Beyond Medelian Genetics

1. Beyond Medelian Genetics • Pedigree Charts • Multiple Alleles • Incomplete Dominance • Co-dominance

2. PEDIGREE CHARTS Pedigree charts show a record of the family of an individual. It can be used to study the transmission of a hereditary condition. It is particularly useful when there are large families and a good family record over several generations. Symbols used in pedigree charts

4. Organizing the pedigree chart Below is a pedigree chart of a family showing four generations. A total of 20 individuals. Generations are identified by Roman numerals. Individuals in each generation are identified by Arabic numerals numbered from the left. Therefore the affected individuals are II3, IV2 and IV3.

5. Multiple Alleles Multiple alleles are different forms of the same gene...that is the sequence of the bases is slightly different in the genes located in the same place on the chromosome. Although an individual organism can have only two different alleles for a characteristic, when multiple allele forms exist there will be a hierarchy of dominance with each allele is identified with a capitol letter and superscripts of letters or numbers.

6. For instance, if mice color can be black, yellow, or agouti (combination of colors), and inheritance is due to multiple alleles, what is the phenotype of the offspring from a cross of heterozygous yellow to a heterozygous black

7. Incomplete Dominance With incomplete dominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype that is a blending of the parental traits. 

8. Co-Dominance

9. The ABO blood system This is a controlled by a tri-allelic gene. It can generate 6 genotypes. The alleles control the production of antigens on the surface of the red blood cells.Two of the alleles are co-dominant to one another and both are dominant over the third. • Allele IA produces antigen A • Allele IB produces antigen B • Allele i produces no antigen.

10. Chromosomal Theory The chromosomal theory of inheritance: • Chromosomes carry genes, the units of heredity. • Paired chromosomes segregate during meiosis. Each sex cell or gamete has half the number of chromosomes found in the somatic cells. (This explains why each gamete has only one of each of the paired alleles)

11. Morgan’s Experiments • Thomas Morgan (1866 – 1945) conducted experiments on fruit flies • He discovered a number of obvious mutations (a heritable change in the molecular structure of DNA) • Morgan noted a white-eyed male mutation (red-eye is normal) • Mated white-eyed male with red-eyed female and F1 generation was all red-eyed

12. He then crossed two from F1 generation and F2 generation was ¾ red-eyed and ¼ white-eyed • On examination Morgan discovered that all white-eyed flies were male and that half of the males were white-eyed and half the males were red-eyed • all females were red-eyed • Examination of chromosomes in cells showed that females had four homologous pairs while males had only three plus two non-homologous chromosomes

13. Sex-Linked Traits • Morgan observed that females has two homologous sex chromosomes (XX) and males had two non-homologous sex chromosome (XY) • From this Morgan concluded that the Y chromosome did not carry the gene for eye colour • Traits located on sex chromosomes are called sex-linked traits

14. Sex-Linked Traits and Males • Sex-linked traits are controlled by genes located on the sex chromosomes • A recessive trait located on the X chromosome is more likely to be expressed in males than in females since males need only one copy of the recessive allele while females need two • Recessive lethal X-linked disorders in humans occur more frequently in males

15. Barr Bodies • Dr. Murray Barr working at the Univ. of Western Ontario identified a dark spot in some somatic cells of female mammals • These dark spots are the results of one of the X chromosomes randomly becoming inactive in each cell • These spots are called a Barr body • The randomness of inactivation can result in different cells expressing alternate forms of a gene if the individual is heterozygous