Chapter 3 - Mendel's Principles

Monohybrid Crosses

Mendel studied single trait inheritance leading to discoveryy of 2 principles:

1. Principle of Dominance

2. Principle of Segregation

Principle of Dominance

Principle states that in a heterozygote, one allele may conceal the presence of another

• Heterzygote: orginsism with 2 different alleles for the same gene

• Dominant Allele: masks the expression of a different allele when present

• Recessive Allele: is masked by dominant allele

Principle of Segregation

In a heterozygote, two different alleles segregate from each other during the formation of gametes.

Each gamete receives only on of the 2 alleles

• due to pairing and subsequent separation of homologous chromosomes during Meiosis 1

Allele

• Allele is a different version of a gene

• Genes occupy specific locations (loci) on chromosomes

• each locus can have 2 or more alleles they code for variations of the same traits

One of a pair, or series, of alternative forms of a gene that occur at a given locus in a chromosome

Gene

A stretch of nucleotide pairs along a DNA molecule that contains coding information

• located on the loci of chromosome

• Each gene can be expressed in different forms called Alleles

  • nucleotide sequence (A, T, G, C) in a gene determines the amino acid sequence

    • DNA → gene → mRNA → protein → trait

Homozygous

• individual carries two identical alleles for a trait (e.g., DD or dd)

Heterozygous

individual carries two different alleles (e.g., Dd)

Dihybrid Crosses

Mendel experimented with 2 different traits establishing:

• Principle of Independent Assortment

Principle of Independent Assortment

• alleles of different genes assort independently of each other

• The rule of genetic transmission is based on the behavior of different pairs of chromosomes during meiosis 1