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

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 human traits is complex. Single-gene traits are important in understanding human 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.

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. X Y 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?

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.

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.

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. Phenotype Genotype Normal Vision XMXm or XMXM or XMY Red-green colorblind XmXm or XmY

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. X Y Interpret: Is this karyotype of a male or female? How do you know?

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)

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