CH 14.1 Human Heredity

Human Chromosomes Each human chromosome consists of a single DNA molecule. Humans have 23 pairs of chromosomes, for a total of 46. Chromosomes are numbered 1 through 23. Chromosome pairs 1 through 22 are called autosomal chromosomes (autosomes). Human males and females share these chromosomes, and the genes they contain, in common. Chromosome pair number 23 are the sex chromosomes. Males have an XY, females have an XX.

Mendel’s laws and other laws of inheritance all apply to humans Mendel’s laws and other laws of inheritance all apply to humans. However, since researchers cannot control human matings, they must analyze patterns of inheritance within families using pedigrees.

A pedigree is a family tree that records and traces an inherited trait through several generations. The traits examined in human pedigrees are generally diseases or disorders caused by either recessive or dominant alleles. Ordinary non-disease traits such as earlobe shape, cleft chins, widow’s peak, and others can also be traced to illustrate how alleles get inherited.  

Types of human inheritance: Autosomal traits are those controlled by genes located on the autosomes. Autosomal dominant traits are inherited by receiving two dominant alleles or one dominant and one recessive allele. Autosomal recessive traits are inherited by receiving two recessive alleles. Sex-linked traits are mainly inherited on the X chromosomes. Although sex-linked traits can be dominant, the sex-linked disorders discussed in class are sex-linked recessive.

Autosomal recessive trait Autosomal dominant trait Sex-linked trait (recessive on X chromosome) Genes on which chromosomes? Chromosomes # 1 -22 X chromosome Genotype to show trait (use A, a) aa AA, Aa XaXa, XaY Who can inherit the trait? Males and females equally More common in males How do they inherit it? Recessive allele from each parent Dominant allele from either parent Males inherit recessive allele from mother only; females inherit a recessive allele from both parents

Autosomal recessive trait Autosomal dominant trait Sex-linked trait (recessive on X chromosome) Are there carriers? Who can be a carrier? Yes, males or females No carriers possible since expression of trait is dominant Only females can be carriers Can males pass allele for trait to sons? daughters? Can pass to both Can pass to daughters, but not to sons Can females pass allele for trait to sons? daughters? Examples Albinsm, cystic fibrosis, phenylketonuria, Tay-Sachs disease, galactosemia Achondroplasia, Huntington’s disease, hypercholesterol-emia Colorblindness, hemophilia, Duchenne muscular dystrophy,

Chromosomal Disorders Chromosomal disorders lack an inheritance pattern because they are caused by “mistakes” that occur during meiosis. Sometimes a pair of homologous chromosomes can fail to separate resulting in gametes with the wrong number of chromosomes. This is called nondisjunction.

Down Syndrome: caused by trisomy, or three copies of chromosome #21

Klinefelter’s Syndrome: extra X chromosome(s) inherited.

Rhesus (Rh) factor is an inherited trait that refers to a specific protein found on the surface of red blood cells. If your blood has the protein, you're Rh positive — the most common Rh factor. If your blood lacks the protein, you're Rh negative.

Turner’s Syndrome: only one X chromosome is inherited.