D’YOUVILLE COLLEGE
BIOLOGY 102 - INTRODUCTORY BIOLOGY II
LECTURE # 22
MENDELIAN & RELATED PATTERNS OF INHERITANCE
4.
Dihybrid crosses: 9: 3: 3: 1 ratio (fig. 14 – 8 & ppt. 1)
• law of independent assortment: each pair of alleles segregates independently
of the other pair in gamete formation; distribution of alleles (in Mendel's crosses)
parallels distribution of chromosomes in meiosis (fig. 15 - 2 & ppt. 2)
• review of concepts of probability:
- product law: probability (P) of two independent events occurring
together is the product of the probability of one multiplied by the probability of the
other (P1 x P2)
- sum law: probability of two events occurring together when there is more
than one way in which the two events can combine, is the sum of the probabilities of
all the ways they can combine (P1 + P2 + ... + Pn)
- Punnett square or forked line methods can analyze outcomes of crosses
5.
Extensions of Mendelian genetics:
• pleiotropy: genes may affect more than a single character in the
phenotype; the syndrome of afflictions in sickle anemia is an example: one mutant
allele can alter many aspects of the phenotype (fig. 5 - 21 & ppts. 3 & 4)
• traits governed by more than one allelic pair: many phenotypic traits are
the end product of a series of enzyme-catalyzed reactions; thus many genes (one for
each enzyme) are often involved in expressing a particular phenotype, e.g., seed
color in corn
• collaborative interaction: involves two gene loci that both contribute to
same phenotypic character, e.g. chicken combs
Bio 102, spr. 13
lec. 22 - p. 2
• epistasis: suppression of one gene's action by another gene's action (2
entirely different genes), e.g. coat color in rodents (fig. 14 – 12 & ppt. 5), also corn seed
color
• modifier genes: control pattern of color expression; e.g., spotting of coat in
beagles, human eye color
• polygenes: act in large numbers contributing equally to phenotypic traits which
show almost continuous variation between extremes (fig. 14 – 13 & ppt. 6); likely
basis of most continuously variable traits like height, skin color, etc.
• environmental influences: temperature & pH may affect gene expression;
e.g., Himalayan rabbit’s black fur; hydrangea flower color (fig. 14 - 14 & ppt. 7)