Inheritance Unit I- Genetic Continuity Sections: 14-1, 14-2 Mr. Connors Biology 12 Human Chromosomes Recall that humans contain 23 pairs of chromosomes (46 total). Two of the 46 chromosomes are known as sex chromosomes, because they determine an individual’s sex. The remaining 44 chromosomes are known as autosomes. Autosomal inheritance typically involves pairs of genes, with gender being irrelevant to gene expression. Sex-linked inheritance involves pairs of genes on the X chromosome in the female, and a single gene on the X in the male. Gender is important in gene expression and must be considered a part of the phenotype. Chromosomal Linkage Morgan Drosophilia melanogaster XX (female) vs. XY (male) Sex-linkage: genes located on a sex chromosome Linked genes: genes located on the same chromosome that tend to be inherited together Human sex-linkage SRY gene: gene on Y chromosome that triggers the development of testes Fathers= pass X-linked alleles to all daughters only (but not to sons) Mothers= pass X-linked alleles to both sons & daughters Sex-Linked Disorders: Colour-blindness; Duchenne muscular dystropy (MD); hemophilia X-linked genes: alleles are carried on the x chromosome. When an allele is carried on the sex chromosome we call this…. Sex linkage: presence of a gene on a sex chromosome. If the trait is dominant we see it in males and females. If the trait is recessive, we will mostly see it in males….WHY? Sex-Linked Genes • The X chromosome and the Y chromosomes determine sex. • Genes located on these chromosomes are called sex-linked genes. • More than 100 sex-linked genetic disorders have now been mapped to the X chromosome. The Y chromosome is much smaller than the X chromosome and appears to contain only a few genes. Why are sex-linked disorders more common in males than in females? – For a recessive allele to be expressed in females, there must be two copies of the allele, one on each of the two X chromosomes. – Males have just one X chromosome. Thus, all X-linked alleles are expressed in males, even if they are recessive. Colourblindness – Three human genes associated with colour vision are located on the X chromosome. – In males, a defective version of any one of these genes produces colourblindness. – Colour Blindness and Baldness in People Colourblindness.. X-linked recessive trait The allele for colorblindness is carried on the x-chromosome. It is recessive. Do the following cross…XBXb x Xby (a non-colourblind female who is a carrier and a colourblind male) B = regular colour b = colourblind What will the children be? Remember… If the trait is carried on the x- chromosome, there is NO allele for the trait on the y chromosome. Tell me the genotypes and phenotypes that will result in the following crosses. (R =normal vision, r = colorblind) XRXr x Xry XrXr x XRy XRXR x Xry Hemophilia • The X chromosome also carries genes that help control blood clotting. A recessive allele in either of these two genes may produce hemophilia. • In hemophilia, a protein necessary for normal blood clotting is missing. • Hemophiliacs can bleed to death from cuts and may suffer internal bleeding if bruised. Duchenne Muscular Dystrophy • Duchenne muscular dystrophy is a sex-linked disorder that results in the weakening and loss of skeletal muscle. • It is caused by a defective version of the gene that codes for a muscle protein. Hormonal & Environmental Factors Environmental Factors might cause a change in the expression of some of the genetic information in an organism. Examples: The effect of temperature on fur colour on Siamese cats. The effect of temperature on Drosophila wing development. Pedigree Analysis A pedigree is a diagram of family relationships that uses symbols to represent people and lines to represent genetic relationships. These diagrams make it easier to visualize relationships within families, particularly large extended families. Pedigrees are often used to determine the mode of inheritance (dominant, recessive, etc.) of genetic diseases. A sample pedigree is below.