Ch. 4 Sex Determination, Sex Chromosomes, X-linked inheritance Flashcards
Biological Sex
An individual's combination of sex chromosomes, hormone levels, external & internal reproductive anatomy and characteristics. Usually binary (male or female)
Gender
An individual’s concept of who they are, based on social and cultural ideas of what it means to be a man or woman.
Not necessarily binary
Not necessarily matching with biological sex
What organisms do asexual reproduction?
bacteria, archaea, unicellular eukaryotes
What is asexual reproduction?
Split yourself into 2
No meiosis
Don’t necessarily have a sex
Sexual reproduction who does it and how?
Most diploid eukaryotes
Involves meiosis, formation of gametes (egg and sperm), fertilization
Sexual differentiation
Male and female
Happens in animals, including humans
Differentiation of sexes is evident via phenotypic dimorphism. (can clearly distinguish males and females by looking at them)
Sex chromosomes
Characterize one sex or the other in a wide range of species
Heteromorphic chromsomes
Dissimilar in their morphology (can distinguish them)
EX: Sex chromosomes X and Y in mammals
The underlying basis for sex determination are?
Specific genes. But NOT the entire chromosome.
Most of these genes are located on the sex chromosomes, but some are autosomal (located on autosomes)
Sex chromosomes - male and female
Females are XX
Males are XY
During meiosis in males, the X and Y chromosomes pair how?
Like homologous chromosomes
Allosomes
sex chromosomes (they are heteromorphic)
Autosomes
all other chromosomes that are not sex chromosomes (they are homomorphic - partners in homologous pair look alike)
Inheritance of sex chromosomes in animals with XX and XY
Female - will pass on an X
Male - can pass on an X or a Y
Homogametic sex
Produces like gametes.
Zygotes with 2 X chromosomes: Results in female offspring.
Heterogametic Sex
Produces unlike gametes.
Zygotes with one X and one Y chromosome: Results in male offspring
Examples of sex chromosomes in diff species
The X-0 system - insects - female xx, male x.
The Z-W system - birds - female ZW, male ZZ.
The haploid-diploid system - bees - F 32 diploid, M 16 haploid.
Worm - XX is hermaphrodite (male/female), XO is male.
Fruity fly - XX is female, XY is male. Sex determination mechanism is not the same as in mammals.
Fish - no morphologically distinct sex chromosomes, but sex linked genes on other chromosomes; also temperature dependent.
Sex determination in Fruit fly
Males are the heterogametic sex (XY); but it is the ratio of X chromosomes to autosomes that determines the sex.
1X2A - 0.5 - male
2x2A - 1.0 - Female
Temperature-dependent sex determination
Many reptile species, like snakes, have sex chromosomes similar to birds (ZW/WW system).
Crocodiles, most turtles, and some lizards: incubation temperature of eggs during embryonic development determines sex.
Different pattens:
Higher temp, fewer F
Higher temp, more F
Most common- medium temp mostly male, Cold/hot F
Human Karyotype
Chromosomes are labeled by size. Number of chromosomes is constant for a species. Humans have 23 pairs: 22 autosomes 2 sex chromosomes
Inheritance of sex chromosomes in humans
Males: heterogametic sex (produce 2 different kinds of gametes)
Females: homogametic sex
Offspring: 50% F 50% M
Klinefelter Syndrome (47, XXY)
Internal ducts are male, rudimentary testes fail to produce sperm.
Feminine development not fully suppressed - enlarged breasts common, rounded hips
Turner syndrome (45, X)
Phenotypically female - female external genitalia and internal ducts. Ovaries are rudimentary. Underdeveloped breasts.
Cognitive impairments.
What did Klinefelter and Turner syndromes lead to?
Lead scientists to conclude that the Y chromosome determines maleness in humans.
Y chromosome is dominant over the X.
In Klinefelter syndrome (XXY), presence of Y is enough to determine maleness, even though male development is incomplete.
In Turner syndrome (X0), Y chromosome is absent and no masculinization occurs.
Human embryos with Y0 are not viable.
What genes are on human sex chromosomes?
Human Genome Project - more than 1000 genes on the female X chromosome 397 possible genes on the male Y chromosome, but fewer than 100 seem to be functional (genes required for male fertility)
PARs (Pseudo-autosomal regions)
Present on both ends of Y chromosome.
Share homology with regions on X chromosome.
Are X and Y chromosomes homologous?
Not truly homologous, they act like homologous chromosomes during meiosis. They can even synapse and recombine through crossover.
MSY: Male-specific region of Y
Nonrecombining region
SRY: Sex-determining region Y gene
Located adjacent to PAR of the short arm of Y chromosome.
Controls male development.
At 6-8 weeks of develop, SRY gene becomes active in X Y embryos.
Encodes protein: Testis-determining factor (TDF) which triggers testes formation.
Development of the early embryo
Up until 6 weeks after fertilization: Mullerian ducts (precursors to the female reproductive system) and Wolffian ducts (precursors to the male reproductive system) are present.
