2Dihybrid CrossesHave space provided for 4 gamete alleles from two parents.YYRR Pure DominantyyrrPureRecessiveY – yellow R – round seedsF1 – 100 % 4:0 dominant for both traitsPhenotype – 100% Yellow and RoundYRyrYyRr
3F1 Hybrid Cross YyRr YR Yr yR yr YYRR YYRr YyRR YyRr YYrr Yyrr yyRR Phenotypes: Yellow and Round 9Yellow and Wrinkled 3Green and Round 3Green and Wrinkled 1Phenotype 9:3:3:1 but each trait by itself is still 3:1YR 9:3 GR GR 3:1 GW
4Test CrossAn individual showing a dominant trait is crossed with a recessive.If any offspring show the recessive trait, the test individual must be a hybrid.This is done by breeders because it is not possible to tell from appearance alone whether an individual showing a dominant trait is pure for the trait (homozygous) or hybrid (heterozygous).Breeders of plants and animals often need this information about their parent stock (and keep detailed breeding records).
5Incomplete DominanceIn some organisms both alleles contribute to the phenotype of a heterozygous individual. This is also known as blending inheritance.In a hybrid you will get a new colour which is a blend of the dominant trait a recessive trait.
6Co-dominanceTwo dominant alleles are expressed at the same time. Because both alleles are expressed there is no blending of traits.RR- red shorthorn cattleWW – white shorthorn cattleRW – roan coat ( a mixture of both red hairs and white hairs).Capital letters with superscripts are often used to represent genotypes in co-dominance.CR red coat CW white coatCRCR homozygous red CWCW homozygous whiteCRCW roan coat
7Co-dominance occurs in human heredity Co-dominance occurs in human heredity. Sickle cell anemia is controlled by co-dominance.Blood type inheritance is another example od co-dominance.
8Multiple AllelesFor some traits more than two alleles exist in the species. These are referred to as multiple alleles.An individual cannot have more than two alleles for each trait, but different individuals can have different pairs of alleles when multiples exist.The alleles for human blood type is an example of multiple alleles.
9ABO Blood Groups There are four major types of human blood. There are antigens (a substance that can cause a response of the immune system) on the surface of red blood cells.1) A antigens (type A blood)2) B antigens (type B blood)3) Both a and B antigens (type AB blood)4) Neither A nor B antigens (type O blood)There are 3 alleles that control blood type: A,B and O.O Is recessive. A and B are both dominant over O but neither is dominant over the other making them co-dominant.
10The usual way to represent alleles in a multiple allele system is to use the capital letter I to represent a co-dominant allele and a lower case i to represent a recessive allele.A superscript letter then identifies each particularco-dominant allele.IA represents dominant allele AIB represents dominant allele Bi represents recessive allele OSince there are 3 alleles, there are 6 possible genotypes:1) IAIA ) IAIB ) IAi ) IBIB ) IBi ) ii
11The possible human phenotypes for blood group are type A, type B, type AB, and type O. Type A and B individuals can be either homozygous (IAIA or IBIB, respectively), or heterozygous (IAi or IBi, respectively).A woman with type A blood and a man with type B blood could potentially have offspring with which of the following blood types?
12Blood type and genotype The relationship between bloodtype (phenotype) and genotypeis shown in the table to the left.The mother (blood type A) andfather (blood type B) could beeither homozygous or heterozygous.
13Type A and Type B cross Four different genetic crosses are possible. All four crosses must beconsidered to determineall potential offspring.
14How are blood types related to the six genotypes? A blood test is used to determine whether the A and/or B characteristics are present in a blood sample. It is not possible to determine the exact genotype from a blood test result of either type A or type B.If someone has blood type A, they must have at least one copy of the A allele, but they could have two copies. Their genotype is either AA or AO. Similarly, someone who is blood type B could have a genotype of either BB or BO.Blood TypePossible GenotypesAAAAOBBBBO
15A blood test of either type AB or type O is more informative A blood test of either type AB or type O is more informative. Someone with blood type AB must have both the A and B alleles. The genotype must be AB. Someone with blood type O has neither the A nor the B allele. The genotype must be OO.Blood TypePossible GenotypesABOOO
16Blood TransfusionsRule: Match the antigen of the donor with the antibodies of the recipient.Blood TypeCan Donate ToCan Receive FromAA, ABA, OBB,ABB, OABAB, A, B, OOO, A, B, ABBlood TypeABABOIn Anti-A SerumclumpingNo clumpingIn Anti-B Serum