Download - 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
• ExampleB = black mouseb = brown mouseC = makes the pigment melanin necessary for either black or brownc = 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