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Inheritance

 

Topic 3
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loki) 
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The particular position of a gene on homologous chromosomes 
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Inheritance

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Biology SL 

Blood type  
➢ 4 possible phenotypes A, B, AB, 0 
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Three different alleles that can produce 6 different genotypes 

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The gene for the blood type is represented by the letter I (I and i) 

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I​A ​: allele for producing proteins called ​type A antigens 
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Gives type A blood 
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I​A​ I​A ​or I​A​i  

➢ I​B​: allele for producing proteins called ​type B​ ​antigens 
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Gives type B blood 
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I​B​I​B ​or I​B​i 

➢ i: recessive allele that does not produce neither A nor B antigens 
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Gives type 0 blood 
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ii  

➢ When I​ I​ , then type AB blood, since A and B are codominant 
A​ B ​

➢ “+” and “-” are controlled by another gene 
➢ Blood type determines the surface of the red blood cells 
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What carbohydrates there are on the surface 

 
Autosomal genetic disease 

➢ The locus of their gene is found on one of the first 22 pairs of chromosomes, and 
not on the sex chromosomes 
➢ Examples:  
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Albinism  
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Little or no pigmentation 

Cystic fibrosis  
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Affected individuals produce excessive amounts of mucus 

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Thalassemia  

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Phenylketonuria (PKU) 

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Tay Sachs disease 

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Sickle cell disease and trait 

➢ A few autosomal genetic diseases are also coded for by a dominant allele 
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Example: Huntington’s disease 
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Affected individuals suffer worsening involuntary movement and 
dementia 

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Disease breaks out at the age of 30-40 

 
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Inheritance

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4

Biology SL 

Mutations, genetic diseases and cancer 
➢ DNA is not supposed to be modified, but exposure to radiation or carcinogens can 
modify the code and cause serious health threats 
➢ Mutations can also occur due to an error in replication 
➢ Products with cancer-causing or mutagenic properties are restricted or forbidden 
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Joint effect:​ using/being exposed to many different things that cause 
mutations or cancer that are not dangerous on their own, but become when 
used together 

➢ Radiation knocks one or more base pairs out of place modifying the genetic code 
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Sometimes they can be benign, but others are harmful 

➢ Large amounts of radiation leaked in the Hiroshima bombing (1945), the Chernobyl 
(1986) and Fukushima (2011) incidents 
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Radiation can still be detected in those places 

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Nature seems to be doing fine, but there are animals and plants whose DNA 
has been modified 

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Cancer studies have shown an increase in cancer frequency in people who 
lived 30 km from the reactor 

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