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Lecture

LEc 28

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Department
Biology
Course
BIOL 1010
Professor
Igor Kovalchuk
Semester
Fall

Description
Mendelian Genetics Lecture 28 Polygenic inheritance of skin color  Three separately inherited genes affect the darkness of skin.  For each gene, an allele for dark skin is incompletely dominant to an allele for light skin  Thus, an individual who is heterozygous for all three genes (AaBbCc) has inherited three “units” of darkness Polygenic inheritance  Human polygenic traits include o Height o Weight o Eye Color o Skin Color o Many forms of behavior Pleiotropy  Pleiotropy is the effect of a single gene on more than one characteristic  The primary result of this gene is the production of defective feathers  Secondary results are both good and bad; good include increase adaptation to warm temperatures, bad include increase metabolic rate, decreases egg-laying.  Sickle-cell anemia is a human disease originating in warm lowland tropical areas where malaria is common  Sickle-cell individuals suffer from a number of problems, all of which are pleiotropic effects of the sickle-cell allele The chromosomal basis of Mendel’s laws  The arrangements of chromosomes at metaphase I of meiosis and their movement during anaphase I account for the segregation and independent assortment of the alleles for seed color and shape  The alternatives are equally likely; F1 plants will produce numbers of eight kinds of gametes.  In the F2 generation, we see the physical explanation for Mendel’s 9:3:3:1 ratio of phenotypes Finding the genes  Between 1884 (the year Mendel died) and 1888; o Details of mitosis and meiosis were reported o The cell nucleus was identified as the location of the genetic material o “Qualities” were even proposed to be transmitted on chromosomes to daughter cells at mitosis  In 1903 Walter Sutton and Theodore Boveri formally proposed that chromosomes contain the genes  The Chromosome Theory of Inheritance is one of the foundations of genetics and explains the physical reality of Mendel’s principles of inheritance.  The location of many genes (Mendel’s factors) was determined by Thomas Hunt Morgan and his coworker in the early 1990s.  Morgan experimental organism was the fruit fly  Fruit flies are ideal organisms for genetics, having a small size, ease of care, susceptibility to mutation and short (7-9 day) generation time  The role of chromosomes in determination of sex was deduced by Morgan from work on fruit flies  During Metaphase I, homologous chromosomes will line up. o A karyotype can be made by cutting and arranging photomicrographs of the homologous chromosomes thus revealed at Metaphase I o Two types of chromosome pairs appear  Autosomes resemble each other in size and placement of centromere, for example pars of chromosome 21 are the same size, while pairs of chromosome 9 are different size from pair 21  Sex Chromosomes may differ in their size, depending on the species of the organism they are from.  In humans and fruit flies, males have smaller sex chromosomes, termed the Y, and a larger one, termed the X.  Males are thus XY and termed heterogametic  Females are thus XX, and termed homogametic  Morgan disc
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