Yaritza has 46 chromosomes, just like her mom and her dad. If the egg and the sperm also had 46, the zygote would have 92, the next generation 184, and so on without end. That doesn't happen, because there is a special division that cuts the number of chromosomes in half before fertilization: meiosis. At the end of the page you will come back to this question.
What meiosis accomplishes
Skin cells have 46 chromosomes, but a mature sperm or egg has only 23. How does a cell manage to reduce its chromosome count to exactly half?
Look at the diagram. Meiosis I separates the homologous chromosomes (one from the mother and one from the father). Meiosis II separates the sister chromatids of each chromosome.
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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.
Human meiosis takes place in the gonads. The formation of gametes is called gametogenesis, and it has two types.
Spermatogenesis (in the testes): each cell that enters meiosis produces four functional sperm. It occurs continuously from puberty on.
Oogenesis (in the ovaries): each cell that enters meiosis produces only one functional egg and three small polar bodies that degenerate. The cytoplasm is divided unequally so that the egg keeps the reserves the embryo will need.
Why each gamete is genetically different
Two siblings have the same parents and yet they are not alike (except identical twins). Meiosis has two mechanisms that mix genes.
Genes on different pairs and linked genes
Look at an $AaBb$ individual. If genes $A$ and $B$ are on different pairs of homologous chromosomes, independent assortment gives four different gametes in equal proportion: $AB$, $Ab$, $aB$, and $ab$ (1:1:1:1). If the two genes are on the same pair (linked genes, with $AB$ on one chromosome and $ab$ on the other) and there is no crossing over, they travel together. Only two types of gametes result, $AB$ and $ab$, in a 1:1 ratio.
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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.
Both begin the same way, with a cell that has already replicated its DNA. But they end very differently.
Mitosis
Meiosis
Divisions
1
2
Daughter cells
2
4
Chromosomes in the daughter cells
$2n$, same as the parent
$n$, half
Variability
none: identical copies
yes (crossing over and independent assortment)
Where it occurs
almost all cells of the body
only in the gonads
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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.
A diploid ($2n$) cell has two sets of chromosomes, one from each parent. A haploid ($n$) cell has a single set. In humans, $2n = 46$ and $n = 23$. If an egg does not reduce its number of chromosomes because of an error in meiosis and is fertilized by a normal sperm, the zygote ends up with three sets ($3n$): this is triploidy, a type of polyploidy. Such individuals, when they survive, are usually sterile. The chromosomes cannot form pairs of homologs with three copies, and meiosis does not distribute the chromosomes properly.
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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.
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.
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.
A baby with Down syndrome has 47 chromosomes instead of 46, because it has three copies of chromosome 21. At what point did the distribution fail?
DNA: the message that is inherited
The chromosomes that meiosis distributes are made of DNA (deoxyribonucleic acid), the molecule where the information that makes you look like your mom and your dad is written. What is that molecule like, and how does it become traits?
The message is in the order of the bases. A gene is a stretch of DNA that contains the instructions for making a protein. Proteins do the work of the cell (enzymes, pigments, structures) and in this way determine traits: gene → protein → trait (phenotype). RNA is a similar molecule that helps read the message: it has a single strand and, instead of thymine, it has uracil (U).
If the order of the bases changes, the message changes. That change is called a mutation. It may have no effect, produce a different protein with a new trait, or cause a hereditary disease. If it occurs in a gamete, it is passed on to the children.
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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.