5.4.1 Genetic Variation

Question 1

Gametogenesis

Answer 1

The production or origin of gametes - a form of meiosis

Question 2

Germline cells

Answer 2

All cells used in reproduction in multicellular organisms

Question 3

Ploidy

Answer 3

The number of sets of chromosomes in a cell, denoted n.

• Haploid: n
• Diploid: 2n

Question 4

Chromosomal crossover

Answer 4

During prophase I, chromosomes pair up with their homologous pair

• Homologous pair in diploid cells: chromosomes that are similar in length, gene position, and centromere location

Question 5

Meiosis and genetic variation

Answer 5

• Gametes may be genetically distinct if crossing over occurs in prophase I, causing the recombination of sister chromatids
• Crossing over provides variety to a population. This is vital to the adaptation and survival of species, as explained in Darwin’s Theory of Evolution by Natural Selection

Question 6

Spermatogenesis and oogenesis

as types of gametogenesis

Answer 6

• A type of gametogenesis
• To become a sperm or ovum, a gametocyte (diploid germ cell) is influenced by the hormones of the body it is in, which is controlled by the organism’s sex chromosomes
• Gametogenesis will specialise as spermatogenesis or oogenesis accordingly

Question 7

Spermatogenesis

process

Answer 7

Specialisation process at the end of meiosis (e.g. swimming mechanisms)

• 4 sister sperm cells are formed in the testes of males
• This process can occur from before birth until death

Question 8

Oogenesis

process

Answer 8

Longer and more complex than spermatogenesis - rather, a form of meiosis

• Oogenesis forms a mature oocyte and 3 polar bodies (sterile gametes that degenerate). This whole process spans birth, puberty and fertilisation in which through Meiosis I and II, majority of the cytoplasm of the parent cell goes into a single gamete, which will become the mature ovum.
  1. Before birth: oogonium (germ cell) undergoes mitosis to produce primary oocytes which are arrested in prophase I. Therefore, females have a set number of ova for their lives
  2. Puberty: Follicular cycle - maturation of a follicle results in one primary oocyte completes meiosis I → secondary oocyte (arrested in Metaphase II) and first polar body
  3. Fertilisation: secondary oocyte released into fallopian tube. If it is fertilised by a sperm cell it completes meiosis II → zygote (2n) and second polar body + two polar bodies from the first polar body.

Question 9

Non-disjunction

Answer 9

Nondisjunction results in an aneuploid zygote (one cell with n+1 or n-1)

Mutations: the spindle fibres separate homologous pairs. However sometimes the homologous pairs or chomatids don’’t separate. This can occur at anaphase I (homologous chromosomes fail to separate) or anaphase II (sister chromatids fail to separate)

Aneuploid: cell has abnormal number of chromosomes

This can result in chromosomal disorders