Mendel’s Legacy

Genetics is everywhere these days – and it will continue as a dominant force in biology and society for decades to come.

Wouldn’t it be nice if people understood it better?

The Fundamental Question

What is the relationship between genes (genotype) and observable characteristics (phenotype)?

The answer?

Phenotype = Genotype + Environment.

Genes and Environment Determine Phenotype

“Phenotype = Genotype + Environment” –

We are not just our genes.

Dangerously Ahead of the Game - A Eugenics Exhibit at the 1920 Kansas State Fair

Will History be Repeated?

A Mendelian Genetic Primer

Genes come in pairs that separate in the formation of gametes.

The members of the pair may be identical (homozygous) or non-identical (heterozygous).

Each form of a particular gene is an allele.

A Mendelian Genetic Primer

One allele is dominant over another (or so Mendel believed).

Only two alleles of a given gene are possible in an individual although many alleles of a gene are possible within a population.

Genes Exist at Physical Locations on Chromosomes

Mendel’s Method

Mendel’s Monohybrid Cross – P to F1

Mendel’s Monohybrid Cross – F1 to F2

Mendel’s Results from MonohybridCrosses

The Explanation – there are two alleles for every trait, one dominant over the other. The alleles separate at meiosis so that half the gametes contain one type of allele, and the other half contain the other type. (Principle of Segregation)

Principle of Segregation

The principle of segregation is explained by the behavior of homologous chromosomes at meiosis.

Segregation

A Consequence of Dominance – Two Different Genotypes Can Produce the Same Phenotype

What Works for Peas Also Works for Humans

An albino

In the cross Aa x Aa, where A is a dominant allele for wild type (standard) pigmentation and a is a recessive allele for no pigmentation (albinism), ¾ of offspring will be wild type and ¼ will be albino.

Are Two Different Characters Like Color and Shape Inherited Together or Independently?

Mendel performed dihybrid crosses to find out.

A Dihybrid Cross Illustrated Though a Punnett Square

The question’s answer?

Different characters are inherited independently (The Principle of Independent Assortment)

Principle of Independent Assortment

The principle of independent assortment is explained by the behavior of homologous chromosomes at meiosis.

The alignment of one pair of homologs is independent of any other.

A cross between two carriers for deafness and albinism.

This is the same 9:3:3:1 ratio seen for the Mendelian cross involving pea color and shape.

What Works for Peas Also Works for Humans

Some Alleles Are Related Through Partial Dominance

Dominance relationships may differ, but the Principle of Segregation remains the same.

One Allele Can Influence Many Traits

This is known as pleiotropy

Anemia, infections, weakness, impaired growth, liver and spleen failure, death.

Traits (phenotypes) associated with the sickle cell allele.

Traits Often are Influenced by Many Genes

This is polygenic inheritance.

Polygenic traits vary continuously.