Biology

Fundamentals of Genetics

Gregor Mendel Father of genetics Monk Austrian Gardener Mathematician

Gregor Mendel

Trait A physical or behavioral characteristic Coded for by at least one gene

(discrete piece of DNA on one chromosome)

But Mendel didn’t know what it looked like

Garden Peas (Pisum sativum) and some Traits

Height long or short stem Pod color green or yellow Seed texture smooth or wrinkled Flower color purple or white

All peas had either one trait or the other, never something in between.

Mendel’s cross pollination Take the pollen from the anther of one

plant and transfer it to the stigma of another

This is sexual reproduction of flowers Each parent transfers half of its DNA

to the offspring

Dominant Traits If an organism has both genes for a

trait (Such as the gene for yellow peas and the gene for green peas), the one that is expressed or shown is the dominant gene (in this case, the pea was always yellow, never green).

Alleles Alleles are different forms of the same

gene. For example there is one gene for pea

color, but there is a yellow pea allele and a green pea allele

A Human Example There are two basic colors of eyes in

human beings: – Dark (brown, black and hazel) – Light (blue, green and grey)

Dark eyes are dominant over light eyes

Eye Color Continued

So, if a woman received the brown eye gene from her father and the blue eye gene from her mother, she would show the brown eye trait, because it dominates over the blue gene.

The blue gene is called “Recessive”, it is still in the woman’s DNA, but is not seen or expressed.

A Code for Genes

Mendel developed an easy code for genes. He used a capitol letter for the dominant gene and a lower case letter for the recessive gene.

Thus:

B stands for the dark (Brown) gene and…

b stands for the light (Blue) gene for

human eye color

Genotype and Phenotype Genotype is the actual alleles that an

organism possesses and is shown with the letters for the alleles. For example: BB, Bb and bb

Phenotype is the gene that is expressed, it is the trait that is seen in the organism. Examples are brown eyes and blue eyes.

Homozygous and Heterozygous

Homozygous organisms have both of the same alleles for a trait, either BB or bb

Heterozygous organisms have one of each allele, Bb (bB is the same thing so we list the dominant allele first)

Homozygous Dominant Two brown eye alleles Genotype is BB Phenotype is brown eyes

Homozygous Recessive Two blue eye alleles Genotype is bb Phenotype is blue eyes

Heterozygous One brown and one blue eye allele Genotype is Bb Phenotype is brown eyes (the

dominant phenotype is expressed when both alleles are present)

Mendel’s pea crosses P1 generation

– Purebreds

F1 generation– All dominant hybrids

F2 generation– 3:1 ratio

Predicting results of monohybrid crosses

Probabilities– The chance or likelihood that something

will happen a certain way– Example:

What is the probability that you will have a child with blue eyes?

Punnett squaresParent # 1 genotype

Parent # 2 genotype

Offspring #1 genotype

Offspring #2 genotype

Offspring #3 genotype

Offspring #4 genotype

Probabilities:

Dominant Phenotype: 4/4

Recessive Phenotype: 0/4

Heterozygous Genotype: 4/4

Homozygous Genotypes: 0/4

More Punnett squaresParent # 1 genotype

Parent # 2 genotype

Offspring #1 genotype

Offspring #2 genotype

Offspring #3 genotype

Offspring #4 genotype

Probabilities:

Dominant Phenotype: 3/4

Recessive Phenotype: 1/4

Heterozygous Genotype: 2/4

Homozygous Genotypes: 2/4

Homozygous Dominant: 1/4 Homozygous Recessive: 1/4

Incomplete Dominance Neither allele is expressed...exactly Example: Pink carnations

Codominance Both alleles are expressed (not blended) Example: Roan Horses

– Both parent hair colors are visible