Transcript Genetics and Heredity

Genetics
Gregor Mendel– the “father” of
genetics
A genetic characteristic
generally has two (or
more) possible
varieties– known as
traits.
Ex. Plant height: long or short. Flower color: purple or white
Along every chromosome, each
specific location contains a specific
gene for a specific characteristic.
Different varieties of each gene are
called alleles.
Genes exist in pairs, like
chromosomes. For every
characteristic, you have two genes
that determine how that
characteristic will appear.
The dominant allele of the gene almost always
completely masks the recessive allele of the
gene.
Genotype: the combination of genes that the
organism has
Phenotype: the actual appearance of the
organism
Heterozygous: when the two alleles are different
Homozygous: when the two alleles are the
same
If the pea plant is purple,
purple is dominant over white
in flower color.
Could be PP (homozygous
dominant) or Pp
(heterozygous).
If the pea plant is
white, it must be pp
(Homozygous
recessive).
Exceptions to “normal” scenarios of
inheritance
Incomplete dominance: in the heterozygous
genotype, the dominant allele does not
completely mask the recessive
Co-dominance: in the heterozygous genotype, two
alleles are visible in the phenotype at the same
time
Epistasis: one gene modifies the appearance of
another gene
Incomplete
dominance
Codominance
Sex-linked traits
• The gene is usually located on the xchromosome.
• Sex-linked traits are most common in
males.
• Usually passed from mother to son.
• Female carriers get one gene from either
mother or father.
• Examples: Colorblindness, hemophilia,
Duchenne Muscular Dystrophy
Alternative Patterns of Heredity
( in addition to incomplete dominance, codominance, and
multiple alleles)
• Epistasis– a gene at one locus alters the
expression of a separate gene
• Pleiotropy– a gene with multiple
phenotypic effects
• Polygenic inheritance– an additive effect
of two or more genes on a single
phenotype
epistasis
Fur pigment color
B= black (complete)
b= brown
Pigment deposition gene
C= pigment deposited
c= pigment NOT deposited
The pigment deposition
gene is epistatic to the
pigment color gene.
Pleiotropy
• Cystic fibrosis– one gene codes for a
defective protein in certain cell membranes;
causes multiple effects: thicker & stickier
mucus membranes, poor nutrient
absorption, chronic bronchitis, recurrent
bacterial infections
• Sickle-cell disease– one gene for a
single amino acid is altered in the
hemoglobin molecule; causes multiple
effects: blood clots, physical weakness,
pain, organ damage, paralysis, lowered
blood oxygen, brain damage
Polygenic inheritance (simplified)
3 pairs of genes
determine skin color
and the combination of
those genes determine
the phenotype. The
more dominant genes,
the darker the skin.
Sample question: describe the
pattern of inheritance of
Huntington’s disease.
• Dominant, autosomal, lethal (presents at
relatively older age, so inheritance is
possible in younger generations)
• Any child born to a parent with
Huntington’s has a 50% chance of
inheriting the gene