Genetics: The Study of Biological Heredity

Do Now:

If you flip a coin twice, what is the probability it will come up heads twice?

What is the probability it will come up heads once and tails once?

What is the probability it will come up tails twice?

Basic Definitions – not on notes

Genetics is the study of heredity in living organisms.

Heredity is the transmission of traits or characteristics from parents to their offspring

Chromosomes = DNA

Genes are found on chromosomes

A gene is the instructions for making a single protein

DNA is a CODE

DNA is a chemical code that carries information.

Genes are pieces of DNA

Chromosomes are DNA

Chromosomes are long pieces of DNA that contain many genes.

Humans have 46 chromosomes

Homologous Chromosomes

• Every cell contains two copies of each chromosome.

To put it another way, autosomes come in pairs. These two chromosomes are calledHOMOLOGUS CHROMOSOMES.

Specific genes are found in theSame location on each homolugousChromosome.

Consider me for example…

Genes for eye color are found atThe same location on both

Chromosomes. The gene for brown Eyes I got from my dad is in exactly

The same location on one chromosomeAs the gene for blue eyes I got from my

Mom on the other homologous Chromosome.

Diploid and Haploid Cells

• Cells are considered diploid when they have 2 copies of each chromosome. Most of your body’s cells are diploid.

• Cells are haploid if they have only one copy of each chromosome pair. This is the case for gametes, like sperm and egg cells, used for reproduction.

Diploid vs. Haploid

Normal Karyotype

• Humans almost always have 23 pairs of homologous chromosomes.

• Is this karyotype of a male or female?

Basic Genetics

You, like most animals, are diploid.

To reproduce, you make haploid cells (sperm & egg) through meiosis.

During reproduction, haploid gametes from two parents combine (fertilization) to make a new diploid organism.

Thus, only half of an organism’s genetic information is passed on to its offspring

Genes, Alleles, and Traits

A trait is a characteristic. For example, brown eye color is a trait most humans have.

A gene is a part of a chromosome that carries information for one trait. For example, humans have one major eye color gene.

An allele is a certain form of a gene. In humans, the most common eye color alleles are brown and blue.

Meiosis

Meiosis is cell division that makes haploid gametes.

The chromosome number is reduced by ½.

Only 1 of each homologous pair of chromosomes gets passed on to the offspring.

Follow the alleles

The 2 chromosomes shown are a homologous pair.

The chromosomes carry different genes.

What % of the haploid cells have the red allele?

What % have the blue allele?

Probability

Since there is an equal chance of each chromosome ending up in an offspring, it’s similar to flipping a coin.

Make A Prediction

Please Write:

If the two pea plants shown were crossed (mated), what % of the offspring do you predict

will be purple? Why?

Anyone Predict 100% Purple?

• The first plant was PURE BREEDING for purple flowers – meaning when it was SELF-FERTILIZED all of the offspring had purple flowers.

• The second plant was pure breeding for white flowers.

• Which flower color would you describe as DOMINANT in pea plants?

Gregor Mendel1822-1884

• Monk from Austria• Studied statistics at the

University of Vienna• Garden enthusiast• High school teacher • The importance of his

discoveries was only understood and appreciated years after his death

Mendel“Father of Genetics”

Mendel’s Experiments

• Make another prediction:– If two of the offspring from the first cross (the

ones that were all purple), what % of the offspring would be purple? Why?

A Word About Plants (Angiosperms = flowering plants)

From Seed to Seedling

Back to the Genetics• Approximatly 75% of the offspring will have

purple flowers.

Mendel’s study of probability and statistics led him to hypothesize that each plant had two “factors” controlling flower color, one from each parent. How did he know???

Probability• If you flip a coin, what is the probability it will

come up heads?• What is that probability expressed as a

decimal?• If something has a probability of 1.0 will it

happen?• You just flipped a coin 4 times and it came up

tails all 4 tries. What is the probability it will come up tails if you flip it again?

• If you flip a coin twice, what is the probability it will come up heads both times?

Classical Genetics is Based on Probability!

• You have 2 copies of each gene, but only one copy or the other is passed on to your offspring.

• Which copy of the gene that gets passed on is random.

• This is exactly what was happening in the pea plants – see if you can come up with an explanation tonight!