Chapter 4: part 2

1
Chapter 4 part 2
2
Modification from Mendel X linkage

• Y chromosome
• Has some male specific genes
• Lacks most genes on the X chromosome
• Males hemizygous
• Have only one X
• If get a mutant X, will be affected
• Mutant X gets passed from mother to son

3
Drosophila used to study genetics

• Red is dominant to white eye
• Get different phenotype if gene is in males or
  females
• females need 2 genes
• Males need one gene
• Percent of children with white eyes, depends on
  whether the mother or the father had white eyes

4
Different phenotype of offspring depending on
whether the mother or father had white eyes
5
Colorblindness in humans female passes the
trait to all of her sons

• Sons children are all normal
• Daughters children can be affected or normal
• Sons are colorblind
• Daughters are carriers

6
Modification from Mendel individuals sex
influences the phenotype

• Inheritance of autosomal genes modified by sex
  hormones
• Sex limited inheritance
• Only one sex can express the phenotype
• Sex influenced inheritance
• Expression of the phenotype depends on hormone
  constitution of the individual
• Example baldness females can have the
  phenotype but it is not as pronounced

7
Modification from Mendel penetrance and
expressivity

• Penetrance
• of individuals that show at least some degree
  of expression of a mutant genotype
• Expressivity
• Range of expression of the mutant genotype

8
Example of expressivity
9
Modification from Mendel genetic background

• Suppressor genes in Drosophila
• Influence other gene effects
• Position effects
• Physical location of a gene in relation to other
  genes may influence its expression
• Moving genes next to heterochromatin regions,
  inhibits expression of genes

10
Modification from Mendel Environmental influences

• Temperature affects phenotype

• Nose, ears, paws lower temperature, dark color
• Rest of the body higher temperature, lighter
  color

11
Temperature affects phenotype

• Conditional mutations
• Temperature sensitive
• Grow bacteria at one temperature or another to
  see mutation
• High temperature heat shock genes
• Protective function to heat stress

12
Nutrition

• Microorganisms
• Have mutations that prevent synthesis of nutrient
  molecules
• Example Leucine- mutants
• Mutation prevents growth of organisms

13
Nutrition

• Humans
• PKU cannot breakdown phenylalanine
• Screen infants if alter diet, alter phenotype
• Galactosemia cannot break down galactose
• Lactose intolerance

14
Modification from Mendel Onset of genetic
expression differs with different diseases

• Tay Sachs disease
• Autosomal recessive, normal at birth up to a few
  months, death by age 3
• Developmental retardation, paralysis, blindness
• Lesch-Nyhan Syndrome
• X-linked recessive, accumulation of uric acid
• Newborns normal for 6-8 months then symptoms
  develop
• Death by age 30

15
Modification from Mendel Onset of genetic
expression differs with different diseases

• Duchenne Muscular Dystrophy (DMD)
• X linked recessive
• Diagnosed 3-5 years of age
• Fatal in early 20s
• Huntingtons disease
• Autosomal dominant
• Onset variable 30-50 years of age
• Death about 20 years after onset

16
Modification from Mendel Genetic Anticipation

• Trinucleotide repeat diseases
• Huntingtons disease
• Fragile-X
• Kennedys Disease
• Myotonic dystrophy
• Increase generations, increase repeat size,
  increase severity and decrease age of onset

17
Modification from Mendel Genomic (parental)
imprinting

• Variation in phenotypic expression depending on
  whether the gene came from the mother or the
  father
• Imprint can be reversed when gene passed mother
  to son to granddaughter, etc.

18
Imprinting

• Example
• Igf2 insulin like growth factor 2
• Heterozygote phenotype influenced by imprinting
• Mouse normal in size if normal allele came from
  the father and dwarf if the normal allele came
  from the mother

19
Imprinting

• Humans 2 diseases, same chromosomal deletion
  (chromosome 15q1), different phenotype if get
  gene from father or mother
• Prader-Willi Syndrome (PWS)
• Get a deleted gene from father
• Angelman Syndrome (AS)
• Get deleted gene from mother