How Sex Alters Inheritance

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Sex Chromosomes

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Play a fundamental role in determining the sex of the individual

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X chromosome

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Bears many genes

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Y chromosome

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Much smaller than X chromosome, only contains 1/4 f genetc info of X chromosome. It is the Y chromosome that has the SRY gene.

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SRY gene

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Codes for strong male determining properties

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Females have ________ autosomal pairs and ____________.

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22 autosomal pairs and one pair of X chromosomes.

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Males have __________ autosomal pairs and ___________

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22 autosomal pairs and one X and one Y chromosome

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Sex Linked

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Most genes on the X chromosome are absent from the Y chromosome, these genes are said to be sex linked. The inheritance characteristics of sex linked genes are completely different from the autosomal genes.

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Females have _________ of sex linked genes.

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2 copies of sex linked genes, they have two X chromosomes.

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Males have _________ of sex linked genes.

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One copy of sex linked genes that is present on the X chromosome that was derived from the mother. Therefore recessive alleles cannot be masked and selection can act on deleterious recessives on the X chromosome.

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Hemizygous

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Used when referring to male genotype XY

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3:1 ratio

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It is different from autosomal genes in that all individuals showing the recessive phenotype are males

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Color Blindness

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X linked disorder. 8% of Caucasian men have red-green color Blindness. That is they see greens as tans, olives greens as browns and reds and reddish brown. Some cannot tell reds from greens at all.

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Muscular Dystrophy

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X linked disorder. Wasting away of muscles. Due to lack of dystrophin causes calcium to leak out of cell that promotes an enzyme top dissolve muscle fibers.

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Hemophilia

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X-linked disorder. Called the bleeder's disease. Lack of clotting factor. Common type, Hemophilia A and B. Hemophiliacs bleed externally and internally. Hemorrhages are stoped by transfusion of fresh blood and clotting factor VIII is now available as a biotechnology product. Hemophilia was prevalent among the royal families in Europe and all affected males could trace their ancestry to Queen Victoria of England.

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Fragile X Syndrom

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X-linked disorder. Is the most common genetic cause of mental retardation second to Down Syndrom. Is due to base triplet repeats of CGC in the X chromosome. A normal person has 6-50 repeats, an affected person has 230-2000 repeats.

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Fragile site

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Breakage of the X chromosome

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X-chromosome inactivation in humans

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Females have two copies of X chromosomes but in each somatic cell, one of the X chromosomes condenses into a tiny dark object called a barr body. Most of the genes in this condensed chromosome are inactive, therefore, each normal female cell contains only one active copy of most of the sex linked genes.

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Barr body

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One of the X chromosomes condenses int a tiny dark object

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Why is it that a female would be a Hemophiliac only if she is homozygous recessive?

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It is not always the same X chromosome that condenses, as long as half the cells are normal such as in a heterozygous woman she would be phenotypically normal

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Nondisjunction

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Occurs during Meiosis I when both members of homologous pairs go into the same daughter cell or during Meiosis II when the sister chromatids fail to separate.

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Trisomy

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The presence of three chromosomes of one type in a diploid individual. Trisomies lead to such servere mental retardation that infants die within a few weeks after birth.

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Monosomy

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The presence of one type of chromosome, Missing a sex chromosome. Is not viable for autosomes.

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Euploid

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True number of chromosomes

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Aneuploid

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missing or extra chromosomes.

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Trisomy 21

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Down Syndrom. Most common autosomal trisomy, extra chromosome 21. Mongolism is used to characterize the disorder. Happens more often for women having children over 35. Oocytes arrested in Meiosis II for a long time, may have been exposed to damaging chemicals and radiation.

Trisomy 13 and 18 also viable- children die very young

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Triplo X or poly X (XXX females)

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Trisomy of the sex chromosome. Distinctive phenotype aside from a tendency to be tall and thin. Most are not mentally retarded. Some may have menstrual difficulties but many menstrurate regularly and are fertile. Has two Barr bodies.

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Klinefelter's Syndrom (XXY males)

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Trisomy of the sex chromosome. underdeveloped testes and prostate gland, no facial hair, some breast development, large hands and feet, long arms and legs; slow to learn but not mentally retarded. One X will become a Barr body

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XYY Syndrom (Jacobs syndrom)

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Trisomy of the sex chromosome. Taller than average, suffer from persistent acne, tend to have speach and reading problems . It was suggested that they were more likely to be criminally aggressive but such behavior is no greater than normal XY males.

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Turner Syndrome

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Is a example of a monosomy (XO Females) . It is a female with only one X chromosome. Short with broad chest, and folds of skin on the back of the neck. The ovaries and uterus are very small and underdeveloped. Do not undergo puberty of menstrurate, no breast development,usually infertile, however some have given birth following in vitro fertilization with donor eggs, normall intelligence.

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Deletions

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Due to a single break that causes the chromoseome to lose an end piece or a double break that results in a loss of an internal piece. Inviable if pieces from both chromosome are missing

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Changes in Chromosome Structure

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It is a chromosome mutation that can be detected under the microscope. Radiation, chemicals, and viruses can cause chromosomes to break.

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Wild type

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The most common form of a chromosome

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Mutant chromosome

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Chromosome that is missing pieces

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Williams Syndrome

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Is a Deletion change of the structure of a chromosome. Loss of a tiny endpiece of chromosome 7, the gene for elastin is missing affecting the cardiovascular system and causing the skin to age early. Have turned up noses, small chin, and large ears.

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Cri du chat Syndrome

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Is a Deletion change in the structure of a chromosome. Chromosome 5 is missing an endpiece, abnormal development of the glottis and larynx results in the infants cry resembling that if a cat. Have small head, mentally retarded, with facial abnormalities.

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Duplication

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A chromosome segment is repeated in the same chromosome or in a non homologoues chromosome therefore a person has more than two alleles for certain traits.

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Inverted Duplication of Chromosome 15

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Is a Duplication change in the chromosome structure. Mental retardation, autistic characteristics such as poor speech, hand flapping, lack of eye contact, poor muscle tone, seizures, curved spine.

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Translocation

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Is the exchange of chromosome segments between two nonhomologous chromosomes; If the translocation results in breakage of a gene.

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5% of Down Syndrome

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Is a Translocation change in the structure of a chromosome. Cases are due to a translocation that occured in the previous generation. A segment of chromosome 21 is translocated to chromosome 14. Therefore the affected person inherits two normal copies of chromosome 21 and an abnormal chromosome 4 that contains a segment of chromosome 21.

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Inversion

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Occurs when segment of chromosome is turned 180 degrees. The reverse sequence of genes may result in altered gene activity. If a gene that needs to be controlled (like growth factor) is moved to region where the gene is activated all the time then it can lead to uncontrolled cell division.

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Principle of Independent Assortment

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Mendel's second Law. When several genes are involved in a dihybrid cross they sort into gametes independently of one another.
•Genes that are present on the same chromosome cannot assort independently during Meiosis unless separated by a crossing over between homologous chromosomes. •Genes on the same chromosome are said to be linked.
•The further apart two genes are on a chromosome, the higher the rate of cross over and will be inherited virtually independently from one another. Two genes that are located near to each other are said to be tightly linked and will be inherited as one unit.

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Hemophilia A

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lack clotting factor IX

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Hemophilia B

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Lack of clotting factor VIII