Diversity, Classification and Variation-Topic 4B Flashcards
What does Meiosis involve?
DNA from one generation is passed on to the next by gametes
What is a zygote?
When a sperm and egg cell (gametes) join together at fertilisation.
Normal body cells have the diploid number (2n) of chromosomes. What does this mean?
Each cell contains two of each chromosome, one from the mum and one from the dad.
Gametes have a …… (…) number of chromosomes
Gametes have a haploid (n) number of cells.
half number of chromosomes as diploid cells
How is genetic diversity increased within a species?
Fertilisation is random which produces zygotes with different combinations of chromosomes to both parents. (mixing of genetic material)
Gametes are formed by…..
Meiosis
Where does Meiosis take place?
In the reproductive organs
Cells that divide by meiosis are ….. to start with, but the cells that are FORMED from meiosis are …..
Cells that divide by meiosis are diploid to start with but the cells that are formed from meiosis are haploid.
What is crossing over?
Homologous pairs of chromosomes pair up and chromatids twist around each other and swap over. They contain the same genes but now different combinations of alleles.
Meiosis produces cells that are……..
Meiosis produces cells that are genetically different
Name the 2 main events during meiosis that lead to genetic variation
Crossing over of chromatids
Independent segregation of chromosomes
What does the crossing over of chromatids in meiosis I mean for the 4 daughter cells.
They contain chromatids with different alleles.
What are chromosome mutations caused by?
errors in cell division
In humans, if meiosis works properly all four daughter cells should end up with …. ………..
23 chromosomes
What is non-disjunction?
a failure of chromosomes to separate properly e.g. non-disjunction of chromosome 21 can lead to Down’s Syndrome.
What is Down’s Syndrome caused by?
A person having an extra copy of chromosome 21. Usually one cell gets an extra copy of 21 and another gets none. This means 3 copies of chromosome 21 due to extra copy fusing with another gamete.
What do gene mutations involve?
A change in the DNA base sequence of chromosomes
Give two examples of gene mutations
Deletion and substitution.
How are gametes produced?
Meiosis
What is independent segregation?
separating chromosomes into random combinations