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Transcript
Spring 2009: Section 3 – lecture 5 Reading – Chapters 8 Chromosomes with specific functions Example: sex chromosomes There are two types of chromosomes in a cell when considering sex determination: sex chromosomes - carry genes that determine the sex of an individual. autosomes - chromosomes that are not involved in sex determination. Example – humans 2N – 46 autosomes – 44 sex chromosomes - 2 In humans have X and Y chromosomes XX = female = homogametic sex XY = male = heterogametic sex Because the female is the homogametic sex, all of the female’s gametes will be X while the male’s gametes are 50% X and 50% Y. So the male is the one who determines the sex of the progeny. In some species the male is homogametic and the female is heterogametic (turkeys, chickens) Could this help explain how a female turkey could produce male progeny even when there are no male turkeys around to provide male gametes? What happens to the extra X chromosome in females? Good example of facultative heterochromatin In most cells one X is extremely condensed. This can be seen at interphase and is called a Barr body In humans: Females will have one Barr body Males will not have a Barr body In cats it is possible to see that condensation of the X in females is random. The way to do this is to have variation in a gene on the X chromosome. Example - Coat color in tortoiseshell or calico cats. 1 X carries a gene for red coat color 1 X carries a gene for non-red color The color of a section of fur will depend on which X chromosome condensed during development. It also indicates that once an X is designated as a Barr body it remains a Barr body in that cell line. Wright and Walters, 1980 The result of random condensing of the X chromosomes is a female cat with a patchwork (red and non-red) coat. Why would you be surprised to see a male tortoiseshell or calico cat? Cloning a calico cat Would you expect the clone of a calico cat to have the same appearance as the nuclear donor? ‘mom’, nuclear donor clone Chromosome modifications Euploidy - Changes in the number of sets of chromosomes. Aneuploidy - changes in the number of chromosomes in a set. Changes within a chromosome Changes within a chromosome a) deletion b) duplication c) inversion d) translocation Euploidy - change in the number of sets of chromosomes. Most species have two sets, one paternal and the other maternal. This is the source of homologous pairs of chromosomes If a species has more than two sets of chromosomes the organism is said to be a polyploid. 2 sets 3 sets 4 sets 6 sets diploid triploid tetraploid hexaploid To describe the number of chromosome sets in a species you use X to equal or represent one chromosome set. 2 sets 3 sets 4 sets 6 sets diploid triploid tetraploid hexaploid = 2X = 3X = 4X = 6X So for each species you will have a somatic chromosome number, a gametic chromosome number and a monoploid chromosome number that indicates the number of chromosomes in one set. Examples: Humans - diploid 2N = 46 chromosomes N = 23 chromosomes 2X = 46 chromosomes X = 23 chromosomes - number of chromosomes in one set Wheat – hexaploid 2N = 42 chromosomes N = 21 chromosomes 6X = 42 chromosomes X = 7 chromosomes Potatoes: tetraploid with 48 chromosomes Rapeseed: tetraploid with 38 chromosomes If all the sets are homologous, i.e. originating from the same species, then the species is called an autopolyploid. Examples of autopolyploids - potatoes, strawberries, bananas, aspen Advantages of an autopolyploid: - plant parts and fruits are larger - plant may be more vigorous Disadvantages of an autopolyploid: - problems in meiosis with chromosome pairing and even distribution of chromosomes. - low or no seed set • Why is poor or no seed set not a problem with potatoes, strawberries and bananas? Another way to have no seed set is to have an uneven number of chromosome sets in a polyploid. Example: banana, an autotriploid Possible origin: 1. Multiple fertilization (not likely) 2. Fertilization of an unreduced (2N) gamete 3. Hybridization between a tetraploid and a diploid. 4X x 3X 2X Problems in gamete formation and seed set are due to uneven chromosome pairing and division in the reduction division of meiosis. (i.e. how do you divided a trivalent equally?) Advantages of autotriploids plants - larger fruit no seeds more vigorous growth oysters - no egg or sperm production fish – more rapid growth Disadvantages of autotriploids?