Download Section 7.1: Chromosomes and Phenotype

Section 7.1:
Chromosomes and
Phenotype
Biology
Objectives
1. How can genes on an autosomal chromosome affect
phenotype?
2. What dominant-recessive patterns of inheritance cause
autosomal disorders?
3. Describe the patterns of inheritance of sex-linked genes.
4. 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
1. How does phenotype depend on the interaction of
alleles?
2. Describe how many genes interact to produce one trait.
3. 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