmental). The model first proposes that approximately
250 million years ago, a mutation in an autosomal gene resulted in a gene
with SRY-like function, triggering the male pathway.

Second, over time, mutations at other loci on that “proto-Y” chromosome occur, and genes
that benefit males (such as those that are involved in sperm production)
accumulate on that chromosome because they are guaranteed to be in male individuals and never in females.

Third, these male-specific genes are
under selective pressure to not recombine with the “non-male”
chromosome. Over time, this allows the two proto sex chromosomes to
further differentiate – in sequence, gene content and size.

In the long-term, this could be problematic. Recombination along a
chromosome is important for it to repair DNA damage that occurs and gain
genetic material from its partner chromosome. This process suggests that the Y chromosome will degrade (i.e., Y chromosome degeneration) and
eventually become extinct over time

However, comparison of Y chromosome size across mammals suggests that
Y degradation has plateaued. Further study has revealed that Y chromosome extinction is unlikely thanks to an interesting feature of the Y
chromosome: sequences on the Y chromosome are often palindromic –
they read the same forwards and backwards. These palindromic regions
allow the Y chromosome to recombine with itself, repairing DNA damage
using the other copy as a template, and “saving” the Y chromosome from its eventual degradation and demise

Question 46

dosage compensation

Answer 46

In species that use chromosomal systems for sex determination,
specific mechanisms have evolved to compensate for differences in the
number of sex chromosomes found in males and females

All species with chromosomal sex determination
have a mechanism in place to equalize expression of (most) sex-linked genes. The specifics of each mechanism vary based on the sex
chromosome content of the organism – because just like many sex determining systems have evolved, many dosage compensation
mechanisms have evolved too.

Question 47

X-chromosome inactivation

Answer 47

the transcriptional silencing of one X chromosome in female mammalian cells that equalizes dosage of gene products

In placental mammals, this inactivation is random in each cell of the individual – some cells will inactivate the maternal X
chromosome, and other cells will inactivate the paternal X. Once a cell has
“decided” between the two X’s, though, that choice is inherited by all of the descendent cells, which will inactivate the same X as their “parent” cell.

This leads to XX mammals being mosaics for the expression of X-linked genes – some patches of cells may exhibit the traits associated with the genes on the
maternal X chromosome, while others will exhibit the traits associated with
the genes on the paternal X

Question 48

how do drosophilia equalize expression of sex linked genes?

Answer 48

In Drosophila, which generally have XX
females and XY males, the expression of X chromosome genes is doubled on the male X chromosome. This means males will produce the same amount of each X chromosome gene as females, even though they have half as many
copies.

Question 49

how does c elegans equalize expression of sex linked genes?

Answer 49

In C. elegans, XX individuals are hermaphrodites and XO individuals
are male. In these worms, expression of all X chromosome genes in
hermaphrodites is decreased by half on each chromosome, so that the level
of each gene product is the same in both sexes. In mammals, XX females
silence the expression from one of their X chromosomes, so that they only
express genes from one X chromosome, resulting in gene expression levels
similar to what is found in males, which have only one X chromosome. This is
called X-chromosome inactivation