Tracing the Inheritance of the Human Y Chromosome Women have sex chromosomes of XX, and men have sex chromosomes of XY. Which of a man's grandparents could not be the source of any of the genes on his Y-chromosome? Father's Mother. Mother's Father. Father's Father. Mother's Mother, Mother's Father, and Father's Mother. Mother's Mother.

Tracing the Inheritance of the Human X Chromosome Women have sex chromosomes of XX, and men have sex chromosomes of XY. Which of a women's grandparents could not be the source of any of the genes on either of her X-chromosomes? Mother's Father. Father's Mother. Mother's Mother. Father's Father. Mother's Mother and Mother's Father

Is there a special pattern of inheritance for genes located on the X or Y chromosome? Yes, they are called sex-linked genes. More than 100 sex-linked genetic disorders have been mapped on the X chromosome. Y chromosomes are smaller than X chromosomes and appear to only contain a few genes.

Red-Green Colorblindness The most common type of red-green color perception defect is due to a mutation on the X-chromosome (i.e. a red-green color blind allele). X-linked red-color blindness is a recessive trait. Females heterozygous for this trait have normal vision. The color perception defect manifests itself in females only when it is inherited from both parents. By contrast, males inherit their single X-chromosome from their mothers and become red green color blind if this X-chromosome has the color perception defect.

Hemophilia Hemophilia in humans is due to an X-chromosome mutation. What will be the results of mating between a normal (non-carrier) female and a hemophilac male? half of daughters are normal and half of sons are hemophilic. all sons are normal and all daughters are carriers.

Chromosomal Disorders In some disorders, entire chromosomes, or large segments of them, are missing, duplicated, or otherwise altered. The most common error is nondisjunction, when homologous chromosomes fail to separate during meiosis. If nondisjunction occurs, the number of chromosomes in the gametes can be abnormal, and a disorder may result.

Homologous chromosomes fail to separate Nondisjunction Section 14-2 Homologous chromosomes fail to separate Meiosis I: Nondisjunction Meiosis II Go to Section:

Homologous chromosomes fail to separate Nondisjunction Section 14-2 Homologous chromosomes fail to separate Meiosis I: Nondisjunction Meiosis II Go to Section:

Homologous chromosomes fail to separate Nondisjunction Section 14-2 Homologous chromosomes fail to separate Meiosis I: Nondisjunction Meiosis II Go to Section:

Down’s Syndrome Occurs in 1/800-1,000 births Caused by nondisjunction Trisomy 21 = three copies of chromosome 21

Frequency of Down Syndrome Per Maternal Age   Age (years) Frequency of Fetuses with Down Syndrome to Normal Fetuses at 16 weeks of pregnancy Frequency of Live Births of Babies with Down Syndrome to Normal Births 15 - 19 ---- 1 / 1250 20 - 24 1 / 1400 25 - 29 1 / 1100 30 - 31 1 / 900 32 1 / 750 33 1 / 420 1 / 625 34 1 / 325 1 / 500 35 1 / 250 1 / 350 36 1 / 200 1 / 275 37 1 / 150 1 / 225 38 1 / 120 1 / 175 39 1 / 100 1 / 140 40 1 / 75 41 1 / 60 1 / 85 42 1 / 45 1 / 65 43 1 / 35 1 / 50 44 1 / 30 1 / 40 45 and older 1 / 20 1 / 25 Return to Prenatal Testing for Down Syndrome Return to Down Syndrome: Health Issues Homepage

Down’s Syndrome The image shows a karyotype of a person with Down’s Syndrome, Trisomy 21

Edward’s Syndrome Occurs in 1/6,000 births Caused by nondisjunction 80% female – most die before birth Caused by nondisjunction Trisomy 18 = three copies of chromosome 18

Sex Chromosome Disorders Turner’s Syndrome (XO) underdeveloped ovaries, short stature, webbed neck, and broad chest. Individuals are sterile, and lack expected secondary sexual characteristics. Mental retardation typically not evident.

Sex Chromosome Disorders Klinefelter’s Syndrome (XXY) Nondisjunction in males Some development of breast tissue, little body hair is present; typically tall, with or without evidence of mental retardation. Males with XXXY, XXXXY, and XXXXXY karyotypes have a more severe presentation, and mental retardation is expected.

Some Autosomal Disorders in Humans Type of Disorder Disorder Major Symptoms Disorders caused by recessive alleles Albinism Lack of pigment in hair, skin, and eyes Cystic Fibrosis Excess mucus in lungs, digestive tract, liver; increased susceptibility to infections; death in childhood unless treated Phenylketonuria Accumulation in brain cells; lack of normal pigment; mental retardation Tay-Sachs Disease Lipid accumulation in brain cells; mental deficiency; blindness; death in early childhood Disorders Caused by dominant alleles Achondroplasia Dwarfism (one form) Huntington’s Disease Mental deterioration and uncontrolled movements; appears in middle age Disorders caused by codominant alleles Sickle Cell Anemia Sickled red blood cells; damage to many tissues

Albinism

                                                                     Phenylketonuria

                           Achondroplasia

Sickle Cell Anemia

Causes of Genetic Disorders Dominant Alleles Recessive Alleles Sex-linked genes Autosomal genes Nondisjunction Gene mutations Chromosomal mutations

Gene Mutations Changes in the DNA sequence that affect genetic information. Result from changes in one gene. Types: Point mutations Frameshift mutations

Chromosomal Mutations Mutations involving changes in whole chromosomes. Changes in the number of copies or structure of a chromosome. Types: Deletions Duplications Inversions Translocation

Deletion Duplication Inversion Translocation