Meiosis, peas and sex

Question 1

character

Answer 1

heritable feature varying among individuals (e.g. color)

Question 2

hybridisation

Answer 2

crossing of two true-breeding varieties

Question 3

P generation

Answer 3

true breeding parents (parental generation)

Question 4

F1 generation

Answer 4

1st filial generation

Question 5

F2 generation

Answer 5

2nd filial generation

Question 6

Mendel’s model - 1st law

Answer 6

alternative versions of genes (alleles) account for variations in inherited characters

Question 7

Mendel’s model - 2nd law

Answer 7

for each character, an organism inherits two versions (alleles) of a gene, one from each parent

Question 8

Mendel’s model - 3rd law - law of dominance

Answer 8

if the two alleles at a locus differ, then one, the DOMINANT allele determins the organism’s appereance; the other, RECESSIVE allele has no noticeable effect on the organism’s appereance

Question 9

Mendel’s model - 4th law - the law of segregation

Answer 9

two alleles for a heritable character segregate (separate) during gamete formation and end up in different gametes; an egg or sperm gets only one of the two alleles present in the diploid cells of the organism making the gamete

Question 10

homozygote

Answer 10

organism that has a pair of identical alleles for a gene encoding a character

Question 11

heterozygote

Answer 11

organism that has two different alleles for a gene

Question 12

alleles

Answer 12

alternative versions of a gene

Question 13

phenotype

Answer 13

an organism’s appereance or observable traits (PHYSIOLOGICAL)

Question 14

genotype

Answer 14

an organism’s genetic makeup

Question 15

law of (in)dependent assortment

Answer 15

two or more genes assort independently - that is, each pair of alleles segregates independently of any other pair of alleles during gamete formation - applies only to allele pairs located on different chromosomes (NOT homologous) or very far apart on the same chromosome

same chromosome - dependent
different chromosome - independent
(YR yr) vs (YR Yr yR yr)

Question 16

fertilisation

Answer 16

the union of gametes, culminating in fusion of their nuclei

Question 17

zygote

Answer 17

fertilized egg (is diploid, because it contains two haploid sets of chromosomes)

Question 18

meiosis

Answer 18

reduces the number of sets of chromosomes from two in the parent cell to one in each gamete, counterbalancing the doubling occuring at fertilization (diploid → haploid)

Question 19

meiosis I

Answer 19

separates homologous chromosomes

Question 20

prophase I

Answer 20

• duplicated homologous chromosomes pair up and exchange segments
• centrosome movement, spindle formation and nuclear envelope breakdown - as in mitosis

Question 21

crossing over

Answer 21

DNA molecules from nonsister chromatids are broken by proteins and rejoined to each other

Question 22

step 1 of crossing over

Answer 22

after interphase, the chromosomes have been duplicated and sister chromatids are held together by proteins called cohesins;
each pair of homologs associate along their length;
the DNA molecules of two nonsister chromatids are broken at precisely corresponding points;
the chromatin of the chromosomes starts to condense

Question 23

step 2 of crossing over

Answer 23

a zipper-like protein complex, the synaptonemal complex begins to form, attaching one homolog to the other;
chromatin continues to condense

Question 24

step 3 of crossing over

Answer 24

the synaptonemal complex is fully formed;
the two homologs are said to be in synapsis;
during synapsis, the DNA breaks are closed up when each broken end is joined to the corresponding segment of the nonsister chromatid, producing crossovers

Question 25

step 4 of crossing over

Answer 25

after the synaptonemal complex disassembles, the homologs move slightly apart from each other but remain attached because of sister chromatid cohesion, even though some of the DNA may no longer be attached to its original chromosome;
the points of attachments where crossovers have occurred show up as chiasmata;
the chromosomes continue to condense as they move toward the metaphase plate

Question 26

chiasmata

Answer 26

X-shaped region where crossing over occurs

Question 27

metaphase I

Answer 27

• chromosomes line up by homologous pairs
• independent assortment!

Question 28

anaphase I

Answer 28

the two homologous chromosomes of each pair separate

Question 29

telophase I and cytokinesis

Answer 29

two haploid cells form, each chromosome still consists of two sister chromatids

Question 30

meiosis II

Answer 30

separates sister chromatids

Question 31

prophase II

Answer 31

• spindle apparatus forms
• chromosomes are moved by microtubules toward the metaphase II plate

Question 32

metaphase II

Answer 32

• chromosomes positioned at plate - as in mitosis
• because of crossing over, two sister chromatids of each chromosome are not genetically identical

Question 33

anaphase II

Answer 33

breakdown of proteins holding the sister chromatids together allows them to separate and move toward opposite poles

Question 34

telophase II and cytokinesis

Answer 34

• nuclei form
• chromosomes begin decondensing
• cytokinesis occurs
• meiotic division of one parent cell produces four daughter cells, each with a haploid set of unduplicated chromosomes
• the four daughter cells are genetically distincs from one another and from the parent cell

Question 35

synapsis

Answer 35

the pairing of two chromosomes that occurs during meiosis; it allows matching-up ofhomologous pairsprior to their segregation, and possible chromosomal crossover between them (prophase I)