Chromosomes and GeneticsHuman ChromosomesSex DeterminationA few odditiesHermaphroditesChromosome VariationsChromosome Number VariationsSome Sex Chromosome AneuploidiesTurner SyndromeOther Number VariationsChromosome Structure VariationsStructure Variation ExamplesSex-linked GenesColorblindnessInheritance of ColorblindnessHemophiliaSex-Influenced TraitsChromosomes and Genetics•Basic review:• Chromosomes are long pieces of DNA, with supporting proteins.• Genes are short regions of this DNA that hold the information needed to build and maintain the body• Genes have fixed locations: each gene is in a particular place on a particular chromosome• Diploids have 2 copies of each chromosome, one from each parent. This means 2 copies of each gene.• The interactions between the 2 copies of each gene give rise to the various forms of dominance.Human Chromosomes•We have 46 chromosomes, or 23 pairs.•44 of them are called autosomes and are numbered 1 through 22. Chromosome 1 is the longest, 22 is the shortest.•The other 2 chromosomes are the sex chromosomes: the X chromosome and the Y chromosome.•Males have and X and a Y; females have 2 X’s: XY vs. XX.Sex Determination•The basic rule: if the Y chromosome is present, the person is male. If absent, the person is female.•In meiosis, the X and Y chromosomes separate and go into different sperm cells: ½ the sperm carry the X and the other half carry the Y. All eggs have one of the mother’s X chromosomes, so when they are fertilized, ½ of the zygotes are XX (female), and ½ are XY (male).•The Y chromosome has the main sex-determining gene on it, called SRY.•About 4 weeks after fertilization, an embryo that contains the SRY gene develops testes, the primary male sex organ. The testes secrete the hormone testosterone. Testosterone signals the other cells of the embryo to develop in the male pattern.•If the embryo does not have the SRY gene, it develops ovaries instead, which secrete estrogen and causes development in the female pattern.A few oddities•It is possible to be XY and female. Two ways this can happen:• 1. the SRY gene can be inactivated by a mutation. If SRY doesn’t work, testes don’t develop and the embryo develops as a normal female.• 2. In a condition called “androgen insensitivity”, the person is XY with a functional SRY gene, but her cells lack the testosterone receptor protein, so the cells don’t ever get the message that the testosterone is sending. Testes develop in the abdominal cavity, and no ovaries, fallopian tubes, or uterus develop. At puberty, the internal testes secrete testosterone, which gets converted into estrogen and the body develops as a normal (but sterile) adult female.Hermaphrodites In some cases, androgen insensitivity is only partial: the cells respond a little bit to testosterone produced by the testes. The embryo develops with ambiguous genitalia, neither completely male not completely female. Such a person is sometimes called a “hermaphrodite”. Another condition, congenital adrenal dysplasia, causes the adrenal glands to produce an abnormally large amount of testosterone in a female embryo, This can also cause development of ambiguous genitalia, a hermaphrodite.•Another rare condition: a chimera occurs when two separate embryos fuse together. This can result in a person with some XX cells and some XY cells. Such a person can have both testes and ovaries, a “true” hermaphrodite. This condition is extremely rare: more people say they have it than actually do.Chromosome Variations•Changes in number and structure are possible: first look at number variations.•Aneuploidy: having an extra or missing chromosome– is fairly common in sperm and eggs. Non-disjunction in meiosis causes chromosomes to not separate equally into the gametes.•The rate of non-disjunction in males is constant: 1-2% of sperm have an extra of missing chromosome. But in females, the rate increases marked with age. This is illustrated by the frequency of Down syndrome births at different ages of mother. Down syndrome is the most frequent result of non-disjunction.Chromosome Number Variations•Except for the X and Y, humans don’t survive with only 1 copy of any chromosome. Also, 3 copies is lethal in most cases. Aneuploidy is a major cause of spontaneous abortion in early pregnancy.•Down Syndrome is the most common human aneuploidy. It is also called trisomy-21, meaning 3 copies of chromosome number 21. •People with Down’s have a characteristic appearance: flattened face, turned up nose, epicanthal folds at the outer corners of the eyes. In most cases the diagnosis is made immediately at birth. Heart defects, protruding tongue, and mental retardation are also found in most people with Down’s. Occurs about 1 in 1000 births.•There are also translocational and mosaic forms of Down syndrome. • Translocational involves a chromosome structure change (2 chromosomes get hooked together) and is inherited. With translocational Down’s, if one child in a family has it, others are likely to also get it. Occurs in about 5% of Down’s cases.• Mosaic Down’s means having some cells with trisomy 21 and other cells normal. The person’s physical appearance and mental condition depends on exactly which cells are which. About 3% of all Down syndrome cases.Some Sex Chromosome Aneuploidies•Non-disjunction can also result in a person with 2 X’s and a Y: 47,XXY. This is called Klinefelter Syndrome.•The Y chromosome makes a person with Klinefelter’s male: possessing testes.•Symptoms: female body hair pattern, breast development, sterile, can be some developmental delay or retardation, especially for verbal skills. •Often not diagnosed, or diagnosed only accidentally.•Most symptoms are helped by testosterone treatment.Turner Syndrome•Also called XO, because people with Turner’s have only 1 X chromosome: 45, X.•No Y means Turner’s people are female. However, no ovaries develop, so they don’t undergo the body changes of puberty and they are sterile.•Hormone treatment cures all but the sterility.•Other symptoms: short stature, webbed skin and low hairline at the neck, some oddities of spatial perception. Not retarded.Other Number Variations•Triplo-X, having 3 X chromosomes. No Y chromosome means female. Many with this syndrome are undiagnosed because they have no symptoms. Some have slight social