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Monohybrid Inheritance

Monohybrid Inheritance. Forms of a Gene. Genes control a characteristic e.g. hair colour. There are two forms of a gene called ALLELES e.g. brown, blonde. Dominant alleles mask the other allele and are always shown in the person’s phenotype. (BB or Bb)

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Monohybrid Inheritance

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  1. Monohybrid Inheritance

  2. Forms of a Gene • Genes control a characteristic e.g. hair colour. • There are two forms of a gene called ALLELES e.g. brown, blonde. • Dominant alleles mask the other allele and are always shown in the person’s phenotype. (BB or Bb) • Recessive alleles only show in the phenotype if both alleles are recessive. (bb)

  3. If the alleles are the same e.g BB or bb the gene is said to be homozygous or true breeding. • If the alleles are different e.g. Bb the gene is said to be heterozygous.

  4. Monohybrid Cross • Cross between two parents who possess different forms of a gene. • One characteristic is studied and the number of offspring is counted. • Mendel was a pioneer in this area. His most famous cross is that of pea plants.

  5. Use of Fruit Fly in Breeding Experiments • Short life-cycle • Vast number of offspring. • Characteristics used are usually wing-type and eye colour.

  6. Pedigree • This is when particular traits are traced through several generations of a family • It can be used to detect the probability of genetic disorders such as cystic fibrosis, PKU and albinism which are recessive (aa). • Huntington’s chorea is different in that the allele for this disease is dominant i.e. a person with Hh would suffer from the disease.

  7. Male affected Male unaffected Female unaffected Female affected

  8. Rhesus D-Antigen • In addition to the ABO blood group antigens, red blood cells can possess further antigens on their surface. • These are called ANTIGEN D. • People who possess the antigen are said to the RHESUS POSITIVE (Rh+) and those lacking the antigen RHESUS NEGATIVE (Rh-)

  9. Rh- people lack the antigen, but react to the antigen by forming anti-D antibodies. This would happen if Rh+ blood was transfused to a Rh- person. They are said to be SENSITISED. • A subsequent transfusion would result in agglutination because the antibodies formed would bind to the antigens.

  10. Having the antigen present is dominant (allele D). • Lacking the antigen is recessive (allele d) dd x DD dd x Dd F1 all Dd F1 Dd & dd Rh+ Rh+ & Rh-

  11. Incomplete Dominance • When one allele of a gene is not completely dominant over the other. • The phenotype can be different from the alleles e.g. sickle cell anaemia. Sickle shaped red blood cells

  12. Sickle Cell Anaemia • Mutation in the gene causes an unusual type of haemoglobin called haemoglobin S which is inefficient at carrying oxygen. • Homozygous SS suffer from the disease and will eventually die. • Heterozygous HS have the sickle cell trait where the mutant allele is partially expressed. • This is useful as they are resistant to malaria and can still do moderate activity. • See Torrance pg 83

  13. Co-dominance • Both alleles are expressed in the phenotype of the heterozygote. • Each is represented by an upper case letter e.g. MN

  14. Multiple Alleles • Gene possesses three or more different alleles for a characteristic e.g. ABO blood group. • Alleles A and B are dominant over O alleles, but are incompletely dominant over each other.

  15. ABO Blood Group

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