Basic principles of meiosis are found in both ________ and __________
oogenesis and spermatogenesis
Oogenesis generates _____ gamete per meiosis and spermatogenesis generates _____ gametes per meiosis
In all sexually reproducing animals, the germ line (define) undergo a series of mitotic divisions that yield a collection of specialized _______ cells, which subsequently divide by meiosis to produce ______ cells
- germ line: the embryonic germ cells
Gametogenesis gives rise to _______ gametes marked not only by the events of meiosis per se but also by ______ events that precede and follow meiosis.
The end product of egg formation in humans
a large nutrient-rich ovum with stored resources that can sustain an early embryo
- oogonia: diploid germ cells in the ovary
- primary oocytes: Germ line cells in which Meiosis I occurs
Begins when diploid germ cells in the ovary, called oogonia (sing.: oogonium), multiply rapidly by mitosis and produce a large number of primary oocytes, which then undergo meiosis
For each primary oocyte, meiosis I results in the formation of ____ ______ cells that differ in size, so this division is _________. The larger of these cells, the ________ oocyte, receives over ______ of the cytoplasm. The other ______ cell is known as a the first _____ ______.
- two daughter cells
- secondary oocyte
- sister cell
- polar body
During meiosis II, the secondary oocyte undergoes another ________ division. What is the product?
- asymmetrical division
- a large haploid ovum and a small, haploid second polar body.
The first polar body usually _______ in development. What are the roles of the two small polar bodies and the large haploid ovum?
- The two small polar bodies apparently serve no function and disintegrate
- The one large haploid ovum serves as the functional gamete
Only one out of the three (rarely four) products of a single meiosis serves as a female _______. A normal human ovum carries ___ autosomes and an ___ _____ chromosome
- 22 autosomes
- X sex chromosome
Oogenesis begins in the _____. By ____ months after conception, the fetal ovaries are fully formed and contain about _______ primary oocytes arrested in the _______ sub-stage of prophase I. These cells, with their homologous chromosomes locked in synapsis, were thought of for decades to be the only oocytes the female will produce. What is the consequence for girls?
- 6 months
- A girl is born with all the oocytes she will ever possess.
Evidence to the contrary that a girl is not born with all of the oocytes she will ever possess (in context)
Scientists have shown that germ-line precursor cells removed from adult ovaries can produce new eggs in a petri dish. However, it is not yet known whether these eggs are viable nor if these germ-line cells normally produce eggs in adults
From the onset of puberty at about age 12, until menopause some 35-40 years later, most women release one _____ ______ each month (from ______ ovaries), amounting to roughly _____ oocytes released during the reproductive years.
- primary oocyte
What happens to the remaining primary oocytes during menopause?
- At ovulation, a released oocyte completes meiosis I and proceeds as far as the metaphase of meiosis II
- If the oocyte is then fertilized (penetrated by a sperm nucleus) it quickly completes meiosis II
- The nuclei of the sperm and ovum then fuse to form the diploid nucleus of the zygote
- The zygote divides by mitosis to produce a functional embryo
What happens to unfertilized oocytes?
They exit the body during the menses stage of the menstrual cycle
How do we go from Oogonia to mature ovum
The production of sperm, or __________, begins in the male testes in germ cells known as ___________.
Mitotic divisions of the spermatogonia produce many _______ cells. What are these cells called
- diploid cells
- primary spermatocytes
Unlike primary oocytes, primary spermatocytes undergo a _________ meiosis I. What is the product?
- symmetrical meiosis I
- two secondary spermatocytes
Each of the secondary spermatoyces undergoes a _______ meiosis II. At the conclusion of meiosis, each original primary spermatocyte thus yields four equivalent ______ _______.
- haploid spermatids
These spermatids then mature by developing a characteristic whip-like tail and by concentrating all of their __________ material in a head, they become functional ______.
- chromosomal material
A human sperm, much _______ than the ovum it will fertilize, contains how many autosomes (pairs) and how many/which sex chromosmes
- 22 autosomes
- Either an X or a Y sex chromosome
How do we go from spermatogina to sperm?
Thomas Morgan's Drosophila melanogaster experiment (Cross A) (5-story) (keep in mind all mating in crosses was consanguineous)
- Morgan fed flies mashed bananas and housed them in empty milk bottles capped with wads of cotton
- In 1910, a white-eyed male appeared among a large group of flies with brick-red eyes.
