Methods of Inheritance

The Different Forms of Inheritance Mendel’s Inheritance (Review) Sex linked (Review) Codominance Incomplete dominance

Mendel’s Inheritance (Review) Mendel discovered how most of our autosomal chromosomes inherit genes. Recall: Autosomal cells are all the non sex related cells in the human body

Mendel’s Inheritance (Review) In Mendelian inheritance, one of the alleles is dominant while the other is recessive. Each offspring will inherit two alleles for a particular trait (one from the mother and one from the father) Recall: Father HFather h Mother HHHHh Mother hHhhh

Sex Linked Inheritance (Review) Unlike Mendelian inheritance, sex linked inheritance involves our sex chromosomes. Recall: There are two sex chromosomes (X and Y) and that each of these chromosomes can have sex linked traits. However, the Y chromosome is degenerate and thus contains very few traits.

Sex linked Inheritance

Codominance Inheritance Some traits work opposite to what Mendel believed For example: Codominance occurs when both alleles are fully expressed This means that both alleles are dominant and therefore both traits will be present in the organism E.g. a Roan animal

E.g. Roan animal A roan animal is a heterozygote from white and red parents. It will have both red and white hairs on its body because both alleles are dominant I.e. Genotype Phenotype WR = White and Red

E.g. 2 Codominance

E.g. 3 Codominance

Incomplete Dominance Incomplete dominance is another example of how some traits are inherited different from Mendel predicted. In incomplete dominance neither of the two alleles for the same gene can dominate the other. The result is that the phenotype is a blend of both parents This is similar to the old theory of blended inheritance.

Incomplete dominance In incomplete dominance we do not use upper-case and lower-case letters because neither allele is dominant over the other. Instead we use superscripts: In this example of the snapdragon flower colour, both red and white alleles affect flower colour (denoted by “C”). The red allele is therefore C R and the white allele is C W.

Practice Incomplete Dominance What would be the result if we crossed two pink snapdragon flowers (C R C W ) together. What percentage of the offsrping would be pink? Red? White?

Pedigree Charts Recall: Mendel would selectively breed different plants to determine what the offspring would look like. This is not possible with humans because there are ethical issues Instead genetic researchers use pedigree charts to track traits from parents to children Pedigree charts are similar to family trees

Pedigree Charts

X-Linked Recessive Disease Example Assume d = diseased and D = normal What is the genotypes for the F1 generation? F1 F2 P

Questions due at the end of class 1. Distinguish between incomplete dominance and codominance. 2. A plant that produces white flowers is crossed with a plant that producess purple flowers. Describe the phenotype of the offspring if the pattern of inheritance is: a. incomplete dominance b. codominance 3. What would be the result if a pure bred red plant fertilized a pure bred yellow plan. Assume that the plant colour undergoes incomplete dominance. Use a punnett square to show your answer.

If Time: X-linked Dominant Here is an example of a pedigree chart for an X-linked dominant disease. Let us pretend that the X-linked dominant disease is Fragile X syndrome X F Y (where F is fragile X syndrome dominant allele) Solve the genotypes for the II generation