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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. 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
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
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 bothCan pass to daughters, but not to sons
Can females pass allele for trait to sons? daughters?
Can pass to both Can pass to both Can pass to both
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.
