Genetics Since Mendel

I. Independent Assortment

A. Monohybrid crosses• 1. Mendel’s 1st crosses using

the genes for one trait

B. Dihybrid crosses• 1. Mendel’s 2nd test, crossing

2 pure plants. Plant A had 2 dominant traits over Plant B who had homozygous recessive for both traits

• 2. He then crossed the offspring

Mendel wondered if alleles segregate independently. In other words, does the fact that the plant has a gene to be tall make it more likely to have a purple flower gene

I. Independent Assortment• C. The heterozygous offspring from the 1st crossed

showed that the genes for different traits segregate independently during gamete formation(sperm/egg– 1. This account for the wide variation between

organisms of the same species

Review Mendel’s Principles• 1. Inheritance is determined by individual units called

genes. Passed on from parent to offspring.• 2. With traits that have more than 1 form, some forms

of that gene are dominant while others are recessive. (Law of Dominance)

• 3. Sexually reproducing organisms, each individual gets a copy from each parent.(Law of individual assortment)

• 4. Alleles segregate independently from one another (Law of segregation)

Most genes are beyond Mendel’s work since not all genes show simple dominant and recessive alleles.

Most traits are determined by multiple alleles and

multiple genes

II. Incomplete Dominance • A. When progeny of 2 homozygous parents show an

intermediate phenotype this inheritance is called incomplete dominance. No allele is completely dominant (All Caps for cross)– 1. Example: Red flower plant crossed with a White

flower plant produced Pink flowered plants

III. Codominance• A. Both alleles contribute to the phenotype, the

appearance is a speckled animal– Ex. Ermitte chicken, Roan cow, Geckos, AB Blood

IV. Multiple Alleles • A. Mendel studied traits in pea plants where only

two alleles controlled them.• B. Many traits however, are controlled by more

than 2 alleles and are said to be multiple allele that produce more than 3 phenotypes– 1. Example: blood type (IA, IB & i)

V. Polygenic Inheritance• A. A trait that is produced by a combination of

genes is called polygenic inheritance– 1. Examples: Skin color (4 genes), eye color, height

VI. Linked Genes• A. Some genes on the same

chromosome stay together during assortment & move as a group. This group of genes are considered linked genes & are usually inherited together. – 1. The frequency of crossing-over

between any 2 linked alleles is proportional to the distance between them. • Farther apart more likely to separate

during crossing-over more recombination

VII. Genetics & the Environment

• A. Characteristics are determined by interaction between genes and the environment.– 1. Temperature can alter how genes can be expressed– 2. Diabetes, heart disease, cancer bipolar disorder all

have a genetic component with environmental influence. (nature vs. nuture)

Dihybrid cross:• In certain bacteria, an oval shape is dominant

over round and thick cell walls are dominant over thin. Cross a heterozygous oval, thick cell walled bacteria with a round, thin cell walled bacteria. Describe the phenotype of the offspring.