Chapter 18- Linkage and Extensions to Mendelian Inheritance

Explain the tenets of the Chromosomal Theory of Inheritance.

• genes have specific loci on chromosomes

• chromosomes undergo segregation and independent assortment

Describe Thomas Hunt Morgan’s experiments; recognize how to write fruit Aly genotypes; wild-type (w+) vs. mutant (w) - Fruit Aly phenotypes (eye color, body color, wing shape, etc.).

reasons for convenient:

• many offspring 

• generation can be bred every 2 weeks

• they only have 4 pairs of chromosomes

wild type- normal 

mutant- alternative traits to the wild type

wild type: w+

mutant: w

Work through X-linked traits and transmission to offspring; why X-linked phenotypes show more frequently in male offspring; females are often carriers of X-linked mutations but are unaffected themselves; explain why Y-linked transmission only affects males.

• X-linked traits are caused by genes on the X chromosome.

• Males (XY) inherit one X chromosome from their mother and one Y chromosome from their father.

• Females (XX) inherit one X chromosome from each parent.

• Males have only one X chromosome—if they inherit a defective gene, they express the trait because there is no second X to compensate.

• Females have two X chromosomes—a normal copy can often mask the effects of a mutated gene (for recessive traits).

Examples of X-linked recessive disorders:

• Hemophilia (blood clotting disorder)

• Duchenne muscular dystrophy (progressive muscle degeneration)

• Red-green color blindness

Be able to do a sex chromosome Punnett square and calculate the probability of offspring from a sex chromosome Punnett square.

Understand what it means for genes to be linked (same chromosome nearby or far away – how do they behave?); unlinked genes - genes on different chromosomes. How did Morgan’s test cross revealed that his Aly genes were linked?

linked genes: genes that are located on the same chromosome that tend to be inherited together 

unlinked genes: genes that are located on different chromosomes

• the farther apart two genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency. 

• physically linked but genetically unlinked, and behave as if found on different chromosomes

• genes that are on different chromosomes are unlinked and have a recombination frequency of 50%

• anything below 50% are linked genetically and physically 

Know Parental vs non-parental phenotypes/genotypes.

50% recombination – genes are on different chromosomes; less that 50% recombination – genes are on same chromosome; the lower the recombination frequency, the closer the genes are to each other (the harder it is to recombine them away from each other)

Be able to calculate percent recombination using numbers of parental individuals (nonrecombinants) and non-parental individuals (recombinants)

Example: the total number of recombinant flies divided by ALL the flies counted will tell you the recombination frequency between the two genes.

Understand how to read a pedigree and perform pedigree analysis to determine the genotypes of affected/unaffected individuals in the pedigree. Linkage in pedigrees: a gene is linked to an autosome or a sex chromosome; a gene can be dominant or recessive.

Know what a ‘carrier’ is (heterozygous).

heterozygous individuals who carry the recessive allele but are phenotypically normal. 

Review how to tell what type of transmission is occurring in a pedigree (dominant, recessive, X-linked, Y- linked, etc.).

dominant: affected individuals will have inherited the gene from at least one affected pattern

recessive: two normal heterozygous individuals will have inherited the gene from at least one affected pattern. 

x linked: traits are more common in males and transmission may appear to skip a generation through female carriers

y linked: traits appear to effect males in each generation. 

Distinguish complete dominance, codominance, incomplete dominance (Alower color, blood group alleles).

complete dominance: when phenotype of heterozygote are identical (PP or Pp are both purple flower)

codominance: two dominant alleles affect the phenotype in separate distinguished ways

incomplete dominance: the phenotype of F1 hybrids is somewhere between the phenotypes pf two parental varieties.

Define the following (pay attention to the examples I gave): pleiotropy, polygenic inheritance, epistasis, epigenetics (X-inactivation, imprinting, etc.), cytoplasmic/extranuclear inheritance.

pleiotropy: one gene has multiple phenotypic affects (sickle cell anemia)

polygenic inheritance: two or more genes influence a single phenotypic characteristic (skin color)

epistasis: a gene at one locus alters the phenotypic expression of a gene at a second locus. 

epigenetics: mechanisms that lead to changes in gene expression that can be passed from cell to cell and are reversible but do not involve a change in DNA. 

• x inactivation: in mammals females, one of the two x chromosomes in each cell is randomly deactivated  

• imprinting: DNA is inherited marked and maintained for the life of the organism 

extranuclear inheritance: 

cytoplasmic genomes: 

• derived from bacteria 

• contain housekeeping genes 

• replicate, translation and transcription 

• largely maternally inherited, some parental and some biparental