Section 7.1: Chromosomes and Phenotype Biology

Objectives How can genes on an autosomal chromosome affect phenotype? What dominant-recessive patterns of inheritance cause autosomal disorders? Describe the patterns of inheritance of sex-linked genes. How are sex-linked genes expressed as phenotypes?

Many Factors Affect Phenotype Specific chromosome on which gene is located Located on autosome or sex chromosome Different alleles on each chromosome Dominant or recessive allele

Disorders Of Automosomal Chromosomes Recessive Alleles: Two copies must be present for a person to have the disorder Often occurs in offspring of parents who are both heterozygotes Parents are “normal” Carriers: Does not show disease symptoms, but can pass on the disease-causing allele to offspring Example: Cystic Fibrosis Recessive disorder that affects the mucus glands

Disorders of Autosomal Chromosomes Dominant Alleles: Less common than recessive disorders Heterozygotes and homozygotes can present with the disease Two Reasons: Either dominant allele causes death, and it is not passed on Or, allele does not present itself until later in life, and it does get passed on

Sex Chromosomes Expression of genes on sex chromosomes differ from the expression of autosomal genes Sex-linked genes: located on sex chromosomes XX (female), XY (male) Females can only pass on an X Males can pass on X or Y X Chromosome has more influence over phenotype Has genes related to more than sex characteristics

Expression of Sex-Linked Genes Different pattern of expression than autosomal genes Males: XY One copy of each gene, therefore all of the genes are expressed Recessive alleles are expressed Females: XX X Chromosome Inactivation: one of the two X chromosomes is randomly turned off Randomly turned off independently in each cell Therefore, females are a patchwork of X chromosomes turned off and on

7.2: Complex Patterns of Inheritance Biology

Objectives How does phenotype depend on the interaction of alleles? Describe how many genes interact to produce one trait. How does the environment interact with genotype?

Phenotype & Alleles Not all genes work with a straight dominant-recessive relationship Some work with a RANGE of dominance In pea plants, the color of the flower was a simple dominant-recessive relationship This means that enough of a protein is made to make the flower purple, or not enough is made and the flower is white But, in many instances, a phenotype comes from more than one allele

Incomplete Dominance In which the heterozygous phenotype is somewhere between the two homozygous phenotypes The alleles are not completely dominant or recessive The different generations show different ratios of phenotypes

Codominance In which both traits are fully and separately expressed

Polygenic Traits Traits produced by two or more genes Skin color and eye color are two traits that are influenced by multiple genes

Epistasis In which one gene affects the expression of other genes In mice: One gene determines general color One gene affects the shading One gene affects whether spots appear One gene can overshadow all of the above

Labradors Black, chocolate, yellow Two genes affect coat color E gene: affects the presence of dark pigment in the coat B gene: affects degree of pigment presence

Environment & Genotype Phenotype is a mixture of genes and environment Example: If the gene for tallness is present, but not enough nutrition is received, then that person will not be as tall as someone with the same gene who received proper nutrition