3. Genetics 1

Question 1

Genetics

Answer 1

the study of genes, heredity, and variation. Can be molecular, classical (laws of inheritance), and populational.

Question 2

Gene

Answer 2

a heritable factor that consists of a length of DNA and controls the synthesis of one polypeptide chain
- the number of genes and chromosomes in body cells is unique for each species
- genes are linked in groups and each group corresponds to a specific chromosome
- genes annotated with capital letters are dominant, and with lowercase recessive.

Question 3

Gene locus

Answer 3

the specific and fixed position of a gene along a chromosome/DNA

Question 4

Which part of a tinted chromosome contains genes and how do we know that?

Answer 4

the lighter regions because where genes are located the DNA is less condensed (to enable DNA replication) so less color is absorbed.

Question 5

Homologous chromosomes

Answer 5

or a homologous pair have the same length, shape (position of the centromere), bending pattern, and they (can) carry different variations of genes on the same gene loci.
They are arranged in pairs according to this property.
Humans have 23 pairs of homologous chromosomes (23 groups of linked genes).

Question 6

Sister chromatids

Answer 6

they are identical chomatids (just a copy made during DNA replication – have the exact same genes, attached at the centromere
two sister chromatids cannot be in the same daughter cell (Only one copy (allele) of each gene should be present in each cell)

Question 7

Allele

Answer 7

a variation of a single gene that differs from other alleles of the same gene by a few base pairs. It occupies the same locus as the other alleles of the same gene.
For example, the gene for the Rh factor on RBC (two alleles – Rh+ and Rh-) and the gene for the ABO system of the erythrocyte blood group.

Question 8

Heterozygous and homozygus

Answer 8

• having different versions of the same gene
• having only one gene version on the chromosome

Question 9

Single nucleotide polymorphism (SNPs)

Answer 9

• positions inside the specific gene where more than one bp can be found - a different variation of the same gene is present due to a change in one or more base pairs (the slight difference doesn’t affect the structure)
• they are a result of base-substitution mutations (the majority are negative/lethal)

Question 10

results of base substitution mutation

Answer 10

negative – allele eliminated from the population
neutral – mutation persists (not subject to natural selection)
positive – allele persists and becomes widespread

Question 11

Highly repetitive sequence (HRS), chimp example, cox

Answer 11

telomeres and the centromere (gets lost in the DNA replication).
e.g. chimp chromosomes have one telomeric and centromeric region each. Human chromosome number two has two extra telomeric regions and one extra centromeric region. Chromosomes 12 and 3 form chimps merged into chromosome 2 in humans – human ancestors and chimps have 48 chromosomes, and humans 46 because of this.
- cytochrome C protein (cox gene) is a conservative gene involved in CR

Question 12

Genome, genome size

Answer 12

• the complete DNA of a cell (includes both coding and non-coding regions).
• genome size is expressed as the number of base pairs and it varies among species: humans – genome size = 3*10^9, number of genes = 23 000

Question 13

Ploidy

Answer 13

the number of complete sets of chromosomes and their genetic information – a cell can be haploid (gametes), diploid (somatic cells)…
n is the number of chromosomes in one set and it is specific for each species.
If none of the chromosomes have their homologous pairs, the cell is haploid.

Question 14

Karyotype
Karyogram
Karyotyping

Answer 14

• describes the number and appearance of the chromosomes in an eukaryotic cell, members of the same species usually have the same karyotype
• micrograph of metaphase chromosomes where they are arranged into homologous pairs beginning with autosomes and ending with gonosomes (humans - 44 autosomes and two gonosomes). Used for gender prediction and detection of chromosomal abnormalities.
• the procedure of obtaining the karyogram.

Question 15

Meiosis

Answer 15

the division of diploid somatic cells in sex glands to produce haploid gametes – uses a mechanism of reducing the number of chromosomes to half the number found in somatic cells and promoting genetic variation.

Question 16

Prophase I

Answer 16

• the longest phase of meiosis (90% of time required, lasts for hours or days).
  Synopsis – pairing up of not completely condensed homologous chromosomes to exchange genes between their non-sister chromatids – tetrads/bivalents formed – this is called crossing over or recombination (results in the recombination of linked genes or the production of new combinations of linked genes in gametes – meiosis can therefore produce an almost indefinitely large number of genetically different gametes.
  Chiasma – the place where non-sister chromatids cross over.

