Every person has 46 chromosomes in their cells, just like their mom and dad. If gametes also had 46, the zygote would have 92 and the next generation 184: the number would keep doubling. That doesn't happen because there is a special kind of division, different from mitosis, that halves the chromosomes before fertilization: meiosis. At the end of the page you will see how this problem is solved.
What meiosis accomplishes
If you look at a skin cell under a microscope, you count 46 chromosomes; in a mature sperm cell or egg cell, only 23. How does a cell cut its chromosome set in exactly half?
A cell is diploid ($2n$) when it has two sets of chromosomes, one inherited from each parent, which form pairs of homologous chromosomes. It is haploid ($n$) when it has only one set, one chromosome from each pair. In humans, $2n = 46$ and $n = 23$.
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Explain what meiosis is and in which organs of the human body it takes place, stating which cells it produces in males and females.
Explain what meiosis is and in which organs of the human body it takes place, stating which cells it produces in males and females.
Look at the diagram of meiosis, in which an initial cell with four chromosomes ($2n = 4$) divides twice in succession. Using the diagram, state how many cell divisions…
Look at the diagram of meiosis, in which an initial cell with four chromosomes ($2n = 4$) divides twice in succession. Using the diagram, state how many cell divisions meiosis includes, how many daughter cells are produced from the initial cell, and how many chromosomes each one has compared to it.
Explain the difference between a haploid cell and a diploid cell, and state how many chromosomes a human diploid cell has and how many a human gamete has.
Explain the difference between a haploid cell and a diploid cell, and state how many chromosomes a human diploid cell has and how many a human gamete has.
Independent assortment works between different pairs of homologs. Imagine an individual $AaBb$. If gene $A$ is on one pair of chromosomes and gene $B$ on another, each pair is distributed at random without depending on the other. Four types of gametes result, $AB$, $Ab$, $aB$, and $ab$, all in the same proportion (25% each).
If the two genes are on the same pair of homologs (linked genes), they travel together on the same chromosome. With $AB$ on one homolog and $ab$ on the other, meiosis I separates the whole homologs and, without crossing over between the two genes, only two types of gametes result: $AB$ and $ab$, 50% each.
When there is crossing over between the two genes, the homologs exchange the segment that separates them and recombinant gametes also appear, $Ab$ and $aB$, usually in a smaller proportion. The closer together the genes are, the less likely it is that crossing over will separate them.
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During prophase I of meiosis, homologous chromosomes exchange fragments in a process called crossing over. Explain what this consists of and what consequence it has for…
During prophase I of meiosis, homologous chromosomes exchange fragments in a process called crossing over. Explain what this consists of and what consequence it has for the genetic variability of gametes.
In a species with $2n = 16$ chromosomes, two individuals heterozygous for two different traits ($AaBb$) are analysed: In Individual 1, gene $A$ and gene $B$ are located…
In a species with $2n = 16$ chromosomes, two individuals heterozygous for two different traits ($AaBb$) are analysed:
In Individual 1, gene $A$ and gene $B$ are located on different pairs of homologous chromosomes.
In Individual 2, both genes are located within the same pair of homologues (linked genes), with alleles $AB$ on one chromosome and $ab$ on the homologue.
a) Deduce how many genetically distinct types of gametes for these two traits Individual 1 can produce and in what relative proportions.
b) If no crossing over occurs during prophase I in Individual 2, predict what types of gametes it will form and compare this with Individual 1.
c) Justify which meiotic event allows Individual 2 to generate recombinant gametes ($Ab$ and $aB$) and explain why the probability of their occurrence depends on the physical distance between both genes.
State two fundamental differences between mitosis and meiosis: one referring to the number of divisions and resulting cells, and another to the chromosome complement of…
State two fundamental differences between mitosis and meiosis: one referring to the number of divisions and resulting cells, and another to the chromosome complement of the daughter cells.
Why is it essential for gametes to be haploid? Reason what would happen to the number of chromosomes in a species, generation after generation, if gametes were diploid.
Why is it essential for gametes to be haploid? Reason what would happen to the number of chromosomes in a species, generation after generation, if gametes were diploid.
Sometimes a pair of homologs (in meiosis I) or a pair of sister chromatids (in meiosis II) does not separate and both go to the same pole. This error is called nondisjunction. Some gametes end up with an extra chromosome ($n + 1$) and others with one fewer ($n - 1$). It is a numerical chromosomal mutation that originates in the separation, not in the DNA itself. If a gamete with all its chromosomes duplicated ($2n$) takes part in fertilization, the zygote has three sets ($3n$): this is triploidy; the general term for a number of complete sets other than two is polyploidy.
Where did the separation fail? Clues from genetic markers
When a person is born with a trisomy, it is possible to find out which parent the extra chromosome came from and in which division it failed. A marker is used: a region of the chromosome that has several alleles (for example 1, 2, 3…) and that can be identified in the lab. The reasoning has two steps.
Step 1: Which parent does the extra chromosome come from? The child receives one chromosome from each parent and one more from the parent who had the error. If the baby has three different alleles, or repeated alleles, that can only come from one parent, that parent is the one that failed. The alleles the other parent does not have cannot come from that parent.
Step 2: Meiosis I or meiosis II? Compare the two chromosomes contributed by the parent who failed:
If they are both homologs (different alleles, one from each chromosome of the pair), the homologs did not separate: the error was in meiosis I.
If they are two identical copies of the same allele, they are sister chromatids that did not separate: the error was in meiosis II.
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In an amphibian population with $2n = 28$ chromosomes, a female is detected whose eggs do not reduce their chromosome number due to a cytokinesis failure in meiosis I,…
In an amphibian population with $2n = 28$ chromosomes, a female is detected whose eggs do not reduce their chromosome number due to a cytokinesis failure in meiosis I, giving rise to viable diploid eggs with 28 chromosomes.
a) If one of these abnormal eggs is fertilised by a normal sperm of the species, calculate the chromosome endowment of the resulting zygote and indicate what genetic term this numerical chromosomal alteration receives.
b) Although the resulting individual may be viable, it is usually completely sterile when attempting to undergo meiosis to form gametes. Justify why an odd (triploid) chromosome endowment prevents proper meiosis.
c) Based on this case, argue why the chromosome reduction of meiosis and subsequent fertilisation are not just a reproductive mechanism, but an essential barrier for the stability of sexually reproducing species.
The karyotype of a newborn is analysed and a trisomy in pair 21 ($47, XX, +21$) is detected. To find out the origin of the error, a genetic marker on chromosome 21 with…
The karyotype of a newborn is analysed and a trisomy in pair 21 ($47, XX, +21$) is detected. To find out the origin of the error, a genetic marker on chromosome 21 with three possible alleles (1, 2, and 3) is studied:
The mother is heterozygous and presents alleles 1 and 2.
The father is homozygous and presents only allele 3.
The baby presents all three alleles simultaneously: 1, 2, and 3.
a) Determine with justification in which of the two parents the meiotic non-disjunction occurred.
b) Reasonedly deduce whether the failure occurred in anaphase I or in anaphase II of meiosis, taking into account the difference between the separation of homologous chromosomes and that of sister chromatids.
c) If non-disjunction had occurred in anaphase II of the same parent, predict which maternal allelic combinations could have appeared in the baby instead of presenting alleles 1 and 2 at the same time.