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Extensions of Mendelian Genetics

Mendelian genetics seems to be relevant to only a small set of heritable features For only a few characters there are…

Only 2 versions of an allele (green or yellow) 1 gene codes for a single external character 1 allele is completely dominant to the other

The basic patterns of segregation & independent assortment apply to more complex patterns of inheritance

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Different Types of Dominance

Complete dominance Phenotype of heterozygote & HomoD are

indistinguishable The pattern with which you are already familiar

Incomplete dominance Phenotype of heterozygote is in between the 2 Homo

phenotypes Example: pink snapdragons

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Snapdragons – Incomplete Dominance

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So incomplete dominance does NOT provide evidence for “blending” theories

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Codominance

Codominance Phenotype of

heterozygote is separate & distinguishable from Homozygous Dominant & Homozygous Recessive

Example: AB blood type or Rhododendron flower

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Dominance & Phenotype

The observed dominance/recessiveness of alleles depends on the level of the investigation

Consider Tay-Sachs disease Brain cells of the baby do not metabolize certain lipids As lipids accumulate, seizures, blindness, and mental

degeneration Death occurs within a few years of conception

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Tay-Sachs Disease

At the Organismal level, the disease is recessive Only children with 2 copies of the recessive trait will have the

malady

Heterozygote is not afflicted – they produce some lipid-metabolizing enzyme, though not as much as in HomoD So intermediate enzyme production

This suggests that @ the biochemical level, the disease is an example of incomplete dominance

Which is Tay-Sachs: dominance or incomplete dominance?

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Prevalence & Dominance

Polydactyly Extra fingers or toes

1 of 400 in the US

The allele for polydactyly is dominant, but rarely present

Recessive homozygotes (HomoR) are found 399 out of 400 instances

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Blood Typing

Only 2 alleles existed for Mendel’s peas, but this is not typical for most traits

Consider ABO blood group in humans A refers to the “A” membrane carbohydrate & type A

blood B refers to (seriously, I’m not writing this down) O means neither A or B carbohydrate is found AB means both A & B are found

BUT the A and B alleles are codominant and are both expressed if an individual inherits both alleles

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Epistasis

A gene at one locus alters the phenotypic expression of a gene at a second locus

Example: Mouse fur color Bb or BB = Black bb = brown

If HomoR for (C) gene [cc], then no fur color (albino or white) Regardless of fur color specified by brown-black gene

If NOT HomoR for (c) gene [Cc or CC], then can be brown (bb) or black (Bb or BB)

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What is the phenotype of…

BBcc?

BbCc?

bbCC?

Bbcc?

BBCC?

bbcc?

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Pleiotrophy

Single gene has multiple effects

Should be unsurprising given intricate molecular and cellular interactions for development of an organism

Phenylketonuria Mental Retardation Reduced skin and hair pigmentation

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Polygenic Inheritance

The additive effect of 2 or more genes on 1 phenotypic character Called quantitative characters since there is a continuum of gradations Normal curve of phenotypes Example: human skin pigmentation is determined by at least 3 separately inherited genes

AABBCC = Dark AaBbCc = Intermediate aabbcc = Light

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Pedigree Analysis

Family tree describing the interrelationships of parents & children across the generations

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Recessively Inherited Disease

Requires 2 copies of the recessive allele (Homozygous Recessive) to express the mality

Heterozygotes are called carriers Normal phenotype, but may transmit disease to

offspring

Examples: Cystic Fibrosis Tay Sachs disease Sickle-cell disease

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Cystic Fibrosis

Recessive autosomal disease

Common in those of European descent

1 of 2,500 affected, but 1 of 25 are carriers

Affects Chloride ion transport between a cell and extracellular fluid

If untreated, most die before 5th birthday Typically, patients live until their 20s or 30s with

efficacious treatment

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Sickle-Cell Anemia Recessive autosomal disease African descent Affects Hemoglobin protein in RBCs Low blood oxygen = hemoglobin molecules

clump together forming sickle shaped RBCs Sickle-celled RBCs clump together creating

chronic vascular occlusion of small vessels Example of incomplete dominance

Heterozygotes are usually normal but will show some symptoms during prolonged periods of reduced blood oxygenation

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Dominant Alleles

Most harmful alleles are recessive, but some human diseases are due to dominant alleles

Only require one copy of the allele to be expressed Hypothesis: if there is a lethal disease carried on a

dominant allele, it would have burned out its carriers by now.

UNLESS, the lethal disease carried by a dominant allele is one that affects organisms of advanced age Like Huntington’s disease

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Examples of Dominant Allele Disease

Achondroplasia – form of dwarfism Heterozygous individual = dwarf 1 in 25,000 have achondroplasia, so 99% of the

population are HomoR

Huntington’s disease Caused by a lethal dominant allele Degenerative disease of nervous system Usually only affects those > 40 yrs old

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Genetic Testing

Pedigree analysis gives some info about risk to offspring There are also tests to identify carriers of certain

genetic diseases Amniocentesis – amniotic fluid is removed and then

cells contained in the fluid are cultured to identify certain chromosomal defects via karyotype

Chorionic villus sampling (CVS) – placental tissue is removed for same purpose as amnio, but results are available far faster, & can be performed earlier in pregnancy Karyotype can be immediately obtained

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