Extending Mendelian Genetics Section 7.1: Chromosomes and Phenotype • As you learned last chapter, some traits depend upon dominant and recessive alleles. • However, we have learned post-Mendel that many other factors affect phenotype. • The expression of a trait is often determined by what chromosome a gene is on. Section 7.1: Chromosomes & Phenotypes • Remember there are two types of chromosomes: – Sex chromosomes – Autosomes • All of Mendel’s traits he studied were located on autosomes. (Most traits are!) Section 7.1: Chromosomes & Phenotypes • Some disorders are a result of autosomal genes. • Disorders are caused by either: – 2 recessive alleles – 1 dominant allele • Recessive allele disorders must have two copies of the gene. Section 7.1: Chromosomes & Phenotypes • Cystic Fibrosis is a disorder that affects lung function and is noted by the gene (cc). • Cc is a carrier of this disorder. • Carriers do not have the disease, but can pass the gene on. Section 7.1: Chromosomes & Phenotypes • Dominant genetic disorders are much less common. • Huntington’s disease is a disorder that attacks the nervous system. • If you have one copy of the gene, you have the disease. • This disease doesn’t occur until later in life, so people have already reproduced before they know they have the disease. Section 7.1: Chromosomes & Phenotypes • Genes located on the sex chromosomes are called sex linked genes. • The “Y” chromosome simply determines maleness, but there are other types of genes on the “X” chromosome. Section 7.1: Chromosomes & Phenotypes • Sex-linked traits are expressed differently because there is not always two copies of a gene. • Males, only have one chromosome that carries genes (X). • Therefore, for some disorders, a male only needs 1 copy of a gene. • This means males will show all recessive traits because there is no other allele to mask. • In females, their sex-linked traits act more like regular traits. Section 7.2: Complex Patterns of Inheritance • Although Mendel was correct in his findings, he could not explain all the variation within a population. • For example, in flowers Mendel found purple dominant to white, but there are hundreds of colors. • How often does this occur?? Section 7.2: Complex Patterns of Inheritance • Sometimes, alleles don’t completely mask other alleles. • In this cause, the heterozygous condition yields a third phenotype. • Examples: – cross red roses (RR) x white roses (WW) = pink rose (RW). – Cross a green betta fish (B1 B1) x steel betta fish (B2B2) = royal blue (B1B2) Section 7.2: Complex Patterns of Inheritance • Incomplete Dominance can be identified by the presence of a third phenotype that is a mixture of the other two. • Co-Dominance is like incomplete dominance in that there is a third phenotype, however, that third phenotype is both genes being expressed: – Red flower (RR) x white flower (WW)= flowers that are both red and white (RW) Section 7.2: Complex Patterns of Inheritance • One type of gene that we have studied already shows Co-Dominance: Blood type Polygenic Traits: • Traits produced by 2 or more genes are polygenic traits. • For example, human skin is controlled by 4 genes that interact and produce a range of colors. • Eye color is the same way! • Scientists also believe that not all genes for eye and skin color have been found. Polygenic Traits: • Some genes are epistatic. • These types of genes override any other gene for a given trait. • For example, albinism, is caused by one gene that overrides all other genes that produce pigments. Polygenic Traits: • Phenotype is usually a combination of genes and environment. – Identical twins that don’t look identical • Scientists have mapped the human genome. • They have built what is called a Karyotype • This is a picture of all of the chromosomes in a cell, • Karyotypes can show abnormalities is chromosomes shape or number.