Lecture 2: Mendelian Genetics

Question 1

compare genome organisation between diploids and haploids

Answer 1

Humans (Diploids)
- 2 matching homologous sets of chromosomes i.e. 2 copies of every gene
- chromosome complement: 2N
except XY chromosomes in males

Haploids

• 1 set of chromosomes i.e. 1 copy of every gene
• chromosomes complement: 1N

Question 2

what is a gene?

Answer 2

• a unit of heredity
• located on a specific part of a chromosome
• comprises DNA sequence that codes for a protein

Question 3

what is an allele

Answer 3

one form of a gene
a specific change in DNA sequence

i.e. can have different alleles for 1 gene (eye colour gene examples of different alleles include blue or brown eyes)

Question 4

what is meant by homozygote and heterozygote?

Answer 4

homozygote: possess identical alleles for same gene
heterozygote: posses 2 different alleles for same gene

Question 5

what is a genotype? what is a phenotype?

Answer 5

genotype: genetic composition of an organism which is the set of alleles the organism has
phenotype: observable properties of an organism produced by combination of genotype and environment

Question 6

what is meant by haploid and diploid?

Answer 6

haploid: refers to the presence of a single set of chromosomes in an organism’s cells
diploid: Sexually reproducing organisms are diploid (having two sets of chromosomes, one from each parent).

Question 7

what is meant by the term ‘wild type’ in genetcs?

Answer 7

• the genotype or phenotype that is most commonly found in nature or in standard lab stock of an organism i.e. “normal”

Question 8

alleles can be dominant or recessive, what does this mean?

Answer 8

dominant: phenotypically expressed in homozygous or heterozygous state
recessive: phenotypically expressed only in homozygous state (i.e. when there are 2 copies of this allele)

Question 9

Dominance and recessive alleles are only meaningful when?

Answer 9

when comparing 2 specific alleles

e.g. INSᴬ is dominant to INSᴮ but recessive to INSᴰ

Question 10

what are the two types of cell division and give a brief overview of their purpose

Answer 10

mitosis:
- 1 diploid cell to 2 diploid cells
i purpose: makes more cells

meiosis:

• 1 diploid cell to 4 haploid cells
• recombination (can take place)
• purpose: make gametes

Question 11

state five similarities between mitosis and meiosis

Answer 11

• Mitosis and meiosis take place in the cell nuclei.
• Both involve cell division.
• Both the processes occur in the M-phase of the cell cycle.
• In both cycles, the stages are common – metaphase, anaphase, telophase and prophase.
• Synthesis of DNA occurs in both.

Question 12

state four differences between mitosis and meiosis

Answer 12

• meiosis undergoes 2 rounds of cell division while mitosis only 1
• meiosis produces 4 daughter cells, mitosis produces 2
• mitosis produces genetically identical daughter cells while meiosis produces genetically different ones
• meiosis produces haploids, mitosis produces diploids

Question 13

outline the main stages of mitosis

Answer 13

(a) interphase - chromosomes are extended and uncoiled, forming chromatin
(b) prophase - chromosomes coil up and condense; centrioles divide and move apart
(c) prometaphase - chromosomes are clearly double structures; centrioles reach the opposite poles; spindle fibres form
(d) metaphase - centromeres align on metaphase plate
(e) anaphase - centromeres split and daughter chromosomes migrate to opposite poles
(f) daughter chromosomes arrive at the poles; cytokinesis commences

Question 14

Dominant alleles show what type of activity?

Answer 14

Heightened protein activity or inhibitory protein activity to the wild type.

Question 15

Recessive alleles show what type of activity?

Answer 15

Reduced protein activity compared to wild type.

Question 16

What is mitosis?

Answer 16

Somatic cell division.

Question 17

Describe prophase I of meiosis.

Answer 17

One round of DNA replication and two rounds of cell division producing four copies of a single chromosome in a tetrad (group of 4)

Question 18

what is recombination in meiosis?

Answer 18

• process by which pieces of DNA are broken and recombined to produce new combinations of alleles

Question 19

Describe the monohybrid cross.

Answer 19

Occurs when one gene controls one phenotype.

The ratio for the F2 is 3:1

Question 20

Describe a dihybrid cross.

Answer 20

Occurs when two genes control one phenotype.

The ratio for the F2 is 9:3:3:1

Question 21

Where on a chromosome does homologous recombination take place?

Answer 21

Between chiasmata

Question 22

when does recombination occur in meiosis?

Answer 22

prolonged prophase of meiosis I

Question 23

what is the goal of meiosis I and meiosis II?

Answer 23

goal of meiosis I: meiosis I is to separate homologous chromosomes
goal of meosis II: to separate sister chromatids
note: no DNA is duplicated in prophase II as it is in prophase I

Question 24

outline the stages of meiosis

Answer 24

prophase I: chromosomes coil and condense
metaphase I: chromosomes align along equator of cell (homologous chromosomes (tetrad/bivalent) align parallele to each other)
anapahse I: the homologous chromosomes are moved to opposite ends of the cell
telophase I: chromosomes may uncoil and the nucleus forms
in cytokinesis cells split apart to form two daughter cells
prophase II: nuclear envelope breaks down and the spindles reform
metaphase II: the chromosomes are moved to the equator of the cell
anaphase II: chromosomes divide and each chromatid moves to the opposie poles of the cell

Question 25

what is a tetrad/bivalent

Answer 25

Set of paired sister chromatids that form a group of four chromatids