Up until 8 weeks after fertilization: gonads are immature and bipotential in the embryo - can develop either way.
They can develop into either male (testis) or female gonads (ovaries)
Urogenital development.
Visible on ultrasound around 20 week after fertilization
Sex Determination - Female
- In the absence of a Y chromosome, no TDF is produced.
- The lack of TDF allows the cortex of the embryonic gonads to develop into ovaries.
- In the absence of testosterone, the embryo develops female characteristics.
Sex Determination - Male
The presence of the Y chromosome causes an individual to be male.
1. The testis-determining factor (TDF) is produced by a gene on the Y chromosome.
2. TDF induces the medulla of the embryonic gonads to develop into testes.
3. The testes produce testosterone, a hormone that initiates development of male sexual characteristics.
Presence of Y chromosome (SRY) causes a person to be male.
Androgen-Insensitivity Syndrome
The SRY gene is primary determinant of sex in human embryos, but several other genes also influence sex development.
Females with AS syndrome develop female external sexual characteristics, but ovaries and uterus are absent (diagnoses after F fails to menstruate).
Females are XY; the gene which encodes the androgen receptor is defective (no androgen receptor means that the cells are inactive to testosterone).
Autosomal gene mutation.
SRY -> TDF -> testes -> testosterone -> cells not sensitive to Testosterone.
Dosage Compensation of X-linked Genes
Each gene is present in two copies on the homologous chromosomes.
Animal development is usually very sensitive to an imbalance in gene number.
If females have 2 sex chromosomes and males only one, there must be mechanisms to adjust for the unequal dosage of X-linked genes in male and female animals.
1. X-linked genes work half as hard in females than in males (F: XX, M:XO)
2. X-lined genes work twice as hard in males than females: Drosophila (hyperactivity)
3. One copy of the x chromosome is inactivated: mammals
The Lyon Hypothesis
If one chromosome is inactivated (XX), which one is it? The paternally derived one or the maternally derived one?
In the early 1960s, Lyon, Russell, and Beutler proposed that the inactivation occurs randomly during early embryonic development.
Support for the hypothesis came from fur coloration genes located on X chromosomes in mammals, and later clones of human fibroblasts with mutant genes (synthesis of the enzyme glucose-6-phosphate dehydrogenase is controlled by an X linked gene)
Barr body
Inactive chromosomes condense
Mechanisms of Dosage Compensation
Embryos at time of X-inactivation (color indicates which X is active). Randomly 1 X is active and other is inactive. Developing embryo showing clones of cells that will produce either light or dark pigment.
Mitosis- all daughter cells are identical so all daughters of dark X will be dark, and vise versa. Results in multi color cat w/patches - Mosaicism
Coat colors in cats are determined by?
Through several genes, one of them is on the X chromosome.
Female cat - XX
Male cat - XY
Gene on the X chromosome has 2 alleles o (orange), b (black)
Coat colors in cats
Males can be orange (XoY) or black (XbY)
Females can be orange (XoXo), black (XbXb), or tortoiseshell (XoXb) (mosaics)
In every single cell, one X is active and one is inactive. Which one is active/inactive is due to chance.
Calico cats
Have 3 colors - black, orange, white
Calico cats are usually female; black and orange are determined through the alleles on the X chromosomes, the white patches are determined through another gene on one of the autosome chromosome.
Are there ever male tortoiseshell or calico cats?
Yes but they are very rare (1 in 3000)
Chromosome aberration: male with 2 X chromosomes XoXbY
Cat with Klinefelter syndrome - not possible to breed male calico cats bc Klinefelter’s are sterile.
Impact of X inactivation in humans
Color blindness
Females are mosaics for all heterozygous X-linked alleles.
Red-green color blindness: X-linked recessive disorder;
Hemizygous males are fully color-blind in all retinal cells.
Heterozygous females display mosaic retinas, with patches of defective color perception and patches of normal perception (however the eye is able to compensate, women with mosaic retinas have normal color vision - unless there are too many patches with defective color perception)
Inheritance of Sex-linked genes in humans
Mendelian?
If genes are located on the X (or Y) chromosomes, inheritance deviates from Mendelian principles.
Inheritance of Sex-linked genes in humans
Males
A male needs to inherit only one recessive allele to be affected by a X-linked trait (males are hemizygous) - only have 1 X so they can’t be homo or hetero. Allele present on gene on X will be expressed even if recessive.
Inheritance of Sex-linked genes in Humans
Females
A female needs to inherit 2 recessive alleles to be affected because they have 2 X chromosomes.
Females can be carriers: if they inherit one recessive allele, they are not affected, but they can pass on the allele to their sons and daughters (with 1/2 probability)
Carriers
The heterozygous genotype for a sex-linked trait is a carrier.
Bc the recessive allele is usually the one that causes problems.
This means that the heterozygotes are unaffected, but they can pass on the recessive allele.
Individuals that are “carriers” are always female - can’t be male bc if male gets it, it will be expressed.
A female must receive the allele from both parents to be affected.
The probability of affected females is higher with inbreeding.
X-linked recessive traits appear more frequently in?
