Mendel and Meiosis

What is the Chromosome Theory of Inheritance proposed by Boveri and Sutton?

It correctly explains the underlying mechanism of Mendelian genetics by combining chromosomes with the paired factors postulated by Mendel.

What did Thomas Hunt Morgan contribute to genetics?

He showed that inheritance can be explained by genes located at specific sites on chromosomes.

How does DNA relate to genes and chromatin?

DNA exists as chromatin and must be in a loose configuration to be accessed; the DNA is exactly the same in every cell.

What are homologous chromosomes?

Paired chromosomes that have the same genes for the same traits located at the same loci on the chromosomes.

How are homologous chromosomes similar and different?

They contain genes in the same order, place, and loci, but their alleles are not always identical, such as B and b.

What is the cell cycle?

The series of events leading to cell division and DNA duplication: G0 = resting, G1 = growth, S = DNA replication, G2 = growth and preparation for mitosis, M = division.

What is mitosis and which cells undergo it?

Mitosis is cell division in which somatic cells divide; the chromosome number is maintained, producing 2n daughter cells from a 2n parent cell.

Why would repeated mitosis without meiosis cause chromosome numbers to increase?

Chromosome pairs in offspring would continually double, such as 28 pairs (4n) becoming 56 pairs (8n).

What is meiosis?

A process in which the number of chromosomes in a cell is halved during the production of gametes.

Where was meiosis discovered according to the notes?

It was discovered in sea urchin eggs, and the chromosomal level was studied in roundworms.

Which cells undergo meiosis?

Only germ cells.

Why does meiosis require two divisions?

Meiosis requires two division steps, meiosis I (Homologous) and meiosis II (Sister chromatids), each containing prophase, metaphase, anaphase, and telophase.

What happens at the beginning of meiosis I?

The cell starts as diploid (2n), and DNA undergoes replication.

What are tetrads?

Homologous chromosome pairs formed during meiosis I; each chromosome contains a pair of sister chromatids that are identical copies.

What happens during metaphase I?

Tetrads align toward the middle of the cell; the law of segregation occurs as homologous chromosomes align randomly and independently.

What happens after metaphase I?

Homologous chromosomes separate and move to opposite ends of the cell, producing the first division into two haploid cells.

What happens at the beginning of meiosis II?

There are two haploid daughter cells with half as many chromosomes as the original cell.

What happens during meiosis II?

Sister chromatids separate and move toward opposite ends of the cell.

What is the final result of meiosis?

One diploid starting cell produces four haploid cells (2n → 4 × n).

How can the number of possible gametes be calculated?

The number of possible gametes is 2^n, where n is the number of chromosome pairs; 2 chromosomes give 2^2 = 4, 3 give 2^3 = 8, and 23 chromosome pairs give 2^23 = about 8 million.

How many possible zygotes can result from fertilization with 23 chromosome pairs?

2^23 possible gametes from each parent can combine, giving 2^23 × 2^23 = about 70 trillion possible zygotes.

How do you calculate possible gametes from a genotype?

Use 2^n, where n is the number of heterozygous pairs; for AaBBCcDd there are three heterozygous pairs (Aa, Cc, Dd), giving 2^3 = 8 possible gametes.

What are the two major purposes of meiosis?

It halves genetic information and creates genetic variation.

How does meiosis create genetic variation?

Through random segregation and crossing over (genetic recombination).

What is crossing over?

The exchange of genetic information between non-sister chromatids during prophase I. (Chiasmata)

How does anaphase I explain Mendel's Law of Segregation?

In anaphase I, homologous chromosomes separate randomly to opposite sides, showing segregation of the chromosome pairs.

How does anaphase II relate to Mendel's results?

Sister chromatids separate, with each parent ultimately providing one allele randomly.

How does prophase I explain Mendel's Law of Independent Assortment?

Crossing over during prophase I creates genetic recombination, while different traits have an independent and equal chance of occurring.

How does meiosis explain Mendel's Law of Segregation overall?

During meiosis I, homologous chromosomes separate, so the two alleles for a trait segregate into different cells and offspring receive one allele from each parent.

How does meiosis explain Mendel's Law of Independent Assortment overall?

During meiosis, alleles for different traits can assort independently, giving different traits an independent and equal chance of occurring.

Why is meiosis important for maintaining chromosome number across generations?

It halves chromosome number in gametes so that fertilization can restore the diploid chromosome number rather than continually doubling it.