Key Terms Imporant People Key Topics Diseases Examples Key Ideas
Chapter 5: Sex Chromosomes
o Sexual Reproduction: development of offspring that are genetically different from the parents of the offspring.
o Sex (noun): determined by the phenotype of an individual, not determined by the genotype.
o Monecious: organism has both male and female reproductive systems. Meaning that both reproductive systems is within a single individual.
o Dioecious: an organism has either a male or a female reproductive systems. Meaning that only one reproductive system is within a single individual.
o Heterogametic Sex: the sex (male or female) that produces two different gametes.
▪ Example: in humans, males have an XY sex chromosome. During meiosis, the chromosomes are divided in the gametes equally, meaning that half
gets the X while the other half get the Y. This produces two different
gamete types (one with X and one with Y) and would be the
o Homogametic Sex: the sex (male or female) that produces identical gametes. ▪ Example: in humans, females have an XX sex chromosome. During Don't forget about the age old question of What is the debate of nature vs nurture?
meiosis, the chromosomes are divided in the gametes equally, but since
each chromosome is the same, it results in all gamete having the same sex chromosome (X). Which produces identical gametes with the X
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chromosome. This is the homogametic sex.
o Nondisjunction: When chromosomes fail to separate during anaphase in either mitosis or meiosis (I or II). We also discuss several other topics like What is nuclear magnetic resonance used for?
∙ Important People:
o Henking: discovered that male insects have a “body” in the nuclei and named it the “X” body.
o McClung: determined that the “X” body is the chromosome
o Stevens and Wilson: discovered that female grasshoppers have two “X” bodies, or in other words, two X chromosomes.
o Morgan: looked at tons of flies that he crossed, in the F1 generation. All but three flies had red eyes. These three flies were males with white eyes (should have had red eyes). Marked this as a random mutation. If you want to learn more check out How do you find solubility from ksp?
o Bridges: student of Morgan’s. Decided the male flies with white eyes was too high of a number to be due to a random mutation. Came up with disjunction. ∙ Key Topics: We also discuss several other topics like Why do you believe and behave the way you do? why do other people believe and behave differently from you? what are the consequences of the answers to those two questions?
We also discuss several other topics like What types of crime are most common?
o Mechanisms for sex determination (ways to determine the sex of an organism) ▪ Chromosomal sex-determining system: way of determining the sex of an organism through the chromosome structure.
∙ What was discovered through insects: Males have X chromosomes
while females have two X chromosomes. The males have a second
chromosomes that is smaller and is called the Y chromosome.
o During meiosis for males, the XY chromosomes split up and each gamete is given an X or a Y chromosome, this is a 50/50 split.
o During meiosis for females, the XX chromosomes split up, but since they are two of the same chromosomes, the
female gametes all get the same chromosome (X).
∙ XX-XY: some plants, insects, reptiles, and all mammals are distinguished in this manner. Males = XY. Female = XX
∙ XX-XO: simple system. Females = XX. Males = XO. NOTE: “O” in the males is a place holder, not a chromosome, meaning there is no second chromosome in the males.
o How it works in meiosis:
▪ For females: the XX chromosomes split and each
gamete gets one.
▪ For males: the X goes to half of the gametes and the
other half does not get one due to the lack of a
∙ ZZ-ZW: Females = ZW. Males = ZZ. This results in the females being heterogametic and the males are homogametic. This is common in some birds, moths, amphibians, and fishes.
∙ Haplodiploidy and the Social Insects: some types of bees have no sex chromosomes, meaning that males are developed from an unfertilized egg.
o Males are haploid: one set of chromosomes
o Females are diploid: two sets of chromosomes
o The Process: A female will undergo meiosis and develop haploid gametes. If a gamete is fertilized by the haploid
gamete of a male, it creates a diploid zygote which will
become a female. If a gamete is not fertilized, it will remain haploid and develop into a male.
▪ Genetic Sex Determination System: determines the sex of the individual by the genes
∙ There is no difference in the sex chromosomes, so the genes can be used to determine the sex of an individual.
∙ This is similar to chromosomal sex determination: NOTE: genes are on chromosomes and control the determination of the sex ∙ This can be found in some protozoans and plants
▪ Environmental Sex Determination System: The environment will pay a factor in determining the sex of an individual.
∙ The environment will output a signal that will effect certain genes to be activated which will make the sex of the individual.
∙ Temperature Dependent: temperatures will trigger which genes to be in affect which will decide what the sex will be of an individual. Example: the male gene of an alligator is activated during warm temperatures. The female gene of an alligator is activated during
cool temperatures. (This process occurs while the alligator is in the egg).
∙ Common Slipper Limpet: will undergo sequential
hermaphroditism. Example: the sac will float in the water and
eventually land on a rock. If no other sac is located on this rock it will develop into a female. The female will then transmit a signal that will attract other limpets. Once theses limpets get to the female they will land on the rock and become male. If something happens to the female, the next male will turn into a female. This process
will continue (this is the sequential hermaphroditism).
o Nondisjunction: Occurs when the chromosomes fail to separate during anaphase. ▪ Sex chromosomes nondisjunction: some eggs can have two copies of the X chromosome while other eggs can result in having none.
