Chapter 9 Part 2

Mendel’s laws apply to humansPedigrees 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

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.

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.

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.

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.

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

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.

• 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

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

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

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