2. Mendel’s laws apply to humans
Pedigrees can reveal the patterns of inheritance
• Autosomal recessive – 2 recessive alleles will
result in disorder
• Autosomal dominant – 1 or more dominant
alleles will result in disorder
• Pedigrees – used to determine if condition is
dominant or recessive
– Males – squares
– Females – circles
– Affected – shaded
– Carriers – appear normal but capable of having
children with genetic disorder
3. Some human genetic disorders are
autosomal recessive
• Tay Sachs Disease – causes severe
developmental problems and eventually death
• Cystic Fibrosis – excessive mucus in the lungs,
which frequently become infected. New
treatments have extended the lifespan to 35.
• PKU – affects nervous system development. A
special diet is required until the brain has fully
developed (age 7). Cannot process the amino
acid phenylalanine.
• Sickle-cell Disease – produced sickle shaped
blood cells.
4.
5. Some human genetic disorders are
autosomal dominant
• Neurofibromatosis – formation of tumors
• Huntington Disease – eventual loss of
body control during middle age, usually
after they have already had children.
Death comes 10-15 years after the onset
of symptoms.
• Achondroplasia – common form of
dwarfism. Homozygous dominant is lethal
and death occurs shortly after birth.
6.
7. Genetic disorders may now be
detected early on
• Testing after implantation
– Amniocentesis – genetic tests performed on fluids surrounding
fetus
– Chorionic villi sampling – suctioning fetal cells from uterus to use
for genetic tests
• Preimplantation genetic diagnosis – can test embryo or
egg
– Testing embryo – fertilization occurs in lab, one cell is removed
to test, only embryos that test negative for the genetic disease of
interest are placed in the uterus
– Testing egg – for heterozygous females, tests polar bodies
(meiosis in females results in one large egg and multiple small
polar bodies with little cytoplasm and haploid number of
chromosomes). If polar body has mutated allele, the egg does
not. Only normal eggs will be used for IVF. There are no
embryos being rejected if the egg is tested.
8. Complex inheritance patterns extend
the range of Mendelian genetics
Incomplete dominance still follows the
law of segregation
• Incomplete dominance –
heterozygous has an intermediate
phenotype
• R1R1 – double dose of pigment – red
• R1R2 – single dose of pigment – pink
• R2R2 – no pigment – white
• F1 all pink
• F2 1:2:1 ration red:pink:white
• They are not blending, the alleles
are still there in full.
9. A gene may have more than two
alleles
• Codominance – both alleles are fully
expressed
• IA – A antigens on red blood cells
• IB – B antigens on red blood cells
• i – neither A nor B antigens on red blood
cells
• IA IB – A and B antigens present on red
blood cells
10. Several genes and the environment
influence a multifactorial trait
• Polygenic inheritance
– trait is governed by
2 or more genes or
sets of alleles
• Each dominant allele
has a quantitative
effect on the
phenotype, and these
effects are additive.
11. • Multifactorial traits – controlled by
polygenes but are also subject to
environmental influenced
– Exposure to sun influences skin color
– Diet can alter risk for diabetes, heart disease,
and cancer
– Exposure to chemicals can alter risk for
cancer
12. One gene can influence several
characteristics
• Pleiotropy – genes have more than one effect
– Martan syndrome
• Abnormal connective tissues
• Results in individuals that have disproportionately long arms, legs, hands,
and feet, a weakened aorta, poor eyesight, and other characteristics
– Porphyria
• Photosensitivity, abdominal pain, paralysis in the arms and legs, epileptic
convulsions, bizarre behavior, and coma
• In the late 1700s and early 1800s, many members of the British royal
family had porphyria
– Sickle cell
• Anemia, weakness, poor circulation, kidney and heart failure and many
others
• Heterozygous individuals have an advantage against malaria since the
parasite cannot complete its life cycle when sickle-shaped cells form
13. The sex chromosomes also carry
genes
Traits transmitted via the X
chromosome have a unique
pattern of inheritance
• Red vs. white eyes in fruit flies
• F1 all were red eyed
– Red – dominant
– White – recessive
• F2 3:1 ratio, but all white eyes
were male
• Males always receive X
chromosome from mother and
Y chromosome from father
14. Humans have X-linked recessive
disorders
• Color blindness
– Affects ability to see certain colors
– Males are more often color blind than females
because males only have to receive one recessive
allele while females have to receive two.
• Muscular dystrophy
– Wasting away of muscles
– Males are rarely fathers since they die by the age of
20
– Alleles remain in the family, carrier mothers pass it on
to carrier daughters
• Hemophilia
– Affects ability to clot
