Lec 6 Bio 308
Lec 6 Bio 308 Bio 308
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This 4 page Class Notes was uploaded by Kiara Reyes on Friday September 9, 2016. The Class Notes belongs to Bio 308 at Northern Illinois University taught by M. Johns in Summer 2016. Since its upload, it has received 6 views. For similar materials see Genetics in Biology at Northern Illinois University.
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Date Created: 09/09/16
Lecture 6: Sex-Related Topics Sex Determination Mammals have the X and Y chromosomes: XX= females and XY= males All other chromosomes are called Autosomes: changed genes is located, any chromosomes that isn’t a Sex Chromosomes Ex] Humans have 46 chromosomes, 44 autosomes plus 2 sex chromosomes “X and Y Chromosomes” X has many genes like autosomes and has nothing to do with Sex Y has very few genes and consisting of mostly inactive DNA One gene on the Y is very important: SRY- gene is the primary determinant of Sex -SRY present, testes develop in early embryo. The test secrete the hormone testosterone, which causes development as male -SRY absent (no Y chromosome), ovaries develop instead of testers and the embryo develops into female Ex] Sex Determination in Birds -Birds use a system of sex chromosomes very similar to mammals: X is Z, Y is W and SRY is DMRT 1 gene. Then ZZ= male and ZW= female Homogametic0- Both Sex chromosomes the same Female mammals (XX) and male birds (ZZ) Heterogametic- Different Sex chromosomes Male mammals (XY) and female birds (ZW) Ex] Sex Determination in Drosophila -Drosophila also have X and Y chromosomes, with XX females and XY male. Doesn’t use SRY gene to determine sex, use ratio of X’s to sets of autosomes -1X plus 2 sets of autosomes is normal diploid male -2X plus 2 sets of autosomes is diploid female -The difference between sex determination mechanisms comes in odd cases Sex Linkage Genes that are sex-linked are on the X-chromosomes. Because male (mammals) have only 1X, any gene on the X in a male is expressed, whether dominant or recessive. In contrast females have 2X’s, so recessive traits are often covered up by the dominant normal (wild type) allele. Hemizygous- Having only 1 copy if a gene “Common Sex-Linked Traits” Red/green colorblindness. Genes for the red and green receptors are on the X. The blue receptors in an autosome Hemophilia (Blood doesn’t clot) two of the genes for proteins involved in clotting are on the X “Sex-Linked Inheritance Patterns” Father gives his X to his daughters only, son gets his Y chromosomes instead as sons get their X from their mother Reciprocal crosses are crosses with the same phenotype in the parents, but with reversal sexes. Usually give different results with sex-lined traits Ex] Colorblind male and Normal female give all normal offsprings. However, a normal male crossed with a colorblind female gives colorblind male children and normal female children Colorblind females can occur as a result of a cross between a colorblind male and a heterozygous (carrier) female “Dosage Compensation” In mammals, males have 1X while females have 2. Having only 1 copy of any other chromosomes would be lethal Question: How can the X be present in 1 copy or 2 copies and produce normal offsprings in either case? -Only 1X is active in each female cell Lyon Hypothesis It has been a long know that female cells contain “Barr Bodies”, blobs of chromatin located on the inside of the nuclear membrane. Each female call has 1 Barr Body, male cells don’t have Barr Bodies Barr Bodies- condensed, inactive X chromosomes found in the nucleus of individual with more than one X chromosome Mary Lyon proposed that Barr Bodies are inactive X chromosomes, and that mammalian cells inactive all but one of their X’s, converting the extras into Barr Bodies Proof: XXY individuals are males, but have Barr Body; OX individuals are female but have no Barr bodies; XXX individuals are female with 2 Barr Bodies in each cell Specifics of Inactivation When the embryo had about 200 cells, each cell randomly inactivates one of its X’s, independently of the other cells. The inactive X stays inactive throughout the individuals life, through many cell generations Ex] Tortoiseshell cats have patches of Black and orange fur, almost all Tortoiseshells are females. Heterozygous for the X-linked coat color gene: Sex-Influenced Traits Sex- Influenced traits- autosomal traits that is dominant in one sex and recessive in the other [Dominance depending on sex] Ex] Pattern baldness is forced in both sexes, but its rarer in females. Females usually get very thin hair all over, instead of the classic receding hairline and bald spot on top that men get Sex-Linked Traits Sex-linked traits- expressed in one sex but not the other. Usually due to anatomical or physiological limitations Ex] Ability to produce milk is sex-limited, because only females have breasts and the milk producing glands Mitochondrial Genes Mitochondrial genes- organelles that produce most energy for eukaryotic cells. Mitochondrial passes a small circle of DNA like bacteria but unlike the linear eukaryotic chromosomes Endosymbiont hypothesis- mitochondrial are descended from free- living bacteria, which developed an intercellular symbiosis with primitive eukaryotic cells “Mitochondrial Inheritance Pattern” Mitochondrial are inherited strictly from the mother, the father’s mitochondrial aren’t passed to his offsprings Any mitochondrial traits will be found in the mother will be found in all her children Heteroplasmy Heteroplasmy- an individual has more than on kind of mitochondria Since mitochondrial are divided randomly during the cell division, different cells get different proportions of the two types If one mitochondria types is mutant and the other is normal, severity of symptoms will vary in different tissues depending on the proportions of the two types Maternal Effect Genes During production of the egg, the mother puts many proteins and RNA into the egg that are produced by diploid maternal cells -Rule: “Mother’s genotype determines offsprings phenotype”
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