Section 7.1: Chromosomes and

PhenotypeBiology

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

Click icon to add pictureClick icon to add picture

Click icon to add picture

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