The Human Genome Chapter 14

Human Heredity Human chromosomes – There are 23 pairs of chromosomes. Half the chromosomes are from each parent. Autosomal chromosomes – Chromosome numbers 1 thru 22. Sex Chromosomes – chromosome pair #23. There are two options XX for female or XY for male.

Human Chromosomes Human Karyotype These human chromosomes have been cut out of a photograph and arranged to form a karyotype. Photo credit: ©CNRI/Science Photo Library/Photo Researchers, Inc.

Karyotype of Down syndrome (trisomy 21)

Boy with Down syndrome

Girl with Cri-du-Chat syndrome

Nondisjunction Trisomy Nondisjunction causes gametes to have abnormal numbers of chromosomes. The result of nondisjunction may be a chromosome disorder such as Down syndrome. Trisomy

Nondisjunction (not coming apart) – Causes chromosomal disorders Incomplete separation of chromosomes during anaphse (I or II) during meiosis.

Nondisjunction continued Chromosomal Disorders: Down Syndrome – Trisomey (3 copies of a chromosome) on chromosme 21. Turner;s Syndrome – Females with only 1 X chromosome (females are sterile).

Nondisjunction

Human Sex Determination

Males and females are born in a roughly 50 : 50 ratio because of the way sex chromosomes separate during meiosis.

Female X X XX XY X Male Y 50:50 XX:XY

All human egg cells carry a single X chromosome All human egg cells carry a single X chromosome. Half of the sperm carry and X chromosome & half carry a Y chromosome.

Pedigree Patterns A pedigree chart shows the relationships within a family.

Human Traits A square represents A circle represents a male. A circle represents a female. A vertical line and a bracket connect the parents to their children. A horizontal line connecting a male and a female represents a marriage. A shaded circle or square indicates that a person expresses the trait. A circle or square that is not shaded indicates that a person does not express the trait. This drawing shows what the symbols in a pedigree represent.

Autosomal chromosomes (#1-22) Dominant autosomal disorder Disease is present in EVERY generation. Equal chance for males or females to have the trait.

This table shows the major symptoms of some well-known genetic disorders.

Autosomal continued Recessive autosomal disorder Disease skips generations. Equal chance for male or female.

Sex-linked genes Genes located on X & Y chromosomes are called sex-linked genes. Most sex linked traits are linked to the X chromosome Males have just one X chromosome. So all X-linked alleles are expressed in males, even if they are recessive.

X Chromosome Duchenne muscular dystrophy The Y chromosome is much smaller than the X chromosome contains only a few genes. Melanoma X-inactivation center X-linked severe combined immunodeficiency (SCID) Colorblindness Hemophilia Genes on X and Y chromosomes, such as those shown in the diagrams, are called sex-linked genes. Y Chromosome Testis-determining factor

Colorblindness Three human genes associated with color vision are located on the X chromosome. In males, a defective version of any one of these genes produces colorblindness.

Father (normal vision) Possible Inheritance of Colorblindness Allele Father (normal vision) X-linked alleles are always expressed in males, because males have only one X chromosome. Males who receive the recessive Xc allele all have colorblindness. Females, however, will have colorblindness only if they receive two Xc alleles.