Day 31 Father Mendel Intro to Genetics

Gregor Mendel is known as the father of genetics

-Mendel spent years working on genetics experiments with pea plants

-He is the one that discovered how traits are shared between parents in their offspring

What are Genes?

§Genes are sequences of nucleotides at specific locations on chromosomes

–Inheritance is the process by which the characteristics of individuals are passed to their offspring

–A gene is a unit of heredity that encodes information needed to produce proteins, cells, and entire organisms

–Genes comprise segments of DNA ranging from a few hundred to many thousands of nucleotides in length

–The location of a gene on a chromosome is called its locus (plural, loci)

–Homologous chromosomes carry the same kinds of genes for the same characteristics

–Genes for the same characteristic are found at the same loci on both homologous chromosomes

–Genes for a characteristic found on homologous chromosomes may not be identical

–Alternative versions of genes found at the same gene locus are called alleles

§Mutations are the source of alleles

–Alleles arise as mutations in the nucleotide sequence in genes

–If a mutation occurs in the cells that become sperm or eggs, it can be passed on from parent to offspring

-Every cell carries two of every allele (Because you are diploid)

-The two alleles can either be the same or different

-If the two alleles are the same then the organism is homozygous for that trait

-If the two alleles are different then that organism is heterozygous for that trait

When we talk about breeding

-True-breeding (homozygous) - is like a pure breed. A true breeding pair can only make that trait. In other words they both have the same allele

-Hybrid (heterozygous) - When a breeding pair both have different alleles for a trait

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So How did Mendel discover genetics?

-Mendel was very lucky he chose the pea plant. It was the perfect organism to test

-The Stamen-Male part of the flower

-The Carpel- Female part of the flower

In nature pea plants usually self-fertilize

-But Mendel would cut the stamen off of some flowers and the carpel off of others so they could not self-fertilize.

-By doing this Mendel was able to cross-fertilize the flowers and choose which plants were being fertilized.

Another aspect to Mendel's success was he would study one trait at a time

-Remember that a good science experiment only has one variable being tested

-Mendel would breed two plants that had one characteristic different and then document what would happen with their offspring

-For example he would breed a white pea plant with a purple pea plant and see what color their offspring were

Offspring terminology

-The first generation that you breed in your experiment is called the P generation (Parent)

-Their offspring are called the F1 generation (Filial)

-Their offspring is known as the F2 generation and so on

When Mendel cross breed a purple pea plant with a white pea plant all of the offspring (F1 generation) were purple. What happened to the white gene?

Afterwards Mendel allowed the F1 generation to self fertilize and the F2 generation ended up being 3/4 purple and 1/4 white. This is know as a ratio of 3:1

-This meant that the white trait did not disappear it was only hidden for a whole generation

Now Mendel was on to something.

-Mendel then let the F2 generation self fertilize

-The white F2 generation plants only produced white offspring. They never produced purple offspring. This meant that the white F2 plants were "true-breeding"

-From the F2 generation 1/3 of the purple plants only produced purple F3 plants

-2/3 of the purple plants produced purple and white F3 plants in a 3:1 ratio

After Mendel took careful notes he had to come up with some new hypotheses and conclusions

-He concluded that in the F2 generation there was 1/4 true-breeding purple plants, 1/2 hybrids, and 1/4 true-breeding white plants

The Rules of Dominant and Recessive traits

1. Each trait is determined by pairs of genes; each organism has two alleles for each gene, one on each homologous chromosome

-True-breeding white-flowered plants have different alleles than true-breeding purple-flowered plants

2. When two different alleles are present in an organism, the DOMINANT allele may make the RECESSIVE allele, even though the recessive allele is still present

-In edible peas the purple-flower trait is dominant to the white-flower trait

3. The pairs of alleles on homologous chromosomes separate, or segregate, from each other during meiosis, which is known as Mendel's law of segregation

4. Chance determines which allele is included in a given gamete--because homologous chromosomes separate at random during meiosis; the distribution of alleles to the gametes is also random

5. True-breeding organisms have two copies of the same allele for a given gene and are homozygous for that gene; hybrid organisms have two different alleles for a given gene and are heterozygous for that gene

-A parent that is homozygous can only make one type of gamete for that trait

-A parent that is heterozygous can make two different gametes for that trait

The genes you have are called your genotype

The genes that are expressed (shown) are your phenotype

When we write a genotype we use a capital letter for the dominant trait and a lowercase letter for the recessive trait. The letter chosen is usually the first letter of the dominant trait

-For example when talking about purple and white pea plants

-Purple is the dominant trait and white is recessive

-So the possible genotypes for flower color are:

PP= Homozygous dominant purple

pp= Homozygous recessive white

Pp= Heterozygous purple