If a gamete produced following non disjunction of a chromosome at second meiotic division was fertilized by a normal gamete, what is the expected frequency of trisomic progeny?
1/4
Meiosis and Gamete Formation
Meiosis is a type of cell division that reduces the number of chromosomes in the parent cell by half and produces four gamete cells. It involves two sequential divisions: Meiosis I and Meiosis II.
- Meiosis I: Homologous chromosomes separate, reducing the chromosome number from diploid (2n) to haploid (n). Each chromosome still consists of two sister chromatids.
- Meiosis II: Sister chromatids separate, similar to mitosis. This results in four haploid cells, each with a single set of chromosomes (each chromosome having one chromatid).
Non-disjunction at Meiosis II
Non-disjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division. If non-disjunction occurs during Meiosis II, it affects the separation of sister chromatids in one of the two cells produced by Meiosis I.
Let's consider a single pair of homologous chromosomes. A diploid cell (2n) enters meiosis.
- After Meiosis I: Two cells are formed, each containing 'n' chromosomes, but each chromosome has two chromatids. Let's call these cells Cell A and Cell B.
- During Meiosis II:
- Suppose Cell A undergoes normal Meiosis II. The sister chromatids separate, resulting in two normal gametes, each with 'n' chromosomes (one chromatid per chromosome).
- Suppose Cell B undergoes non-disjunction for the specific chromosome during Meiosis II. The sister chromatids fail to separate. This cell will produce:
- One gamete with an extra chromosome (n+1), where both sister chromatids went to the same pole.
- One gamete missing that chromosome (n-1), where the sister chromatids failed to move to this pole.
Therefore, from a single diploid cell undergoing meiosis with non-disjunction in one cell during Meiosis II, the following four gametes are produced:
- Two normal gametes (n) from the cell that divided normally in Meiosis II.
- One gamete with an extra chromosome (n+1) from the cell with non-disjunction in Meiosis II.
- One gamete missing a chromosome (n-1) from the cell with non-disjunction in Meiosis II.
Frequency of Trisomic Progeny
The question states that a gamete produced following this non-disjunction event is fertilized by a normal gamete. A normal gamete has the chromosome number 'n'. Let's look at the possible outcomes when each type of gamete from the non-disjunction meiosis is fertilized by a normal gamete (n):
| Type of Gamete from Meiosis with Non-disjunction | Fertilized by Normal Gamete (n) | Resulting Zygote (Progeny) Chromosome Number | Classification of Progeny |
|---|---|---|---|
| Normal (n) | + (n) | = 2n | Normal diploid |
| Normal (n) | + (n) | = 2n | Normal diploid |
| Gamete with extra chromosome (n+1) | + (n) | = 2n+1 | Trisomic |
| Gamete missing a chromosome (n-1) | + (n) | = 2n-1 | Monosomic |
Out of the four possible types of progeny resulting from the fertilization of the four types of gametes produced (from one meiotic event with Meiosis II non-disjunction) by a normal gamete, only one is trisomic (2n+1).
The expected frequency of trisomic progeny is the number of trisomic outcomes divided by the total number of possible outcomes:
\text{Frequency of trisomic progeny} = \frac{\text{Number of trisomic outcomes}}{\text{Total number of possible outcomes}}
\text{Frequency of trisomic progeny} = \frac{1}{4}
Therefore, the expected frequency of trisomic progeny is 1/4.
Important Questions from Alterations of chromosomes
A species of plant (species 1) is diploid (2n = 6) with chromosomes AABBCC and a related species (species 2) is also diploid (2n = 4) with chromosomes PPQQ. The following statements were given by students regarding the chromosome numbers involving these plant species:
A. Autotriploid of species 1 will have 12 chromosomes
B. Allotetraploid involving species 1 and 2 will have 16 chromosomes
C. A monosomy in species 1 will generate 5 chromosomes
D. A double trisomy in species 1 will generate 8 chromosomes
E. A nullisomy in species 2 will generate 2 chromosomes
The combination of statements with all correct answers is:
Fertilization of gametes containing chromosome with duplications or deletions often results in children with disabilities. What is the probability of a couple where the male is karyotypically normal and the female has a pericentric inversion in heterozygous condition producing a child with disabilities if crossing over took place within the pericentric inversion in 26% of meiotic divisions?
In this case consider that fertilization with a gamete containing chromosomes with duplications or deletion will result in disabilities.
Chromosomal inversions are balanced rearrangements and thus do not change the overall amount of genetic material. While inversions can exist in homozygous condition, some only exist as heterozygotes. In the latter condition, the breakpoint disrupts:
A colour blind father has a daughter who is also colour blind and has Turner's syndrome. The genotype of the daughter is due to:
A cruciform structure of chromosomes during meiosis is a characteristic feature of:
