8- Single gene inheritance.pdf

Single Gene Inheritance Gregor Johann Mendel, founder of the science of genetics. Prepared by Dr. Somaya Elbushra MBBS MSc MPHE  Mendel developed a simple theory to explain the transmission of hereditary traits from generation to generation. Mendel performed hybridi...

Single Gene Inheritance Gregor Johann Mendel, founder of the science of genetics. Prepared by Dr. Somaya Elbushra MBBS MSc MPHE  Mendel developed a simple theory to explain the transmission of hereditary traits from generation to generation. Mendel performed hybridizations, which involve mating two true-breeding individuals that have different traits 2 Mendel’s Principle of Segregation: Mendel’s first law states that the two members of a gene pair of alleles segregate during meiosis and that each offspring receives only one allele from each parent. 3 Mendel's law of dominance states that in a heterozygote, one trait will conceal the presence of another trait for the same characteristic. The effects of a dominant allele (S) are seen even if it is pair with the recessive allele (s) Recessive allele is only visible when paired with another recessive allele (ss) 4 Punnett squares, devised by Reginald Punnett, can be used to predict the possible outcomes of a genetic cross or mating and their expected frequencies. 5 Incomplete Dominance: is a form of Gene interaction in which both alleles of a gene are partially expressed, often resulting in an intermediate or different Phenotype. It occurs because neither of the two alleles is completely dominant over the other. 6  Mendel’s second law, the principle of independent assortment, states that pairs of alleles for genes on different chromosomes segregate independently in the formation of gametes 7  Inheritance Patterns Autosomal Dominant Inheritance: Individuals affected in every generation Caused by alterations in only one copy of a gene pair. The condition occurs in the heterozygous state, despite the presence of an intact copy of the relevant gene. A is a dominant allele that cause a disease Males and females are equally affected. 8 Genetic disorders that follow a pattern of autosomal dominant inheritance include: Huntington's disease. Marfan syndrome and Achondroplasia 9 Autosomal Recessive Inheritance: AR alleles are clinically expressed only in the homozygous state (SS). The offspring must inherit one copy of the disease causing allele from each parent. Carriers typically do not have condition (Heterozygous carrier) Autosomal recessive disorders include: cystic fibrosis, sickle cell anemia 10 Autosomal Co-Dominant Inheritance: In codominant inheritance, two different alleles of a gene are expressed, and each version makes a slightly different protein. Both alleles influence the genetic trait or determine the characteristics of the genetic condition ABO blood group: There are three alleles that determine the type of the blood group;( A,B & O alleles) Both A & B alleles are Dominant over the O allele (the Recessive allele). 11 X-linked Recessive Inheritance: X-linked = chromosome gene transmitted by the X Recessive = required two copies (Homozygous) of the gene to express the disease. Males are hemizygous for the X chromosome. So disease is seen much more commonly in males than in females. Male-to-male transmission is Not seen in X-linked inheritance. 12 X-linked recessive conditions include red-green color blindness and hemophilia A 13  X-Linked Dominant Inheritance: Male-male transmission of the disease is not seen. As the disease is dominant (one copy of the abnormal gene cause the disease), so Heterozygous females are affected. X-linked dominant diseases are seen about twice as often in females as in males. Because females have two X chromosomes (and thus two chances to inherit an X-linked gene) and males have only one, 14 15 X inactivation (the Lyon hypothesis): Because the Y chromosome carries only about 50 protein-coding genes and the X chromosome carries hundreds of protein-coding genes, a mechanism must exist to equalize the amount of protein encoded by X chromosomes in males and females. This mechanism, termed X inactivation, occurs during the development of female embryos. 16 17