Appear more frequently in males that females
X linked recessive traits in females vs males
Female must inherit 2 copies of the allele, males need to inherit only one copy.
X-linked recessive traits
Affected males are usually born to?
Affected males are usually born to unaffected others (who are carriers)
Do X-linked recessive traits skip generations?
Yes, tend to skip
X-linked recessive traits.
Father to son
Trait is never passed father to son - bc fathers always pass Y to sons
X- linked recessive traits
Daughter of affected male
All daughters of an affected male will be carriers - bc male will pass on affected X to daughters
X-linked recessive traits
An affected male does not pass the trait to his sons, why?
Bc he passes Y
X- linked recessive traits
An affected male to daughter?
Can pass the allele to a daughter, who is unaffected. Bc they get normal X from mom and carrier X from father.
Color blind dad x Normal mom
XbY x XBXB
All daughters are carriers and all sons are normal
Normal dad x carrier mom
XBY x XBXb Normal female Carrier female Normal male Affected male
Color blind dad x carrier mom
XbY x XBXb Carrier female *Affected female - homozygous XbXb Normal male Affected male
X-linked dominant traits
Appear in both males and females
Each person with an X-linked dominant trait must have an affected parent (unless the trait had reduced penetrance)
Does NOT skip generations
Affected men pass the trait to all daughters but sons
A male can inherit the trait only from his mother (same for recessive traits) - this is what distinguishes X linked dominant from autosomal dominant (here, a male can inherit the trait from his father, can’t in recessive).
A female can receive the trait from either parent.
Ex: Hypophosphatemia (familial vitamin D resistant rickets)
X linked dominant traits
Affected males pass the trait to?
All of their daughter, and none of their sons. Bc sons always inherit Y from dad.
X linked dominant traits Affected females (heterozygous) pass on trait to?
Pass the trait on to about half of their sons and about half of their daughters.
Y linked Traits
Who is affected?
ONLY males
Y linked traits
How is it passed?
Trait is passed from father to son
Y linked traits
Does it skip a generation?
NO
Y linked traits
Dominant or recessive?
Neither dominant nor recessive (bc males only have one Y)
Ex: Hypertrichosis of the ears (hair ears): growth of hair on the outside rim of the ear
Dihybrid crosses- independent assortment?
Can we treat sex and “trait” as 2 independent characters in a dihybrid cross, following Mendelian rules?
Yes, if the traits in question are located on autosomes
No, if the traits are located on the sex chromosomes - do not assort independently
Females can be homozygous or heterozygous, but males not, they are hemizygous (XY).
Dihybrid crosses- independent assortment?
Female tortoiseshell cat x male black cat
XbXo x XbY Black female Tortoiseshell female Black male Orange male (can't have orange female) Fur color and sex DO NOT assort independently
Inbreeding
When inbreeding occurs, probability of being homozygous for a recessive, harmful allele is increased.
In many plants and animals inbred lines suffer from inbreeding depression (loss of vigor)
The founder effect: Polydactyly in the Amish population in Pennsylvania
In the early 1700s, a small group of perhaps 200 German immigrants reached North America.
The Eastern Pennsylvania Amish people are descendants of this group.
One of these immigrants carried a mutation for Ellis-van Creveld syndrome: this syndrome can result in polydactyly (recessive allele).
Today, 5 of 1000 Amish babies in Pennsylvania have this syndrome.
Compared to 1 in 60,000 in the general populations.
Genetic Analysis of inbreeding
Individuals A and B are half-siblings.
Their offspring, I, is inbred, and inherited one copy of her genes from A and one copy from B.
These copies may be identical by descent if they are identical copies inherited from individual C.
C is the common ancestor of I.
Sex ratios in the real world?
In animals and plants?
If sex determination in humans (XX-XY) results in 50% sons, 50% daughters among offspring, is there a 1:1 ratio of males and females in human populations?
There is no.
Some reasons are unknown and some are known.
One sex might have a higher mortality (also dependent on age groups).
Since ancient times, people have been trying to influence the sex of babies.
Medieval Europe: place a hammer under the bed for son, and scissors for a girl.
Sex selection in Humans?
Female infanticide in ancient Greece: male: female ratio of 4:1 (desirable to have sons)
Infanticide has been replaced by sex-specific abortion in some countries.
In vitro fertilization with PGS (preimplantation gender selection), “family balancing” : legal and ethical issues.
Medical reasons: families with x-linked disorders.
Non medical reasons?
United Nations and WHO oppose PGS; banned in most of the EU, Australia, Canada, UK ; not banned in the US, Mexico, Italy, Thailand
Sex linked
the gene controlling the trait is located on an allosome (hemophilia)
Sex influenced/ sex limited
the gene controlling the trait is located on an autosome, but the sex of the organism influences the phenotype
Sex limited
Only expressed in one sex (milk yield in dairy cattle)
Sex influenced
Expression of the gene is influenced by sex hormones (horn formation in some sheep)
Male pattern baldness
Found to be both sex-influenced autosomal and x-linked inherited
