Key Terms Imporant People Key Topics Diseases Examples Key Ideas

What determines if an individual will be male or female and is located on the y chromosome

Chapter 5: Sex Chromosomes 

∙ Terms:  

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.

How androgen-insensitivity syndrome happens?

▪ 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  

heterogametic sex.  

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  

What is the role of sex chromosomes?

If you want to learn more check out How are proteins localized within a cell?

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  

second chromosome.  

∙ 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  

autosomes

o Diseases:

▪ 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  

chromosome

∙ 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  

affected.

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  

impairments

▪ 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  

reproductive tracts.

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  

chromosome.

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.