LECTURE
CONNECTIONS
5 | Extensions and Modifications of Basic
Principles
© 2009 W. H. Freeman and Company
Sex-influenced and sex-limited characteristics
• Sex-influenced characteristics are determined by
autosomal genes and are inherited according to
Mendel’s principles, but they are expressed differently in
males and females.
• The trait has higher penetrance in one of the sexes.
Example: the presence of a beard on some goats is
determined by an autosomal gene Bb that is dominant in
males and recessive in females.
Sex-influenced and sex-limited characteristics
• Sex-limited characteristics is encoded by autosomal
genes that are expressed in only one sex.
• The trait has zero penetrance in the other sex.
Example: In domestic chicken, some males display a pattern
called cock feathering; other display a pattern called hen
feathering.
Cock feathering is an autosomal recessive trait that is
sex limited to males.
Genotype
HH
Hh
hh
Male
Phenotype
Female
Phenotype
Sex-influenced and sex-limited characteristics
• Cytoplasmic inheritance: some characteristics are
encoded by genes located in the cytoplasm.
Example: Chloroplasts and mitochondria.
Mitochondria in humans:15,000 nucleotides that encode
37 genes
• For most organisms, all the cytoplasm is inherited from
the egg.
Sex-influenced and sex-limited characteristics
• Genetic maternal effect: the phenotype of the offspring
is determined by the genotype of the mother.
• The genes are inherit from both parents but the
offspring’s phenotype is determined not by its own
phenotype but by the genotype of the mother.
• The substances present in the cytoplasm of an egg are
pivotal in early development.
Example: shell coiling of the snail Limnaea peregra.
Sex-influenced and sex-limited characteristics
• Genomic imprinting: differential expression of genetic
material depending on whether it is inherited from the
male or female parent.
• Epigenetics: phenomena due to alterations to DNA that
do not include changes in the base sequence; often
affects the way in which the DNA sequences are
expressed (DNA methylation).
5.7 Anticipation Is the Stronger or Earlier
Expression of Traits in Succeeding Generations
• Anticipation: A genetic trait becomes more strongly
expressed or is expressed at an earlier stage as it is
passed from generation to generation.
Example: The mutation causing myotonic dystrophy consists
of an unstable region of DNA that can increase or decrease in
size as the gene is passed from generation to generation.
5.8 The Expression of a Genotype May Be
Influenced by Environmental Effects
• Temperature-sensitive allele: an allele whose product
is functional only at a certain temperature
In rabbits, the Himalayan allele is temperature dependent.