▪ Autosome chromosome nondisjunction: an egg or sperm cell can either have two copies or no copies of a single chromosome.
▪ Either of these cases can result in a genetic disorder.
o Sex Determination in Drosophilia
▪ Females = XX. Males = XY. The Y chromosomes does not determine if it will be a male.
▪ The autosome ratio to the X chromosome determine the sex of the fly. This allows for a genetic balanced system.
▪ The X chromosomes are female producing while the autosomes have genes to produce the males.
▪ Ratio to determine if male or female: X:A
∙ The number of X chromosomes : number of haploid sets of
▪ Turner Syndrome: effects females, have a single X chromosome ∙ Symptoms: do not undergo puberty, immature female sex
characters, low hairline, folds of neck skin are pronounced,
cognitive impairment, sterility.
∙ Sex Chromosomes: XO
o “O” is used as a place holder, there is no second X
∙ Probability: 1/2000 females effected, pretty common
∙ 45, X
o The “45” represents the number of chromosomes the
individual has and the “X” represents the chromosome
affected (shows that the individual only has one X)
▪ Klinefelter Syndrome: effect males, have multiple X chromosomes and a single Y chromosome
∙ Symptoms: small testes, enlarged breasts, reduced facial and/or pubic hair, some have cognitive impairment, and often sterile.
Symptoms are usually not severe enough to know that something is wrong with the individual genetically. Usually discovered when
the individual realizes they have fertility issues.
∙ Sex Chromosomes: XXY, XXXY, XXYY
o The individual can have multiple X chromosomes and will
be affected at the same level, however, the more X
chromosomes an individual has, the more likely the
symptoms will be more prominent and noticeable and will
be easier to diagnose.
∙ Probability: 1/500 males affected, often males are not diagnosed due to not knowing something is wrong due to the symptoms not being severe.
∙ 47, XXY
o The “47” represents the number of chromosomes the
individual has and the “XXY” is the chromosome that is
o NOTE: if the individual is XXXY or XXYY this will
affect the count. The individual will be 48, XXXY or 48,
XXYY. If there are multiple “X” chromosomes and a “Y”
chromosome then the individual will have Klinefelter.
▪ Poly-X Females: females affected have extra X chromosome(s) ∙ Symptoms: tall, thin, fertility varies, no cognitive impairment, normal if XXX, cognitive impairment will appear in individuals
with more than three copies (XXXX, XXXXX)
∙ Sex Chromosomes: XXX, XXXX, XXXXX
o The individual will have more than two X chromosomes,
the severity of cognitive impairment will increase with the
extra number of X chromosomes.
o Females that are XXXX, or XXXXX will have cognitive
▪ Androgen-insensitivity Syndrome: XY females (Look below at SRY) ∙ SRY or other genes can have an effect on the development of a sex ∙ Symptoms: female characteristics on the outside, no uterus,
oviducts, or ovaries, testes are found in abdominal cavity
∙ Explained how down below under SRY**
o Role of Sex Chromosomes
▪ The X chromosome stores genetic information that is essential and needed for both sexes, any individual needs AT LEAST one X chromosome. ∙ NOTE: if no X chromosome is present it is lethal
▪ The gene that determines if an individual is male is on the Y chromosome. If there is a Y chromosome, the individual is a male.
▪ The absence of the Y chromosome will produce a female individual. If there is no Y chromosome, the individual is a female.
▪ Genes needed to be fertile are located on the X and Y chromosomes. Females need at least two X chromosomes to be fertile while males need an X and Y chromosomes to be fertile.
▪ Extra X chromosomes can be harmful.
o Why the Y chromosome is important for producing male phenotypes: ▪ There are a few males that are XX this is because….
∙ During development all human individuals have gonads that are not classified as male or female yet (this is early development). o For males: testes develop and they secrete testosterone which will trigger the male characters to develop and for and an inhibiting substance to destroy the female
o For females: when the male determining gene is absent, then the neutral gonads will develop into female ovaries. The humans’ default is to be female, when there is a lack of male gene, then it will become female.
∙ SRY: Sex-determining region Y
o Where/when is it found?
▪ This is found in all males that are XX genetically
▪ This is absent in females that are XY genetically
▪ Causes XX genetic mice to become males if
engineered into the genome of the mice.
o What does this mean?
▪ This is what determines if an individual will be
male or female and is located on the Y
o It is the first trigger to create a male
o How Androgen-insensitivity syndrome happens:
▪ All females affected are XY and all have the SRY
▪ The SRY triggers and starts the development of
testes. The testes then secrete testosterone, but the
affected women do not have the receptor for the
testosterone. This means that the further
development of male characters do not take place,
creating a female on the outside.