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Biology: GeneticsBiology: Genetics

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Autosomal (body cells) Autosomal (body cells)

DominantDominant InheritanceInheritance

� Dominant gene located on 1 of the “regular cells”

� Letters used are upper case… “BB or Bb”� Affected individuals have to carry at least 1

dominant gene (heterozygous or homozygous)� Passed onto males and females� Every person affected must have at least 1

parent with the trait� Does not skip generations� E.g. Huntington’s disease, Marfan syndrome

Autosomal Autosomal DominantDominant--

Marfan syndromeMarfan syndrome

�� Marfan syndromeMarfan syndrome (or Marfan's syndrome) is a genetic disorder of (or Marfan's syndrome) is a genetic disorder of the the connective tissueconnective tissue..

�� It is inherited as a It is inherited as a dominantdominant trait. It is carried by a gene called trait. It is carried by a gene called FBN1FBN1, which encodes a connective protein called , which encodes a connective protein called fibrillinfibrillin--1. 1. Because it is dominant, people who have inherited one affected Because it is dominant, people who have inherited one affected FBN1 gene from either parent will have Marfan's. Parents have a FBN1 gene from either parent will have Marfan's. Parents have a 50/50 chance of passing on the gene to their children.50/50 chance of passing on the gene to their children.

�� People with Marfan's are typically tall, with long People with Marfan's are typically tall, with long limbslimbs and long and long thin fingers.thin fingers.

�� The most serious complication is defects of the The most serious complication is defects of the heart valvesheart valves and and aortaaorta. It may also affect the . It may also affect the lungslungs, eyes, dural sac surrounding the , eyes, dural sac surrounding the spinal cordspinal cord, skeleton and , skeleton and hard palatehard palate..

Autosomal (body cells) Autosomal (body cells)

RecessiveRecessive InheritanceInheritance

� The recessive gene is located on 1 of the autosomes� Letters used are lower case…. “bb”� Unaffected parents (heterozygous) can produce

affected offspring (if they get both recessive genes ie homozygous)

� Inherited by both males and females� Can skip generations� If both parents have the trait then all offspring will also

have the trait. The parents are both homozygous.� E.g. cystic fibrosis, sickle cell anaemia, thalassemia

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�� ThalassemiaThalassemia (from (from GreekGreek θαλασσαθαλασσα, thalassa, , thalassa, sea + sea + αίµααίµα, haima, blood; British spelling, , haima, blood; British spelling, "thalassaemia") is an inherited "thalassaemia") is an inherited autosomal autosomal recessiverecessive blood diseaseblood disease. In thalassemia, the . In thalassemia, the genetic defect results in reduced rate of synthesis genetic defect results in reduced rate of synthesis of one of the of one of the globinglobin chains that make up chains that make up hemoglobinhemoglobin. This causes . This causes anemiaanemia. .

�� ThalassemiaThalassemia is a quantitative problem of too few is a quantitative problem of too few globins synthesized, whereas globins synthesized, whereas sicklesickle--cell anemiacell anemia(a (a hemoglobinopathyhemoglobinopathy) is a qualitative problem of ) is a qualitative problem of synthesis of an incorrectly functioning synthesis of an incorrectly functioning globinglobin. .

�� The disease is particularly prevalent among The disease is particularly prevalent among Mediterranean peoples.Mediterranean peoples.

Incomplete dominanceIncomplete dominance

� In a heterozygous organism, neither gene is dominant, both genes are expressed equally

� Capital letters used for both alleles� Snap dragons- red = RR, white= WW,

pink = RW� Cows- brown = BB, white= WW, roan=BW

Incomplete dominanceIncomplete dominance

CoCo-- dominancedominance

� Occurs when alternative alleles are present in the genotype and fully observed in the phenotype

� E.g. ABO blood grouping system, where a single gene locus features multiple alleles- IA, IB, and i. Individuals carrying alleles for both A and B express both in the phenotype AB.

CoCo-- dominancedominance

Oii

ABIAIB

BIB IB, or IBi

AIA IA or IAi

Phenotype (blood group)Genotype

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Sex linked inheritanceSex linked inheritance

� Genes are carried on the sex chromosomes (X or Y)

� Sex-linked notation� XBXB normal female� XBXb carrier female� XbXb affected female� XBY normal male� XbY affected male

Sex linked inheritance Sex linked inheritance

DominantDominant

� Dominant gene on X chromosome� Affected males pass to all daughters and

none of their sons � Genotype= X AY

� If the mother has an X- linked dominant trait and is homozygous (X AXA) all children will be affected

� If Mother heterozygous (X AXa) 50% chance of each child being affected

� E.g. dwarfism, rickets, brown teeth enamel.

Sex linked dominant Sex linked dominant

disordersdisordersDwarfism

Sex linked Inheritance Sex linked Inheritance

RecessiveRecessive

� Gene located on the X chromosome� More males than females affected (males inherit X

from mother)� Females can only inherit if the father is affected

and mother is a carrier (hetero) or affected (homo)� An affected female will pass the trait to all her s ons

� Daughters will be carriers if father is not affecte d� Males cannot be carriers (only have 1 X so either

affected or not)� Can skip generations� E.g. colour blindness, haemophilia, Duchene

muscular dystrophy

Sex linked Sex linked recessiverecessive

problemproblem� Red-green colour blindness in men is caused by the presence of a

sex-linked recessive gene c, whose normal allele is C.a) Can two colour blind parents produce a normal son?b) Nob) Can they produce a normal daughter?c) Noc) Can two normal parents produce a colourblind son or daughter?d) Son only…gets it from momd) Can a normal daughter have a colourblind father or mother?e) Yes… but not bothe) Can a colourblind daughter have a normal father or mother?f) Mother only

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Sex linked Inheritance in Sex linked Inheritance in

DrosophilaDrosophila

General PedigreeGeneral Pedigree Autosomal Autosomal DominantDominant

PedigreePedigree

Look for:� Trait in every

generation� Once leaves the

pedigree does not return

� Every person with the trait must have a parent with the trait

� Males and females equally affected

Autosomal dominant Autosomal dominant

pedigreepedigree

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Autosomal Autosomal RecessiveRecessive

PedigreePedigree

Look for:� Skips in generation� Unaffected parents can

have affected children� Affected person must

be homozygous� Males and females

affected equally

Autosomal Autosomal recessiverecessive

Sex linked Sex linked DominantDominant

pedigreespedigrees

Look for:� More males being affected� Affected males passing onto all

daughter (dominant) and none of his sons

� Every affected person must have an affected parent

Sex linked Sex linked recessiverecessive

pedigreespedigrees

� More ales being affected� Affected female will pass onto all her

sons� Affected male will pass to daughters

who will be a carrier (unless mother also affected)

� Unaffected father and carrier mother can produce affected sons

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Sex linked Sex linked recessiverecessive