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  P 1 generation –Purebreds  F 1 generation –All dominant hybrids  F 2 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 squares Parent # 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 squares Parent # 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: 1/4Homozygous 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