Meiosis, peas and sex Flashcards
character
heritable feature varying among individuals (e.g. color)
hybridisation
crossing of two true-breeding varieties
P generation
true breeding parents (parental generation)
F1 generation
1st filial generation
F2 generation
2nd filial generation
Mendel’s model - 1st law
alternative versions of genes (alleles) account for variations in inherited characters
Mendel’s model - 2nd law
for each character, an organism inherits two versions (alleles) of a gene, one from each parent
Mendel’s model - 3rd law - law of dominance
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
Mendel’s model - 4th law - the law of segregation
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
homozygote
organism that has a pair of identical alleles for a gene encoding a character
heterozygote
organism that has two different alleles for a gene
alleles
alternative versions of a gene
phenotype
an organism’s appereance or observable traits (PHYSIOLOGICAL)
genotype
an organism’s genetic makeup
law of (in)dependent assortment
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)
fertilisation
the union of gametes, culminating in fusion of their nuclei
zygote
fertilized egg (is diploid, because it contains two haploid sets of chromosomes)
meiosis
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)
meiosis I
separates homologous chromosomes
prophase I
- duplicated homologous chromosomes pair up and exchange segments
- centrosome movement, spindle formation and nuclear envelope breakdown - as in mitosis
crossing over
DNA molecules from nonsister chromatids are broken by proteins and rejoined to each other
step 1 of crossing over
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
step 2 of crossing over
a zipper-like protein complex, the synaptonemal complex begins to form, attaching one homolog to the other;
chromatin continues to condense
step 3 of crossing over
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