Topic 11:

Introduction to Genetics

The Father of Genetics:

Gregor Mendel

CHROMOSOMES, GENES, & ALLELES

So you've found the one! Yay! Congratulations! And now that you're married and settled down, you decide it's time to start thinking about having a family. You start to wonder: "Will my child have my eyes or my partner's eyes?" "Will my child be tall like me or short like my partner?"

These questions and more can be found in the study of GENETICS! Typically during sexual reproduction. both partners contribute half of their genetic information (DNA) to their future child.

In humans, each parent contributes 23 chromosomes so that their child can have 23 homologous pairs of chromosomes aka 46 chromosomes in total in all of their body cells.

So why even mention that these chromosomes come in pairs, why is that important? Well, we pair our chromosomes when we count them because the chromosomes in a homologous pair are pretty much identical! How so?

The chromosomes found in that homologous pair both carry GENES (sections of DNA) that give instructions for the same TRAITS.

The locations of these genes on homologous chromosomes is called the locus or loci (plural)

These two genes, while they both provide instructions for a single trait, they don't necessarily provide the same instructions for that trait. The two genes may have slightly different sets of instructions.

The different varieties of a gene are called the ALLELES of that gene.

Example: Attached vs. Detached Earlobes

Both chromosomes in a homologous pair code contain a gene that codes for a trait that determines earlobe shape.

A person may have two genes for detached earlobes, or two genes for attached earlobes, or even one gene for detached earlobes and one for attached earlobes.

Both alleles give instructions regarding the same trait (earlobes), but they give different instructions on the form that trait should take on (detached vs. attached).

There are two possible alleles for this trait: an allele for detached earlobes and an allele for attached earlobes.

DOMINANT & RECESSIVE ALLELES

Another cool thing about alleles is that they all aren't on equal standing. Some have more 'power' and 'influence than others.

A DOMINANT ALLELE is one that is present in a pair of alleles, will always give the instruction that shows up in an individual's appearance. A RECESSIVE ALLELE is one that will give the instruction that will show up in an individual's appearance if it's paired with another recessive allele.

Example: Attached vs. Detached Earlobes

A pairing of two D (dominant) alleles will produce someone with detached earlobes. A pairing of two d alleles (recessive) will produce someone with attached earlobes.

So what happens when you have a pair of alleles one being the D (dominant) allele and the other being the d (recessive) allele?

A person with Dd alleles will have detached earlobes just like a person that has two D (DD) alleles. Even though that person has both the D and d alleles in their genetic makeup, they do not influence the person's appearance equally. The dominant allele overpowers the recessive one.

There are two possible alleles for this trait: an allele for detached earlobes (D) and an allele for attached earlobes (d).

The allele for detached earlobes (D) is dominant over the allele for attached earlobes (d).

The genes that make up an individual is called the GENOTYPE.

For example, the possible genotypes from our earlobe example wereDD, dd, and Dd.

The physical appearance of an individual is called the PHENOTYPE.

For example, the possible phenotypes from our earlobe example are only detached and attached earlobes.

An individual with identical alleles aka the same alleles for a trait is said to be HOMOZYGOUS for that trait. (Homo- means same.) A person with the genotype DD is homozygous dominant for detached earlobes. Another individual who has the genotype dd is homozygous recessive for attached earlobes.

Someone with different alleles for a trait is said to be HETEROZYGOUS for that trait. (Hetero- means different.) So a person with the genotype Dd is heterozygous for the earlobe trait.

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