- A mutation had altered a gene determining eye color, changing it from the normal wild type allele (red) to a new allele that produced white
- When the white eyed male mated with red eyed females, all the flies of the F1 generation had red eyes
- The red allele was clearly dominant to the white
The normal wild type allele (fly eye color experiment) was abbreviated ____ for the red eyes, while the counterpart mutant white eye color allele was abbreviated ____. The superscript signifies?
Why was the abbreviation w in lowercase? What would be the significance of the uppercase?
- It was lowercase to signify that the w allele is recessive to the wild-type w+.
- If it was capitalized it would signify that the mutation results in a dominant non-wild-type phenotype
Explain cross B (4-story)
- Morgan crossed the red-eyed males of the F1 generation with the red eyed females and obtained an F2 generation with the predicted 3:1 ratio of red to white eyes.
- *Something was off*: Among the red eyed offspring, there were two females for every one male, and all the white-eyed offspring were males.
- This result was surprisingly different from the equal transmission to both sexes of the Mendelian traits expected.
- In the ratio of eye color phenotypes was not the same in male and female progeny
Explain cross C (2-story)
- By mating F2 red-eyed females with white eyed males, Morgan obtained some females with white eyes
- This allowed him to mate a white eyed female with a red eyed wild type male
Inheritance pattern in which males inherit a trait from their mothers, while daughters inherit the trait from their fathers
Explain cross D (3-story)
- Morgan mated a white eyed female with a red-eyed wild-type male
- The result was exclusively red-eyed daughters and white-eyed sons
- The pattern seen in cross D is known as crisscross inheritance because the males inherit their eye color form their mothers, while the daughters inherit their eye color from their fathers.
From the data, Morgan reasoned that the white gene for eye color is ______ (Explain)
- X-linked, that is, carried by the X chromosome.
- *Note that while symbols for genes and alleles are italicized, symbols for chromosomes are not
In this X linked gene, the Y chromosome carries ____ allele of this gene for eye color. How many copies of the gene do males have and where does it come from? Where does the Y chromosome come from?
- no allele
- Males therefore, have only one copy of the gene, which they inherit from their mother along with their only X chromosome
- Their Y chromosome must come from their father.
Describes the genotype for genes present in only one copy in an otherwise diploid organism, such as X-linked genes in a male
Males are _______ for this eye color gene, because their diploid cells have half the number of alleles carried by the female on her two X chromosomes
If the single white gene on the X chromosome of a male is the wild-type w+ allele, what color eyes will he have and what will be his genotype?
- He will have red eyes and a genotype that can be written Xw+ Y
- *We designate the chromosome, X or Y, together with the allele it carries, to emphasize that certain genes are X-linked
In contrast to an Xw+Y male, a hemizygous XwY male would have a phenotype of _____ eyes. Females with two X chromosomes can be one of three genotypes, name them
- white eyes
- XwXw (white-eyed)
- XwXw+ (red-eyed because w+ is dominant to w)
- Xw+Xw+ (red-eyed)
Morgan's assumption that the gene for eye color is X-linked explains the results of his breeding experiments. How (genotypically) did crisscross inheritance occur?
Crisscross inheritance occurs because the only X chromosome in sons of a white eyed mother (XwXw) must carry the w allele, so the sons will be white-eyed. In contrast, because daughters of a red-eyed (Xw+Y) father must receive a w+- bearing X chromosome from their father, they will have red eyes
Then Calvin Bridges, one of Morgan's top students found another key piece of evidence. Bridges repeated the cross Morgan had performed between white-eyed females and red-eyed males, but this time he did the experiment on a larger scale. What was the result?
As expected, the progeny of this cross consisted mostly of red-eyed females and white eyed males. However, about 1 in every 2000 males had red eyes, and about the same small fraction of females had white eyes
What was Calvin Bridges' hypothesis for the results?
Bridges hypothesized that these exceptions arose through rare events in which the X chromosomes fail to separate during meiosis in females. He called such failures in chromosome segregation nondisjunction
Explain the figure (2-story/4-list-1story)
- The figure shows nondisjunction would result in some eggs with two X chromosomes and others with none.
