Genes on the X Chromosome:
*1909 Thomas Hunt Morgan
II
III
XY
IV
or
XX
*Sex chromosomes
*Autosomes
Example: In Drosophila and all mammals
sex chromosomes designated as X and Y
XX=female
XY=male
Modifications of Mendelian Ratios
X-linkage - transmission and expression of genes on the X chromosome
X-linkage in Drosophila:
white mutation (eyes)
sex-linkage - transmission and
expression of genes on sex
chromosomes
Sex-limited Inheritance:
*Sex-limited trait – trait that is expressed in only one sex even though
the trait may not be X-linked.
*holandric genes: genes on the Y chromosome
Example: hypertrichosis (excess ear hair)
*autosomal genes
Example: milk production in mammals; L=lots, l=little
Sex-influenced Inheritance:
*Sex-influenced trait – phenotypic expression that is conditioned by the
sex of the individual
Examples:
*cleft palate in humans
*horns in sheep
*pattern baldness in humans
Summary:
Sex-linked: on X or Y sex-chromosome
Sex-limited: all or none expression by sex
Sex-influenced: genotype + sex determines phenotype
Phenotypic Expression:
Gene expression often governed by genotype and environment
*Penetrance – frequency with which individuals with a given genotype
exhibit some degree of a phenotype associated with that trait.
If 9/10 of individuals
carrying an allele
express the trait, the
trait is said to be 90%
penetrant
*Expressivity – the degree or
range in which a phenotype for a
given trait is expressed
*Temperature
*Onset of genetic expression
Chapter 7: Sex Determination
Life cycles and reproductive modes
*Asexual reproduction – formation of new individuals from the
cell(s) of a single parent.
*Sexual reproduction – reproduction through the fusion of gametes.
*Alternation of generations – alternation of forms or of mode of
reproduction in the life cycle of an organism.
Some additional terms:
*Primary sexual differentiation
*Secondary sexual differentiation
*Unisexual, dioecious and gonochoric
*Bisexual, monoecious and hermaphroditic
Chlamydomonas
Caenorhabditis elegans (C. elegans)
X and Y
Chromosomes:
Early Studies
1906 Edmund B. Wilson
and Protenor
X and Y
Chromosomes:
Early Studies
Wilson and Lygaeus
X and Y Chromosomes: Early Studies
Homogametic sex – sex which produces uniform gametes with
respect to sex chromosomes.
Heterogametic sex – sex which produces gametes with unlike sex
chromosomes.
*The male is not always the heterogametic sex…
Examples:
Moths, butterflies, most birds, some fish, reptiles, amphibians, at
least one species of plants
Chromosome Composition in Humans
Karyotype
XX
XY