Extending Mendelian Genetics Chapter 7 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.