Polygenic inheritance of skin color
Three separately inherited genes affect the darkness of skin.
For each gene, an allele for dark skin is incompletely dominant to an allele for
Thus, an individual who is heterozygous for all three genes (AaBbCc) has
inherited three “units” of darkness
Human polygenic traits include
o Eye Color
o Skin Color
o Many forms of behavior
Pleiotropy is the effect of a single gene on more than one characteristic
The primary result of this gene is the production of defective feathers
Secondary results are both good and bad; good include increase adaptation to
warm temperatures, bad include increase metabolic rate, decreases egg-laying.
Sickle-cell anemia is a human disease originating in warm lowland tropical areas
where malaria is common
Sickle-cell individuals suffer from a number of problems, all of which are
pleiotropic effects of the sickle-cell allele
The chromosomal basis of Mendel’s laws
The arrangements of chromosomes at metaphase I of meiosis
and their movement during anaphase I account for the
segregation and independent assortment of the alleles for
seed color and shape
The alternatives are equally likely; F1 plants will produce
numbers of eight kinds of gametes.
In the F2 generation, we see the physical explanation for
Mendel’s 9:3:3:1 ratio of phenotypes
Finding the genes
Between 1884 (the year Mendel died) and 1888;
o Details of mitosis and meiosis were reported
o The cell nucleus was identified as the location of the
o “Qualities” were even proposed to be transmitted on
chromosomes to daughter cells at mitosis In 1903 Walter Sutton and Theodore Boveri formally proposed that
chromosomes contain the genes
The Chromosome Theory of Inheritance is one of the foundations of genetics and
explains the physical reality of Mendel’s principles of inheritance.
The location of many genes (Mendel’s factors) was determined by Thomas Hunt
Morgan and his coworker in the early 1990s.
Morgan experimental organism was the fruit fly
Fruit flies are ideal organisms for genetics, having a small size, ease of care,
susceptibility to mutation and short (7-9 day) generation time
The role of chromosomes in determination of sex was deduced by Morgan from
work on fruit flies
During Metaphase I, homologous chromosomes will line up.
o A karyotype can be made by cutting and arranging photomicrographs of
the homologous chromosomes thus revealed at Metaphase I
o Two types of chromosome pairs appear
Autosomes resemble each other in size and placement of centromere, for
example pars of chromosome 21 are the same size, while pairs of chromosome 9
are different size from pair 21
Sex Chromosomes may differ in their size, depending on the species of the
organism they are from.
In humans and fruit flies, males have smaller sex chromosomes, termed the Y,
and a larger one, termed the X.
Males are thus XY and termed heterogametic
Females are thus XX, and termed homogametic