Transcript Going Beyond Mendel. Chapter 11

Going Beyond Mendel. Chapter 11
Introduction
• Sometimes one gene simply controls a one
trait (another reason why Mendel was
lucky—genotypes matched phenotypes)
• However, sometimes a genetic trait can be
controlled by:
–
–
a number of genes that interact with each other
with some being dominant over other and…
the “conditions” the gene is under to finally
produce the phenotype.
Dominance Relationships and Gene
Interactions
• There are various ways in which two alleles of a
gene can affect the phenotype.
– Often they are simply dominant or recessive
– Other times, there are situations where the
recessive allele may function a little or
interact with the dominant allele.
1. Incomplete dominance (partial
dominance)
• phenotype of a heterozygote is between the two
phenotypes of the two homozygotes
• Example
– cR cR = red flowers
– cW cW = white flowers
– cR cW = pink flowers
2. Codominance
• the equal expression of both alleles
resulting in a mixed phenotype.
• examples
– Cattle –
– Blood type/group –
3. Multiple alleles
• Himalayan Rabbit example
• Blood type/group example
4. Lethals
• alleles that cause the death of the organism
• recessive lethal alleles
– Corn:
– Huntington’s disease
5. Epistasis
• one gene interferes with the expression of
another gene. Fig 11.8 in text
• Example
B = black mouse
b = brown mouse
C = makes the pigment melanin
necessary for either black or brown
c = can’t make pigment necessary for
either color
Conditional Phenotypes
• How genes are expressed depends on the
conditions under which they exist
• The same genotype can result in different
phenotypes in:
– separate individuals
– different tissue of the same individual
Environment
Development
• As an organism grows from fertilized egg to a
mature organism, certain nerves and tissues
may or may not be “completed”
– incomplete penetrance – variable expressivity –
• Ex.) Polydactyly: tendency to have extra
fingers or toes
Sex
• a sex-limited or linked trait shows up in
only one sex or the other
– Ex.) uteran cancer.
• a sex-influenced trait can affect both sexes
but the effect is different.
– Ex.) middle-aged male go bald but women’s
hair only thins.
Age
• Some genetic traits can have variable
ages of onset
– Ex.) muscular dystrophy, Huntington’s
disease, and baldness.
Pleiotropy
• Example
– Phenylketonuria (PKU)
• Victims can’t metabolize a.a.
phenylalanine which then builds up and
leads to brain damage.
• Victims also have light hair and skin color.
Polygenetic inheritance
– Examples
• skin color:
• height:
• intelligence
• birth weight