CH 11: Genetics (p.262)
Date:
11-4: Meiosis (p.275)
 Chromosome Number
o homologous chromosomes: pair of corresponding chr. from each parent
 carry alleles (dif. forms of genes) for same traits
o diploid cell: contains both sets of homologous chr. (1 set from each parent)
 represented as 2N (for humans, 2N = 46)
o gamete: sex cell (sperm or egg)
 haploid: contains only 1 set of chr. ( ½ the # in body cells)
 represented as N (for humans, N = 23)


Meiosis: process in which homologous chr. separate, cutting the # of chr. in cell in ½ (fig 11-15)
o
Meiosis I
 homologous chr. pair up to form tetrads during prophase I
 crossing-over occurs when hom. chr. exchange portions of their chromatids (fig 11-16)
 recombination of alleles
 hom. chr. line up in metaphase I & separate to form 2 haploid (N) cells
 sister chromatids do NOT separate like in mitosis
o
Meiosis II
 each new cell divides
 sister chromatids of each chromosome separate during anaphase II
 results in 4 haploid (N) daughter cells (NOT identical!)
Gamete Formation (sexual reproduction)
o sperm (male) & eggs (female) are produced thru meiosis
o fertilization occurs when a sperm & egg join (N + N = 2N)
o a fertilized egg (2N) is called a zygote
 contains 1 set of chromosomes from each parent
Date:
11-1: The Work of Gregor Mendel (p.263)
* genetics: study of heredity



Gregor Mendel
o pea flowers fertilize themselves (self-pollination)  offspring are identical to parent
 called true-breeding plants
o crossed dif. true-breeding plants for his expts
 offspring have 2 parents
Genes & Dominance
o trait: a specific characteristic that varies from 1 individual to another
o P generation: original plants
o F1 “
” : offspring of P gen.
 offspring of cross btwn 2 parents w/ dif traits are called hybrids
 results: F1 plants were all the same
o led to 2 conclusions: (fig 11-3)
1. genes (DNA sequences that determine traits) are passed from 1 gen. to the next
2. principle of dominance: the dominant allele will always mask the expression of the recessive allele for a
given trait
Segregation (fig 11-4)
o Mendel allowed the F1 plants to self-pollinate & produce the F2 offspring
o ¼ of the plants showed recessive traits
 segregation (separation) of alleles occurred during gamete formation
 each gamete carries ONE allele for each trait
Date:
11-2: Probability & Punnett Squares (p.267)
*probability: the likelihood that a particular event will occur
 used to predict outcome of genetic crosses

Punnett Squares: predict gene combinations from a cross (fig 11-7)
o homozygous: having 2 of the same alleles for a trait
 true-breeding for that trait
 Ex: TT, tt
o heterozygous: having 2 dif. alleles for a trait
 hybrid for that trait
 Ex: Tt
o genotype: genetic makeup (Ex. TT, Tt, tt)
o phenotype: physical characteristic (Ex. tall, short)
* probability can predict average outcome, but not precise outcome of events
Date:
11-3: Independent Assortment (p.270)
 Two-Factor Cross: F1 (fig 11-9)
 Two-Factor Cross: F2 (fig 11-10)
* results of expt  principle of independent assortment: genes for dif. traits segregate independently during meiosis
 …unless they are on the same chromosome!
 chromosomes actually assort independently, not individual genes
 crossing over can separate genes on same chromosome