Understanding Inheritance

Understanding Inheritance

Chromosomes, Genes, Alleles and all the rest!

Chromosomes

Inside of a nucleus are thread like structures called Chromosomes.

Cells contain chromosomes from both parents, organized in pairs

A gene is a section of a chromosome that holds genetic information for one trait.

Genes and Alleles

So, genes are a section of a chromosome responsible for one trait.

An Allele is a form of that gene that you get from one parent. They are for these same gene, but can differ in their varient.

Genotype and Phenotype

Whats the difference?

Phenotype

Is how a trait appears, or is expressed. For example, the color of your eye.

Genotype

The two alleles that control the phenotype of a trait. (The alleles that control how a trait appears).

Genotype and Phenotype

We cannot see a genes genotype. But we can understand the genotype by observing the phenotype.

The two alleles that control the eye color are the genotype. The brown eye is the phenotype.

Symbols for Genotypes

Scientists use symbols to represent the alleles in a genotype. Uppercase letters represent dominant alleles.

Lower case letters represent recessive alleles.

Homozygous (ho-muh-ZI-gus)

When two alleles of a gene are the same, its genotype is Homozygous

Heterozygous (He-tuh-roh-ZI-gus)

If the two alleles of a gene are different, its genotype is heterozygous

Punnett squares model possible genotypes of offspring based on the traits we inherit from our parents.

In this picture, the Y (capital) represents a dominant allele for yellow peas. a y (lowercase) represents

recessive.

Because this pea has inherited an allele for both traits from each parent, the offspring has a a ratio

of 3:1 chance of being yellow/green

Pedigrees (not like the dogs)

A Pedigree is a model that shows phenotypes of genetically related family members.

In the pedigree in this picture, three offspring have a train (attached earlobes) that their parents do not have.

How can the offspring have different traits to their parents?

In the case of the earlobes, the offspring inherited two recessive alleles from their parents genes. Their parents would have both inherited these from their own parents. However, each one also inherited a more dominant trait.

This means, even if your earlobes are unattached (dominant), you may still pass on a recessive genotype that you have, but just dont know about!

Incomplete Dominance and Codominance

Incomplete dominance is when traits of alleles are mixed (a red and a white parent mix to nmake a pink child)

Codominance is when we can observe the phenotype of both alleles at the same time

Genes and the environment

Some alleles are responsive to enironment! For example, some plants change color based on the acidity of the soil it grows in.