Monohybrid Inheritance Section 11

Monohybrid Inheritance Gregor Mendel ( ): Gregor Mendel ( ): - Used varieties of pea plant - Used varieties of pea plant - Ensured large numbers were used - Ensured large numbers were used - Studied only 1 characteristic - Studied only 1 characteristic - Observed 3:1 phenotypic ratio - Observed 3:1 phenotypic ratio Principle of segregation: ‘ the alleles of a gene exist in pairs but when gametes are formed, the numbers of each pair pass into different gametes. Thus each gamete contains only one allele of each gene. Principle of segregation: ‘ the alleles of a gene exist in pairs but when gametes are formed, the numbers of each pair pass into different gametes. Thus each gamete contains only one allele of each gene.

Homozygous (TT) or (tt) – 2 of the same allele Homozygous (TT) or (tt) – 2 of the same allele Heterozygous (Tt) – 2 different alleles Heterozygous (Tt) – 2 different alleles

Law of probability Most monohybrid crosses give roughly 3:1 phenotypic ratio Most monohybrid crosses give roughly 3:1 phenotypic ratio 1 in 4 chance of a recessive allele being expressed 1 in 4 chance of a recessive allele being expressed 200 animals – roughly 150 have 1 phenotype, 50 will have another 200 animals – roughly 150 have 1 phenotype, 50 will have another Since fertilisation is random – rarely 3:1 exactly Since fertilisation is random – rarely 3:1 exactly

Monohybrid Inheritance in humans Family trees used to predict genetic trends Family trees used to predict genetic trends E.g. tongue rolling E.g. tongue rolling Rhesus D antigen: Rhesus D antigen: D-antigen – found on some blood cells D-antigen – found on some blood cells Present = Rhesus positive blood (Rh+) Present = Rhesus positive blood (Rh+) Absent = Rhesus negative (Rh-) Absent = Rhesus negative (Rh-) Rh- people produce anti-D antibodies –’sensitised’ Rh- people produce anti-D antibodies –’sensitised’ Antigen-D present (DD/Dd) Antigen-D present (DD/Dd) Antigen-D absent (dd) Antigen-D absent (dd)

Monohybrid Inheritance in humans Albinism: Albinism: Inability to make melanin – pigment for skin, eyes, hair etc. Inability to make melanin – pigment for skin, eyes, hair etc. Is a recessive trait Is a recessive trait AA or Aa = normal AA or Aa = normal aa = albino aa = albino Other examples Other examples - Cystic Fibrosis (mucus secretion) - Cystic Fibrosis (mucus secretion) - Phenylketonuria (PKU) - Phenylketonuria (PKU) - both are recessive traits - both are recessive traits

Monohybrid Inheritance in humans Huntingdon’s Chorea: Huntingdon’s Chorea: Determined by a dominant allele Determined by a dominant allele Only expressed after average of 38 years old Only expressed after average of 38 years old 50% chance of children of Huntingdon’s sufferer getting the disorder 50% chance of children of Huntingdon’s sufferer getting the disorder 1 in 20,000 cases 1 in 20,000 cases

Incomplete dominance – sickle cell anaemia Mutation of haemoglobin gene – haemoglobin S synthesised Mutation of haemoglobin gene – haemoglobin S synthesised Homozygous (SS) – Sickle cell anaemia Homozygous (SS) – Sickle cell anaemia - reduced oxygen carrying capacity - reduced oxygen carrying capacity - blood cells malformed (sickle- shaped) - blood cells malformed (sickle- shaped) Heterozygous – (HS) – Sickle cell trait Heterozygous – (HS) – Sickle cell trait - both types of haemoglobin present - both types of haemoglobin present - neither H or S are dominant (incomplete dominance) - neither H or S are dominant (incomplete dominance) - sickle cell = malaria resistance - sickle cell = malaria resistance

Incomplete / Co-Dominance Incomplete – both alleles expressed in the phenotype, but unevenly Incomplete – both alleles expressed in the phenotype, but unevenly - sickle-cell: more H than S expressed - sickle-cell: more H than S expressed Co-Dominance - both alleles equally expressed in the phenotype Co-Dominance - both alleles equally expressed in the phenotype - AB blood – exactly 50:50 split - AB blood – exactly 50:50 split - MN blood - another blood type antigen - MN blood - another blood type antigen - no antibodies produced - no antibodies produced - equal expression (equal dominance - equal expression (equal dominance

Multiple Alleles 3 or more alleles for a characteristic 3 or more alleles for a characteristic 3 alleles = 6 possible genotypes 3 alleles = 6 possible genotypes No. of phenotypes depend on the type of dominance No. of phenotypes depend on the type of dominance – complete, incomplete or co-dominance – complete, incomplete or co-dominance E.g ABO blood – 6 genotypes, 4 phenotypes E.g ABO blood – 6 genotypes, 4 phenotypes