BILD 1 Chapter Notes - Chapter 14: Gynoecium, Allele, Chromosome
18 Dec 2018
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14.1 Mendel used the scientific approach to identify two laws of inheritance
● Blending hypothesis: genetic material contributed by two parent mixes
○ Over many generations → freely mating population will give rise to uniform
population of individuals
Mendel’s Experimental, Quantitative Approach
● Why work with peas
○ Many varieties
○ Short generation time
○ Large number of offspring from each mating
○ Strictly control mating between plants
■ Pollen producing organs (stamens)
■ Egg bearing organ (carpel)
● In nature is self pollination → mendel dusted pollen instead
○ Developed into pea
● Character: heritable feature that varies among individuals
● Trait: each variant for a character
● True breeding: over many generations of self pollination → plants produced only
same variety as parent plant
○ Mendel cross pollinated two contrasting, true breeding pea plants
● Hybridization: mating or crossing of two true breeding varieties
● P generation: parents generation
● F1 generation: hybrid offspring
○ Allowed to self pollinate → produce F2 generation
The Law of Segregation
● If blending model of inheritance was correct → F1 hybrids would have produced pale
purple flowers → as a result all F1 offspring were purple and only in F2 generation
did white flowers come out
○ Mendel reasoned that white flower trait was hidden
○ Purple = dominant, white = recessive
● Crossed true breeding variety → smooth & round vs wrinkled = F1 hybrids produced
round seed
○ In the F2 generation → 3:1 with 1 being wrinkled
Mendel’s Model
● Alternative version of genes account for variations in inherited characters
● Alleles: alternative versions of a gene
○ Gene is a sequence of nucleotides on a particular part of a chromosome →
DNA at the locus can vary → which will affect function of protein
● For each character → an organism inherits two copies of a gene → one from each
parent
○ Somatic cell in diploid organism has two sets of chromosomes → inherited
from each parent
● Two alleles at locus differ then dominant allele determines organism’s appearance;
recessive allele has no noticeable effect on organism’s appearance
● Law of segregation: two alleles for a heritable character segregated during gamete
formation and end up in different gametes
○ Egg or sperm gets only one of the two alleles that are present in somatic cells of
organism making the gamete
○ Two members of pair of homologous chromosomes to different gametes in
meiosis
○ Organism has identical alleles for particular character → allele is present in all
gametes
● Two different alleles present in F1 indi will segregate into gametes such that half the
gametes will have purple and have will have white
● Self pollination → gametes are random
● Punnett square
Useful Genetic Vocabulary
● Homozygote: organism that has a pair of identical alleles for a gene encoding a character
○ Homozygous for that gene
○ Breed true because gametes contain same allele
● Heterozygote: organism that has two different alleles for a gene
○ Heterozygous for that gene
○ Produce gametes with diff alleles; not true breeding
● Phenotype: appearance or observable traits
● Genotype: genetic makeup
Test Cross
● Cross the two parent plants → each unknown genotype will determine the
appearance of offspring
○ Offspring of a Pp x pp will be expected to have 1:1 ratio
● Testcross: breeding organism of unknown genotype with recessive homozygote; reveal
genotype of that organism
The Law of Indi Assortment
● Monohybrids: all F1 progeny produced in test crosses of true breeding parents →
heterozygous for one particular character being followed in cross
○ Monohybrid cross
● Crossed plants with YYRR and yyrr
Document Summary
14. 1 mendel used the scientific approach to identify two laws of inheritance. Blending hypothesis: genetic material contributed by two parent mixes. Over many generations freely mating population will give rise to uniform population of individuals. Large number of offspring from each mating. In nature is self pollination mendel dusted pollen instead. Character: heritable feature that varies among individuals. True breeding: over many generations of self pollination plants produced only same variety as parent plant. Mendel cross pollinated two contrasting, true breeding pea plants. Hybridization: mating or crossing of two true breeding varieties. Allowed to self pollinate produce f2 generation. If blending model of inheritance was correct f1 hybrids would have produced pale purple flowers as a result all f1 offspring were purple and only in f2 generation did white flowers come out. Mendel reasoned that white flower trait was hidden. Crossed true breeding variety smooth & round vs wrinkled = f1 hybrids produced round seed.
