1. KEY CONCEPT A combination of methods is used to study human genetics.

2. Human genetics follows the patterns seen in other organisms. • The basic principles of genetics are the same in all sexually reproducing organisms. • Inheritance of many humantraits is complex. • Single-gene traits areimportant in understandinghuman genetics. Apply: Why can the genetics of pea plants and fruit flies be applied to humans? Fig. 4.1 - The widow's peak, or pointed hairline, is a phenotype produced by a dominant autosomal gene.

3. Y X Females can carry sex-linked genetic disorders. • Males (XY) express all of their sex linked genes. • Expression of the disorder depends on which parent carries the allele and the sex of the child. • The X chromosome has about 1100 known genes, including many that cause genetic disorders. The Y chromosome is about 1/3 the size of the X and has only about 250 know genes. These X and Y chromosomes are duplicated and condensed. (color SEM; magnification about 15,000X Contrast: How can carriers differ between autosomal and sex-linked disorders?

4. A pedigree is a chart for tracing genes in a family. • Phenotypes are used to infer genotypes on a pedigree. • Autosomal genes show different patterns on a pedigree than sex-linked genes. Tracing Autosomal Genes Figuring out genotypes from phenotypes requires you to use a process of elimination. You can often determine which genotypes are possible, and which ones are not.

5. If the phenotype is more common in males, the gene is likely sex-linked. Tracing Sex-Linked Genes Figuring out genotypes from phenotypes requires you to use a process of elimination. You can often determine which genotypes are possible, and which ones are not.

6. Fig. 4.4 – Red-Green Colorblindness • A person with normal color vision can easily distinguish between different colors. A person who is red-green colorblind cannot.

7. X Y Several methods help map human chromosomes. • A karyotype is a picture of all chromosomes in a cell. Fig. 4.5 - A karyotype can help show chromosomal disorders. This is a normal karyotype. Interpret: Is this karyotype of a male or female? How do you know?

8. Karyotypes can show changes in chromosomes. • deletion of part of a chromosome or loss of a chromosome • large changes in chromosomes • extra chromosomes or duplication of part of a chromosome A karyotype can help show chromosomal disorders, such as the deletion in chromosome 1. (LM; magnification 8000 X)

9. A karyotype can help show chromosomal disorders, such as the extra chromosome 21 in Down syndrome. (colored LM; magnification 11,000 X)