Meiosis and genetic variation Flashcards
What is meant by the terms haploid and diploid?
Haploid - there is only one of each type of chromosome in the nucleus of a cell.
Diploid - there are two copiesof each chromosome in the nucleus - one from the mother and one from the farther.
How are zygotes formed? State whether they are haploid or diploid.
Zygotes are formed by the fusion of two haploid gametes during fertilisation - - one ovum and one sperm. This makes zygotes diploid.
The diagram on the right shows a cell preparing to undergo meiosis. Describe what is happening between stages A and B.
First, the DNA unravels and replicates so there are two copies of each chromosome. The DNA then condenses to form double-armed chromosomes, each made from two identical copies called sister chromatids. The sister chromatids are joined in the middle by a centromere.
Describe what happens during the first division of meiosis (meiosis I).
During meiosis I, the chromosomes arrange themselves into homologous pairs and then separate, halving the chromosome number.
There are 38 chromosomes in an adult tiger cell. If a cell in a tiger’s reproductive organs undergoes meiosis, how many chromosomes will there be in each cell produced after meiosis I?
How many chromosomes will there be in each cell produced after meiosis II? Explain your reasoning.
There will be 19 chromosomes in each daughter cell after meiosis I and 19 chromosomes in each daughter cell after meiosis II. This is because, before meiosis I, the DNA replicates so that each of the 38 chromosomes is composed of two sister chromatids. The chromosome number is then halved to 19 in meiosis I as a member of each homologous pair is separated into the two daughter cells. Meiosis II then separates the sister chromatids, maintaining only one copy of each of the 19 chromosomes in each gamete.
During meiosis, homologous pairs of chromosomes ‘cross over’. Explain what is meant by this and how this leads to increased genetic variation.
Crossing over is where homologous chromosomes pair up and the chromatids twist around each other, causing parts of the chromatids to swap over. This leads to increased genetic variation because each daughter cell has a different chromatid with a different combination of alleles.
During sexual reproduction, fertilisation between gametes is random. Explain how this could lead to an increased genetic diversity within a species.
Random fertilisation increases genetic variation because it produces zygotes with different combinations of chromosomes to both parents. This mixing of genetic material increases genetic diversity within a species.
During which division of meiosis does crossing over take place?
Crossing over happens during the first division of meiosis (meiosis I).
What is a homologous pair of chromosomes?
A homologous pair of chromosomes is made up of two chromosomes that are the same size and carry the same genes, but they may have different alleles. One chromosome is inherited from the mother and one from the father.
Haplodiploidy is where the sex of an organism is determined by whether it is haploid or diploid. For example, in bees, the males are haploid and the females are diploid.
The diagram on the right shows how gametes of each sex are produced in honey bees. Identify which sex undergoes meiosis to create its gametes.
Explain your reasoning.
Females undergo meiosis to create their gametes because they create four daughter cells with half the number of chromosomes as the parent cell.
Suggest how male might produce their gametes, if not through meiosis. Explain your answer.
Males must undergo mitosis to create their gametes because they create two daughter cells, each with the same number of chromosomes as the parent cell.
What is the independent segregation of chromosomes? Explain how this leads to increased genetic variation.
Independent segregation of chromosomes is the random division of chromosomes from each homologous pair during meiosis I.
It means that each daughter cell ends up with a different combination of maternal and paternal chromosomes and therefore different alleles.
This ‘shuffling’ increases genetic variation in any potential offspring.
Edwards’ syndrome is a genetic condition caused by having three copies of chromosome 18. Explain how chromosome non-disjunction could cause Edwards’ syndrome.
Chromosome non-disjunction is where a homologous pair of chromosomes fails to separate during meiosis.
It could cause Edwards’ syndrome if non-disjunction of chromosome 18 occurs, creating a gamete with an extra copy of chromosome 18.
This gamete would then fuse with another gamete during fertilisation, making a zygote with three copies of chromosome 18.
