1. Section 7.1: Chromosomes and Phenotype
Biology

2. 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?

3. 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

4. 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

6. 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

8. 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

9. 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

11. 7.2: Complex Patterns of Inheritance
Biology

12. 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?

13. 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

14. 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

16. Codominance
In which both traits are fully and separately expressed

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

20. 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

21. 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

23. 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