Question 17

Metaphase I

Answer 17

• spindle microtubules attach to the centromere and bivalents line up at the equator
• because the orientation of each pair is random, each bivalent can orient itself in two ways – that’s why there are 2^n genetically different gametes (ignoring crossing over) that can be produced in each organism - n is the number of bivalents (2^23 in humans) – this increases genetic variability.

Question 18

Anaphase I

Answer 18

• the two chromosomes of each tetrad move to the opposite poles (this ensures the reduction of the chromosomal number). In anaphase and anaphase II sister chromatids separate, while in anaphase I bivalents separate.

Question 19

Telophase and cytokinesis I

Answer 19

• chromosomes get segregated into two clusters, the nuclear membrane reforms, and chromosomes partially uncoil.
  At the end of cytokinesis I daughter cells will enter brief interphase (with no DNA replication) or immediately enter 2nd meiotic division.

Question 20

Prophase II
Metaphase II
Anaphase II
Telophase II
Cytokinesis II

Answer 20

• nuclear envelope breaks down and 2nd meiotic division begins
• spindle fibers bind to both sides of the centromere
• spindle fibers pull sister chromatids to the opposite poles
• nuclear envelope reforms
• produced four haploid gametes that are genetically different from each other

Question 21

what is the consequence of meiosis and sexual reproduction and what is it due to?

Answer 21

• increased genetical variation of the offspring due to:
  I| Crossing over
  II| Random orientation of bivalents
  III| Random fertilization of gametes

Question 22

draw mitosis and miosis and identify the number of chromosomes in each phase of each division

Answer 22

…

Question 23

distinguish and compare between mitosis and meiosis

Answer 23

both can be used as a means of reproduction – difference mitosis for unicellular, meiosis for multicellular organisms and mitosis is asexual while meiosis is sexual reproduction. Only meiosis has crossing over/synopsis and bivalents (homologous pair recognize each other). Mitosis produces two identical daughter cells (somatic) and meiosis four genetically different gametes. In mitosis homologous pairs are blind to each other while they recognize and pair up in meiosis.

Question 24

linked genes, how is gene loci connected with crossing over and linked genes

Answer 24

The closer the gene loci are the more linked their genes are – linked genes are those most likely to be inherited together (usually not separated in crossing over because of their closeness).
The further apart genes on a chromosome are the more likely it is for a successful crossing over to happen between them (less likely to entangle)

Question 25

parental vs recombinant gamete production

Answer 25

Since the crossing over doesn’t always have to happen it is more likely that a gamete with the same genotype as the parental (non-recombinant) gamete will be produced than that with a recombinant genotype (50%>25%).

Question 26

Nondisjunction, risk factor

Answer 26

• when bivalent (homologous chromosomes)/sister chromatids don’t separate during anaphase I/II and it leads to aneuploidy (mutated gametes)
• when a parent (especially the mother) is older – eggs “frozen” in prophase I - the longer the homologous chromosomes are joined, the more difficult it is to separate them – if an older egg gets fertilized, the probability of it being mutated is greater.

Question 27

Chromosomal mutations, aneuploidy, polypoloidy

Answer 27

• changes in structure/number of chromosomes (human organism can only survive a chromosomal mutation on the 13th, 18th, 21st, and sex chromosome pairs), aneuploidy, or polyploidy.
• when one pair of a chromosome has a chromosome surplus or missing so the number of chromosomes in a gamete is n+1 or n-1 instead of n.
• when each homologous pair has an extra chromosome so a gamete has 3n, 4n…number of chromosomes.

Question 28

chromosomal mutations examples

Answer 28

cry of a cat syndrome - one arm of a chromosome (5) missing
Down syndrome - or trisomy 21, aneuploidy in the 21st chromosomal pair

Question 29

Mutations

Answer 29

• permanent structural changes to the DNA of a gene that can be inherited
  (only coding parts – the non-coding part mutations are not subject to natural selection and are therefore more common as they can be passed down)

Question 30

Types of mutations, outline each

Answer 30

1| addition (adds a codon into the DNA – new amino acid, detrimental)
2| deletion (removes a codon from the DNA – detrimental)
3| substitution (replaces just one base pair – doesn’t have to be detrimental because multiple codons code for the same a-a (degeneration property))
4| translocation (a codon is taken from one region and placed in another – detrimental)
5| inversion (a codon is taken out, flipped, and reinserted in the same place – detrimental).