Lecture 15 - Inheritance and sexual reproduction

Question 1

what is the animal life cycle and reproduction?

Answer 1

adult (2n) –> meiosis –> egg or sperm (n) –> fusion –> zygote (2n) –>mitosis –> adult

Question 2

what is the plant life cycle and reproduction?

Answer 2

sporophyte (2n) –> meiosis –> spore (n) –> gametophyte (n) –> female gamete or male gamete (n) –> fusion –> zygote (2n) –> mitosis –> sporophyte.

Question 3

explain meiosis

Answer 3

> produces haploid gametes - sperm or egg cells
one round of DNA duplication in a diploid cell (s phase) - as for mitosis
two cell divisions
- i. reduction division (haploid)
- ii. separation of chromatids (haploid)

Question 4

describe the process of meiosis

Answer 4

meiosis I
> first division
  - reduces chromosome # from diploid to haploid
  - by separating homologous chromosomes
Meiosis II
>second division
  - separates chromatids
  - four haploid gametes per starting cell.

Question 5

how does sex contribute to genetic variation?

Answer 5

> homologous chromosomes segregate and the non-homologous chromosomes assort independently during gamete formation, this creates different outcomes.
in humans: 2^23 = ~ 8 million unique gametes from one individual

Question 6

what is independent assortment of alleles?

Answer 6

> variants of the same gene (i.e., alleles) account for trait variation
variation between individuals results from different combinations of alleles
~4^10,000 possible unique gametes!

Question 7

Explain gentic variation via recombination

Answer 7

> recombination events (crossovers) occur at pachytene (a stage in prophase I of meiosis I) at different sites along the chromosomes. Some sites, so-called “hotspots” are used more often than other sites.
mixes paternal and maternal regions between homologous chromosomes
gives rise to new combinations of alleles on a chromosome.
point of crossing over is called a chiasma

Question 8

what are the four main sources of variation in cells?

Answer 8

> mutations
- new alleles
independent assortment of homologous chromosome pairs (in meiosis)
- different combinations of parental chromosomes in different gametes
recombination (crossing over in meiosis I)
- new combinations of alleles on chromosomes
fusion of gametes (in fertilisation)

Question 9

What happens when meiosis goes wrong

Answer 9

> non -disjunction of homologous chromosomes in meiosis I or sister chromatids in meiosis II

• chromosomes sometimes fail to separate properly during meiosis I or II
• gametes +/- chromosome
• zygote will be aneuploid (often lethal) (2n +/- 1)
• e.g, down syndrome, trisomic chromosome 21 ( 47, +21)