D’YOUVILLE COLLEGE
BIOLOGY 102 - INTRODUCTORY BIOLOGY II
LECTURE # 21
BASIC PATTERNS OF INHERITANCE
1.
Heredity & Variation:
• genetics must account for faithful transmission of genetic information
(heredity) & departures from identical inheritance (variation)
• blending hypothesis: popular in early 19th century, the hypothesis
proposed a mixing of heritable traits from parents; could not account for why
population doesn't become uniform after many generations, nor why some traits
'skip' a generation
• particulate inheritance: current theory (established by Gregor Mendel - fig.
14 - 1 & ppt. 1) proposes that inherited traits are determined by discrete particles
(genes) that do not mix, but are maintained independently
- alternative expressions of a given gene are caused by mutations and are
known as 'alleles' (fig. 14 - 4 & ppt. 2)
• sexual exchange: shuffling of alleles resulting from chromosomal behavior
during meiosis is main source of variation
- crossing over extends variation possibilities & random mating multiplies
variant possibilities
- meiotic daughter cells differ from parental cell – receive only one
member of each chromosome pair (either a maternal or paternal homologue); this
reshuffling is called recombination, and is a powerful source of variation: e.g., for
species #, n = 2, number of possible gamete types is 4 (22) (fig. 13 – 10 & ppt. 3); for
species # = n, 2n gamete types are possible, e.g. in humans (n = 23), 223 = about 8
million types
- random fertilization (gamete combination): any of 8 million sperm types
may fertilize any of 8 million egg types: 64 trillion possible variants in offspring!
Bio 102
lec. 21 - p. 2
- crossing over extends the # of variant possibilities to much larger # (fig.
13 - 11 & ppt. 4)
Bio 102
lec. 21 - p. 3
2.
Mendelian Genetics:
• Mendel: conducted first systematic experiments on inheritance (with
meticulous mathematical analysis) using controlled matings: self fertilization or cross
fertilization (hybridization) between different true breeding varieties (P generation);
observed offspring of at least two successive generations (first filial, F1, and second
filial, F2 generations) (fig. 14 – 2 & ppt. 5)
- suggested particulate inheritance (gene idea)
- found seven true breeding characters showing pairs of contrasting traits
in pea plants (table 14 – 1, ppts. 6 & 7)
3.
Monohybrid crosses: demonstrated dominance in F1
• self crosses (F1 x F1) demonstrated reappearance of recessive trait in
offspring (F2)
- dominant to recessive ratio is 3 : 1 (fig. 14 – 3 & ppt. 8)
• genotype & phenotype:
- pair(s) of “factors” (genes) constitute genotype (alternative alleles)
- expressed trait for a given character = phenotype, e.g., for the character of
flower color, traits were purple and white
• homozygous & heterozygous:
- true breeders have two representatives of same allele (= homozygous); hybrids
have one representative of each of two different alleles (= heterozygous) (fig. 14 – 6 & ppt.
9)
Bio 102
lec. 21 - p. 4
• law of segregation of characters: each member of an allelic pair goes to a separate
gamete; correspondence with chromosome behavior at meiosis (fig. 14 – 5 & ppt. 10)
- laws of probability, e.g., coin flip ( fig. 14 - 9 & ppt. 11): probability of a
'head' = 1/2 = probability of a 'tail'
- product rule: probability of two events occurring together = their
individual probabilities multiplied together
- sum rule: if a given outcome can occur in more than one way, the
probability of the outcome is the sum of probabilities of each way, e.g., probability
of a 'head' & a 'tail' may be 1/4; however, probability of a 'tail' & a 'head' (reverse
order) is also 1/4; thus, the probability of a 'head' with a 'tail' in either order is 1/4 +
1/ = 1/
4
2
• confirmations of genotypes: F2 selfed: 25% breed true for dominant trait;
25% breed true for recessive trait; 50% show pattern of hybrid (heterozygous) F1
• test cross (unknown crossed with homozygous recessive) produces 1 : 1
ratio of dominant to recessive (fig. 14 - 7 & ppt. 12)
4.
Plasticity of dominance: dominance ranges from complete to incomplete to
codominance
• incomplete dominance: flower color in snapdragons (fig. 14 - 10 & ppt. 13)
Bio 102
lec. 21 - p. 5
• codominance: in M/N blood groups, heterozygotes have equal #s of M
molecules and of N molecules on their red cells
• level of phenotypic analysis: whether dominance is complete, incomplete,
or a case of codominance is contingent upon level of phenotypic analysis: e.g. TaySachs disease in humans:
- organismal level, homozygotes with 'dominant' allele & heterozygotes
are both disease-free (= complete dominance)
- biochemical level, homozygotes who are disease-free have complete
activity for a lipid-metabolizing enzyme, homozygotes who suffer from Tay-Sachs
disease have no activity for the enzyme, while heterozygotes have one-half the usual
activity (= incomplete dominance)
- molecular level, homozygotes have either the normal enzyme or the
faulty enzyme, while heterozygotes have equal amounts of both (= codominance)
5.
Multiple alleles: more than two alternative expressions for a single gene
• ABO blood groups have four main phenotypes: type A, type B, type AB, &
type O, based on pairs of three separate alleles (IA, IB, & i)
- IA& IB alleles are both completely dominant to i, but codominant to
each other (fig. 14 - 11 & ppt. 14)