ch.5 beyond mendel’s laws 5.1. lethal alleles lethal genotypes- death before alleles are passed...
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Lethal Alleles Lethal genotypes- death before alleles are
passed on. Before reproduction. Exception- Huntington’s disease-
Begins at age 30-40. Lethal alleles- spontaneous abortions If both parents are heterozygous-
25 % chance of being homozygous recessive. Homozygous dominant- lethal in embryos
Ex. Mexican hairless dogs.
Incomplete Dominance
Heterozygous phenotype is intermediate between each homozygote.
Human Ex. TaySachs- intermediate enzyme level in
heterozygotes. Human hair curliness
SS=straight, ss=curly, Ss=wavy FH-Familial hypercholesterolemia
A heterozygote has half the number or receptors for LDL cholesterol in the liver with intermediate plasma cholesterol levels.
Classic Snapdragons use C= color
RR= Red CRCR rr= white(WW); CWCW Rr (RW)= pink; CRCW
P1 CRCR x CWCWF1 CRCW x CRCWF2 CRCR CRCW CWCW 1:2:1
red pink white Same genotypic and phenotypic
ratios
Codominance: Both alleles are experessed in a
heterozygote. AB Blood Type ABO Blood types:
Determined by cell surface antigens Antigen- protein with attached sugar on plasma
membrane of red blood cells Type A- A antigen Type B- B antigen Type O- no sugar antigen Type AB- both A and B antigens
Codominance examples-
Both alleles are expressed equally Example-
is red and white striped flowers Roan cattle
Multiple alleles 1 gene; more than 2 allele forms Examples-
Acyl CoA dehydrogenase deficiency Mitochondria- affects heart and skeletla
muscles. Other alleles lessen severity producing some
enzyme. PKU- buildup of phenylalanine
More than 300 mutant alleles-4 phenotypes Classic PKU- profound mental retardation Moderate PKU Mild PKU Excrete excess phenylalanine in urine Correlation of genotypes with phenotypes – to
monitor diet and predict outcome.
Cystic Fibrosis-
Several hundred alleles 70% of cases- F508 homozygous Many variations Pancreatic function correlates better
than lung symptoms.
Blood Typing: ABO blood group:
Antigens are on the rbcs. Antibodies to other antigen sugar types circulate in
the plasma portion of blood. Blood Type A- has anti B in plasma. Blood Type B- has anti A in plasma Blood Type O- has anti A,B in plasma Blood type AB- no anti A or anti B
Blood Type O- universal donor because there are no A or B antigens.
Blood type AB- universal recipient, since no antibodies to A or B.
To type a person’s blood- the rbcs are mixed with different antibody sera. Clumping indicates the presence of the corresponding antigen on the red blood cells. Ex Antigen A clumps with antiA sera.
Rh Factor- Named for rhesus monkeys where found. The main factor is a D antigen found on the red blood
cells. Unlike ABO blood types, antibodies are only present if
an Rh negative person is exposed to Rh positive blood cells.
85 % pop. Has the D allele=Rh + 15% pop. Is recessive dd= Rh- Rhogam- anti D is given to Rh- mothers to bind any
fetal Rh + red blood cells if there is a bleed to prevent the mom from making circulating antibodies to the fetus’ red blood cells.
Hemolytic Disease of the newborn results from this imcompatibility. – can result in death.
EpistasisOne gene affects the expression of a second gene
Example: H gene is epistatic to the ABO gene.• H protein attaches a molecule to the cell surface
to which the A or B antigens are attached• hh genotype = no H protein= • Without H protein the A or B antigens can not be
attached to the cell• All hh genotypes have the phenotype of type O,
although the ABO blood group can be anything (A, B, AB, or O)
Bombay phenotype-identified in 1952- Bombay, India Very rare example of epistasis Individuals phenotype= Blood type O However they have no H antigen so they
have antibodies to H (anti-H), antibodies to A (anti-A) antibodies to B (anti-B) in their circulating plasma.
Bottom line- they will be incompatible with all blood types; even O
Can only receive their own blood; or another Bombay’s.
Incomplete Penetrance The phenotype is not always observed
among individuals carrying the genotype – express or not DD or Dd - only 80% show polydactyly
Variable Expressivity
• A phenotype that varies in intensity Polydactyly two extra digits on each hand and foot vs. one extra digit on one foot Individuals with the same genotype for familial hypercholesterolemia have varying levels of symptoms
PleiotropyOne gene has many
symptoms or controls several functionsExample: porphyria
variegata
Photo © North Wind Picture Archives
Figure 5.5a
Figure 5.5b
Genetic Heterogeneity
• Different genes can produce identical phenotypes
Hearing loss Osteogenesis imperfecta
• Genes may encode for different enzymes in a biochemical pathway
Clotting disorders
Phenocopy Appears inherited but is caused by the
environment May have symptoms that resemble an inherited
trait or occur within familiesExamples: Exposure to teratogens
Thalidomide causes limb defects similar to inherited phocomelia
Hydroquinone exposure looks like alkaptonuria
Infection AIDS virus can be passed from mother to
child, looking like it is inherited
Importance of Human Genome Sequence
Complications to Mendelian inheritance more common than originally thought
Overlapping of definitions – Marfan syndrome has both epistasis and genetic heterogeneity
Mitochondrion Organelle providing cellular energy Contains small circular DNA No crossing over or DNA repair High exposure to free radicals Mutation rate is greater than nuclear DNA 37 genes without noncoding sequences Mitochondrial genes are transmitted from
mother to all of her offspring
Mitochondrial Disorder Examples
Mitochondrial myopathies – weak muscles
Leber optic atrophy – impairs vision
Ooplasmic transfer technique can enable woman to avoid transmitting a mitochondrial disorder
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