Genetics - 04 Non-Mendelian Genetics

HUMAN GENETICS WITH CYTOGENETICS • EXTENDING NON-MENDELIAN GENETICS INTRODUCTION • • • Mendelian inheritance - emphasizes that one allele is completely dominant over its paired allele. Hence, in the monohybrid, dihybrid, trihybrid as well as in polyhybrid crosses that you solved, the dominant allele always masked off the expression of the recessive allele in heterozygotes. But this mode of transmission, known as the complete dominance is not true for all traits. Some traits are transmitted in a different way. NonMendelian inheritance results in different phenotypes as well as phenotypic ratios. LESSON 1: INCOMPLETE DOMINANCE AND CODOMINANCE LESSON 3: MULTIFACTORIAL INHERITANCE • • • Codominance o Both alleles for a gene are expressed equally in the phenotype of heterozygotes o Both alleles are expressed in the offspring o If you cross a red flower and white flower, the offspring will be flowers with red and white patches on them. Both parent phenotypes are shown together on the offspring. o The most common example of codominance is the AB blood type. If a person with A type blood and a person with B type blood have a child, that child could have type AB blood where both phenotypes are fully expressed. Incomplete Dominance o Dominant allele is not completely expressed o Intermediate phenotype/ mixture results in heterozygotes who inherit the two alleles o There is a blending of the two alleles resulting in a third phenotype which is a mixture of both alleles. T o Phenotype produced does not look like either of the parents. o For example, when you cross, a white flower and red flower, all their offspring will be pink flowers, a completely new phenotype. The two parent phenotypes are blended together into a new phenotype You cannot see either of the parent phenotypes which are white or red in the offsprings. LESSON 2: MULTIPLE ALLELE, LETHAL ALLELE & EPISTASIS • • Multiple alleles - described when a trait is controlled by more than two alleles, and that even if a trait is controlled by more than two alleles each offspring still only inherits two. Epistasis - occurs when the expression of a gene is controlled by the expression of another gene Lethal alleles o Known as lethal genes o Alleles that cause the death of the organism that carries them. o Lethal alleles are recessive, dominant, or conditional depending on the genes involved. o It can lead to the death of an organism and it can manifest early in development. Multifactorial inheritance o Involves many factors in causing a birth defect. o The factors may include genetic and environmental. o One gender (either males or females) may often be affected frequently than the other or one gender is more likely to show and express the problem over the other gender. o Hip dysplasia, height and neural tube defects. LESSON 4: SEX LINKED, SEX INFLUENCED AND SEX LIMITED INHERITANCE • • • Sex linked o Occurs when a gene is located on a sex chromosome o These traits are influenced by genes on the X chromosome in humans because the X chromosome is larger and contains more genes than the Y chromosome. Sex-limited traits o Determined by genes located on autosomes and express or visible only in one sex. o They are responsible for sexual dimorphism. Sex-influenced traits o Autosomal traits that do not show distinctive expression between women and men and influenced by sex & internal environment (sex hormones) o Difference in degree of expression bet. male & female