Download Ch.5

Ch.5
E.Baker
Genetics
Fall 2012
Circumstances that appear to contradict
Mendel’s law---although the law actually still
apply.
1)Lethal alleles-an allele that causes early death
• Some allele combinations cause such severe
problems in an embryo or fetus that
development stops
• Sometimes considered lethal if they cause death
before a person is old enough to reproduce
• Can cause miscarriages
• Sometimes a double dose of a dominant allele is
lethal
• Ex: Mexican hairless dogs
2)Multiple alleles-a gene that exists in more
than 2 allele forms, although a diploid
individual only has 1 or 2 of them.
• Although each person has 2 alleles for any
autosomal gene (one on each
chromosome) a gene can exist in more
than 2 allelic forms
• Ex: Blood Types
3)Co-dominance-alleles that are codominant to each other are both
expressed in a heterozygote, such as a
person who has AB blood type
4)Incomplete dominance-heterozygous
phenotype is intermediate b/t that of either
homozygotes. Ex: Snapdragons (red,
white, pink)
5)Epistasis-refers to one gene’s interfering with
the expression of a different gene.
Ex: Labrador Retrievers,Bombay Phenotype
6)Penetrance and Expressivity
Penetrance-refers to whether a phenotype is
expressed for a particular genotype
a.Completely penetrant-everyone who
inherits the combination has some symptoms
b.Incompletely penetrant-some individuals
do not express the phenotype or they have
no symptoms Ex: Polydactyly (having extra
fingers or toes)
Penetrance is expressed numerically as a
percentage
Expressivity-refers to the degree to which a phenotype is
expressed when it is expressed; intensity or severity of
symptoms (ex. Do they have extra fingers and/or toes on
1 foot, both feet, etc?).
7)Pleiotropic-refers to the appearance of several
apparently unrelated phenotypic effects caused by a
single gene
a. Marfan syndrome -autosomal dominant gene
produces a defect in an elastic connective tissue.
b.Porphyria-inherited as a dominant trait that leads to
intermittent attacks of pain and dementia with symptoms
first appearing in adulthood.
c.Sickle cell anemia -caused by double dose of the
recessive allele
8)Phenocopy-an environmentally caused trait that appears
to be inherited, the trait either resembles a Mendelian
disorder or mimics inheritance by occurring in certain
relatives. Ex: children with AIDS
9)Genetic Heterogeneity-occurs when
different genes cause the same
phenotype, different genes can produce
phenotypes that are clinically
indistinguishable.
Ex: 132 forms of deafness
When Mendel’s Laws do NOT apply.
1)
2)
Mitochondrial genes & maternal inheritancemitochondrial genes are maternally inherited b/c only
females transmit them; Ex: mitochondrial illnesses
tend to affect cells w/ an abundance of mitochondria
(such as muscle cells)
Uniparental Disomy-rare inheritance of a double dose
of genetic material from 1 parent but none from the
other; the term literally means “2 bodies from one
parent”; caused by nondisjunction during meiosis II of
both parents.
Genomic imprinting-a phenotype that is different
depending upon the sex of the parent who passes it
on.
Ex: a) Prader-Willi syndrome- child is obese, small hands & feet,
does not develop signs of puberty (double dose of mom’s DNA or
missing DNA in 1 parent’s gene to be expressed)
b) Angelman’s syndrome- child has extended tongue, poor
muscle coordination, large jaw, laughs uncontrollably, & arm
flapping (double dose of dad’s DNA).
3)
Linkage- refers to transmission of genes on
the same chromosome; linked genes do not
assort independently and do not result in
predicted Mendelian ratios for 2 or more genes;
the closer 2 genes are on a chromosome, the
greater the probability of them being inherited
together, the farther apart 2 genes are on a
chromosome, the greater the probability that
crossing over will occur b/t them.