- Fertilization of these chromosomally abnormal eggs could produce four types of zygotes:
- XXY (with two X chromosomes from the egg and a Y from the sperm)
- XXX (with two Xs from the egg and one X from the sperm)
- XO (with the lone sex chromosome from the sperm and no sex chromosome from the egg
- OY (with the only sex chromosome again coming from the sperm)
These results also suggested that the 2 abnormal sex chromosome karyotypes expected from nondisjunction in females (XXX and OY) die during embyronic development *so no progeny from them*
What did Bridges learn after examining the sex chromosomes of the rare white-eyed females produced in his large-scale cross?
He found that they were indeed XXY individuals who must have received two X chromosomes and with them two w alleles from their white-eyed XwXw mothers
What did Bridges learn about the exception red eyed males from the cross?
They were XO, their eye color showed that they must have obtained their sole sex chromosome from their Xw+Y fathers
Explain the figure
Because XXY white-eyed females have three sex chromosomes rather than the normal two, Bridges reasoned they would produce four kinds of eggs: XY and X, or XX and Y
The figure helps us visualize the formation of these four kinds of eggs by imagining that when the three chromosomes pair and dis-join during meiosis, ____ chromosomes must go to one pole and ____ to the other. With this kind of segregation, only two results are possible:
- Either one X and the Y go to one pole and the second X to the other (yielding XY and X gametes)
- Or the two Xs go to one pole and the Y to the other (yielding XX and Y gametes)
When do XY and X gametes occur? When do XX and Y gametes occur? Which is more likely?
- The XY and X gamete comes about when the two similar X chromosomes pair with each other, ensuring that they will go to opposite poles during the first meiotic division.
- The XX and Y possibility happens only if the two X chromosomes fail to pair with each other
- The XY and X is more likely
Bridges also predicted that fertilization of these four kinds of eggs from an XXY female by normal sperm would generate an array of sex chromosome karyotypes associated with specific eye color phenotypes in the progeny. How was this verified?
By analyzing the eye color and sex chromosomes of a large number of offspring.
- For instance, he showed cytologically that all of the white-eyed females emerging from the cross in the fig had two X chromosomes and one Y chromosome, while one-half of the white-eyed males had a single X chromosome and two Y chromosomes.
Why did E.B. Wilson state that the gene for red-green color blindness is found on the X chromosome?
Because the condition usually passes from a maternal grandfather through an unaffected carrier mother to roughly 50% of the grandsons
6 factors of X-linked Recessive Trait
- The trait appears in more males than females because a female must receive two copies of the rare defective allele to display the phenotype, whereas a hemizygous male with only one copy will show it
- The mutation will never pass from father to son because sons receive only a Y chromosome from their father
- An affected male passes the X-linked mutation to all his daughters, who are thus carriers. One-half of the sons of these carrier females will inherit the defective allele and thus the trait
- The trait often skips a generation as the mutations passes from grandfather through a carrier. If she is, one-half of her sons will be affected
- With the rare affected (homozygous) female, all her sons will be affected and all her daughters will be carriers
5 factors of X-linked Dominant traits
- More females than males show the aberrant trait
- The trait is seen in every generation because it is dominant
- All the daughters but none of the sons of an affected male will be affected. This criterion is the most useful for distinguishing an X-linked dominant trait from an autosomal dominant trait
- One-half the sons and one-half the daughters of an affected female will be affected
- For incompletely dominant X-linked traits, carrier females may show the trait in less extreme form than males with the defective allele
An inactive X chromosome observable at interphase as a darkly stained heterochromatin mass
X chromosome inactivation
In mammals, a mechanism of dosage compensation in which all X chromosomes in a cellular genome except one are inactivated at an early stage of development through the formation of heterochromatic Barr bodies
Mechanism that equalizes levels of X-linked gene expression independent of the number of copies of the X chromosome; in mammals, the dosage compensation mechanism is X chromosome inactivation
Explain the figure (2)
- Early in embryogenesis, each XX cell inactivates one randomly chosen X chromosome by condensing it into a Barr body (black oval).
- The same X chromosome remains a Barr body in all desendants of each cell XM = maternal X chromosome; XP = paternal X chromosome
Explain the figure (3)
- Human females have patches of cells in which either the maternal or paternal X chromosome is inactivated
- The twins shown here are heterozygotes (Dd) for the recessive condition anhidrotic ectodermal dysplasia, which prevents sweat gland development
- Patches of skin in blue lack sweat glands because the chromosome with the wild type allele (D) is inactivated